Max Wing Loading for Indoor R/C

[JonB]

Hi Guys:
I'm contemplating building an indoor R/C flying version of the "Miles Sparrowhawk", 185 sq. in wing area. 34 in span. Projected all up flying weight..less than 5.0 oz. This will yield approx. wing loading of 3.85 to 4.00 oz/sq. ft. The airfoil I intend to use would be a "Clark Y" at about 15% thickness, flying at about 3 degrees angle of attack. I really could use some advice from you guys on whats been your experience on flying indoors..perhaps what is the upward end of wing loading spectrum to keep an airplane slow enough to fly comfortably...in a confined enviornment. My "Field" of choice is where I fly indoor rubber duration ...Hanger No.1 at Lakehurst Naval Airstation...Even though this site is huge by standards, I still want my airplane to fly at a slow pace....your opinions would be greatly appreciated...
Sincerely,
Jon B. Shereshaw:)

[Sail 'n Soar]

JonB,

I'll admit that I am out of my league when advising on indoor, but at a wing loading of about 4 oz/sq ft and an average wing cord of about 5.5", you are going to be flying at a Reynolds number on the order of 40,000 to 50,000. That means that you will be WELL below the critical Reynolds number for a 15% foil. :( I'd recommend going with a maximum thickness of around 10%. If you stick with the 15% Clark Y-like foil (the Clark Y was 12%, I believe), be sure your wing structure provides turbulators in one form or another.

In terms of your basic question about a wing loading upper limit, your 15% flat-bottomed turbulated foil wing at 3 degrees should fly around 10 mph. A thin highly cambered high lift foil should lower the min speed to about 8 mph - based on an average lift coefficient of 1.4. Increase the wing loading to 6 oz/sq ft and these numbers become 10 and 12 mph.

[JonB]

Sail'n soar...Thanks for the advice..It is VERY helpful...But I need some clarification of your statement about being "Well Below" the critical reynolds number....Can I construe that as acceptable...what might be some implications of not using a "turbulator"
Thanks..
Jon

[strouse]

You will be fine. Thin, undercambered airfoils are best for slow flight, but a flat bottom or Clark-Yish will work well also. Small wings are very inefficeint, and most of the lift is derived from air deflecting downwards off the wing. One of the most popular true slow fly models, the Kolibri just uses a airfoil derived from bowing a carbon fiber frame to some sort of camber(ie-not precise). I read somewhere where designer Tom Hunt I believe, even put a wing from one of his models on backwards to demonstrate the airfoils are not critical on these small models. I'm sure there are benifits to using the optimal airfoil for a given models and turbulator strips and such, but they are not critical, I can only see where they enhance adequate perfromance to a optimal level. I'm no expert, but have scratch built about a dozen or so sub 5 oz. models that fly fine by my standards. My flat airfoils are always the closest 1/16" to 12% of chord. Just my $.02.

[Sail 'n Soar]

A couple of things. Who's talking about optimal airfoils? A couple of lines with a straight edge and French curve will work just fine. At slow speeds the arbitrary French curve and straight edge foil should out perform most "real" foils. Single surface foils also work well, e.g. all the thin foam park flyers.

In terms of "well below" critical Reynolds number, for your 15% Clark-Y-like wing you are probably at about half. As for practical turbulators, a stringer or two between the leading edge and spar on an open wing structure will work just fine. Keping the leading edge radius small or even sharp also helps. In the past I've used tape trim on my ~8 oz/sq ft glider designs and flown normal D-box wing structures up to 15% at the root with no problems. You can get away with about anything but a smooth, featureless top surface with a blunt leading edge.

wstrouse mentions something about Tom Hunt putting the wing on one of his models on backwards. I have no doubt that it flew. That is one way to really demonstrate the value of a sharp leading edge for very slow flight. At really slow speeds I would bet that most wings will work BETTER backwards! Basically, the sharp trailing edge, now leading edge, trips the boundary layer helping the air stick to the upper wing surface longer. Try it! Take one of your normal blunt edge thicker wings and swing it around then give it a quick angle of attack change by tilting your wrist. Now do the same thing swinging the wing backwards. You should notice a much more pronounced tendency to climb, i.e., lift, for the wing moving backwards. If you get more lift indicated swinging the wing forward than backward, swing it more slowly and try it again. The backwards wing will keep working as a wing at the point the other creates nothing but drag.

Still, unless you have some reason for doing so I'd still go thinner than your suggested 15%. wstrouse successfully uses ~12% foils on his models in your target weight class. Sounds good to me. There is nothing better than building on the experience of others.

[Tom Hunt]

Hi Gang

just a clarification on the flying the wing on backwards.

