building a glider

[hello183]

I have almost finished with the fuselage of it. The vertical stabilizer is on the side. I used some Legos to keep the sides square. Right now it looks ok. I am planning on throwing it off the largest hill I can find when it is done, and see how far it can go. Do you think I could just make the nose out of foamboard? Because it would be lighter. The sides alone together weighed about 3.6 grams, and the stabilizer weighed about .8 grams. The fuselage is .6 meters, and the wings should span 1.2 meters. I have a picture.

 

[quorneng]

Nice!
There is no reason why it should not fly well but "How far it will go" will depend on how straight it flies. A plane will never maintain a true straight path without some form of control. It will always turn eventually.
The wind strength will of course have an even bigger impact of where the plane ends up!

 

[hello183]

Thank you. I might not be able to get it to fly straight, what I meant is I'd like to see how long I can get it to fly for. Now I finished the fuselage. with all the uprights in, it weighs about 4 grams. A mistake I made was that the longerons had a slight curve to them after I built the sides, so when I was pinning stuff down, some of the uprights stuck out from the fuselage, and I had to sand them down quite a bit so they wouldn't bulge out anymore, but I think it is still strong. The picture on the top is what it would look like when the vertical stabilizer is glued on.

The moment arm for this airplane is quite tiny, so I was thinking about making the horizontal stabilizer with a lifting airfoil. Does anybody know any flat bottom airfoils that are suited for this? Could I just get any airfoil and slice it in half down the line where you plot it to make a flat bottom airfoil?

Also, I think I will make the nose sometime soon. I have no clue how to do that, I am thinking about gluing some sheets of foamboard together, I'm confused on how it gets sanded from a square into a circle.

 

[Tench745]

Thank you. I might not be able to get it to fly straight, what I meant is I'd like to see how long I can get it to fly for. Now I finished the fuselage. with all the uprights in, it weighs about 4 grams. A mistake I made was that the longerons had a slight curve to them after I built the sides, so when I was pinning stuff down, some of the uprights stuck out from the fuselage, and I had to sand them down quite a bit so they wouldn't bulge out anymore, but I think it is still strong. The picture on the top is what it would look like when the vertical stabilizer is glued on.

The moment arm for this airplane is quite tiny, so I was thinking about making the horizontal stabilizer with a lifting airfoil. Does anybody know any flat bottom airfoils that are suited for this? Could I just get any airfoil and slice it in half down the line where you plot it to make a flat bottom airfoil?

Also, I think I will make the nose sometime soon. I have no clue how to do that, I am thinking about gluing some sheets of foamboard together, I'm confused on how it gets sanded from a square into a circle.

I'm confused about what you're describing doing with the nose. From what I see of the fuselage, it also looks like it has a pretty reasonably sized moment-arm for the tail. Do you have any pictures of what this is supposed to look like when it's finished so I can better understand?

 

[hello183]

here is a picture of the plans I drew in sketchup

The wing mounts right in front of the second former, I thought the moment arm was quite small, but I am not exactly sure. I think the horizontal stabilizer coefficient should be around .5. I figure I could make the airplane balance with less weight in the nose with a lifting stabilizer. For the nose, what I was saying is that they are usually carved out of balsa, but I think that solid foamboard is lighter than solid balsa. So for the nose, I was wondering if I could glue a couple of squares of foamboard and carve that down to the nose. The problem is that tissue would have to stick over it, and I don't know whether it can do that or not.

 

[danskis]

Nice build. I fly gliders and here are a few things I've learned. About the nose. The nose does not have to be round - it just needs to have a taper for the final 2 - 3 inches. Also, don't worry about making the nose light as you will need to add a bunch of weight to get it to balance correctly on the CG. Make the nose strong as thats what's going to hit first. You might consider sheeting the nose with balsa, foam board or something else. Glider kits just laminate a bunch of sheet balsa together and then you carve it down. You said the wing goes behind the second former - usually gliders have a fairly long nose. If you thermal this plane you should be able to stay up for 10 minutes easy. Read more on how to thermal an RC glider. Take a look at these exceptional flyers.

Sailplane Kits – Sonoran Laser Art

 

[quorneng]

Helo183
Just a point but almost any surface can provide lift. There are simple planes with 'flat plate' wings that fly quite well.
Thus the term 'lifting tail' refers to the condition where the incidence of the tail plane results in it creating lift in normal flight however to achieve any degree of longitudinal stability the tail has to create proportionally a bit less lift than the wing as the plane speeds up so the nose tends to rise causing the plane to slow down to the desired speed. And vice versa.
This is not easy to set up. Get it wrong, the plane becomes unstable and a crash is inevitable.
The conventional set up for adequate stability is for the wing to be set to create lift and the tail set to create zero or negative lift in steady flight.
With an own design it can take quite a few attempt before a 'good' combination is obtained to give a smooth constant glide.

 

[hello183]

I decided to make the outside foam, but to have a sheet of balsa in the center that is the same as the profile of the nose viewed from the side so it will be easy to do that after I have sanded down its profile from the top.
@quorneng My idea with the lifting tail was that, when the airplane was flying, at its normal speed, it would generate just enough lift so the airplane would have perfect balance. It would mean less weight for the nose, so less weight for the airplane in general. But I am confused with what you mean by the tail has to create proportionally less lift than the wing. Why proportionally?

