SMALL - espritmodel.com SMALL - Telemetry SMALL - Radio
Reply
Thread Tools
Old Aug 12, 2014, 09:42 AM
I NEED a micro seaplane lrf!!!
fabersum's Avatar
Torino
Joined May 2007
322 Posts
Help!
Suggested airfoils for a 18"-20" WWII warbird?

Hello everybody

I am designing a Fiat G 50 (Italian WWII fighter) to be powered with all the gear out of an Eflite UMX Sbach 3D.
I am aiming at a 1:24 scale that would give me exactly 18" ws. The model will be CNC routed out of blue polistyrene foam. I hope to end up with a 75 g AUW but this is just a guesstimation.

I could go with scale airfoils (NACA 2415 at root, NACA 2410 at tip) but probably the Reynolds number and so on would make some other profiles more suitable... and I would love to find a biconvex profile to respect the lower profile of the fuselage (I'd be sorry to have to "flatten"it with a Clark Y)

So: which airfoil would you suggest to use for such a model?

Many thanks!

Andrea
fabersum is offline Find More Posts by fabersum
Last edited by fabersum; Aug 12, 2014 at 09:45 AM. Reason: forgot one thing.. :-)
Reply With Quote
Sign up now
to remove ads between posts
Old Aug 12, 2014, 09:48 AM
Registered User
Gunbarrel, CO
Joined Dec 2006
114 Posts
Does the airfoil on an 18" wingspan model really make any difference? I would think the wing and tail incidence would be the biggest factor.
alset is offline Find More Posts by alset
Reply With Quote
Old Aug 12, 2014, 10:45 AM
Registered User
Canada
Joined Nov 2000
7,071 Posts
Airfoil likely won't make much perf difference.
But If a Billet model a curved single surface foil won't be overly scale in appearance.
Try a 'True" Clark Y (different that what most everybody thinks what a clark actually is though).
It will work as well as Any and better than most other foils.
Post piccies of the rascal during construction?.. it looks very interesting.

PS: a flying stab eliminates Incidence issues.
Bare is offline Find More Posts by Bare
Reply With Quote
Old Aug 12, 2014, 11:05 AM
I love my ID.
Fairfax VA
Joined Aug 2004
664 Posts
It is simple. thinner is always better than thicker. I am telling you this from my experience and report from high budget research results from uav company. And there is a reason profile models fly better. I would keep it thin as possible but try not to ruin the scale look. Wing incidence wise, low main wing incidence will force your plane to fly in tail down attitude at slow speed. I usually set mine at positive 3 degree. Tails can be adjusted after maiden for best result.
jingjingjing is offline Find More Posts by jingjingjing
Reply With Quote
Old Aug 12, 2014, 12:02 PM
I NEED a micro seaplane lrf!!!
fabersum's Avatar
Torino
Joined May 2007
322 Posts
I understand airfoil doesn't matter that much at these sizes..

I think I will do a couple of test wings - it doesn't take much to CNC one at this size..

I suppose my tests will be Clark Y and NACA 2415-2410. I will give some positive incidence (3° or 4°) at the root and 2° washout at wingtip to reduce the infamous G50 tendency to tipstall..

Of course I will post some photo here when I'll have something to show! :-)
fabersum is offline Find More Posts by fabersum
Reply With Quote
Old Aug 15, 2014, 02:41 PM
Registered User
Joined Mar 2014
72 Posts
At that size and wieght I would take Clark Y.

I usually set the lower surface (the rear 60% of it) parallel with the axis of the plane (streching over the whole wing span) and the tail surface parallel as well.
If You do so and give the Calrk Y 110% of its original thickness at the root and just 80% thickness at the tip there is a built in aerodynamical offset which helps to avoid tip stall.
The thick root airfoil helps with the scale look and the over all thickness is a bit thinner than original Clark Y which helps with small models.

