building a glider
- Thread starter hello183
- Start date
I've been busy so not much stuff has been done, but maybe it will pick up now.
@Piotrsko, Is this true for airplanes with ailerons? When I did some research, I found a calculator:
http://www.charlesriverrc.org/articles/design/eda1.xls
It says for anything under 2 meters, he was fine with anything with 8 to 9 degrees dihedral. Also, this glider is free flight, and the 15 degrees is polyhedral, which makes its equivalent dihedral angle about 10.8 degrees. Even with the area the polyhedral cancels out, the airplane still seems to have a lot of wing area for its weight, so I thought it would be fine.
Out of curiosity, how is equivalent dihedral angle calculated for curved dihedral wings? Like those designs with a curved spar and the wing bends elliptically instead of with flat breaks?
@Piotrsko, Is this true for airplanes with ailerons? When I did some research, I found a calculator:
http://www.charlesriverrc.org/articles/design/eda1.xls
It says for anything under 2 meters, he was fine with anything with 8 to 9 degrees dihedral. Also, this glider is free flight, and the 15 degrees is polyhedral, which makes its equivalent dihedral angle about 10.8 degrees. Even with the area the polyhedral cancels out, the airplane still seems to have a lot of wing area for its weight, so I thought it would be fine.
Out of curiosity, how is equivalent dihedral angle calculated for curved dihedral wings? Like those designs with a curved spar and the wing bends elliptically instead of with flat breaks?
Piotrsko
Master member
Aileron effect is somewhat negated by dihederal, so you need more powerful surfaces to compensate. Stunt aircraft tend to have flat wings. Rudder only controlled aircraft have big angles or polyhideral because that enhances rudder control by lifting the outside wing and starting a roll moment with a bit of slip leading the turn. Free flight uses that same effect for self stabilising and turn in into a thermal.
Big dihederal also leads to more structure at the joint because you are moving forces in a non linear way. Not overly significant on our stuff, but gets hairy on heavy flying items. In my experience, wings break at the joint when overstressed in flight. It also reduces efficiency by reducing the aspect ratio. Obviously I'm not a big dihederal fanboy.
On curved spar wings the effective dihederal is taken the same as a multi: the amount of tip up offset from a reference plane (tabletop). A bit trying on a fully curved wing because it wont sit flat on the table. You can also level the tips and measure that
Big dihederal also leads to more structure at the joint because you are moving forces in a non linear way. Not overly significant on our stuff, but gets hairy on heavy flying items. In my experience, wings break at the joint when overstressed in flight. It also reduces efficiency by reducing the aspect ratio. Obviously I'm not a big dihederal fanboy.
On curved spar wings the effective dihederal is taken the same as a multi: the amount of tip up offset from a reference plane (tabletop). A bit trying on a fully curved wing because it wont sit flat on the table. You can also level the tips and measure that
@Piotrsko Well it is already done so I guess I will try it with 15 degrees and it's awful I might try to build it again. I don't think the wings will break at the polyhedral breaks though. I decided just to cut the center ribs manually. I didn't feel like making holes in them for a template. I cut one rib out of 1/16th balsa to see how light it is, and I think it will be light enough. These are the 15 center ribs.
Piotrsko
Master member
As I said, it is my preference and reasons for low dihederal. Your airplane and choice, but you did ask. If you want to go lighter still without much penalty, lightly sand the blanks with 320 grit sandpaper.. 1/16 was pretty much the standard back in the day for everything, 3/64 wouldn't be that far off for strength.
I actually forgot about posting. I have made some progress.
I try to crop out all of the clutter, but it is too big for that anymore. In the back is the original horizontal stabilizer I made for this airplane, but I made a mistake of using 3/32 square basswood spars with it. It is a bit too heavy. It weighs about 17 grams as it is in the picture, without the nose block. The wing seems to warp a lot. I guess it is because I haven't glued in the top spar yet.
I try to crop out all of the clutter, but it is too big for that anymore. In the back is the original horizontal stabilizer I made for this airplane, but I made a mistake of using 3/32 square basswood spars with it. It is a bit too heavy. It weighs about 17 grams as it is in the picture, without the nose block. The wing seems to warp a lot. I guess it is because I haven't glued in the top spar yet.
CthulhuJon
Member
Regarding warped wing. Are you pinning it down onto a flat surface while gluing it together ? It should stay fairly flat and non-warped if you do this. Usually worry about warp when shrinking the covering.
