Snapping 232

[errolw98]

Hello all,

I am fairly new to RC Aircraft and wanted to get into pattern and aerobatics. I bought Real Flight G2 and spent about 200 hours flying everything in the library before deciding on a CAP 232 as my starter plane. Yes, I am an "A" type.
I bought the Hanger 9 Sport CAP 232 .40 ARF, OS 46FX with props from 10 x 7 to 12 x 4, and a JR 8103 and put it all together following the instructions the best I could. I do have extensive background in precision crafting of formula race car parts so I am pretty well suited to the task.
After setting the control throws to Hanger 9 recomendations and using my Great Planes CG Machine to insure the CG was right it was time for the first flight. I have had a ball so far and after a couple harrowing first flight saves, reduced the elevator throw so the CAP won't turn over on full elevator input. About 5/8 of an inch is all she will take before snapping. I have moved the CG around fore and aft about 1/2 of an inch each way with very little change. Forward weight seem marginally better but slower responding.
I did not get this charicteristic from the Great Planes CAP 232 model in RF G2 untill I really boosted the Elevator throw. Is the GP 232 that much better? do I have a seriuos assembly problem that I have overlooked?

My question is this:
What causes one wing to stall and snap a plane over so quickly with the input of the elevator and other than reducing throws, how can this be eliminated or reduced?

[Mike Taylor]

If it always is the same wing that stalls first, look for a warp in the wing...

[Dave Robelen]

Hi,
First, Ignore the stall/spin/snap characteristics in the simulators. The programming would be far too complex toproperly model this area of flight.
The cause of the model stalling more completely on one side first may be from any one of several reasons. Minute differences in the wing twist, small differences in the leading edge contour, ailerons with dissimilar gaps, covering tighter one one side-- The list can be extensive. Almost all airplanes where the wing can be fully stalled will "drop a wing" at this point.
Good luck with the 232.
Regards, Dave

[errolw98]

Mike and Dave both,
Thanks for your input to my problem. I can tell now that this subject has many variables. I also understand that the G2 simulator, while being in my opinion very good, can't reproduce all parameters of flight.
That being said, I should point out that this snap does not seem to follow a particular wing but rather in level flight will stall randomly one wing or the other on maximum elevator throw with Up elevator input or when inverted with down elevator input. In a gentle loop the plane does not seem to favor one wing or the other as if there were a side to side weight problem. Wind dirrection, Roll attitude and Airspeed seem to influence it more.
I was reading a review of a Global Freestyle (RC Universe) that displayed the same charicteristics when entering a popup or wall or at the bottom of a loop at higher airspeeds. The reviewer had the good fortune of an IMAC champion as a test pilot partner. They mixed in 60% negative flap or spoiler with positive elevator input, which I have the capability of doing, that seemed to calm it down a lot. Have you ever tried such a remedy? Any other Ideas?

Thanks,
Bud Wilkinson

[Dave Robelen]

Hi Bud,
I too have flown a 232, and found the desire to snap under high loads a rather troublesome trait. Where I would normally pull a model through the tight corners of a square loop, the 232 would roll abruptly at the first corner. The one I was flying was a bit overweight, and the tip airfoil was on the thin and sharp side. I was pleased that it was not my model and could hand it back to the owner, who does not like to fly aerobatics anyhow :rolleyes:
Having the ability to split the controls and mix in flaps would be a big help in getting the wing to stall near the center first. The one I was flying did not have that capability.
Cheers, Dave

[errolw98]

Dave,
Thanks for the reply. Your experience with the 232 is very near to mine.
I plan to finally explore the mixing capabilities of my JR 8103 tommorrow morning. I could either be really happy or really sad in the afternoon. I have already set up the radio and will try it at a high altitude with small inputs first. The article stated that negative flap or spoiler that they added was contrary to most curent logic. I suppose I will have to try both to find out what works best.
Just to make clear in my mind, how does the addition of flaperon help shift the stall point to the center of the wing rather than the tips if the aileron is used for this function?

Bud Wilkinson

[Dave Robelen]

Hello Bud,
I mistakenly assumed you had seperate flaps inboard of the ailerons. When flaps of this sort are deflected to increase the lift, the area ahead of the flap assumes a higher angle of attack, and that portion of the wing stalls first. If I were trying to achieve this result by mixing elevator and ailerons, I would raise the ailerons slightly with up elevator creating a modest washout effect. My problem with working the problem this way is the decrease in lift that results from the deflected aileron. I was tempted to put some sharp edged stall strips on the inboard leading edge ala full scale lightplane practice, but I did not have access to the model for more experiments.
Regards, Dave

[errolw98]

Dave,
That is something I will have to research (Leading edge stall strips). I have the radio set now for raising ailerons with up elevator and will try that.
I do a lot of Aero work with our Toyota Atlantic car and had a thought of putting upside down Gureny flaps on the outer ailerons trailing edge to increase lift on the outer tips of the wing. We put them on facing up to increase downforce, on this aplication I would put them facing down to increase lift. Size and length will have to be worked out. An interesting experiment though.

