Suggest an Airfoil, Help please

[Edwong]

Hello, currently I'm working on a big project to recycle a wing I have, here is the link to it,

http://www.rcgroups.com/forums/showthread.php?t=1035234&highlight=wing+cap+232

If you just skim read it all you'll see the design changes I've been doing.

Basically the thing is, I know the balsa wing I have now will die eventually. So I'd like to replace it with an Epp wing that I can customize completely. It is a pusher aircraft, 36" wingspan. The airfoil on the wing at the moment is semi symmetrical so there is no reflex and therefore I have to have a horizontal stabilizer, rather than manually put reflex in on control surfaces.

So I'd like to ask what kind of foil I should use. My aim for the project is to get above 100mph, I'm gonna be running over 400w through it. CG at the moment I've got by where it was when the wing used to be on the Cap it used to belong to. But this is just a ball point figure, I'm gonna be doing some very extensive test with AA batteries as weight to get the CG perfect with the toss and glide thing.

Anyway, here is the site that does the custom wings,

http://www.foamwings.co.uk/Pages/Wings/BespokeWingDetails.asp


I would really appreciate any help.

Thanks,
Edward

[MCarlton]

BW 050209 looks like a candidate, thats the only one I've read about in any detail.

http://www.b2streamlines.com/BW050209.pdf

Someone else will help more, I am sure.

You could probably do worse than to just use the a normal cambered section, given the width of the "elevons" on the wing as a percentage of chord. Give them a chunk of reflex and bring the CG right forward to about 15% MAC or something.

I'm guessing though, so, probably better not to take too much notice ;)

[Edwong]

thanks very much, so you'd recommend me having reflex? I always though that this made it very sensitive to elevator input and less control throw for ailerons.

So do you reckon this will get me to 120mph?

[Edwong]

So I wouldn't need a horizontal stabilizer with that foil? Cause I was aiming for a JW60 look and a anything horizontal sort of destroyed that. I'm gonna be getting enough epp to make a lot of fuselages all for different purposes, and I have grown fond of the V-tail idea so I'll be trying different things out for sure. The V-tail increased the length from the thrust point to the CG so having the vertical fin will be far better for stability and efficiency.

[MCarlton]

The section I put up is auto stable in pitch, as it was designed for "plank" type flying wings with no sweep, so no horizontal stabiliser required.

If you have no horizontal stabiliser, you'll always need reflex, because thats where the stability comes from. Basically, the wing wants to pitch "forwards" ie LE down, and is prevented from doing so by the reflex. Its a bit more complex than that, but thats the basic idea.

With a normal section, on an unswept flying wing, there is nothing to stop the wing wanting to pitch, so it won't fly. Now, if the wing is swept, the sweep adds pitch stability, so you can get away with a section with a very low CMo (Pitching Moment - ie, how much the wing "wants" to pitch forwards), because generally, washout is applied to the tips from about 50% of the span outwards. In effect, that washout creates a reflexed effect. But a plank needs a stronger CMo coupled with reflex to keep things stable and generally speaking, the higher the aspect ratio, the more reflex is required. Hence a low AR delta can fly with very little reflex, due to the wing being, in effect, being highly swept (look at the 25% chord line on something like a "Diamond Dust")

The reason reflex sectioned "planks" are sometimes twitchy on elevator is because there is virtually no pitch damping (ie, nothing is stopping the wing wanting to pitch) so small changes at the TE have an exagerated effect.

Hence CL combat models tend to be "planks" because the loop radius can be tiny in comparison to a conventional layout.

If you want to experiment, you can use a V-tail and keep the reflex section. Basically you can use the V-tail as a control surface, so set it up at the same incidence angle as the wing, so it is effectively unloaded aerodynamically until you move a control surface on it.

But, you will still have quite rapid response to pitch control, so keep the movements less than you would normally use, or use a bit of expo to keep things manageable around neutral.