 

[quorneng]

If a disturbance causes a gliding plane to speed up for any reason its natural aerodynamics have to automatically react to this by trying to raise the nose so the plane slows down. As it slows down the nose lifting effect will reduce so it will go back to flying as it was before the disturbance.
In other words the plane has to exhibit stability to maintain a set flying speed.
This is what the tail plane does. It provides a down ward force at the tail to keep the nose from dropping. If the plane speed up the down ward force it creates also increases, adding to the nose up effect so the plane will try to return to its 'normal' gliding speed.
If on the other hand the tail is providing a lifting force than if the plane speeds up that lifting force will increase trying to push the nose down more causing the plane to speed up further. The only way this can be avoided is for the wing to create more lift as the plane speeds up than the tail plane does so there is a resulting nose lifting effect.
The problem is with a lifting tail is the correcting force comes from a difference in positive lift rather than a much bigger difference between a positive and negative force.
In other words with a lifting tail arrangement the change in lift created by the tail from a change in speed must not be in the same proportion as that created by the wing. It is not easy to arrange for this to be the situation.

 

[danskis]

Most of the gliders I fly have a flat tailplane. Or slightly tapered on top. I wouldn't worry about a lifting tail.

 

[hello183]

I'll forget about the lifting tail idea for now. I was able to roughly carve out the nose today.

I was thinking about making a nose skid, but the nose feels so strong I don't think I will need that anymore. I want to cover the airplane in tissue, and I found an article that said that tissue can go over foam. But I think I might try to paint the nose separately and then glue it to the rest of the airplane after covering.

 

[CthulhuJon]

re:- flat bottomed airfoils.... E205 Airfoil generates good lift and has lowish drag. good for slope soarer. good all round airfoil.
http://airfoiltools.com/airfoil/details?airfoil=e205-il
http://heli-air.net/2016/02/10/e205/

https://www.rcgroups.com/forums/sho...the-Classic-Clark-Y-Airfoil-or-Profile-Sought

You can alter params, and print out airfoils from airfoiltools.com, see above.
If you are making a non-tapered wing.. you can make 2 ply or plastic airfoils, from your print out. The sandwich a bunch of rectangular pieces of balsa in-between and bolt them together, then sand all of them at the same time... or maybe do 1 wing's worth at a time...

 

[hello183]

thanks, Cthulhujon, but I don't really need a flat bottom airfoil for the horizontal stabilizer anymore. I think I will work on the wing now. I want to use some basswood in it for the spars because it feels tougher for its size, and the airfoil is going to be thin. Because it's basswood, I think 1/16 in sq sticks will be fine for the spars. But I'll test it and see. I want to make the tip chord airfoils a bit thicker so then the spar wouldn't have to taper downwards down the wing. I will have to glue the spars together at the correct angles to make the polyhedral before assembling the rest of the wing, and I don't want to have to worry about those angles a lot.

 

[CthulhuJon]

You are welcome.
I actually sent that in case you needed it for the main wing.
It’s best to make the polyhedral sections separately on a flat surface so they remain true and flat before gluing the completed sections together. You can angle one rib on each section so they will glue together at the desired angle later. Use a block of balsa or stacked books to lift wingtip to get the same angle on each wing when gluing.
Looking forward to seeing how it turns out.

 

[hello183]

For the main wing I was planning on using the s7055 airfoil. Also, I actually won't make the tip airfoils thicker. Instead of the basswood, I decided to cut the spars out of heavier 1/8 in sq balsa. Before I go on, how do people find the polyhedral angles. Its stabilizing effect depends on how far out the breaks are on the wings, right? Does it really matter? Would it be fine if I went with 10 or 12 degrees polyhedral on each break? The center section is 60 cm and the tips are 30 cm each.

 

[danskis]

10-12 degrees on each side is good

 

[hello183]

I found an article about dihedral and equivalent dihedral angle and decided to go with 15 degrees on each tip. I am working on the bottom spar right now, but I don't have any pictures on that yet. Since end-to-end glued sticks are supposedly not very strong, I cut some polyhedral braces out of a soda bottle. Here is the airplane with better light.

I have not really used balsa as thin as 1/32 before, but I know that it is a thing. Is it like paper? Would it be strong enough for wing ribs?

 

[Tench745]

I found an article about dihedral and equivalent dihedral angle and decided to go with 15 degrees on each tip. I am working on the bottom spar right now, but I don't have any pictures on that yet. Since end-to-end glued sticks are supposedly not very strong, I cut some polyhedral braces out of a soda bottle. Here is the airplane with better light.
View attachment 227468
I have not really used balsa as thin as 1/32 before, but I know that it is a thing. Is it like paper? Would it be strong enough for wing ribs?

I’d probably opt for 1/16” balsa for ribs. You can do thinner, but it becomes difficult to carry around without accidentally breaking something.

 

[danskis]

You could also cap each rib if you wanted to use 1/32. But its way easier to use 1/16.

I think the soda bottle dihedral braces are iffy. I would use a credit card - easy to cut and strong.

 

[hello183]

@danskis I might have considered that more but I already started gluing things together before I read that.

If I went with the cap strip idea, should they also go on the bottom of the ribs? If they were 1/16 x 1/32 inch would that be fine? I have also heard of 1/20 balsa. If I could find that, I might like to try to use that for the ribs also.