I actually did pin it to my building board, a piece of drywall. I don't think that is the problem. I think it will be really stiff when I glue in the top spar. I can't do it now, because I still have to make polyhedral braces for the top spar. I will do that after I glue all the tip ribs on.
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quorneng
Master member
hello183
Just a point but an un covered 'open' structure like your wing will have little torsional strength and not that much bending strength either with just top and bottom spars. The covering, if taut, obviously gives torsional rigidity to the wing but also provides the necessary shear resistance between the top and bottom spars.
The covering (tissue and dope?) is an important structural element for both the wing and fuselage when built in this way.
Just a point but an un covered 'open' structure like your wing will have little torsional strength and not that much bending strength either with just top and bottom spars. The covering, if taut, obviously gives torsional rigidity to the wing but also provides the necessary shear resistance between the top and bottom spars.
The covering (tissue and dope?) is an important structural element for both the wing and fuselage when built in this way.
Thank you quorneng. I was thinking about using tissue covering over it. I was thinking about shear webs or turbulator spars. But I think I might reinforce it another way. I heard that you could get some decorating tissue at craft stores or Walmart somewhere. Is that good?
I was able to finish cutting the ribs for the wing tips. Since the tips are trapezoidal, it is not hard to find the length of each rib. I will see where the spar cutouts should go when I am putting the rest of the wing together. I have a question, compared to any airplanes anybody else has built, would this airplane be seen as light? The 1/16 balsa I had for the ribs seems heavy. Does anybody know some other airplane to compare it to?
Piotrsko
Master member
My 4meter glider weighed 20lbs including 2 pounds of lead nose ballast, my 1meter was about 1-1/2 lb and 1/16 was typical for ribs back in the day. your beer scale says 3.3 grams, you have another 460 grams to make a pound so another hundred ribs. Get the first one to fly, make improvements on the following ones.
thanks for that info @Piotrsko. Also, those 4m gliders seem really cool. I read about the one that Jackson T made. Anyway, I am aligning one of the wing tips' spars so that I can start gluing the ribs on. I should have drawn a plan, aligning the spar would be easier with one, even if the plan is just a line that I could use for reference.
Tench745
Master member
You could still draw a line on something.
quorneng
Master member
Decorators tissue can be used but it is basically not water proof, it tends to disintegrate if it gets wet but ideally you have to water shrink it once it is covered to get it taut. Getting decorator tissue damp enough to shrink but not to disintegrate takes a bit of experience.
There are 'water resistant' tissues that remain strong when wet but they are many times the price.
After water shrinking to get it taut it is painted with 'dope', a type of sealing varnish that make the tissue properly water proof and 'fixes' it in its taut condition.
It may surprise you but the distance a glider can travel from a given height is, within reason, not effected by its weight. Heavier just means it flies faster along the same path but it will be sinking faster. It is going to depend on the lift conditions where you fly as to how slowly you need the glider to sink, in other words its weight, in order for it to be able to stay up.
I would expect the type of glider you are building would not weigh more than 6 to 8 ounces ready to go.
There are 'water resistant' tissues that remain strong when wet but they are many times the price.
After water shrinking to get it taut it is painted with 'dope', a type of sealing varnish that make the tissue properly water proof and 'fixes' it in its taut condition.
It may surprise you but the distance a glider can travel from a given height is, within reason, not effected by its weight. Heavier just means it flies faster along the same path but it will be sinking faster. It is going to depend on the lift conditions where you fly as to how slowly you need the glider to sink, in other words its weight, in order for it to be able to stay up.
I would expect the type of glider you are building would not weigh more than 6 to 8 ounces ready to go.
@quorneng although the distance is not affected by weight, doesn't the flight time get affected by it? Because the lighter one sinks slower? But thanks for that information. The wing tip is nearly done.
@Tench745 I actually ended up cutting a little template out of foamboard, that slips over the polyhedral rib and pressed that to the spar to keep it straight. I then propped the wing with some books.
@Tench745 I actually ended up cutting a little template out of foamboard, that slips over the polyhedral rib and pressed that to the spar to keep it straight. I then propped the wing with some books.
danskis
Master member
Assuming the air is dead i.e. no thermals, what predominantly determines how long it stays up is wing loading. Wing loading is determined by the weight of the plane and the size of the wing. Just imagine your glider with twice the wingspan or half the wingspan. The way your building it the weight wont change much but wingspan will effect wing loading.