Bud Wilkinson

[Dave Robelen]

Hello Bud,
The stall strips would be used to control the local stall pattern. They have a very small effect on the lift prior to the stall. The outboard flap would load up the tip and likely cause it to stall even earlier than the clean wing. It would also act as a spoiler for any inverted maneuvers. There is at least on more option, and that would be to blend a more rounded, thicker LE on the outer wing panel. This is all assuming that you have sealed the aileron H/L.
Regards, Dave

[errolw98]

Well I think I have found the definitive answer to my snapping H9 Cap 232.The symptoms were that as soon as I
boosted the elevator input to a value over about 5/8 of an inch either up or down, the plane would snap roll out of it's
intended line. Air speed had some but little effect on the symptom. Also, I found that the direction of travel in verticle,
(i.e. Flying straight and level and trying to pull up or flying a 90 degree up or down vertical line) made no difference.
Very unnearving when trying to save a low altitude vertical decent as the plane would just snap out of the command to pull up. The same was true for inverted flight with down elevator commands.

I have tried mixing both Spoilerons and Flaperons with elevator input in varying degrees. I also tried adding stall
strips to the leading edge from the fusealage out for anywhere from 1/3 to 2/3 of the wing length. I tried varing the CG both
fore and aft. Fore CG did help slow the snap to a point untill the plane became less responsive and hard to fly but did not
cure the problem.

With this experimentation done at the field, I went home to my Real Flight G2 simulator and began experimenting with things that you just can not do in real life. After exhaustive research on G2 I found that only three things had a positive
affect on the snapping.
The first is unrealistic in that the wing position was moved back about three to four inches without changing CG.
The second was to lighten the plane to 3 or less pounds. This would be difficult to achive.
The third is to increase the wing area. This coupled with the 12 degrees of elevator setting as the Cap 232 comes delivered
on G2 indicate that the plane in its current design will not take the elevator at 3D levels.

The bottom line as it turns out is that the H9 CAP 232 has 594 Sq. In of area and weighs around 7.5 Lbs wet for a
rather high wing loading of around 29 Oz per Sq. Ft. When the G2 plane was set around 12.5 Oz per Sq. Ft. or less, Using a
lighter plane or a larger wing the symptoms vannished and CG was able to be moved aft and elevator surfaces used at 45
degrees with complete stability at any elevator throw setting. This along with the evidence that my new Flip 3D has around
11.5 Oz per Ft wing loading and has no such bad habbit should lead me in a positive dirrection with the CAP.

I know that the G2 simulator is not a real world circumstance and that the results will not necessarily be repeatable. But at least I think I have an answer that gives me a dirrection. If I want the plane to perform at 3D elevator levels without an unerving snaping out of the command I think the guidelines of less wing loading will apply. Now to decide how much wing to build and how much weight to loose. This exercise may help others that are looking for the same answer to countless other posts I have read over the past two weeks on the subject of Snapping and Wing Tip Stall for many models.

WING LOADING, WING LOADING, WING LOADING. Keep it as low as possible.

Let me know if anyone else has any other thoughts or if I am completely out to lunch.

Bud Wilkinson

[Dave Robelen]

Hi Bud,
It sounds like you are on track. It is a definite shame that so many of the scale ARF aerobatic models end up too heavy to perform even a basic routine with safety.
The stall you are encountering originates at the LE due to heavy spanwise flow on the tapered wing, so it is unlikely that any TE manipulation would be of help.
There is a LE modification developed by NASA where the outer 1/3 of the wing LE is fitted with a glove that extends the chord, lowers the LE radius and blends into the upper and lower surface. This works only one way, so it would present still more problems for the negative G maneuvers. While the wing loading is a major player, the taper rate is responsible for some of this nonsense as well. Rather recently I had the chance to log time on a large scale Stadaucher, and I found much the same result when I went for a square loop. The tip would drop right at the moment of peak loading, spoiling the maneuver. In this case, the model has a rather moderate wing loading, but the LE is fairly sharp, and the wing has a rapid taper.
On the other hand, I fly an Ultra Stick 120 with a Zenoah G26, and the rectangular wing with a fat LE lets me fly really tight corners. In fact, I had to rework the vertical tail and rudder to get acceptable positive snaps with full inputs.
Just for kicks, how about putting a rectangular wing with a NACA 0018 airfoil on the 232 in your G2 program and see how it behaves. In real life it would look some kind of wierd, but I bet it would clean up the unwanted snapping.
Regards, Dave

