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Jan 07, 2002, 06:43 AM
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airfoils on tiny jets? undercambered or airfoil?

the micro jet that we have seen worked with under cambered wings so i you put an airfoiled wing it thier place would it have any effect besided adding weight?

i have heard that airfoils lose thier effenecy as the size goes down after a sertain piont(really small) it no more effeicent then a flat wing(no cambed ,no airfoil)

what do you think?
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Jan 07, 2002, 06:36 PM
Thanks for the Fish!
aerogel's Avatar
Hi I-fly Here is a sight with some good numbers
Thin is It In my Book
Jan 08, 2002, 01:03 AM
Electric Coolhunter
Thomas B's Avatar
Based on competition results and long time practice, Ifly, you have heard wrong.

Even on small indoor hand launch gliders, there is lots of evidence that an airfoiled wing performs significantly better than a flat plate.

The undercambered single surface airfoils used by lots of slow indoor models (the little jets, Lite Sticks, Tiger Moths, Bleriots IIs, etc, etc, ) work well, even at the low Reynolds numbers they are operated at. They develop a nice amount of lift at fairly low speed.

You could easily exchange the airfoil on one of the small jets to a normal Clark Y or RG-15 or E-214 or whatever and the model will fly. It will also fly a little faster (less drag at higher speeds than the single surface wings) and be less suitable for indoor venues. Anything but a bare foam (1 lb/cubic foot foam) wing will end up heavier than the molded single surface undercamber wing, and NEED to fly faster.

Go to the Jet forum and view the video of the little Sabre flying indoors that is linked to a post there. That little bird has about 1.1 sq feet of wing area and weighs about 7.3 oz. Call that about a 6.5 oz/sq foot wing loading. That model is about at the pratical limit of as heavy and as fast as an indoor model can be.
You need to keep this in mind as you work on an indoor jet.
Last edited by Thomas B; Jan 08, 2002 at 09:53 PM.
Jan 08, 2002, 01:15 AM
Tom is right, but maybe I can add a few things...

The undercambered(UC) airfoils function better at lower Reynolds than a full section airfoil. It is commonly though that UC airfoils are ONLY for low RE applications, but that isn't necessarily true. The UC airfoils are also very effective at higher RE, so they are something to be considered for a wide range of subjects, where most people only consider full section airfoils.

When most airfoils operate out of their design limits, they tend to lose effectiveness, but the UC airfoils don't succumb to this as most other types do. Essentially they just keep getting better(in most areas at least).

Jan 08, 2002, 09:51 PM
Electric Coolhunter
Thomas B's Avatar
Excellent point on using undercambered airfoils on larger machines. They were as common as could be in the first couple of decades of flight on full size aircraft.

My experience with UC airfoils does show one aspect that tends to get worse as you use the typical UC airfoil outside the design limits, even if the airfoil is working well in some ways.

Drag can go up a huge amount if you try to fly an undercambered airfoil at high speeds. I fondly recall flying the old M.E.N. Trainer and the original Olympic 99, both with undercambered 'foils.
Great for slow speed cruising around, but NOT good for speeding up and flying fast. In fact, the UC airfoil pretty much was its own built- in speed brake,
Jan 08, 2002, 10:51 PM

YES, that's what I was referring to, but didn't mention it directly. The UC airfoils get VERY draggy, but they still maintain the lift component, as opposed to some airfoils when flown out of their design specs, the lift drops, and the drag rises. With the UC, the lift AND drag rises.

It could be fairly beneficial in some designs, since it does have a tendency to limit top speed as you've mentioned.

Really, you can't go wrong with the UC airfoils, the plane WILL fly(possibly with more drag though), but the same may not necessarily be true for a full section airfoil.

When you're flying faster(design speed), then you can move to the full section airfoils easily.

Jan 10, 2002, 01:16 PM
Our Daddy and Heli Junkie
Fred Bronk's Avatar
UC wings were very common on earlier planes because they tested models and just scaled them up. They did not take into account the air molocule size so the wings acted very different between the model and the full size, but they did fly.

I read an article that had a nice breakdown on wing area vs airfoil that was something like this (now this is from memory)

Under 250 sq/in undercambered
250-600 sq/in Clark Y-FB
Over 600 sq/in semi sym

Wing loading can also go up with each step up and the plane will still fly great.

My small planes fly great at 4-8 oz a sq ft W/L, but a 30-40 oz loading can be the norm for large planes, and full size W/L are usually in the lb sq ft!
Jan 10, 2002, 04:52 PM

That's very interesting information, hopefully it's been tested enough to provide a rough guidline to follow.

