Stumped By BD-5

[nolasco]

I have built a profile BD-5 that is giving me problems. Here are the specs:

23" wingspan
17" long
5.5" root chord
4.5" tip chord
3/4" dihedral at the tips
115 sq.in. main wing area
18 sq.in. tail area
Undercambered airfoil (curved plate)
AstroFlight Firefly 4:1
GWS 4.5x4 prop
8,9-cell 50mah Nicd, 120mah Nimh
Taileron control (Fully-Flying)
4.5oz AUW (on 8-cells)
CG at 1/3 of root chord from the leading edge
5.63 oz/sq.in. wingloading

The plane takes a long ROG run before abruptly jumping into the air at a high angle of attack. And just as abruptly, it suddenly points the nose down and dives into the ground.

Hand launch results in a dive into the ground.


Some questions:

(1) Is the wingloading too high for the Firefly/GWS prop combo?
(2) Am I nose or tail heavy?
(3) Are my control throws too large?
(4) Are fully-flying tailerons a really bad idea for the BD5?
(4) Are there any suggestions for the incidence angle, decalage, motor thrust line?

In the meantime, it might be wise for me to build a few fuselages. Everything else is detachable.

Jun Nolasco

[phuffstatler]

That zipping along the ground and leaping into the air sounds like your nose gear may be a bit short, causing your wing not to create any lift until you really haul back on the elevator. You might want to lengthen it (nose gear) a bit to get the wing into a positive lift position for takeoff. Of course, it wouldn't stick to the ground quite as well on landing and rollout....

The abrupt stall may just be because it runs out of airspeed too quickly. The dive to the ground on hand launch says that it shouldn't be tail heavy (a cause of stalls too) But a 1/3 back on the wing CG shouldn't be nose heavy at all.

I think full flying tailerons should be fine, though the throws of the surface become much more effective with it. You didn't list your actual throw angles... I know the full scale BD-5J's have all gone to that (the ones that have survived) and it works well, plus I know of a scratch built 1/3 scale one with a turbine in it that flies just fine like that.

I'll let the real experts advise you on incidence and whatnot. There's more design experience on this forum than I have....

Hope you get it worked out,
phil in austin

[Dave Robelen]

Hello juan,
One of the largest challenges is the high thrustline. This is what makes it resist a liftoff until you apply excesive pitch, and then the speed rapidly decays and you get the dive. Sudden changes in power also aggravate this condition. Since you apparently did get a liftoff from the ground, there is probably plenty of power.
If possible, I would find an area of soft grass or weeds, and sort out the trim by tossing it power off and adjusting until you get a smooth glide. This trim will probably result in a dive at full power, so you will need to be ready to change the stick to some up when you launch with full power.
It is interesting that this exact same problem caused several fatal accidents when pilots took delivery of their new machine at Wichita. In these cases the planes were seen to "porpoise" up and down until impact. The lack of a two place trainer has been a real problem for the transition to the BD-5.
Good luck, Dave

[phuffstatler]

Thanks Dave, for reminding me of that thrustline. Although one of my all-time favorite aircraft, I forget about that high line pusher arrangement from a design viewpoint....

oops,
phil in austin

[nolasco]

Here is a progress report ...

I sacrificed speed for thrust and replaced the Firefly 4:1 and GWS 4.5x4 combo with a DC5-2.4 5:1
and GWS 7x3 setup. I also set the angle of incidence of the main wing to around 10 degress
adjustable) and the tailerons to around -10 degrees. Finally, I mounted my gearbox in such a way that I have an adjustable thrust angle. I used 5-10 degrees for the thrust angle.

The takeoff run is now much shorter, around 1/3 of the original. I can now also get a straight
airborne run for a few feet before the BD-5 either spins to the right, left, or drops the nose.

Question: is that left/right spin a stall? If so, then it looks like I either have insufficient thrust, speed, or both. I might try a DC1717 6.3:1 and GWS 7x6 to get more speed and thrust. That
setup makes my 17" Micro-IFO fly like a bat from hell.

Also, having the main wing and/or tail angled closer to the horizontal just makes the BD-5 run on the ground. Hence, I have what looks to me like an insane decalage (20 degrees).

Finally, it looks like the CG position is very sensitive. The same goes for the full flying taileron. Trimming is a real chore on this one.

A picture will be posted once I get get home tonight.

If this one ends up in a bust, I have a Pushy Galore waiting to be built. At least all I have to worry about on this one is the very high tail. :rolleyes:


Jun Nolasco

[Dave Robelen]

Hi Jun,
The post takeoff turns sound very much like a stall/spin. Given the amount of decalage, it seems almost inevitable. That 3" pitch prop is probably the culprit. I expect you will need at least 5" of pitch, maybe more to generate the pitch speed needed to fly.
If at all possible, I would find some soft grass and proceed to toss the bird in glides until you have the trim in the ball park. It may still be nosedown under power, but that should be manageable. If you can get the airframe gliding, you will have established a trim point, and will also have a decent feel for the mimimum flying speed.
Regards, Dave

[nolasco]

Here is a pic. For now it is just a profile version ...

Jun Nolasco

[nolasco]

... and another.