[errolw98]

Dave thanks for your further input. I did not try modifying the wing other than changing the dihedral from positive 2 degrees through neg 2 degrees for kicks and the entire range in there. Also I flattened the taper on the LE with some help. I didn't go further due to stumbling on to the wing loading.
I will end up building a different wing for the CAP at some point without regard to what it looks like. I just want it to fly properly.
Your input will lead me to experiment with wing design on G2 for another 2 or 3 nights before setteling on the final design.
Originally I was going to loosly model it after my Flip's wing but now I will try several iterations of other designs before construction begins. One other thing that helped on G2 was moving the wing back on the fuse. Do you think this is a true scenario or was it masking the real problem?
Your point about a lot of "3D" ARFs coming out way too heavy for the wing design I belive to be true. Just today in my LHS I heard of a new Funtana "Tip Stalling" and going in hard. 3rd flight. Shame if you ask me.
One other observation, as I flew my CAP yesterday. It was the coolest weather we have had here in So. Oregon since first flying the CAP. 35 degrees cooler. No other changes were made to the plane and it would take even less elevator to bring on the snap. Square loops are fairly easy and unless I used low rates and made large square corners and extended the Vert and Horiz lines for appearance, they were snapped out of on the first corner.
Does this make sense with heavier air? Thats all I can figure.

Thanks again for your input. I will let you know how it flies with a different wing.

Bud Wilkinson

[Dave Robelen]

Hi Bud,
That is a very interesting observation about the model snapping easier in cooler air. There must be so many interacting things going on that it would tough to pin that down. From a purely aerodynamic viewpoint, the angle of attack for stall is independent of air density within reason.
As far as the shifting of the wing position on the sim, that sounds like a bogus result. Did you move the CG aft by the same anount? If not, the G2 would think you had a very noseheavy model.
There are an abundance of good role models n the earlier pattern models from the sixties through the eighties. Most had tapered wings, and all could fly nice tight pitch corners without unsanitary behaviour. A common design feature was a fairly thick airfoil, often around 18%, and the leading edge was usually well rounded. Probably just as significant was a reasonably low wing loading.
The trend I was observing with the current crop of models is a tendency for the scale designs to be overbuilt and underpowered. Considering that the full scale counterparts will snap with extreme ease, it may not be in our best interests to model them too closely.
I will confess that I prefer flying precision areobatics to the more abrupt "3D" style of maneuvering. This is what causes me to be a bit critical of many of these models. They can be thrown around with ease, and with ample thrust will hang on the prop, but many of the maneuvers are post-stall thrashing that does not involve a high level of precision.
Take care, Dave

[errolw98]

Dave,
Once again you make me think. That is good. When I moved the wing back in the sim I did not move the CG. I wanted it to be a pure wing movement only. I wonder now if I fooled myself by not moving the CG as well. It would seem as if I have. The idea came from a Canadair Regional Jet that I fly on frequently and with the engines on the rear, I have no idea where it's CG is. The wing however is substantialy rearset, about 2/3 down the Fuse.
The air density thing was just an observation. Probably doesn't mean anything. Just the conditions of the day.
When you mention 18 % with regards to the airfoil thicknes, what is it 18% of? Sorry I am kind of new to model wings and lift as opposed to downforce. I know my Flip is twice as thick and very rounded on the LE as well as not having any taper on the LE.
I have already resigned myself to the fact that the CAP in the 40 size is not a 3D aircraft and still fly it for the pattern work that it is actually good at.
Even in pattern type flying, it would be nice to give the model a good amount of elevator without worrying about saving it's life if that input comes on a downline at too low an altitude for much of a mistake, say the final corner of a square loop.
As for loosing weight from the plane. I dont really see how to do it without a complete removal of the covering and some stratigic surgery. I don't think the average consumer of such an aircraft should have to do that.
This brings me to the point you made about underpowered ARFs. I wonder how the replacement of the current OS 46 FX with a 60 something swinging a larger diameter prop would affect the overall planes handling. First it would seem to increase weight. Also in the G2 sim the performance difference is small if not undetectable when flying.
Anyway these are questions for another day. I have spent two days on my roof stripping it and must finnish with construction ASAP so I can fly. If I can stand up.

Bud Wilkinson, DSTP Motorsports.