What's also interesting is the comment about the tolerable increased loadings as things get bigger. I would assume that has a fair bit to do with increasing Reynolds Number, and the fact that the sections are more efficient. That's what makes sense to me anyway...

Thanks for the info,

Jan 10, 2002, 08:49 PM
Electric Coolhunter
Thomas B's Avatar
I would not automatically use an undercambered airfoil in models of less than 250 sq in. I have a Playtron CAP 21 that is very happy with its 88 sq. in. of wing area and 9 oz weight and NACA 23XX series airfoil (non scale airfoil). Flys good, flys fast. Numerous speed 400 pylon aircraft prove that idea less than perfect as well.

No need to limit yourself to certain types of airfoild just based on size of model.
Jan 11, 2002, 02:43 AM
Registered User
Size , weight, and desired flight envelope I would imagine.
Jan 11, 2002, 03:12 AM
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davidfee's Avatar
Originally posted by Mikey-flies
Size , weight, and desired flight envelope I would imagine.
Yes, the desired flight envelope is key, and it's all about Reynolds numbers. "Oh no... math!!!" Yeah. The difference between an 80sq" pylon racer and an 80sq" FF rubber plane is the Reynolds numbers. The FF plane is much lighter and it's moving much more slowly... so it's seeing fewer air molecules. Because of this, it needs a different airfoil to achieve the same lift. The FF plane would not have very good duration with an MH-30 or RG-14, and the pylon racer would be a dog with a Benedek or Jedelsky undercambered section. Where it gets a bit fuzzy is in the low-powered indoor "fast" planes such as the GWS-powered EDF's. These are never going to fly "fast," so they are still at low Reynolds numbers and need the low Rn airfoil sections. Hence the undercamber.

have fun,
Jan 11, 2002, 06:30 AM
Registered User
jberg's Avatar

Originally posted by aerogel
Hi I-fly Here is a sight with some good numbers
Thin is It In my Book
this report is really very interesting. But I miss some notes about the flight distances. To estimate the needed thrust for level flight you have to know the Cl/Cd ratio or Cd in isolated form. I did not yet try to calculate Cd from the given numbers, so I don't know if it is possible somehow, but the flight distance or the glide path angle would surely help.

One could assume Cl= ~0.8-1.0 (UC) and Cl= ~0.6-0.8 (flat). Then:

Cp = Cl ^3/2 / Cd --> Cd = (Cl=0.6..1.0)^3/2 / Cp

I don't know how accurate that would be.

Anyway, thanks for publishing this!
Last edited by jberg; Jan 11, 2002 at 06:43 AM.
Jan 11, 2002, 04:44 PM
Senior to who? Member
crossup's Avatar
One easy solution to the UC vs Clark Y debate is to use the UC and fly it.
THEN use Reynolds wrap on the bottom to make it into a flat bottom Clark.
Fly and evaluate.....
Jan 11, 2002, 04:51 PM
Registered User
Thread OP

this is what i found during my advantuers with numbers...

i have found that a tiny f-16 would be very possible
i have some scaled down f-16 plans from a friend that were half the size of the really big f-16(91.38 inch long) so i took that stats from that and compared them to the f-16c block 40 5 view drawings...
since the full size(91.38 inch one) has 1022.5 sq inch of wing area(they figured it out not me) i had to bring it down to the micro size

the micro f-16 specs -Length 25.842 inch Wing span 15.6247 inch (no missiles or rails) with a wing area of about 289 sq inch!
thats close to 2 ft of wing area

with that wing area and an auw of 7 oz gives me 3.48 oz per ft!
even with a auw of 9 oz its only 4.48 oz per ft!
with the kp-44 for power and the turbo set up which give 5oz of thrust off 10 cells the thrust to weight isnt that bad
i figured the all up electrics weight would be 4.722 oz the only varible is the airframe weigh which can weigh in any were from 2.278 oz to4.278oz(no way!) and still have good power

there is only one draw back form this...the intake is rather small so there will need to be about a 1 inch cheater hole(thats with exact scale inlet) but the area may be off too since i use the area of an ellipse and the inlet on the f-16 is close but not an ellipse

so the next part is building the plug for the molded depron fuse...any ideas on that?
Jan 12, 2002, 03:11 AM
Our Daddy and Heli Junkie
Fred Bronk's Avatar
Yes, It depends on what you want the model to do. The info is not stating you must do it that way. Just what airfoils produce the best lift for a given area.

For instance take the NASA electric plane they are trying to break the hieght record with. The wing is not UC at all and look how slow it flys.

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