Jun Nolasco

[Dave Robelen]

Hello Jun,
Sending the pictures was a good deal. I did not realize that you had enlarged the wing chord to the extent that you have. It is very likely that the horizontal tail is too small for adequate pitch stability. There is a formula you can use to check this out.
The first step is to find the area ratio between the horizontal tail and wing. Divide the tail are by the wing area. Next, determine the wing average chord. This would be the chord at midspan on a straight taper. Next, determine the moment arm from the .25 chord of the wing to the .25 chord of the tail. Divide the moment arm by the wing average chord. Multiply this by the area ratio. If the resulting number is less than .5, there will be problems with an undercambered wing. .6 would be better still.
Regards, Dave

[nolasco]

Dave,

Ouch! I am way below the "0.5" figure. My main wing has to be much smaller and/or I should have a much larger tail.

Does this also apply to flat bottoms? I am asking because I had this urge to cover the bottom of my wing (ending up with a dual-surface undercamber). When I tried that out, it actually flew ... for while anyway, and only once! It was still a handful though, probably because of my full-flying taileron setup (even with reduced throws).

That small change alone seems to have made a nice improvement. I probably ended up with better lift, reduced drag, or both.

I am now tempted put a larger tail, ditch the full-flying taileron to a "non full" one, and/or just try a more conventional setup (A/E or R/E).

Just how bad/good does the full-size BD-5 fly anyway? It seems to have a really tiny wing area (47 sq.ft. or so).


Jun Nolasco

[Dave Robelen]

Hi Jun,
First, there are two versions of the prop BD-5. The BD-5A has the short wing with a small area. There is also a -B model with a lenghtened wing. The tail volume on the full scale is sized to be conservative for the long wing, and plenty large for the shorter wing.
A player in this tail effectiveness is the type of airfoil as you are noticing. The more camber an airfoil has, the stronger the nosedown pitching moments, and the greater the load the tail has to trim out. An undercambered surface would have the largest pitching moment, while a symetrical one is essentialy zero. There are also reflexed flying wing airfoils that have basically no pitching moment.
The flight reviews from independent pilots were generally very flattering for both models of the prop job. The BD-5J is the same short wing set-up as the -A model, but with a much higher wing loading which does increase the landing speeds, etc. BTW, the BD-5 airfoil is nearly symetrical, freatly reducing the load on the tail.
The use of tailerons can be tough with designs that have relatively short stabilizers and longer wings. the moment arms inherently give a mild roll response for the deflection involved, and it is possible to encounter "tail stall" when the up going surface passes 20 deg. or so. It might be an option in your case to enlarge the stabilizer by increasing the span and kill two birds with one stone.
Take care, Dave

[nolasco]

It flies!!! Increasing the tail area sure helped (thanks Dave!). And it only happened because I dared a hand launch after losing the front wheel retainer.

A DC5-2.4 5:1 and GWS 6x5 prop seems to provide more than enough power to fly this (now) 4.6oz plane. I'm putting back the Firefly 4:1 and GWS 4.5x4 prop to see how that goes.

But since it is already beat up and a bit heavier with all the repairs done to it so far, I am inclined to build a new one. Besides, I have a new supply of foam to work with.:D

Now comes a laundry list of quirks ...

(1)The plane seems to oscillate up and down at high power levels. At medium to low power, I did not notice any such oscillation. I am thinking I have too much upthrust, elevator, am nose heavy, or a combination of the three. Any ideas?

This oscillation seems to explain why I always get that sudden descent into the ground after initial climb out when I attempt to ROG at full power. This also seems to explain why I had a sudden climb out when I first started an ROG at full power and then decided to cut throttle because I was nearing the end of my flying area. Full throttle is pushing the plane into the ground.

(2) I also noticed a big tendency to roll to the left at high power levels. Applying full right barely straightens out the flight path. At low power levels, no such tendency was noticed. I guess I need some right thrust as well.

Once this is all fixed and done, it's on to my next project: a Pushy Galore! Stay tuned for that one.


Jun Nolasco

[Dave Robelen]

Hello Jun,
That is great news that you got it into the air. In trying to digest the rolling and oscillating tendencies that you describe, I am having difficulty sorting out the power effects.
Can you get it high enough that you could get some gliding time, or very low power flight?. If these tendencies improve noticably with gliding or low power flight, it would be power effects. The thrust line may be a player, but it is also possible that the inflow into the prop acros the stab is causing the rolling effect ( maybe also the funny pitch stuff). If all feels normal at low power settings, the thrustline is about all that is left.
Good luck, Dave

[nolasco]

I did try a few low powered flights (with the batteries being close to draining) and did not notice any oscillations or rolls. The powered glide was nice and smooth.

Since I have purposely built an adjustable thrust line, I'll try smaller angles next time.

Jun Nolasco

[PeterSchug]

A quick two cents.

Down thrust at the rear is like up thrust at the nose.

I don't know if that's the problem, but that looks like a large angle of thrust pushing the tail of your BD-5 down.

Pete

[nolasco]

I built my profile BD-5 to have an adjustable thrust angle. I actually use less than what is shown on the picture.

Jun Nolasco