high speed airfoil

[flybike]

Sorry this isn't about electric flight, but I thought some of you could help me anyway. I am designing a plane strictly for high speed, all fiberglass, quickie 500 style engine. This will not be used in any competion, partly because there is none near where I live, so rules about airfoils do not apply. The goal is simply to have some fun. My question- how thin can I make the wing to reduce drag yet still maintain the ability to create lift? 1/2in, 1/4, 1/8? Somewhere around 48" span, 9"chord. Is a symetrical shape the best? At this thickness would it be strong enough to, say, pull out of a 175mph dive? Anyone have experience with these types of engines? Thanks, Hans

[Duane]

Hey, you might try www.rcuniverse.com for big glow stuff. Something like a MH43 airfoil would be nice, you can download Profili for free, I don't remember the site now, I can find it later maybe.

[flybike]

Thanks for the link, I have never heard of RC universe before. I'd like to stick with a conventional plane, but are the delta wings faster? Thanks, Hans

[davidfee]

In my experience, a major consideration in using an ultra-thin wing is structural integrity. My most recent wing has a maximum thickness of 7% (thins to 6% at the tips) and it needs a lot of carbon in the spar to keep it from bending. Symmetrical is not the way to go in most cases. Lift comes from camber or angle of attack, and just a little camber can go a long way at speed.

On a 9" chord, 6% would be just over 1/2" thick. At that thickness, your spar will need to be VERY strong... and it will end up being very heavy as a result. If you went much thinner, I'd definitely suggest two spars in the wing (fore and aft) instead of one.

Also consider your wing loading and the max lift coefficients of the airfoils you're looking at. You might end up building a model which can't launch or land safely. Keep an eye on the stall speed of the design.

MH 30, MH 43, MH 32, MH 22, RG 14 and SD 7003 are all good sections for smaller models, but you'd have to do some calculations to see what would be "best" for your expected Reynolds numbers and flight envelope.

Have FUN!! :)
-David

[flybike]

I just built a wing out of solid doug fir, using a planer, not unreasonably heavy, super strong, 1/4 inch thick. I'll try to make a 'glass copy of it tomorrow, maybe get the weight down. I noticed over at rcuniverse that almost all the planes were deltas, is there a reason for this?

[davidfee]

Deltas are popular with sport pilots... although I can't figure out why. Maybe the looks? They are pretty fast in the straights, but they bleed off a LOT of speed in the turns because they have very high drag at high lift coefficients/angles of attack. This is why pylon racers are never deltas... instead they are usually reasonably high aspect-ratio conventional designs.

If all you want to do is go fast, then a delta (even a flat plate) can do well for you. A conventional racer planform will probably result in a better-flying model, however.

On the plus side for a delta is that the spar can be lighter.

Have fun with whatever you do! :)
-David

[thin wing]

Deltas only give advantage at speeds exceeding 300mph as the atmosphere or air get compressed at the leading edge so are popular for concord or mach1 plus aircraft f16 etc

For models deltas can give the illussion of speed but are the same speed as staight type wings as speeds usually less than 200mph

thinner wing go faster on average but invest in bungees (tricky with oily birds)

Recomend get a fast electic pylon ship and put in oily power 500

try something like aeronaut suprise hotliner and deep pitch prop
should get 175mph in dives easy and still be easy to hand launch


fly low and fast and crash :D :D :D

David in Ireland

[jbourke]

Originally posted by Duane
Hey, you might try www.rcuniverse.com for big glow stuff.

You could, but I would certainly appreciate it if more folks made use of our very own glow forums. Just click on the "Aircraft" tab or the following links: http://www.rcgroups.com/forums/index.php?topic=air

Jim

[Duane]

Sorry, but I never knew there was much glow activity around here. I usually see smart remarks from moderators about no glow questions and stuff like that! Don't worry, nothing can replace this site for e-power! I was just giving and option for many glow modelers. Rc universe has a pretty crappy electric forum. I also recommend those people coming over here. I like to look at all options.