Take care to make the wing surface lightly rough. Balsa just impregnated with dope or even covered with paper gives a roughness which can be essential for the flying behaviour.
If the surface is too smooth You will get into trouble.
no Depron is offline Find More Posts by no Depron
Reply With Quote
Old Aug 16, 2014, 04:02 AM
Weltherrschaft will be mine
Germany, SN, Leipzig
Joined Nov 2012
285 Posts
I do understand why in theory on small models the actual airfoil shouldnt matter.... but in praxis i believe its not that simple I have flown at least 2 models with different foils but same aspect ratio and area, and there are very noticable differences. i only fly sub 100gramm models.
it seems to be a fact that u can get much more lift with an undercamber greater than 4% with a trade off in speed and efficiancy....
depends on what u want: a slow stable bird or and acro-race plane?
borkencorter is offline Find More Posts by borkencorter
Reply With Quote
Old Aug 17, 2014, 06:05 AM
I NEED a micro seaplane lrf!!!
fabersum's Avatar
Torino
Joined May 2007
322 Posts
Nodepron, thanks for your suggestions. My previous model had a Clark Y wing with the rear part set "flat" as you said (with constant thickness and 2° washout at wingtips) and it flew good, nut for the G50 I would love to be able to use a biconvex airfoil.. Anyway I am curious about the 110-to-80% thickness thing: is it working like the washout?
Thanks for the advice about the surface roughness.. I think that the CNC routed foam with not too much sanding will be just right. But what if I build a balsa version - already in my plans - and cover it with so-lite film? that really is smooth..

borkencorter, thanks to you too :-)
I would like a scale-like Flying model. So I suppose a quite slow and stable one. Do you have some suggested profile?

I think I'll end up building 4 or 5 wings to do some tests..

have a nice Sunday everyone!
fabersum is offline Find More Posts by fabersum
Reply With Quote
Old Aug 19, 2014, 03:56 PM
Weltherrschaft will be mine
Germany, SN, Leipzig
Joined Nov 2012
285 Posts
for stability washout and an a little bit of dihedral are more relevant... but for slow flying nothing beats a undercambered airfoil(at least with micros without slats/slot/flaps)
clark y with uc like on planes like um t-28/champ. think of what airliners do with their wing planform when starting or landing .and the bigger the prop, the better the lift specially on micros. it s not the airspeed that gives u lift its the speed of air
borkencorter is offline Find More Posts by borkencorter
Reply With Quote
Old Aug 20, 2014, 01:35 AM
Push
XXXmags's Avatar
Joined Jan 2008
326 Posts
Hi fabersum,

I did some reading on low reynolds number airfoils a while ago for use in a gyroplane.

Gyroplanes have very poor lift to drag ratios, especially at small model sizes, maybe 3:1 at best. This makes rotor blade profile extremely important, especially when working with limited power. I did some simple tests comparing different rotor blades with identical planform and area. The airfoils I tried were:

- A flat plate ( 0% camber )
- A slightly cambered plate ( roughly 4% camber )
- A highly cambered plate ( roughly 10% camber )

Note:
- The maximum camber point was somewhere around 40 - 50 % chord.
- The target reynolds number at the rotor blade tips was around 30,000.

As a gyroplane rotor is pneumatically driven, it extracts power from an airstream. That power extraction is seen as a reduction in the speed of the air behind the rotor disk.

I placed the test rotor behind a fan and turned it on, waiting for the rotor to reach a steady speed. The rotor blades that were most efficient would be those with the slowest air behind them.

To compared the speed of the air behind the rotor, I took a piece of paper and placed it behind the rotor. If it bent a lot, the airspeed was high, if it barely moved, the airspeed was low. The most efficient profile would be the one that bent the paper less.

These were the results:

- The flat blades reached an RPM of 2550 rpm. Although the rotor speed was high, the airflow behind it was noticeably fast. The paper was substantially bent. By placing my hand close to the tips of the blades, I could feel a smooth transition to the free stream created by the fan. By getting closer to the center of the rotor, the airspeed would slow down, up to a point, but overall, it was smooth. This indicated to me a poor lift to drag ratio. The drag of the blades was low allowing them to spin quickly, but the poor lift they produced barely disturbed the air.

- The slightly cambered blades reached an RPM of 2820 rpm. The airflow behind it was very slow. It was so slow in fact that if I moved my hand close to the tips, the transition between the free airstream and the rotor disk was almost abrupt. there was a very sharp increase in airspeed once you left the wake of the rotor. This indicated to me a very good lift to drag ratio as the air swept by the blades was highly slowed down.

- The Highly cambered blades reached an rpm of around 2220 rpm. It had the lowest rpm of all the profiles. The air behind it was much slower than that of the flat blades, about as slow as those of the slightly cambered blades. The transition close to the tips was also abrupt. To me this indicated an improvement in lift to drag ratio over the flat blades, but the increase in drag due to the high camber hindered the lift gains. The overall lift to drag ratio was lower then that of the slightly cambered blades as they did not spin as fast.

In conclusion, the results were roughly what I expected. Camber provides gains in lift to drag ratio of an airfoil, but only to a point. Note that, for the sole purpose of auto-rotation, the only aspect that matters about the airfoil is maximum lift to drag ratio. The better the lift to drag ratio, the more power can be extracted from the air.

