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Old Mar 10, 2012, 03:50 PM
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St Clair Shores, Michigan
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Request good airfoil for slowflying rubber powered Peanuts

I cannot discuss this intelligently, I am not an engineer. (and I am more or less a newbie at all of this)

Anybody have a real favorite? - I am looking for realistic, slow flight for Peanut scale models that do nothing but fly beautifully as they go up and come down. Thats my flight envelope. Indoors or out.

There must be links... and if there are links like "flight 101" or suggestions to move this to another forum, that would be welcome also.

I dont mind building a testbed model and several wings...

thanks!

Ray
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Old Mar 11, 2012, 12:53 AM
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The 'Wack, BC, Canada
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For P-Nuts and other small rubber models the simple flat plate or slightly curved flat plate is fine. The Reynolds numbers are low enough that it's all they need. However it's tough to make a wing with leading and trailing edge and spar and keep it a flat plate. So we generally see thin flat bottom style airfoils used for the majority of these models. And truth be told they work just fine. For example a 3 inch chord works well with a simple flat bottom airfoil of between 3/16 and 1/4 inch of thickness.

And keep in mind that the simple structure of this style of model produces very odd airfoils between the ribs and spars where the tissue stretchs flat for the most part. In some cases you can end up with an airfoil of a triangular shape between the ribs due to the tissue sag instead of the more arched normal shape. But even with that the models are so light that they work just fine.
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Old Mar 11, 2012, 07:26 PM
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Thank you for your reply

I knew these models were forgiving of all kinds of compromises but I had not thought about tissue sag. You make a good point.

Will keep in mind I have to work at low Reynolds numbers. I will think less of airfoils and more of building straight and true and light.

Thank you very much for your reply.

Ray
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Old Mar 11, 2012, 08:52 PM
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Dinser, straight, true and light are the operative goals for any small free flight models. Keep those in mind and you'll succeed in fine form.

And if you have not found it already I'd like to draw your attention to both the FF forum here at RCG and another site;

http://www.rcgroups.com/free-flight-64/

www.smallflyingarts.com
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Old Mar 17, 2012, 05:33 PM
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Ray,

I have had some success with the Neelmeyer airfoil. It's flat bottomed and quite thin at 9% of the chord.

As Bruce has indicated small models are quite tolerant of the actual shape of the airfoil but they don't like thick airfoils or very cambered airfoils. So something flatbottomed with around 2-3% camber and less than 10% thick will work ok. Thinner is better but then you have to confront issues with structural stiffness and also scale looks. That said I have used Clark Y without any issues...

Rubber powered models should be trimmed to fly slow for best duration (at minimum sink speed) so there are no issues with drag at high speeds to think of either.


Quote:
Originally Posted by BMatthews View Post
But even with that the models are so light that they work just fine.
This is the key to success A good peanut is a light peanut! It really makes a difference to duration in FF rubber scale, and in lots of ways it's more important than the precise airfoil used.


Another forum suggestion is the Hippocket Aeronautics site.


Jon
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Old Mar 18, 2012, 02:58 PM
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Sadly the realities of light structure and tissue covering makes the comparison of small shape differences between airfoils a waste of time.

As Yak52 said above the key seems to be the thin and modest camber along with a fairly sharp leading edge for this sort of flying speed. The shape differences between airfoils such as the ClarkY, the Neelmeyer, Go617 or many others I've seen and any number of other options is simply lost on a peanut model due to the shape distortion introduced by the tissue sag between ribs.
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Old Mar 18, 2012, 07:17 PM
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Do you really think any of those shapes -if held exactly, would improve performace of a peanut?
I have some very nice peanut scales and frankly -I don't see how they could.
Any of those airfoils is developed for a finite condition

The peanuts fly in a fairly small speed envelope-to be sure
However at the size , I really don't see how anything other than weight and rigidity are of worthwhile goals -for them
In outdoor aerobats of say 350 square inches , the new molded foam in very thin sym sections work better than the flat winged designs - that is a fact .
Thereason the shaped sections work so well is that the material when molded produces an extremely light and stiff shape
These models weigh- 9 ounces ready to fly and performance is incredibly good
Typically the flat foam even braced well ,simply continues to twist under loads
The very thin molded sections provides an accuracy in maneuvers as good as many large 1000 sq in designs The wings are about a 7 % section .
I really believe that getting the structure such that twisting is completely eliminated is more important that airfoil- in these small models
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Old Mar 19, 2012, 08:26 AM
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Quote:
Originally Posted by richard hanson View Post
Do you really think any of those shapes -if held exactly, would improve performace of a peanut?
I have some very nice peanut scales and frankly -I don't see how they could.
In general terms I agree. But that is because the airfoils use on these models are all as bad as each other and we are used to the poor performance and accept it as normal. This is because at these very low Reynolds numbers most airfoils at 'sub-critical' with seperation and no reattachment, and all the drag that this condition brings.

However if you manage to create an airfoil with a low critical Re (reattached flow), then performance can be startling! A curved plate does work particularly well. Unfortunately the structural issues and resulting weight start to eat away any advantages in a one speed duration model.

Peanut models, as you say, have a small flight envelope. In fact flying at minimum sink (for endurance) means drag is less of an issue as long as there is plenty of lift. This means that the issues of weight and stiffness become the more important factors as long as you keep thickness and camber within reasonable bounds.

