invertmast

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Im beginning the design work of a 1/3 scale model of a Pond racer. Unfortunately, information on the fullscale is extremely limited. Because of this, i was hoping you guys could help with an airfoil selection.


Model specs are:
101.67" span
16.8" root chord
6.8" tip chord
6.8" forward sweep
98" long
rtf estimated 36lbs


I'm planning on stuffing around 12,000 watts of power into this thing with some custom over-squared props (roughly 25-30" of pitch). This should give a speed of around 180-200mph (hopefully).

My initial thoughts are to keep the airfoil thickness at 10% or less. The NACA2410 or MH30 are two airfoils i first pulled up that have the necessary thickness and appear similar to the full scale Roncz airfoil.

Any suggestions? A stab airfoil suggestion would be helpful as well.

Thanks,
thomas

ByLoudDesign

Thomas ~ Very cool project!! I pulled up the NACA2410 and the MH30 in CompuFoil, and the MH30, without pull the two of them into ACAD looks like a slimmer NACA2410. I like the NACA foil better, would think that the tip foil would be a 8% if the root is 10%. The stabs would work well with the NACA00nn foils. I love the sound of the twin engines! It sounded much like a P-38!

speedracerntrixie

My Feedback: (29)

Have you looked at some of the sailplane airfoils such as the RG15 and maybe some of the MH airfoils they are using? I have been running a pylon airplane with an RG15 with the camber removed with great success.

invertmast

My Feedback: (23)

I have looked at the MH series. I would like to use the MH53, but profili doesnt have it listed. MH30 is the next closest to the MH53 and the Naca2410 is slightly thicker and provides a little more Cl at the same alpha and more Cl at slightly more aloha, so it should provide a slightly better approach speed since it should be able to maintain a high AoA for slower speeds.

Im open to suggestions though, as my airfoil selection knowledge is pretty marginal.

UStik

You know Hepperle's website?
I mean you could import the airfoil coordinates from there into Profili.
And you could use the whole 5x series by Hepperle with their different camber.

invertmast

My Feedback: (23)

I hadnt thought of importing the coordinates, that is a fine idea. I also looked into the MH17, it seems it would work as well. Doing the lift calcs, it would provide the necessary amount of lift at .5alpha at a respectable cruise speed and it has a small pitching moment for reduced loads on the small stab.

BMatthews

If speed is your goal then I'd suggest you go with less camber than the 24 series. You'll find that if you run the numbers for the wing size and even for flying at 100mph that the angle of attack in level flight will be with this much camber will end up being negative since the camber provides so much lift that the airfoil ends up needing to fly at a slight negative value.

The MH30 is a better option if you'll be doing hard pylon racing turns but for flat out speed it will actually hold you back as well. Look at the lift/drag graph from the Hepperle website;



See how the Cd for the higher Rn curves is at the minimum up around a Cl of around 0.25? But in level flight at 100mph and higher you'll find that you're going to be flying at Cl values of don in the 0.15 range at 100mph and that'll become even lower as the speed climbs towards 200mph. So you end up running at a higher drag value than required.

Now look at the curves for the MH31;



See how the curves show a low Cd value right down to a Cl of 0.0? This is an option that will allow that last little bit of slipperiness during level flight but it'll still do OK in the pylon turns as long as you don't go nutz on the elevator.

So all in all I'd suggest the MH31 over either of the other two options for your Pond.

Now it is also a thin section. So you're going to need to go to some greater lengths to ensure a STRONG wing to take the G loads of that anticipated 36 lbs pressing into the wing at up around 10 or 12G's. This is a situation where lots of carbon used correctly and good vacuum bagging is pretty well a must if you want the wing to handle the loads without failing. If you're thinking of building with more or less normal balsa skinned foam or built up structure then you'll need to avoid any hard pulling pylon turns.

And in fact if you have any doubts on this at all then going with a similarly performing MH34 which will give you 8.5% thickness to work with or even the MH44 which is up at 9.7% thick become better options. In fact for the pretty much negligible added profile drag the 34 looks like and away top choice. It'll still need some pretty advanced structural design to hold the load. But that added little bit of sectional thickness will help a lot.