This is fine for this special use, but for an airplane, where stall characteristics are also important, the profile has to be more carefully chosen.

I've done some quick tests on the stall of flat plates, and there isn't much to speak of. The lift increases quickly up to around 10 - 15 degrees, then stays roughly constant until around 30 deg, then it smoothly decreases to zero at 90 degrees angle of attack. I did this by sticking a flat plate out of the window of a car moving at 70 km/h. The reynolds number was around 130,000.

I build a 50 cm wingspan model to try this out in practice. The stall was very smooth with no abrupt jumps. Stall angle was relatively low too. Tip stalls weren't a problem either, but I used a rectangular planform, so that probably helped.

I'm not sure how camber affects the stall, so that's up for experimentation. I have an old aerobird challenger ( RTF weight 300g ) with a highly cambered wing. The tips on it are thin and are swept back, with some slight washout. The stall characteristics are horrible, it tips stalls at every opportunity. This is a combination to avoid.

The results of my experimentation coincided pretty well with the stuff I read. Just google low reynolds number airfoils, cambered thin airfoil, flat plate low reynolds number.

I read that a good airfoil for reynolds numbers bellow 50.000 is a thin cambered plate, with 4% maximum camber placed at 40% of the chord, measured from the leading edge. This is a about what the best performing autogyro blade had.

This is a lot to read, but hopefully it's a got something that's of use to you. I attached some of the papers I read to this post.

EDIT: I forgot to add this.

Thick airfoils perform poorly at low reynolds numbers. I had a helicopter where you switch a set of flybar paddles from very thin ones, to very thick ones. The thick ones produced barely any lift. They could barely tilt the flybar. The thin ones had ample amounts of lift and could easily rotate the flybar's plane of rotation.

The drag of thick airfoils is also very high. I made a glider that used a clark Y style airfoil, and it had a glide ratio of around 2:1. The tail feathers and the wing had a thickness of around 17%.
XXXmags is online now Find More Posts by XXXmags
Last edited by XXXmags; Oct 05, 2014 at 10:23 PM. Reason: Bad RPM's
Reply With Quote
Old Aug 20, 2014, 02:12 PM
Registered User
Joined Mar 2014
72 Posts
Hi fabersum,
look at the pictures.
The one is Clark Y with 110% thickness the other one with 80% thickness.
110% means an offset of the chord against the bottom of the airfoil of 2,31° .
With 80% thickness it's just 1,70°.
This diffrence means a lot in terms of airflow.
Indeed these angles are the aerodynamic offests of the wing sections aigainst the tail which is parallel to the axis of the plane.

I made best experiences with this setup in the shown model ... which by the way is a bit too heavy (63 g/dm^2). It doesn't fly like a passenger aircraft form the late '30s but more like a fighter of the '40s.
Doesn't matter it flys very handy provided that the CG is far enough in the front (1/4 of the reference chord).
I can slowly pull the elevator to the end and the bird still flies stable.
Sudden elevator and ruder urges it to an unwilling spin.
no Depron is offline Find More Posts by no Depron
Reply With Quote
Old Aug 20, 2014, 03:23 PM
Registered User
Belgium
Joined May 2012
54 Posts
18" span and AUW of 75 gr means high wing loading (around 35 g/dmē ?) and, as a result high airspeed.

Cambered airfoils produce more lift than symmetrical ones at the same airspeed and angle of attack (AOA). So, cambered airfoils are the better choise for planes with a high wingloading.

Determine the wingloading first. Then estimate the minimal airspeed needed for that wing loading. Then check whether you can achieve the minimal airspeed with the the motor (kV)/battery (Voltage)/prop (pitch) combo you have in mind. E.g. a 2000 kV motor + 1S battery (min. 3,2V) + 3" pitch prop results (without slip) in an airspeed of 8 m/s. Not enough when at least 10 m/s is required... In reality the actual airspeed will be even lower than 8 m/s due to slip of the prop. in case of the setup above the plane will never be able to fly.

