Hi Everybody,

I'm designing a zero-zero glider (incidence=0 and decalage=0). It's my first time trying to design this kind of glider (I'm using XFLR5 to simulate) and I'm having difficulty in obtain enougth lift when Cm=0 (Total Pitching Moment=0). Simulating using deflected elevator give me enought lift but on almost 4 degrees of AoA. Is this common in this kind of design? For this kind of glider what is the usual static margin? I'm working with 7% of SM.

Best Regards

With model gliders the '0-0' theory does not work very well. The model should be flown in two different conditions, minimum drag, for lift hunting, and maximum Cl when circling in lift. These conditions can be related to two different airspeeds. Since, hopefully, most of the time the model will be in lift, then it is best to set the model up with a little decalage (longitudinal dihedral, et al) so that at zero elevator (speed) trim the wing is at its best Cl angle to the FSD. The fuselage should be set at the lowest drag angle with the wing at this AOA so there will also be some positive rigging angle of the wing to the fuselage (incidence). The balance then needs to set by the 'dive' test as frequently explained in these pages, the model must be speed stable in order to allow the pilot to hunt, spot and fly it into lift. I guess you have to fly these things a bit to appreciate what is needed by the guy on the ground to fully appreciate the requirements.
Evan, WB#12

The only airfoils with a Cm=0 or even close to 0 are going to be symetrical or very low camber airfoils. Or a low pitching moment airfoil that has some amount of reflex in the camber line to balance the pitching moment out.

Also to generate lift you require a positive lift coefficient of some measure. That requires either camber or an angle of attack or, far more likely, a combination of both. And other than a cambered airfoil at higher speeds you will require a positive angle of attack to generate the higher lift coefficients. And finally the lift coefficient required for level flight or during higher G loads is dependent on the flying speed. The slower it flies or the heavier it is loaded the greater the angle of attack and the lift coefficient will be. At the speed you trimmed the simulation for this angle apparently needed to be 4 degrees. Double the flying speed and you'd see the angle required drop by a lot. But as long as you need lift you still need a lift coeffient that is over 0.

You state that you're operating with a 7% margin of static stability. That implies that the CG is located ahead of the neutral stability point. That alone will result in your inability to operate at a true 0-0 decalage setting. Any static margin greater than zero will require a reduction in the tail lift so that the combined lift between the wing and tail has the positive pitch torque needed to hold up the nose.

So basically you're trying to do something that just isn't possible. Perhaps tell us more about what you're trying to do and how you want the design to perform and we can work up something.

We used to trim outdoor hand-launched gliders quite close to zero-zero; otherwise they would enter a loop immediately upon the very vigorous launch, instead of attaining the target height of 100 feet or so. With the zero-zero setup, the CG has to be well aft, or the airplane would just dive into the ground from full altitude.

I think that a lot of the secret in handlaunched gliders was in selecting just the right balsa density for the wing. The wings normally had flat bottom airfoils, of about 6% thickness. At high airspeed, diving moment developed by the campered wing would twist the wingtips to a slight negative angle of attack, preventing looping until the airspeed fell off near the top of the climb, whereupon the airplane would enter a stable glide.

Hi everybody,

thanks for all answers... Let me explain why I'm asking about a Zero-Zero RC Glider Design. Some months ago I bought Peformance Tunning DVD from Radio Carbon Art - Produced by Paul Naton (very good DVD!!!). This DVD shows that hi performance gliders should be set to Zero-Zero (or -0.5 decalage). Then I get an old glider and modified it to became a Zero-Zero glider (or near Zero-Zero). This modification speed it up!!! It's a little bit different when flying, but it's ok on a light slope. Then I decided to design a new glider using Zero-Zero, but when I saw the results on XFLR5, I got scared [X(]... using 1 degree of incidence and -0.5 of decalage it's ok (on a regular analysis), but there are Zero-Zero RC Gliders around the world... I was thinking that the analysis of this type of glider should be different from regular gliders...

Best Regards

For a fast sloper where it's flown all the time, zero-zero should be OK.
Thermalling though, having some built-in pitch stability is really a good idea.

Depending on the airfoil it still won't be a true 0-0 setup. But with a strongly set back CG the need for negative angle in the stabliator or elevator becomes very minimal. But there's still SOME angle or a cambered airfoil that produces lift at a 0 degree angle of attack.

Also you say that your model is using a 7% Static Margin for stability. That is FAR TOO MUCH forward a CG to use with a 0-0 or really minimal decalage angle. You'll want to use an online CG calculator (see the sticky thread at the top of this forum) for finding the location of the neutral point and figure on inching your present CG back towards that point in small steps. Some very odd things can happen to some model gliders when the CG is pushed that far back. And they are not always pitch related. Some funky things can happen to the spiral stability of the model as well. So approach it carefully.

Just to complete this post...

BMatthews you are right!!! I tried the simulation using a CG Neutral (few millimeters from the Neutral Point) approach and now my design is able to fly!!!!

Thanks