The only issue with adding a V-tail is that you will need to add more weight in the nose to keep the CG in the right place. If you do that by increasing the nose length, then its worth hanging the V-tail out a bit further back. You don't want more "side" area forward of the CG than behind it, otherwise you'll lose yaw stability (yaw stability does tend to be an issue on planks, hence wings like the JW60 have a comparitively huge fin) as there is no "moment" (leverage basically) for the fin to use, and no side area behind the CG as with a conventional layout.

I don't know if this will get you to 120mph, I'm not that smart. Remember that a plank is less efficient than a normal layout and can be more draggy, so a plank may need a thinner section than you would normally use, plus, when calculating wing loading and area, its as well to adjust the figures by a few % to take account of the less efficient wing section.

I hope someone else chimes in, because all this is just based on things I have read, and I am no expert in any sense of the word, and I wouldn't want to be leading you down a blind alley.

[Joel K. Scholz]

I might suggest the SD8020. I built a 100mph wing with this and was very happy with the results/ Here is the video of the plane:
http://www.youtube.com/watch?v=5Gm9SiI_kj4
Here is a link to the build.
http://www.rcgroups.com/forums/showthread.php?t=804375

[MCarlton]

I'm not sure how well 8020 would work on an unswept wing? Seems to me that the Cmo is a bit too "soft". It'll work well on your model (as it does, brilliant model by the way) because the planform is pretty well swept.

[Edwong]

Yeah my aim was to keep that same shape but thankyou. What about the JW60 airfoil? seems to perform and I'm making something that looks just like it (if I got with the large vertical fin).

[Edwong]

I'm gonna be doing tests as a slope model before I power it I was going to make more than one fuselage for if I want to switch between them but that will be in the future. So I'm gonna be building it as planned, test fit the electronic equipment install my servos, I've already got a couple receiver batteries and I'll try it out at a slope.

If it all goes to plan then I'll install the power setup. The slope is not all that steep but there is a lot of wind most of the time.

I'm gonna scrap the idea of using the Cap wing, I'm wanting a little more out of this project so I'll just use the cap wing for a later project, maybe a beginner plane for a friend, already has good dihedral.

I don't really know where to start with foils, it is a little more important now if I'm gonna be flying unpowered, so what would you recommend?


Thanks,
Edward

[Edwong]

You reckon I could make my fuselage round this wing?

http://www.adamsnet.plus.com/shop.htm

[BMatthews]

MCarlton, the 8020 will work because in the final trim format the pilot will add a little bit of up elevon which will become the reflex needed for stable operation at the same time it balances the CG forces.

Since the JW60 is a plank wing then the airfoil from it would work just fine.

I would skip on that wing kit unless you make it up as it sits. Being a swept wing it likely has some washout cut into the cores. When you recut the root section to make it a plank style wing the design will be all mixed up. It would be like your wingtips have up elevon in them all the time.

[MCarlton]

MCarlton, the 8020 will work because in the final trim format the pilot will add a little bit of up elevon which will become the reflex needed for stable operation at the same time it balances the CG forces.

I understand that, I was just thinking that if the OP was initially looking for an ideal section for a plank wing, I would look for a dedicated plank section. I don't know an awful lot about it.

[BMatthews]

Well, I tend to agree with you on choosing a plank specific airfoil. But oddly enough if it is intended for aerobatics where it'll be upside down as much as right side up then a symetrical airfoil is probably the best and just be prepared to push a bit of down to achieve the reflex reversal as required.

For anything light lift or thermal friendly though a proper "one way" airfoil would be my choice as well. But for a DS glider that'll be seeing significant positive G loads but seldom be flipped over I'd say that an airfoil with a smidgeon of camber that still has the minimum drag bucket extending down to Cl=0 would be an advantage.

[MCarlton]

Yep, I see what you mean, and it makes sense. I suppose that in DS especially, low drag down to Cl=0 would be beneficial, and in that area, it looks like a proper relfexed section would suffer.

Looking again, some other candidates spring to mind for this application;

MH54 - 0.6% camber, 8.0% thickness, very low drag beyond Re.1,500,000

MH17 - Near zero Cmo, ideal Re.400,000+

MH17 looks like it could be nice for a DS model, lots of "thickness" in the read portion of the section to keep things stiff and flutter free.