So in answer to
Quote:
Originally Posted by richard hanson View Post
Do you really think any of those shapes -if held exactly, would improve performace of a peanut?
Not really. Avoid the fat foils and keep the wingloading down.

But for a RC model of that size or something with a wide speed range, then there are aerodynamic gains from the correct airfoil... providing it can be kept accurate and stiff enough. Which precludes stick and tissue
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Old Mar 19, 2012, 09:52 AM
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Quote:
Originally Posted by Yak 52 View Post
In general terms I agree. But that is because the airfoils use on these models are all as bad as each other and we are used to the poor performance and accept it as normal. This is because at these very low Reynolds numbers most airfoils at 'sub-critical' with seperation and no reattachment, and all the drag that this condition brings.

However if you manage to create an airfoil with a low critical Re (reattached flow), then performance can be startling! A curved plate does work particularly well. Unfortunately the structural issues and resulting weight start to eat away any advantages in a one speed duration model.

Peanut models, as you say, have a small flight envelope. In fact flying at minimum sink (for endurance) means drag is less of an issue as long as there is plenty of lift. This means that the issues of weight and stiffness become the more important factors as long as you keep thickness and camber within reasonable bounds.

So in answer to


Not really. Avoid the fat foils and keep the wingloading down.

But for a RC model of that size or something with a wide speed range, then there are aerodynamic gains from the correct airfoil... providing it can be kept accurate and stiff enough. Which precludes stick and tissue
There were some molded foam ,tiny rc designs -notably a Neuport17 by HH
the REAL problem again was that weight simply couldnot bekept low enough to get performance which was acceptable -to me
So a difference in wings were cut from flat foam and undercambered or a perfectly good copy of original shaped undercambered foil was lost. they staggered around at low speeds because of the weight and making a slow approach to landing - out of the question.
My first rule of aerodynamics for small models is simple:
You can not make em light enough
The opposite of my first rule for monster sized models:
you can't get enough power in em.
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Old Mar 21, 2012, 06:08 AM
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First, many thanks to everyone who wrote.

I am rebuilding a peanut Corby Starlet (Walt Mooney plan of 1981) it has a very fat symmetric airfoil.

My model is much too heavy, so this attempt will be a no-cal profile fuselage. I'll use the curve of the plan airfoil but using sliced ribs, covered only on top.

Based on your input I will give that wing a sharp leading edge. That will be interesting. If I can get that trimmed out and flying well I may add some bond paper in front of the spar to minimize tissue sag. I know it will be hard or impossible to conclude much (too many variables involved) but I am intrigued.

I appreciate all the input. Thank you

Ray
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Old Mar 21, 2012, 06:45 AM
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Originally Posted by Dinser View Post
Based on your input I will give that wing a sharp leading edge. That will be interesting. Once that flies I may add some bond paper in front of the spar to minimize tissue sag....
Don't worry tooo much about tissue sag. The Mooney plan has a sheeted LE which would be heavy for a peanut. The other alternative is to add a 'turbulator spar' between the LE and the main spar. Pretty much every airfoil section at this size has big seperation bubbles in all conditions. The trick is to get the seperation where you want it - to permit reattachment and keep the airfoil above it's critical Reynolds number: the sharp LE and turbulator spar helps with this.

Going from a thick foil to single covering will immediately improve things. In spite of the extra drag of the structure, the thin foil should 'remain supercritical' although some trimming will be needed: I would expect you might need to fly it slower and the extra camber might require slightly more negative tail incidence.

Keep us updated!
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Old Mar 21, 2012, 07:02 AM
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Thanks Yak52.

What about two or three turbulators made of fishine? that stuff is very light.

I also need to rethink using the curve of the plan airfoil - the thickness would be 6% of the chord - from everyone's responses, I want about half of that

thanks very much, this is helpful - want to build, but gotta get in to work!

Ray
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Old Mar 21, 2012, 09:15 AM
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4 or 5% camber should be ok with a single covered foil. The 2-3% comment I made was in reference to a full airfoil with double covered surface. The actual airfoil shape between the ribs will have less camber and thickness (depending on what spars there are obviously)

I have heard of people using fishing line to prevent sag but never tried it myself. The pic below shows a Bostonian with a 9% Neelmeyer and an extra turbulator spar, it will be double covered. I don't think I would be neccessary on a single covering. Do you even need the spar on a no-cal?
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Old Mar 21, 2012, 07:01 PM
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Thanks Yak52

I will just use the plan airfoil, then, with no spar nor turbulators. (Turbulators may be the first mad scientist upgrade once it flies)

Will keep you posted, thanks very much. All suggestions (still) very welcome

Ray
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Old Mar 21, 2012, 08:08 PM
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Dinser, since you're doing a No-Cal conversion the model will come out so light that it does not need a normal spar or ribs. For a P-Nut size No-Cal model simply using 1/16 square for the leading and trailing edges is fine. Then glue in some 1/16 deep simple arch shaped ribs cut from 1/32 sheet which give you about a 4 to 6% camber arch and call it good. For a 3 inch chord at 5% that would be 3x.05=.15 inch high arch. Cover only on the top side and yer done. DO NOT SHRINK the tissue. If you do your wing will be doing impressions of a propeller.... Or if you think you'll be flying outdoors with it then pre-shrink the tissue and crumple it up then smooth it out and use it for covering. The resulting "alligator hide" look gives it a little bit of spring so it won't pull overly strongly. But you need to be tidy with the covering to make it look nice since you won't be water misting it to shrink to shape.
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