was makes a glider rolls so slowly ?

[yclui]

Hello everybody,

I am in the process of learning how to design models by building and flying test planes with different airfoils, CG positions, dihedrals, decalage and . . . . tail CLA ( I just learned this term from this forum ). It is a very stimulating process that has revived my interest in the hobby and I am sure I will come back from time to time with aerodynamic mysteries that I cannot resolve.

There is one mystery right now with my 2-meter ASW28 glider from Lenger Modellbau ( http://www.lenger.de/cgi-bin/e_web_store.pl?page=asw21.htm&&cart_id=9317746_31378 ). The specification of the glider is as follows :
Wingspan: 78.74" (2000 mm)
Wing Area: 269 sq in (17,4 dm²)
Wing Loading: 10.1 oz/sq ft (31 g/dm²)
Fuselage Length: 34.37" (873 mm)
Weight: 19 oz (550 g)
Airfoil: S 3010 ( looks like a Clark Y to me, the bottom is very flat )

The aspect ratio of the wing is on the high side, the chord measures about 5 inch at the wing root and gradually tapered to about 2.5 inch at the wing tip which is terminated with a winglet. The aileron spans about 2/5 of the wing with an average width of 6/8 inch.

The plane has got one big problem: the aileron response is TERRIBLY SLOW ! even with an aileron deflection of 25 deg !!! I have put in sufficient differential ( up deflection : down deflection = 2 : 1) and double checked that the CG location is correct. The latter was done by trimming the plane to fly level and straight, put it into an approx. 30 deg. dive, fingers off the sticks, the plane would slowly pull it's nose up as it picks up speed - looks OK to me.

It seems to me that the so-called roll damping for this plane is huge - the largest I have ever come across. Many experienced members of my club have examined the plane but no one can put forward any sensible explainations .

Can anyone shed some light on this ? Anything I can try to find out the answer ?

Thanks,

Y C Lui

[Dave Robelen]

Hi Y C Lui,
as you have indicated the long span of a glider has a very large damping in roll.The aileron size you indicate is on the small size, and with only 2 deg downward deflection, the downgoing aileron is hardly contributing at all. I bet the full size machine used a lot less differential and cranked in rudder to compensate. You might try less differential and blending rudder in, I bet that will perk it up.
Regards, Dave

[Richard Miller]

Lui,

I could sit here for a couple of hours and comment, and when I finished I'm sure Dave would have a few things to add.

Think of any aircraft, dynamically, as resembling a child's jack: three sets of arms on the Cartesian coordinates. Concentrate part of the total mass of this jack, translated to airplane, at the center of gravity, and the rest, six segments, at the end of each of the arms of the jack. Imaginary lines of distance "L" between the C.G. and the [six] centroids. Centroid: a center of mass.

Thus each wing half has its [mass] centroid. It's roughly the point at which the wing panel would balance on a pin. Its resistance to displacement is expressed by its mass-weight X the SQUARE OF the distance "L". Make the distance to that centroid twice the original and 4 times the force will be required.

Number One: Keep everything as light as possible.

What goes for the mass and the force to dispace it goes as well for the force itself, whatever aerodynamic force the elevator, rudder, aileron produces. But there are problems. Adverse yaw and the other adverse roll are a couple of them. In addition to creating extra lift, the outside [lifting up] wing creates extra drag. Solutions: Differential aileron travel; only upward travel on the inboard aileron; bigger ailerons. l

Adverse roll results from the fact that the rudder centroid is almost invariably above the longitudinal axis. It's just about impossible to put it anywhere else. This causes the nose to yaw in the desired direction, but for the induced roll to fight it.

Know what? I'm going to stop now and let you digest this and see if you have questions, if you want to go on with the matter.

-Richard

[yclui]

Dear Dave and Richard,

Thanks very much for the hints. In fact when I took my ASW28 for it's maiden flight, the aileron differential was very small and you know what ? the aileron response was as slow as it is now except that it scared me more by stalling at the tip and going into into a deadly spiral when full aileron deflection was applied !

Some experienced members in the club also adviced me to use the rudder which I did tried. What astonished me was that the rudder was actually quite effective in rolling the plane which hasn't got much dihedral - another mystery that I do not intend to go into for the time being. I have subsequently tried to mix aileron to rudder and it did improve the rolling control a bit but it also brought along yaw motions that I did not feel comfortable with . May be I should try to adapt to it.