Consider this. I've read that AMA and FAI plyon racers typically pull up around 15 to 20 G's in the end turn. So even if you don't pull as tight a turn you're likely still looking at 10 G's as a very likely value. If the model hits your anticipated 36 lbs then this means the wing is supporting 360 lbs of "weight" during the turn. So if you don't feel like the wing is such that you can set it on a couple of chairs out near the wing tips with pillows for padding then have some 200lb person step onto the center section without any worries then it isn't strong enough.

invertmast

My Feedback: (23)

Bmatthews,
thanks for the info. Since this is a big/scale model, i expect to fly it in a scale manner, so razor sharp pylon turns most likely wont be in its flight regime, plus building this thing (easily) to handle a 15-20G gurn at 36lbs would probably make hitting my goal of 36lbs pretty difficult. So max speed with large sized turns (or even cuban eight reversals) is fine by me. Honestly, 200+ mph isnt even a requirement 100% for me, a friend who is interested in one (since i plan to do molds for the booms and center pod) wants it capable of that speed, so i figured i may as well design it that way from the beginning.
Preferably for building the wings i would like to keep it simple, balsa sheeted foam cores, etc. But i do have a pretty sizeable amount of various Carbon fiber (unidirectional and twill weate fabrics, 3,12,50k Tows) and a significant amount of fiberglass cloths. Ive also got a full vacuum bagging setup.
For my model, i suspected a 1/32" balsa sheeting over the Down hiload foam. This would have a 2" wide piece of unidirectional carbon full span on top and bottom and probably a layer of 1oz glass as well on the bias, all vacuum bagged of course.

I still need to do the bagged wing structural calc's with (i believe) Dr. Drela's excel spreadsheet, but first guess makes me think the above would probably be close.

What makes all this tricky, is that the wing has to split into 3 piece. Essentiall the center section holding the entire thing together, then the outer panels that get bolted on, preferably by a wing tube, but a ply/carbon laminate blade spar setup is a possibility as well.

BMatthews

I'd suggest that some of those options are not that great for a thin wing on a model of that size. Especially your description of the 1/32 sheet balsa with some carbon and a lighter sort of fiberglass overlay. Without some additional spar work to tie it all together this would be pretty iffy for any sort of near vertical bank turn.

A wing joiner also really needs a proper spar to tie into. It's not a good idea to have any foam between the slip tube for the joiner rod and the wing skins. And a proper spar is the best way to provide a solid link between any stress carrying skins and a joiner tube or blade.

invertmast

My Feedback: (23)

Yea i understand, a spar of some sort would be integrated, especially with roughly a 1.5" root thickness and 1/2" tip thickness. I need to do the bagged wing calcs and get started on the CAD drawings to come up with a rough estimate on weights.

BMatthews

If much of any sort of higher speed is going to be reached there's also the issue of torsional stiffness to resist wing flutter as well. A really good spar doesn't help much if it allows the wing to twist around it. A scale model of any sort of air racer needs to be a bit faster than most other model designs or it's simply not going to seem "right" in the air. So while you might not be worried about 200mph like your buddy is wanting it's worth making the model up so it can reach and withstand some good airspeeds. After all, even if the model is only set up to fly around at maybe 100mph in level flight who can resist the lure of some high speed dives that ease back into high speed passes? So even your slower version is very likely going to see up close to the 200 mph at some point. And when that occurs you want the wings to survive.

Pylonracr

Yup.
Thomas, if you are going for stiffness in a thin wing, do what I do. Make a top and bottom skin with 2 layers of 2.5 oz Kevlar separated with .125 Nomex honeycomb. You get stiff wing skins with little weight. Laminate your Spar material with diminishing layers of directional Carbon starting about 1/2 chord at the root and tapering to 1/10 at the tip, then 1/4 root to about 0 at 2/3 span, and 12K tow tip to tip. Put all of this in the inner lamination to maintain smooth airfoil shape.

I followed your F 14 build, so I KNOW you can do this simple wing. Find an airfoil you like - ( and I would listen to Shawn here) and make a prototype.

Scott

speedracerntrixie

My Feedback: (29)

Thanks for the vote of confidence Scott. I do have to say my airfoil is working really well. Last race on a 700' course I ended up flying a 1:16.2 for the 10 laps. That was running only 40% nitro. By comparison that is 2 seconds slower then Verano running 60% nitro. The video of that heat is in the scale racing forum in the Valley of speed thread.

BTW, have you flown that wing yet?

Pylonracr

UUUMMMMM.... Not Yet.

pkoury

My Feedback: (7)

The only way I can judge whether you are knowledgeable is based on the content of your postings. Content includes the use of proper grammar and punctuation and based on your postings I do not believe you are knowledgeable. Other than stating your opinion you give no supporting information to back up your opinions which does not help your case.