[Dave Robelen]

Hello Bud,
I apologize for tossing numbers around without setting the background. The percentage thickness of a wing is determined by dividing the thickness by the width. So, a 10" wide wing 1.8" thick would have an 18% thick airfoil. Assuming the usual distribution of curves, this would lead to a rather large radius on the leading edge which will resist stalling rather well in larger models. The fact that a wing this thick has a fairly high cruise drag is why they are reserved for powerful aerobats and bombers that need large amounts of lift.
The placement of the CG is always related to the wing and it's center of lift. On a wing with no taper or sweep, the CG will be between 25-35% back from the leading edge (the percentage being the portion of the wings chord).
On your twin engine jet, the heavy rearset engines would tend to move the CG of the airplane rearward, so the wing is set to accomodate that. In addition, if the wing has sweep, the CG measurement is taken midway between the fuselage centerline and wingtip which moves the point still further aft.
In the G2 program, when you move the wing, move the CG the same amount to see the effect of a single change.
Changing from a very powerful .46 to a more mildly tuned .60 would probably do pretty much as the program implies, not much difference. Pretty much all of the modern engines are designed to produce their peak horsepower at high rpm's with modest torque, so the prop might not increase all that much.
The other issue with changing to a larger engine and prop (aside from weight issues) is the increase in torque and slipstream effects. These could aggravate the sensitive snapping tendency.
Good luck with your CAP tuning project, I am waiting to see if you can whip this thing.
Regards, Dave

[errolw98]

Dave,

Thanks again for the input and info. I have a priority with my roof right now. Spent 5, 12 - 13 hr days so far with just me and my wife. Coming together nicely though. Once done, I will have more time to experiment. I will post once I think I have found the definitive cure and get your input then prior to building whatever that leads me to. I will fix it or crash it trying, darn it.

Bud Wilkinson, DSTP Motorsports

[errolw98]

Well, with my roof complete and time on my hands, I constructed a new wing for my H9 Cap 232. The original wing was a 60 in span quad taper design with 596 Sq in of wing area. The new wing was based loosely on the wing from a Flip 3D as far as construction. At 57.5 inches of span and 18 inches of length and 3 inches of depth at the thickest, it was masive in comparison. Stiffer and lighter as well. No taper was utilized and only a slight 3/8" dihiedral and 1/4"sweep with 5 of the 18 inches being Alieron. The wing area figured uot to about 1007.5 Sq inches at aprox 19% chord thickness.
The plane will now take all the elevator you want to give it and is only limited by the elevator hitting the control rod for the rudder. no hint of a snap or any bad bevavior. I placed the LE at the same point as the original wing and flew it at the Mean Airodynamic Chord 28% point. just behind the original CG point.
Scale looks are cool, but for me, I would rather have a confidence inspiring aircraft instead of one that you have to catch all the time.
The only detraction at all is that in 3D style flying, which it does quite well nw, I feel the wing is a bit large at the tips as snap rolls are more barrel rolls and an inverted flat spin has a harder time rotating with authority at the same rudder throw. still working out the proper throws for this wing. Maybe more rudder is all that is required.
It does the wall, blender, waterfall, tight inside and outside loops, harier rolls and harier landings and more, effortlessly. All of the pattern manuvers are still very crisp as well with less speed required. The only thing that was at all dificult was hovering I think due to the now inadequit rudder and elevator throws and perhaps a more rearward CG to get more weight on the tail will be required.
It is like having a new plane that is fun to fly. wing loading, wing loading, wing loading.

Thanks for all the input and help.

Bud Wilkinson

[Dave Robelen]

Hi Bud,
Thanks for sharing the inspiring success story. The flight qualities you describe very closely track those of my large Ultra Stick 120. Same big, blunt wing with a fat LE radius. As you say, the looks of a sharp taper are cool, but the behaviour ain't. A good portion of the difference between our models and the full scale aerobats is the Reynold number. Their tip numbers at the lowest speed are still way above what we reach. As you ar obseving, when you slow the beast down ot is hard to get enough rudder, let alone too much.
Regards, Dave

[mithrandir]

Well.. I tend to think that most CAPS are ill handling coupling steaming piles of buffalo *****.. other then that, they are OK.. LOL... but take a look at this........ http://www.rcuniverse.com/forum/upfiles/21727/Lh18833.jpg

And in this size class.. this GSP KATANA (Chief Aircraft too) is the funnest plane I have had in years!!!

http://www.rcuniverse.com/forum/m_1050950/mpage_17/key_katana/tm.htm

OK.. a little explanation on the zig zag tape...
(I am an engineer for a UAV company, and spent a good number of years in RC sailplanes, I have a bit of experiance with aero stuff, but I am not an AERODYNAMICIST)

