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"Flat Bottom" Airfoil - 3/25/2012 1:31 AM   
AmishWarlord



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Is there an official name of the "Flat Bottom" airfoil found on most trainers?

The AG36 looks close but a little on the thin side.





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RE: - 3/25/2012 1:48 AM   
BMatthews



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Status: offline 		No there isn't. The average flat bottom airfoil on most trainers is a flat bottom, some sort of circular radius on the leading edge and a top curve done using the TLAR (That Looks About Right) free hand curve or perhaps drawn around the designers shoe sole.

To paraphrase the disclaimer at the beginning of a lot of moves "the airfoil shown is not of any particular type. Any resemblance to a brand name tested airfoil is purely coincidental".

Most of them have fairly high drag when pushed to higher speeds. Now those of us looking for ultimate efficiency may find this disturbing and be inclined to replace it with a "proper" airfoil. But be careful what you wish for. A model which does not like to speed up easily is actually a big blessing to a new pilot. By replacing the "12% Florsheim Special" with a known better performer could end up with the model being too much for the newbie pilot when it speeds excessively at the slightest tendency to drop the nose.

So if the model comes with a well known good reputation just go with the proven recipe and leave it alone.

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RE: RE: - 3/25/2012 1:58 AM   
AmishWarlord



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RE: RE: - 3/25/2012 11:20 AM   
AmishWarlord



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Status: offline 		BMatthews,

I have a question on what I'll call the trailing edge beam on our model wings. Dose this beam have any structural purpose on a modern "un-silked" sheeted wing besides as a place to anchor the hinges? I've noticed on full scale aircraft that when you pull off an aileron or elevator that it's just open space between the ribs with no huge heavy beam running across like that seems to be used on most of the model wings in my plans collection.

I'm going to omit it on the next few models to see what happens.

I'm planing on using a built up hinging system inside the ribs so I don't need all that wood to anchor the hinging to.

I use jigging when I construct wings so I don’t need it to hold the ribs in place wile the sheeting is applied.

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RE: RE: - 3/26/2012 7:12 PM   
mithrandir



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Status: offline 		the aft spar serves the purpose of creating a closed wing structure for torsional stiffness.... it is necessary... don't leave it out...

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RE: RE: - 3/26/2012 8:54 PM   
AmishWarlord



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Status: offline 		How about just a dowel placed back there running the length of the wing panel?

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RE: - 3/27/2012 8:28 PM   
BMatthews



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Status: offline 		Amish, you need a trailing edge cap, beam or whatever you want to call it at the hinge line. It greatly stiffens up the trailing edge of the wing and supports the hinge line. If the hinge line is able to bend, bow or distort and the hinges get out of line then you won't be able to move the aileron. Or at best you'll greatly increase the load on the servo. On top of this it provides you with a hard edge to bond the covering to instead of leaving it floating around or trying to stick it to something less suitable than a balsa cap.

A dowel will both be heavier and not as stiff as a vertical balsa trailing edge cap which the ribs glue to and which the hinges anchor to. You may elect to anchor the hinges at the rib locations and that isn't a bad idea. But you still are better off with a balsa trailing edge cap.

As you say they don't need to be as big as some suggest. And on a wing section like you show where you're using "barn door" style ailerons that are that thick the trailing edge cap can easily be a piece of medium thickness wood just to tie the upper and lower skins together. For example on a 10 inch chord wing with that same section and ailerons shown I'd use a 1/8 sheet vertical cap if the wing was fully sheeted with 1/16 sheet. Or if it was open framed I'd use an open C shape with a 1/8 vertical web to join the top and bottom and something like 3/32 x 1/2 for inset top and bottom caps. Or if I were using cap strips on the wing of 1/16x1/4 I'd just use 1/16x1/2 caps along the top and bottom of that vertical web. It would be light but give excellent rigidity that way. And the 1/8 vertical would give you good bonding for the part of the hinges that weren't actually in the rib at the T joint.

The other consideration is that with any hinge line you need to provide a V or >< style joint to let the surface move without closing the gap and binding. Using a wider balsa trailing edge vertical web gives you the thickness to carve in the angle(s) needed.

I didn't know you were working on a Goldberg Eagle. But it's an excellent example of how a "bad" airfoil can be used to advantage. The Eagle's airfoil slows down nicely but it doesn't like to go fast due to the truly atocious aerodynamics. But for a newbie pilot this disadvantage becomes very much a desireable advantage by acting like a built in automatic speed brake. It's a great example of the sort of accidental or purposeful "mistake" which truly makes the model perfect for the intended use.

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RE: - 3/27/2012 10:33 PM   
AmishWarlord



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Status: offline 		Crap! Why does aerodynamics and structural design have to be so hard and stuff?

LOL

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RE: - 3/27/2012 10:49 PM   
BMatthews



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Status: offline 		It's not hard if you try to copy how an eggshell is made. Use as little as needed around the outside skin and as little as possible inside to hold the outer parts in their position. Simple to see how you can make a little do a lot this way. But not always that easy to accomplish in ways that are easy to build.

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RE: RE: - 3/27/2012 11:40 PM   
AmishWarlord



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Status: offline 		Checks trailing edge of egg shell to see if there is a aft spar in there, Ha! just as I thought!

