building a glider
- Thread starter hello183
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I have shrunk the covering for the wings now, so all that's left is the nose cone, which I will redo because I accidentally sanded too much off of it. Instead of using several pieces of foamboard for it, would it be ok if I used one of those foam blocks or spheres that they sell at the store? Also, the polyurethane glue was difficult to sand, so I might try using white glue for laminating the pieces together.
danskis
Master member
Use whatever is easiest for the nose cone. Keep in mind that the nose doesn't have to be light. You will be adding weight to the nose in order to balance the plane correctly in order for it to glide well. Do you have a sturdy place to add the weight? White glue does sand better than hot glue. You can also thin the white glue to make it easier to sand.
That said - when the wing is covered - and before installing the nose - you should put the wing on the plane and balance the plane - add weight to the nose so it glides well. You don't know the actual balance point since this is an original design. This is a guess - I would start about 3/8 to 1/4 inch in front of the spar. Coins work well for adding weight.
That said - when the wing is covered - and before installing the nose - you should put the wing on the plane and balance the plane - add weight to the nose so it glides well. You don't know the actual balance point since this is an original design. This is a guess - I would start about 3/8 to 1/4 inch in front of the spar. Coins work well for adding weight.
danskis
Master member
@Piotrsko You're right!!! But in english - from the pictures it looks like his spar is at the high point (whadya think?) (OK maybe a little behind the highpoint) so I took a guess.
For the sake of education lets say he's using this one. What would the balance point be?
S7055 (10.5%) Flat-Bottomed (s7055-il) (airfoiltools.com)
For the sake of education lets say he's using this one. What would the balance point be?
S7055 (10.5%) Flat-Bottomed (s7055-il) (airfoiltools.com)
Piotrsko
Master member
Numbers say max thickness at 31.8%, so my estimate of 30% is close enough. Fix any issues concerning cg with tail volume. I had a tandem once where the CG was at 90% and dragonflies are halfway between the wings
I see the spar behind the high point. If I was sure it wouldnt rotate nose high at stall, I would use the spar since that concentrates the forces where the wing is essentially strongest.
I see the spar behind the high point. If I was sure it wouldnt rotate nose high at stall, I would use the spar since that concentrates the forces where the wing is essentially strongest.
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Piotrsko
Master member
Weight to the nose is for pitch down at stall and having shorter noses. Everything stops going up and the heavy stuff tends to fall rotating around the CG. If it's biased too rearward it will tail slide.
My comment about fixing cg problems with tail volume is more about being able to go more rearward say to a max of 40% making it sensitive to thermals without the nose up pitching taken to extremes. Set up properly, the tail contributes a nose down moment past the stall and the plane should bobble slightly, drop the nose, unstall, and continue. Bigger tail the faster it rotates
My comment about fixing cg problems with tail volume is more about being able to go more rearward say to a max of 40% making it sensitive to thermals without the nose up pitching taken to extremes. Set up properly, the tail contributes a nose down moment past the stall and the plane should bobble slightly, drop the nose, unstall, and continue. Bigger tail the faster it rotates
danskis
Master member
So the balance point is somewhere around 1/4 to 1/2 inch in front of the spar. We'll see what Piotrsko's (much more educated guess) will be. Its a delicate glider so probably better to have it a little nose heavy.
That said - if you need to add a lot of weight in order to balance it you should consider reinforcing the nose if the nose cone isn't on yet.
That said - if you need to add a lot of weight in order to balance it you should consider reinforcing the nose if the nose cone isn't on yet.
Piotrsko
Master member
My educated guess is no more than 1/4 " in front of the spar or on the spar with previously mentioned larger tail. Previous picture says the horiz stab is pretty big, maybe 10%. Ratz, no mention of pitching moment in ordinates so how it stalls is anyones guess. So: best first guess is 1/16 x 1/8 wide shim at trailing edge of elevator to start, more if it does trend to dive in test glides (aka nose plant). Move the shim to the front if it immediately stalls or add some taped on pennies to the nose. More than 4 shims thick means tail isn't large enough or CG is too far forward. Im hoping for some serendipity in the nose profile of the airfoil giving you induced positive incidence but it looks like the nose is flat to the bottom and a bit sharp which leads to sharp stalls
Problems we have here is that many variables which are unsolved are going to affect flight characteristics.
Problems we have here is that many variables which are unsolved are going to affect flight characteristics.
quorneng
Master member
My own observation would be to set the CofG initially at a safe position (25% of the wing chord back from the leading edge) regardless of the wing section or tail volume. Balanced at that point and the wing and tail set to a reasonable incidence (say +3 & 0 degrees) it will glide in a stable manner and should show if it has any tendency to turn one way or the other.
When you have a reliable straight glide then you can try adjusting the position of the CofG rearward to achieve an improved glide angle.
Have you decided on how or where to launch it when you have it gliding well?
When you have a reliable straight glide then you can try adjusting the position of the CofG rearward to achieve an improved glide angle.
Have you decided on how or where to launch it when you have it gliding well?
danskis
Master member
So right now it looks like he has glued the tail on and the wing and the tail would sit at 0/0 decalage. So the only way to adjust the incidence (decalage) is to shim the front of the wing - assuming he will attach it with rubber bands. Say with a piece of posterboard or 1/16 balsa to start.