Richard, if I understand your first point right, you are suggesting that the moment of inertia may be too large for my plane. Although I have not measured it, I do feel that the moment of inertia of the plance is quite large because of the long span and the fact that the alieron servos ( Futaba 3102 ) are installed as much as 23 inches away from the wing root . I am sure this factor has some contribution but what puzzles me is that the many other members of my club are flying this kind of scale gliders and none of them had experienced this slow aileron response problem. Among the planes flown by these guys is the Nimbus 4D ( http://www.lenger.de/cgibin/e_web_store.pl?page=nimb1.htm&&cart_id=3476062_12493 ) which has got a long, sword-like wing with an aspect ratio as high as 39.1! ( that of my plane is about 21 ).

Regarding your second point, do you mean if it is possible to shift the vertical position of the rudder ( or fin ? ) downward so that it's centroid lies right on the longitudinal axis of the plane, the aileron response can be expected to improve ? I have seen some pattern ships adopting such fin position for more linear roll but I am not sure whether this is a significant reason behind the slow aileron response of my ASW28 because the position and size of my plane's fin looks quite similar to many others exhibiting normal aileron response.

As you and Dave have indicated that the aileron of my plane may be too small, will it be meaningful for me to try to increase the width of the aileron by sticking a strip of balsa ( say, 1/4 inch wide ) to the trailing edge of the aileron and see how it goes ? In doing so, I will be changing the plan form of the plane. Will it bring any side effects that will make it difficult to draw conclusions on the test results ?

I do look forward to hearing more suggestions and exchanging of ideas until the reason is identified and hopefully proved.

Cheers,

Y C Lui

[Dave Robelen]

Hello Y C Lui,
There is one important point that has not been covered in the aileron installation. Is there any gap between the aileron LE and the wing? A gap along the hingeline can really cut into the effectiveness of any aileron. If you can see a gap, use a plastic tape along the hingeline to seal this. If the ailerons are hinged on one surface, just run the tape along the hingeline. If the hinge axis is in the center of the aileron, crease the tape into a "v" and pressit up into the hingeline. This could really improve the response if thre is any gap.
Regarding the rudder area distribution. The vertical placement of the rudder area has a large influence on the rolling moment produced by the rudder when it is deflected. Consider the rudder as a small vertical aileron. Looking from the rear, a tall rudder above the centerline will produce a yaw in the desired direction, but the roll moment will be in the opposite direction. Placing more of the rudder area below the centerline reverses this effect so that deflecting the rudder will roll the aicraft in the right direction along with the yaw force. This is a bit of a moot point with your glider, since the rudder position is basically set by the scale design.
I would expect that something is amiss with the aileron installation if the response to aileron deflection is really that sluggish. You mentioned that the servos are mounted a good distance from the root out in the wing. If you gently restrain the aileron with your fingers and move the stick for roll, does the servo feel strong enough? somtimes a long thin cable can really reduce the power available to a servo.
Take care, Dave

[yclui]

Dear Dave,

Thanks very much for spending time with me. My ASW28 has got an all-composite, hollow molded wing and just like many other planes of such construction, the ailerons are hinged at the top by the fiber glass cloth making up the wing so there is no through-gap between the aileron and the wing at any time. However, when the aileron is deflected upward, a slit will be openned up at the bottom and a senior member of our club has suggested me to close up the slit by a strip of mica film. One half of the strip was taped to the bottom surface of the wing just next to the sub-trailing and other half was pressing lightly against the LE of the ailerons which can slide freely on the film as it deflects. I tried this method last week but the problem remains.

Thanks for introducing this rudder-initiated rolling mechanism to me. Until now there was just one single mechanism in my mind and wing dihedral is the key element in that mechanism, I know I can learn from you gentlemen. I really appreciate it .

Regarding the mucle of the servo, I can safely say that they are more than enough for a plane of this size. The servos I am using are all-metal-gear Futaba 3102 which as got a torque of 51.4 oz in. Many of our club members flying planes of the similar size are just using Futaba 3013 or 3106 the torque of which is at least 60% lower but all of them are doing just fine. I have also exercised due care in choosing the extension cords. Those installed in my plane has got at least 30% more copper compared with standard servo cords. Finally, I did tried to feel the strength of my aileron servos and I cannot be more comfortable with it .

I am going to try again this week with bigger ailerons and will update you guys on the outcome ASAP. Meanwhile, pls let me know in case any new ideas pop up.

With many thanks,

Y C Lui

[Wells]

When a plane rolls, there is a helix angle created as the wing moves forward and up/down. The aileron design will pretty much determine what this angle will be. The larger the wingspan, the slower the roll rate will be, since the wintips have to travel that much farther for a given angle. Likewise, the slower the model flies, the slower it will roll. It's inherent in the glider's design, slow speed and large wingspan!