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RE: RE: - 3/28/2012 2:37 AM   
MerlinV


 

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Status: offline 		Even a weedy egg like me knows that wing stiffnes is important (now, although when I started in this hobby, I had all sorts of strange ideas).

An eggshell has integral structural integrity. Every part of the egg shell helps to support the rest of the structure... Having said that, if one of my hens can't be bothered moving from the roost rail to the nesting box and just drops one on the floor of the coop, then the shell has no hope of standing up to the stress.

Cheers,

Hugh

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RE: - 4/25/2012 4:40 AM   
larrysogla


 

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Status: offline 		AmishWarlord, Our models really turn and loop at much more than 9 g's..........on full scale aircraft the pilot pulling 9 g's will blackout without an inflatable legging to stop the blood from pooling in the legs and draining the blood in the brain and causing a blackout. Our model aircraft are overpowered, overcontrolled and jerked all over the sky...........the stress on the wing and tail are horrendous and the model wing need all the structural strength to survive our high "G" manuevers. The trailing edge balsa piece gives additional strength to our overstressed model wing. Larry.

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RE: RE: - 4/25/2012 6:39 PM   
Von Ohain


 

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Status: offline 		You all forget one thing here:

(Or, Matthews touches it briefly, but maybe not enough to make someone "outside the loop" understand it)

What Matthews means with to replicate an eggshell is the honeycomb wing, or really, the philosophy behind it.
On a full scale aircraft, the ribs and spars is _NOT_ the load bearing structural compnents!
The skin of the wing is!
The ribs and spars is there only to stiffen up the skin and prevent it from buckling!
Not so on all model planes, the ribs and spars is load bearing, and the outer skin is just thin plastic film covering who's flopping about!

You are right that you don't need the trailing edge beam, IF you got a honeycomb (or related) wing!
An example used on oldtimer performance models, and some sport models today is the sheeted foam wing.
You wouldn't believe foam to be better than balsa ribs and spars, but it actually is if you sheed the foam with balsa.
Then the balsa sheets is actually responsible for the wings structural integrity, and the foam is just there as something to glue the balsa sheets to.
Such a wing needs no trailing edge beam, main beam nor leading edge beam.
Actually, I scratch built my wing for my 2 metre wingspan Composite ARF Extra 330 SHP in this way.
Its a foam wing on a 2 metre petrol powered high performance aerobatic IMAC model!
The hinges is just anchored into wooden blocks, "randomly" glued into the foam, and between the balsa sheets.
Works like a charm, even stronger than the stock composite wing, which I actually managed to break in the air.
Not so with the balsa sheeted foam wing

Ok, that was a digress, but just to illustrate the point.
Is the skin load bearing, or is the skin just flapping about?
With a load bearing skin, you can anchor the hinges to the skin, and forget the trailing edge beam.

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RE: RE: - 4/25/2012 6:50 PM   
AmishWarlord



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Status: offline 		Wing skin is load bearing 3/32 laminated hot molded pressed balsa.

Hopefully there are no shrink wrappers here giving me advice on trialing edge beams.

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RE: - 4/26/2012 12:11 AM   
jester_s1


 

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Status: offline 		BMatthews - do you mean that the trailing edge cap should be vertical grained like a shear web instead of running the span of the wing? What's the reasoning behind that?

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RE: - 4/26/2012 4:08 AM   
MerlinV


 

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Status: offline 		Sheer webs have vertical grain because they are what is holding your top and bottom spars together. It's holding these together that is stopping the two piece spar bending.
If you had the grain running length wise, then the sheer webs would simply split!

I recently built a d-box wing. Even with the sheeting of the leading edge back to the spar complete, and the Training edge (Actually Edge forwad of the Aileron) attached, the wing was very flexy. When I put ythe sheer webs in, all of a sudden, the wing stiffenned up.

Cheers,

Hugh

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RE: RE: - 4/26/2012 9:21 PM   
BMatthews



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Status: offline 		Jester, what Merlin said is 100% correct. But in the case of a split V style trailing edge there's so little load that one can "get by" with spanwise webbing to close the triangle. But for a more load bearing item such as closing a D tube or joining upper and lower spars there's no cheating allowed. Vertical grain for such uses is the best way to go.

Although I've done both control line and RC models with D tube leading edges where there's no spars as such and the "webbing" is something like a 1/8 to 3/16 spanwise "spar" that is slotted half way from above and the ribs slotted half way from below so the spar and ribs egg crate together. It's not technically right but in those cases the designer (me in a couple of cases) gets away with it due to the light weight of the model and reasonably generous wing thickness so the D tube sheeting itself becomes the actual spar.

But in the case of a thin and fairly heavily loaded wing spar I'd never even consider such an option. For those it's flat spruce upper and lower spars with vertical webbing joining them. For the serious stuff such as glider wings the webbing actually should go between these two spars rather than glue to the vertical "face" edges. When only glued to the faces you're relying on the strength of the glue or the balsa surface fibers. Not a wise idea. With the webbing fitted between the spar caps so it forms an actual "T" shape you only use the glue to keep the piece from coming out of place. The whole end grain edges and all the fibers in the webbing cross section share the load in a far more optimum manner.

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