Thomas; There were two Pond Racers that flew in the Galveston Giant Scale Air Races that I believe were Kerry Sterner designs, might try contacting him for input.

invertmast

My Feedback: (23)

Funny you mention that, i did email Mr. Sterner and all of the information pertaining to his model was sold to a kit manufacturer in canada (sky hobbies or something like that).

iron eagel

I would suggest that you look at some of the methods Mark Drella uses on building wings and spars. I think a multiple spar setup 2 or three along with diagonal ribs and stressed skin may be what is called for in your application.
Bruce would a MH 54 be a better choice?

BMatthews

Iron Eagle, given that this wing would more than likely be fully sheeted if a built up method was used I think the sheeting and associated fiberglass and resin for a finish would take care of the torsional stiffness. After all such wings have been done before and this was good enough. No reason it won't again.

I do like your suggestion of going with the proven sort of strong spar construction so often found in thin wings used on larger sailplane models. Serious sailplanes these days thrown into the air by the seriously over powered winches are seeing line loads of 150 lbs pretty regularly. And that means the spar setups used for such wings are going to be appropriate to a project of this sort.

And using such spars in a foam cored bagged wing is going to make things that much more secure.

The MH51 has great low drag at very low angles of attack which would be seen at high speeds. But the drag ramps up a little faster as the Cl climbs to .2 and higher. So for a pure speed model I'd say it would be better. But we still gotta turn. And we need to turn frequently at such speeds to keep the model in sight. So I'd still lean towards something with a hair more camber that slightly favors a wider low drag region that extends up to around a Cl value of .2 to .3 before the drag starts to slide to the right hand side of the scale on the charts. The 8% thickness would put more of a strain on the wing structure as well. So I think I'd still lean towards the MH31 or close to it with a lean towards what the MH51 shows.

iron eagel

For a powered aircraft this design has wings much closer to a sailplane than a P-51 that it was designed to compete against hence the sailplane spar recommendations. Kent Knoll has had some good success using those spars with foam cores and Drella type of spars built into them then applying the skins using bagging. I go with a multiple layer layup for the skins using glass/balsa/glass/ carbon disser layer over the top (I do not think the spars alone will carry the load on this bird).
You are probably right about the MH54 I just figured it was better suited for operation with higher Reynolds numbers, and I was guessing that at 1/3 scale and the speeds he was looking at he would be operating at a lot higher Reynolds numbers that our average models, and I was looking at it more from the pure speed aspect than sport flying 200Mph is pretty darn fast for a model of that size.
Thanks for taking the time to explain the drag issue during turns I didn't realize the 54 CD climbed up so fast (I have never seen a plot of the CL v CD for a MH54).

BMatthews

Hey, I'm mostly taking a semi educated guess at this stuff as I go along too. It might be that the 54 would be a great choice. But the 8% thickness puts me off a little as well as my guessing at how the wing would rise for the Cl value during a steep turn. And at higher speeds just about ALL the turns end up being steep banked to keep the model in sight.

iron eagel

Bruce, you are right on about the 8% wing would be a formidable challenge to build, even with a 10% it is going to have to be fairly robust to carry a 36 lb load. It's either a very steeply banked turn or pull up and a half loop to reverse either way you know there are going to be some good G loads to deal with.
Either way it looks like a really fun project, and with 12KW of power it should be fairly quick given the right prop/motor combo.
Your guesses are a heck of a lot more informed than mine, I am still learning at this point, kinda nice to be able to say that at my age.

speedracerntrixie

My Feedback: (29)

I pretty much went through all this thought process when designing the latest version of my Macchi pylon airplane. Being that years ago I was an avid sailplane guy ( qualified and competed for a spot on the U.S. Soaring team in 1996 ) I looked at some of the requirements for those airfoils. The most important factors for me was an airfoil that worked well with a high load and relatively high angle of attack like what we would expect during a pylon turn. Secondly I wanted an airfoil that would maintain energy in the turns and as an almost contradictory charictaristic one that would slow down well. In the end I took the RG 15 and tweaked it to fit my needs. I took the camber out and at the tip I thinned it to 8%. I am very pleased with the performance. Construction was another hurtle as these airplanes take an estimated 30G on the turns. Here is the A main final heat in our last race. The lighting is poor and it's hard to make out which airplane is which and is best viewed on the largest screen you can. I won this heat with a time of 1:16.2. We fly 10 laps on a course of 700'.


http://youtu.be/pzYotxRC0RA

RAPPTOR

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Im my world ,your the pool guy..

speedracerntrixie

My Feedback: (29)

Thats funny right there. The reality is that I build Atlas V SRBs for a living.

iron eagel

Nice video some tight turns there.