Without the tape, the plane would rock left and right in roll whilst harriering, at about 2 Hz. This is more then likely a result of a "UNSTEADY" flow condition. As you know, at our speeds and sizes, laminer flow tends to persist, and at least the boundary layer is not very robust. (The thin layer of air immediatly against the wing surface is moving slow, has very little enegy, it will seperate easilly) A typical "FIX" might be to transition the flow from laminer to turbulent forward of the trouble area.(Ahead of the adverse pressure gradiant, or ahead of the minimum pressure point). The problem with this is in the case of a sailplane, the minimum pressure point will travel fore and aft according to the AOA. Turbulent Flow = Drag, seperated Flow = more drag then turbulent!! Fortunately we (alcohol burners) are grossly overpowered, care not for high speed or fuel economy!!! To visualize the difference between laminar and turbulent, look at the smoke that rises from a cigarette placed in an ash tray in a calm room. The smoke initially rises in a nice smooth column, laminar. Then it abruptly changes to an unorganized disrupted "Puffy" looking pattern, turbulent.
Back to the unsteady flow thing, If we were to acquire a "Deep Stall" flight attitude in perfect balance, we would have no wing rocking. But, one little cricket farts, that micro burst of flatulance disturbes our perfectly smooth air that we fly in, maybe it causes a bit of yaw to occur, and the cycle begins. One wing is moving a bit faster then the other, and the nature of the flow over each wing is suddenly different. The location of seperation from left wing to right wing is no longer the same, could only be different by say .050" on the first cycle, but... this difference is enough to cause a slightly larger oscillation in the other direction, and each cycle the difference in the flow field at any given point in time between the left and right wings continues to diverge. This distubance could also be not from a yaw upset, but maybe a roll upset. At high alpha, high roll rates will effectively increase aoa of one wing, and reduce aoa of the other, and rarely is there ever a purely roll or yaw upset, usually one couples into the other. (highly 3 dimensional flow is a real **** to predict...) A "turbulator" very close to the leading edge would cause the air to transition from laminar to turbulent, and this transition might create a boundary layer that will stay attached further along the wings cord, but prob'ly not enough. (Actually the vibration of the engine may be causing the flow to be turbulent as soon as the air touches the wing, so turbulator or not, no difference).
Now, why is this ZIG-ZAG tape better? Short answer- it does something to the boudary layer to eliminate/reduce the "UNSTEADY" flow. Both wing tips are stalled, but the actual seperation point, and maybe even the nature of the reverse flow, is much more uniform left to right. The small alpha and beta disturbances don't cause huge differences to result in seperation points, pressures etc. Now the long part- Ever notice the leading edge strakes on an F-18 wing root? At high angles of attack, these features of the wing cause a huge vortex of spinning air to form.

http://www.dfrc.nasa.gov/Gallery/Photo/F-18HARV/Small/EC89-0096-240.jpg

now go check out that link... look how that smoke used for visualization forms a tight vortex that appears to explode over the wing. This vortex is what gives a huge boost to the flow over the wing and keeps it from massive seperation. Also helps the flow over the 2 verticals as well...
In fact, go here...

http://www.dfrc.nasa.gov/Gallery/Photo/F-18HARV/index.html

and check out all the cool snaps at various alphas and different methods of visualizing the flow field, ie. smoke, oil and tufting.
Now, what the zig zag tape does is exactly what the F-18 wing strakes do. It causes a vorticity to develop. Granted on a scale several orders of magnitude smaller, but the same. In fact, a whole sheet of micro vorticity, each little forward facing point is the center of 2 vortices. This adds robustness to the boundary layer, it mixs the low energy air in the lower regions of the boundary layer, with higher energy air (ie. faster), above the boundary layer, maybe 2 or 3 BL thicknesses up. So now we have a flow field that is still seperated, but the location of the seperation is more consistant from left to right. The seperation point still moves fore and aft with alpha, but it is nearly the same for both sides. Small disturbancs dont cause a "China-Syndrome" effect to occur with the seperation point. I guess I should have pointed out that even a stalled wing can make lift, and the further aft the seperation point, the more lift created. (Less lift lost? lol) You can also see a unsteady flow if you take a "CYLINDER" shaped object, say a round shampoo bottle, stick it half way in the tub water in a vertical orientation and move it along the water at a constant speed. Look at the waves created behind the bottle, see how they form an alternating pattern in the water. That would be an example of a unsteady flow condition. (Von Carmen waves I think they are called). I have seen similiar patterns behing big trucks driving down dirt roads, you can see alternating patterns of votices to the sides and behind the big box trailers.
I should also mention that the Zig-Zag tape additionally makes the boundary layer more robust, It is much more difficult to get the plane to snap even when you want it too. The layer just wants to stay attached much better!
I hope this helps!!!! Kinda hard to explain in a paragragh or two!
I might actually go ahead and tuft my wing and get some video to try to get a better feel for this myself! If anyone else tries to do some tufting, remember to put similiar tuft pattrns left to right. The pressence of the tufts could actually alter the nature of the flow field, and better this disruption occur symetrically left to right!!!! (I am writing this at 12:00 at night, so when I re-read it during a more coherent moment, I may find the need to edit it a bit!!!)
the ZIG-ZAG tape can be purchased here.. kinda'expensive. Maybe a bunch of guys get together for this?
http://www.eglider.org/catalog/items/item296.htm