It was on a customers Lowwatt, and it was an honest mistake.

I had been teaching him on the model early morning before work. After the first flight, he took the wing off to change the battery to fly again. The wing was covered completely in yellow, no decals or markings and the wing has a symetrical taper to the planform, so front-to-back looking down is not "obvious".

He inadvertantly put the wing back on TE fwd. Without paying much attention myself, I took the model from him and hand launched it for him. He immediately complained that it seemed to not climb as well this time. I told him just to keep climbing at that rate and I would take it from him once a safe altitude was reached.

Being very familiar of how this model flies, I aggreed with him that it seemed a bit sluggish, but we began to blame it on a soft charge. Once we were well into the flight, it became apparent that the capacity of the pack was ok... maybe we just lost a cell.

When the BEC shut down the sp 400 motor, I brought the model in for a landing. I did my "normal" flair at about three feet and the model dropped in like a lead balloon.... it normally would not do that. The light bulb shone brightly now in the early morning light! :D

I took a look at the wing.... asked the owner if he noticed any "wrong with this picture"? It took a minute or two to find that he had put the wing on backwards.


Moral of the story..... yes, the airfoil shape in small models is not that important if your only interested in commiting "flight".

If you want the model to "handle" well also, make sure the airfoil looks "reasonable". High point not too far aft, LE radius as large as possible, and not too thin if flat bottomed.

"StiK" airfoils fly great with the wing on the right way! but just a tad worse (drag and sharp stall) when made to go the wrong way!

Tom Hunt

[JonB]

Sail n' Soar.....Just one more thing....If I'm reading your latest post correctly, I can use a relatively thick section ...maybe 13 -14% if I keep the leading edge "Sharp"? Can you give me some criteria for leading edge radii? If i'm not mistaken, its a percentage of something...but I dont remember of what? Chord? Camber? Thickness?
what would L/E radius would considered sharp and what would be considered too blunt? If you can be specific...it would be great...I'm going to be using a modified "Clark-Y" section about 13-14% thickness..
Thanks in advance for all your help!
I promise to post some pics as this project progresses...To this date I have pretty much settled on equipment and servos..3 Westecknic servos, FMA M5 RX, 3 FMA-Kokam Li-polymer cells, MicroMo (Fulhauber) 1717 motor with either 11.8:1 or 6:1 planetary gearbox, approx 8.5-9.0 prop, R/A microlite covering - 5 micron thickness, Built up balsa construction 4-6lb max density (on its was from "Lonestar balsa", Castle creations BEC
Sincerely,
Jon B.:D

[Bill Glover]

Originally posted by Tom Hunt
Hi Gang

just a clarification on the flying the wing on backwards.

It was on a customers Lowwatt, and it was an honest mistake.

BTW the wing section on the Stubenfliege indoor slow-flyer is a pure arc from LE to TE and would work exactly the same in either direction (though the wing fixing prevents you doing this by accident).

The camber comes from the combination of elliptical dihedral (carbon rod wing with tension wires on the top surface) and heat shrinking the Mylar covering.

I posted a pic of the section in this thread for someone who thought that carbon rod wings were flat-sectioned!

http://www.rcgroups.com/forums/showthread.php?s=&threadid=22315

Many indoor models use very approximate camber bent 'by eye' into their Depron sheet wings and fly very nicely - it really isn't critical.

[Sail 'n Soar]

Answering your direct question, LE radius is normally presented as a percent of wing cord. What is sharp enough will depend on how you construct the rest of the wing. Since you are building a scale Miles Sparrowhawk, you are wanting to have some reasonable radius just for asthetics.

In way of comparison, all of the low speed Eppler foils I'm aware of but still higher than the low speeds your light-weight will see, have leading edge radii less than 1%. ( Eppler had other full-scale application designs as well) Low speed foil designs newer than the Epplers appear to have the same similarly sharp leading edges.

The 13.72% thick Eppler 209 has a LE radius of right around 1%. Note that I am definitely not advocating the E209, the modified Clark Y is just fine. But that airfoil does illustrate that a 1% LE radius on a 13% to 14% foil looks and is reasonable. I am currently working on a less ambitious project than your sparrowhawk and am using a 1/8" dowel for my leading edge on a 7" cord 12% foil wing. This results in about a .9% radius.

You haven't said what sort of wing construction you are using. If it is an open rib structure, then a stringer between the leading edge and main wing spar will also help.

I've also used the old NACA 2412 to 6412 foils in some of my earlier projects. Their LE radii were 1.58%. Those were D-tube wing construction with a strip of thick trim tape about 15% behind the leading edge when flying at about 8 oz/sq ft.