Some estimated values based on thin airfoil theory:
* flat, uncambered airfoils (flat plate):
AOA +3°, airspeed of at least 5 m/s -> max. wingloading: 5 gr/dmē
AOA +3°, airspeed of at least 10 m/s -> max. wingloading: 20,5 gr/dmē
AOA + 6°, airspeed of at least 5 m/s -> max. wingloading: 10 gr/dmē
AOA + 6°, airspeed of at least 10 m/s -> max. wingloading: 41 gr/dmē

* 25% cambered airfoils with circular crosssection, max camber at 50% chord (flat plate bended):
AOA +3°, airspeed of at least 5 m/s -> max. wingloading: 6 gr/dmē
AOA +3°, airspeed of at least 10 m/s -> max. wingloading: 24 gr/dmē
AOA + 6°, airspeed of at least 5 m/s -> max. wingloading: 11 gr/dmē
AOA + 6°, airspeed of at least 10 m/s -> max. wingloading: 44,5 gr/dmē
HansL is offline Find More Posts by HansL
Reply With Quote
Old Aug 20, 2014, 05:09 PM
Weltherrschaft will be mine
Germany, SN, Leipzig
Joined Nov 2012
285 Posts
I noticed that a good slowfly airfoil doesnt really stall at all think of the champ, even with 65 gramms auw it just parachutes down when u try to stall it. same goes for the sky 500 and several scratcbuilds i have or know.

the more scientifical answers abve second what i believe is true for small planes.
conclusion is to use a high lift wing and a big prop for a nice lifting airstream.

i know easy way to produce highlift foils:
- I dont know what goldener schnitt means in english ,a nyway its math that old motherf*****... just search a picture for goldener schnitt in the web. you will soon find nice pictures of a spiral. print it out to as big as needed. now just take a ruler and draw a line through a half loop wherever it fits your idea of a wing planform. now u can cut formers after that template. always worked for me not really scientfic but me-proof and proven..... i learned it from a friend and he has it from his granfather who told him that s the way they build FF models in his HJ group back in the days whe germans only wanted german bananas(worldwide)
borkencorter is offline Find More Posts by borkencorter
Reply With Quote
Old Aug 21, 2014, 06:12 AM
I NEED a micro seaplane lrf!!!
fabersum's Avatar
Torino
Joined May 2007
322 Posts
Hello everybody

thanks for all the replies!

Borkencorter, I found the goldener schnitt thing.. I think it has something to do with logaritmic profiles like the Simplex.. you draw just one curve and wherever you cut it you always have the same profile, for any chord you choose.. it seems pure magic to me and works great for single surface undercambered wings.

HansL, you say 35 g/dmē wing loading is quite high, but if NoDepron's lovely Atalanta flies like a '40s fighter at 63 g/dmē my Fiat G50 should fly like a slow and underpowered '30s fighter - that would be very scale :-D - or am I getting it wrong? Anyway at 45 cm ws I have 2.76 dmē or 3.25 dmē wing surface (2,76 considering the two separate wings only, 3.25 considering the part of fuselage between the two wings too) so the wing loading @ 75 g AUW should come out 23-27 g/dm2 - that should put me in the 10 m/s range with +3° AOA. A 2500 Kv motor on 2S with a 5030 prop should give me around 16 m/s of pitch speed, correct? So I should be fine..

NoDepron, thanks, I got the different thickness/different AOA thing now :-)

And thanks for the infos and files to XXXmags too..

In the end, luckyly, again I think that many airfoils could work at this size.. thinking about my Eflite/Parkzone Ulta Micros they have everything from single surface undercamber (T28, Spitfire, Mustang..) to flat-convex airfoil (Mig 15) to symmetrical convex airfoil (Polecat, Sbach) and all of them fly well!
fabersum is offline Find More Posts by fabersum
Reply With Quote
Old Aug 21, 2014, 06:57 AM
I love my ID.
Fairfax VA
Joined Aug 2004
664 Posts
Hobby zone Champ wing loading is 9.12g/dmē.
Champ auw: 38 gram

Pkz J-3 cup is little bit less at 9.08g/dmē.
J-3 cub auw:29 gram

Just something familiar to compare with.
jingjingjing is offline Find More Posts by jingjingjing
Last edited by jingjingjing; Aug 21, 2014 at 09:43 AM. Reason: j-3 cub wing loading is less than champ
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Sold WWII scale spinner for Warbirds to Wacos. unclegeorge Aircraft - Fuel - Engines and Accessories (FS/W) 0 Oct 19, 2013 11:14 AM
Sold WWII scale spinner for Warbirds to Wacos. unclegeorge Aircraft - Fuel - Engines and Accessories (FS/W) 0 Oct 04, 2013 07:04 PM
Sold WWII scale spinner for Warbirds to Wacos. unclegeorge Aircraft - Fuel - Airplanes (FS/W) 2 Sep 27, 2013 08:15 PM
Discussion Airfoil suggestions for ASK-18? lrsudog Scale Sailplanes 2 Dec 13, 2008 01:20 PM
Looking for suggestion for an 18-20 cell kit JeffD Sport Planes 1 Nov 27, 2003 07:50 PM