[Dave Robelen]

Hi Mithrander,
Thanks for the discourse on the zig-zag strips and the benefits in deep stall conditions. As you have observed, trying to determine the boundary layer effects of a model in flight can be a bear.
By way of background, I retired from NASA Langley in 1995, the last major program I worked on was the F-18 vortex issue. You are quite correct in the benefits in terms of lift at high angles, however the effect on the vertical tails was another matter. The vortex was causing massive high frequency vibration of the tails, leading to early failure and relacement. Many hours in the wind tunnels led to a compromise solution that is employed on the new F-18EF, slots in the LEX ahead of the wing to shift the vortex away from the tails.
If you are looking for a handy way to tuft a model wing with minimum intrusion, strips of the mylar tape from a recorder cassete taped on the wing will give a pretty good idea of the boundary layer effect. This material is a bit more "2-dimensional" than yarn, but multiple rows still give a fair idea where the stall cells are and their behaviour.
This thread got started as a result of the rotten stall characteristics of some (possibly all) of the CAP 232 models that I have flown. In fact, the typical scale aerobatic machine with a symetrical highly tapered wing can be pretty unpleasant in slow flight situations. Even though the appearance suffers a bit, models with either modest taper or none are much more managable through a wide flight envelop for our purposes.
Thanks for sharing your experience.
Regards, Dave

[errolw98]

mithrandir

You and Dave are both obviously highly educated in the field of aerodynamics whether you are Aerodynamicists or not. My background in aerodynamics is in the downforce generated by wings and tunnels or underwings of formula type race cars. CART Champ Cars, IRL or Indy Racing League Cars, Toyota Atlantic Cars and the like.
Our mission is somewhat different and so many of the techniques do not apply or apply upside down. In the case of the underwings, not much can be translated as this is more of a venturi effect although the study of vorticies is helpfull. One trick that was used a few years back and has since been outlawed, was a sharks tooth strake at the entrance of the underwing tunnel for the purpose of spinning the air through the tunnel for an increase of downforce. The nice thing about tunnels is the downforce created there has little aero drag penalty and only carries the increased sense of wieght penalty as if the car were increasingly heavier as speed increases.
As for the Zig Zag tape. We were unable to use it per the rules against such aero enhancements but found the effects so worthwhile in terms of the boundry layer boost, that these were incorporated into our sponser graphics on the lower side of the wing at the LE and the upper surface about 4 - 5 inches before our Gurney flap. This more robust boundry layer from the top surface allowed us to use a smaller profile Guerney flap and achive less drag while keeping the boundry layer attached to the bottom surface further allong the chord before stall or what we call stall which is actually detachment of the boundry layer and thus the faster moving working layer of air. Less AOA and Gurney flap for the same downforce still equaled less aero drag.
In the case of a rece car, You do not feel or notice a differential from right to left, but I suspect this was occuring as well in corners, which would add a substantial amount of drag where the turbulant air was partialy attached. In the case of a multi stage wing, this effect can be even greater as each detachment affects the next stage of the wing. Zig Zag "Stickers" were helpful here as well.
As it relates to my CAP snaping problem, I never thought of using the Zig Zag tape as a possible solution. That plane is now no more unfortunately. I have a Flip 3D, Goldberg Extreme 330, and an Aeroworks 40 sized Edge 540T now. The Edge displayes the very same charicteristic as the CAP 232 in that at higher speeds, it will only accept a limited amount of elevator before becoming quite unstable. ZigZag tape could be an aid if it does not cure it altogether. The Flip 3D may benefit from the ZigZag tape as well. In the Harrier moves, it will begin to rock when attempting this upright. Inverted however, it is rock solid and can be dirrected with a small amount of rudder input to fly almost anywhere I want it to.
I have not flown my Edge 540T enough to tell where else it may benefit from such treatment. Some benefit is sure to be there as well as it seems to be a tip stalling monster as well.
Glad to hear a good report on the Katana as well. It was to be my next plane as Chief Aircraft is only about 25 miles from my home.

Thanks for the input and suggesting a forgotten aero trick.