Started web brousing on the M5 Sparrowhawk. Neat looking little plane. Here is a site with another modeler's version - probably big bore gas, though. http://www.modelairplanenews.com/motm/pp_01-98.html

Looking forward to your pictures!

SnS

[Sail 'n Soar]

JonB,
The following two posts have coefficient of lift vs drag charts from wind tunnel data for the Gottingen 801 foil. They were published in Martin Simon's book Model Aircraft Aerodynamics. The Gottingen 801 foil has a 1.2% LE radius, 7% camber and about 10% thick. The upper surface is a reasonable approximation to a 12% Clark Y for discussion purposes. The first chart shows the wind tunnel results for the exact contour, smooth surface 801 airfoil. The poor porformance at Reynolds numbers of 63,000 and below, where you will be flying, reflect the section is below its critical reynolds number.

[Sail 'n Soar]

Trying chart 1 again.

[Sail 'n Soar]

Jon B, if it doesn't work this time I'll try through a private message. I guess I've forgotten how to add pictures.
SnS

[Sail 'n Soar]

This second chart shows the Gottingen 801 foil performance with a "paper covered" wing structure, i.e., open wing ribs, stringers and spars. The surface droop between structural elements results in the foil performing well at a Reynolds number of 42,000. This should be consistent with your lowest flying speed for your average cord.

[JonB]

Dear Sail n Soar:
Well I finally think I have an airfoil and sample root rib. I would really value your opinion. Its a NACA 5314 (Generated) with an approx. 1% Leading Edge radius. Max thickness is 14.03% at 29.3%. Max camber is 4.53% at 33.3% of chord. Flying at 3.5 degrees positive incedence. The reynolds number at which I'll be operating is 74,728. All input is appreciated! Please see attached image.
Sincerely,
Jon B. Shereshaw:D

PS. The scanned and posted image looks like it might have undercamber near the front lower spar! In reality, it DOES NOT...its a flat bottomed airfoil...I guess the paper may not have been flat to the scanner glass when processed.

[Sail 'n Soar]

Jon,

Glad to hear back, was wondering how things are going. Before I get carried away, just from its appearance your airfoil seems reasonable. Delving a little deeper, you must be looking at the NACA 5314 because it approximates a flat-bottomed foil. I don't know what plotting program you are using, but you've modified the foil from a 2.16% to a 1% leading edge. Your Reynolds number must be based on a flying speed ~11.65 mph and a root cord of ~8.2". Correct? Based on your 185 sq. in. and 34" span wing, that indicates a tip to root foil ratio of less than .5". That will result in the Reynolds number dropping from ~75,000 at the root to ~37,000 near the tip. With that taper, the max CL for a wing without wash-out will occur more than half way out to the wing tip, which will result in a Reynolds number there ~55,000. From there out is where the wing is most likely to stall first i.e., tip-stall prone.

It also looks like you are using something like 1/32" sheeting top and bottom. The Gottengen 801 graphs I posted earlier show the impact of a smooth vs. semi-faceted, i.e., "paper covered", wing, in that Reynolds number regime. I'd suggest including washout of a few degrees to move the max CL point inboard for a more benign stall behavior. Wings are relatively easy to make. You might also consider having a constant LE radius (higher percentage at the tip). Performance could drop a smidge, but the stall behavior toward the tip would be more benign (CL starts lower, but drops off more slowly with no abrupt change).

You might want to build a set for a stick fuselage glider just to check the design out before you spend all the time, effort, and expense into building the actual model. If you build it as a standard D-tube forward portion and open frame behind you can experiment with different degrees of wash-out. I've even used cardboard wrapping paper tubes as test mule fuselages, with various aluminum or solder blocks and steel nuts and bolts for ballast. That would allow you to try overall geometries, CG position, tail surface areas, etc. If gliding testins indicates some boundary layer turbulation is required, you could test out some similar or contrasting color trim tape strips between the LE and quarter cord on the upper surface.

Gerry

[Sail 'n Soar]

Jon B.

Haven't heard of your SparrowHawk progress lately. I assume you are still pursuing other branches of the hobby. Found this web site on another thread recently and thought you might enjoy. http://soaring.cnde.iastate.edu/calcs/frames.shtml

Enter your airplane wing area, span, and weight and the program interpolates Michael Selig collection overseen low turbulance wind tunnel data and it gives Cl vs Cd for "Reduced Reynolds Number," a very usful term the definition for which is provided in the program technical data.

Quoting, "The code stops drawing when it runs out of data. The wind tunnel data is only available at a few Reynolds numbers. Usually the data was taken at 300,000, 200,000, 100,000 and 60,000. For some airfoils there is less. For a few airfoils, Selig chose to test down to 40,000." So the lower numbers are right in your ball park.

Gerry