Errol Wilkinson

[Dave Robelen]

Hello Errol,
One possibility regarding the different behaviour inverted and uprigt is the amount of "unshielded" vertical tail. Upright in a stalled maneuver, the wake from the stalled stabilizer can blank the vertical and rudder pretty thouroly. When the model is inverted, the vertical tail is in clean air and is a lot more effective in damping these motions.
This has been an issue in tailspin dynamics in many lightplane designs, especially where the vertical tail is swept so that a great deal of it is bathed in this low energy wake.
I still am not fond of thin, sharply tapered wings on models with moderately high wing loadings, they are a great deal less forgiving at the higher angles of attack. If the zig zag tape approach can really help, that could save more than one model.
Regards, Dave

[errolw98]

That the vertical tail surface is in unstable or little air, making it ineffective, makes sense. I agree with you and still think the bigger issue is wing profile and loading on smaller scale planes. For an experiment I enlarged my Flip 3D in RFG2 by 1.41 to get an 80 inch model. I was thinking of building one. Interestingly, with this scale change of 1.41, the wing area nearly doubled with the same scale look. It stands to reason then that when the models get smaller and stay true to scale, the opposite would be true.

Errol

[Dave Robelen]

Hi Errol,
What you ran into was one of the basic scaling relatioships. For a given scale ratio the area will go up (or down) by the square of that ratio. So-- a 2X enlargement will produce a 4X area increase. Of course, going the other way, you lose area real fast with reductions in size.
The same system requires that the weight change by the scale to the 3rd power, so that 2X enlargement would have to weigh 8X as much for a true comparison. Enlarging a model generally leads to a lower effective wing loading, and improved aerodynamics due to the Reynolds number increase, while scaling down works just the opposite. I suspect that our scale aerobatic models are not truly representative of the full scale counterpart for these reasons. Still--- If I were offered a ride in a CAP 232, I belive I would give it some serious thought
;)
Regards, Dave

[errolw98]

Dave,
I would like to know more about the Reynold numbers you refer to in your posts. Is there anywhere you would dirrect me to on the web for such study? Or perhaps a formuls that is used for it's calculation? If you increase all of the flying and control surfaces by the same factor of the 1.41, as used in the case of my Flip, do the Reynold number relationships remain liner with the area gain in the tail surfaces vs the area gain in main wing? My guess is that it depends on the ratio of area gain between these surfaces. Would I be correct in assuming that if the tail surface begins at a ratio of say 0.5:1 to dream up a number, that if the scale change doesn't net the same or better ratio, you would also adversly affect the Reynold number?

The funny thing about the scaling issue is the weight factor. Some of the giant scale planes are boosting wing loadings in the 25 to 30 Ozs per square foot range and they fly beautifully. The 40 sized models seem to like a wing loading around 12 Ozs per square foot for simillar performance. A multiplication factor of 8 would put a light 5 Lb model into the 40 Lb range for a 2X scale of the same plane. I bet the wing for that would have to be quite different than one for a 25 - 26 Lb 33% Model with a 10 HP DA 100 twin.

It does make you wonder though why there aren't more full scale planes with the HP to weight ratio to pull off more of the 3D type manuvers. Maybe the human element is to blame for this though. I suppose if you took 33% of 180 Lbs at 59.99 Lbs and stuffed it into the 33% model at 25 Lbs, you would have a pretty sluggish 85 Lb model indeed. Even if you adjusted this 59.99 Lbs for the removal of the radio, battery and receiver stuff that a pilot would control, you would still have a very heavy bird. The radio stuff is probably lighter than the systems that would be needed to accomodate the human pilot anyway so you can likely use this as a generous comparason.

Even with all this in mind, I think I would still ride in a CAP once at least, if the opportunity ever came up.

Thanks,

Errol

[Dave Robelen]

Helo Errol,
The Reynolds number bit is fairly basic in concept. For airplanes it is the chord length X velocity X a constant that represents atmospheric conditions. Typically for standard atmosphere this constant is 6380 when velocity is measured in feet/second, and chord in feet. Increases in Reynolds number are linear with increases in chord and speed, but the effect of a change in the Reynolds number is generally non-linear.
Below a certain value, the airflow has great difficulty following the aifoil curve, and early seperation leads to large values of drag. This is essentially why my micro models are fussy about the airfoils, and require much more than scale power to fly. As the number increases, say above 500,000, the airflow begins to stay attached pretty well, and the drag goes down while the maximum lift increases. At these higher numbers the effect of increasing the Reynolds number is pretty much linear in the matter of increased lift and decreased drag. All this is why an F-16 or large transport can operate with such astronomical wing loadings.
While I do not know offhand of a good tutorial on the subject, a google search will turn up more info than you care to digest.
The bit about the increase of 1.44 would bring an increase of 1.44X in the Reynolds number X whatever the velocity increased, not a trivial amount.
Finally, the matter of that rapid taper in the wing may be just enough to put the tip in a critical region of the Reynolds number where early seperation is normal. Food for thought.
Regards, Dave

[errolw98]

Dave,
Once again you have made my brain function. Thanks for the info. I have not flown one but now wonder if this symptom is the reason behind H9s Funtanna wing that has a tapered LE and TE but constant chord thickness for the length of the wing. Efectively it would start out at the fuse as say a 14% while at the tip it would be an 18 or 20%.

Errol

[mithrandir]

Back to the F-18...
I saw a video at the nasa dryden site of the high AOA stuff.. I noticed that the verticals appeared to be nearly fluttering!!! I have also noticed the slot or hole in the strake to attenuate this too... I think LOL.
As far as good resources, try the University of Illinois, Urbana web site.. the exact URL is not handy, sorry... Try searching Michael Selig. There is a gaggle of model related aero stuff there... I have often noticed that the smaller scale-aero planes don't seem to utilize another method of tip-stall attenuation. That would be transitioning the airfoil. At the tip, they should transition to a further forward (Percentage wise that is) thick point. Additionally, they could use a thinner section. (Yes, a thinner section stalls at a higher AOA... but it typically stalls more abruptly) Or they could use a thicker section, that might give a little more warning that a snap is impending!! LOL
I am next going to attempt to put a slot right at leading edge on the cord line, and stick some zig-zag tape in the slot, so that there are little triangles sticking forward, kinda like forward facing shark teeth... The zig zag tape steadied the harrier, but when I transition from 45 AOA to 90 AOA, it is a bit unsteady thru that range.
Re: Reynolds Number and effects.
It has been my observation that above 500,000 Rn, all the wierd bubbles and weak boundary layer issues disappear. (That would be a 12" wing cord at about 55 miles per hour) I have even observed a hysterisis in some of the smaller planes I have flown. I once had a .60 sized plane, at 24 oz/ft^2 wing loading, It was a flying manhole cover, snapped good when I wanted, and when I didn't LOL. Then I had a 35% sukhoi 26 at 32 oz/ft^2. It was a kite. I had difficulty getting this one to truly "SNAP".
Happy AV8shun gents!!

mds

[f-16flyboy]

Hello all,

I am fairly new to RC Aircraft and wanted to get into pattern and aerobatics. I bought Real Flight G2 and spent about 200 hours flying everything in the library before deciding on a CAP 232 as my starter plane. Yes, I am an "A" type.
I bought the Hanger 9 Sport CAP 232 .40 ARF, OS 46FX with props from 10 x 7 to 12 x 4, and a JR 8103 and put it all together following the instructions the best I could. I do have extensive background in precision crafting of formula race car parts so I am pretty well suited to the task.
After setting the control throws to Hanger 9 recomendations and using my Great Planes CG Machine to insure the CG was right it was time for the first flight. I have had a ball so far and after a couple harrowing first flight saves, reduced the elevator throw so the CAP won't turn over on full elevator input. About 5/8 of an inch is all she will take before snapping. I have moved the CG around fore and aft about 1/2 of an inch each way with very little change. Forward weight seem marginally better but slower responding.
I did not get this charicteristic from the Great Planes CAP 232 model in RF G2 untill I really boosted the Elevator throw. Is the GP 232 that much better? do I have a seriuos assembly problem that I have overlooked?

My question is this:
What causes one wing to stall and snap a plane over so quickly with the input of the elevator and other than reducing throws, how can this be eliminated or reduced?




a snap is when an aircraf comes to the point of no air or not enuff air moving over the wing so... at this point the next main factor taks place the engen tourq witch causes the aircraft to snap to the left every time, this is sumthing you cannot stop just dont put the aircraft at this point

[Dave Robelen]

Hi,
The snap with neutral rudder and aileron is directly a function of wing stall. These acro planes are frequently designed to have easy "agressive" snap qualities to make the maneuvers involving the snap positive. The flow around the wing is complex at the stall, being influenced by the airfoil, location of the wing on the model, spanwise airflow circulation at high angles, all of which vary with each airplane design. Also, the horizontal tail may be stalling, which would cause the nose to pop up even quicker.
For whatever reason or combination, the CAP 232 is more inclined to snap on approach with excess elevator than most other models.
You cannot use results from our model simulators to predict this characteristic. Their math models are far too simple to handle the complex stalling flow we se in the real world. I have an early Real Flight, and while it demonstrates a lot of responses, true spins and snaps are not part of the package. Do not judge the model by the sim.
Regards, Dave