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Old Mar 17, 2012, 07:53 PM
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Originally Posted by dave1993 View Post
i was always aware that as re decreases so does importance of wing profile
Are you saying that most airfoils (including "blocky" ones and flat plates) have similar characteristics at low Re, or are you saying that these characteristics aren't important?

By characteristics I mean: (L/D)max, CLmax, CDmin, or more generally, the drag polar and lift curve.
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Old Mar 18, 2012, 06:58 AM
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Once you get down to slow speeds and small areas, the airfoil may as well be a flat plate.
you can undercamber it to help at really low speeds but all the futzing around with high points leading edge radii - surface textures etc., is just busy work.
I am referring to speeds of 25 mph or less and wing areas in th 400sq in n under area .
The bumble bee and the hummingbird fly very well with NONE of the aerodynamic characteristics of birds such as seagulls etc..
Really all you have to do is try a number of these small craft to see how the "rules" simply can be bent and or ignored.
actually geting the structure (strength and rigidity) and weight correct is far far more important.
In full scale the problem has always been trying to get lifting devices which work for the power available .
any airfoil is just a compromise for a specific load/speed etc.. No magic involved
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Old Mar 18, 2012, 09:54 AM
Grad student in aeronautics
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Is this coming from observing some designs that fly slowly?

I'm not sure which "rules" you're talking about. Do you have some equations in mind? Are they approximations meant for a certain range of vehicle size?

The mechanisms used to generate lift naturally change with object size because of the fact that weight scales with length cubed and surface area scales with length squared.
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Old Mar 18, 2012, 10:59 AM
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Quote:
Originally Posted by richard hanson View Post
Once you get down to slow speeds and small areas, the airfoil may as well be a flat plate.
you can undercamber it to help at really low speeds but all the futzing around with high points leading edge radii - surface textures etc., is just busy work.
I am referring to speeds of 25 mph or less and wing areas in th 400sq in n under area .
The bumble bee and the hummingbird fly very well with NONE of the aerodynamic characteristics of birds such as seagulls etc..
Really all you have to do is try a number of these small craft to see how the "rules" simply can be bent and or ignored.
actually geting the structure (strength and rigidity) and weight correct is far far more important.
In full scale the problem has always been trying to get lifting devices which work for the power available .
any airfoil is just a compromise for a specific load/speed etc.. No magic involved
.
Low speed airfoils.... although those dragon flies can move!
High aspect ratio, and really really thin!
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Old Mar 18, 2012, 11:01 AM
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Quote:
Originally Posted by DPATE View Post
Is this coming from observing some designs that fly slowly?

I'm not sure which "rules" you're talking about. Do you have some equations in mind? Are they approximations meant for a certain range of vehicle size?

The mechanisms used to generate lift naturally change with object size because of the fact that weight scales with length cubed and surface area scales with length squared.
.
The size/mass of the object relative to air density.
Insects have little size, little mass, and flat wings.
Cambering starts with birds.
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Old Mar 18, 2012, 12:39 PM
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Quote:
Originally Posted by DPATE View Post
Is this coming from observing some designs that fly slowly?

I'm not sure which "rules" you're talking about. Do you have some equations in mind? Are they approximations meant for a certain range of vehicle size?

The mechanisms used to generate lift naturally change with object size because of the fact that weight scales with length cubed and surface area scales with length squared.
You might call it observation - I have designed / built hundreds of models of all sizes - for contests of varied types - movie - fun and my own enjoyment
The requirements are vastly different in a 40 pound TOC aerobat and a small foamies weighing 1 ounce
I can get both to perform the same maneuvers !
As for equasions -- I have no background in the math for this stuff .
I know that approach is interesting to many fliers - I simply preferred a hands on approach.
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Old Mar 18, 2012, 02:10 PM
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Dick, to be fair the flat foamies you are flying use the sharp and deep stall charactaristics of the flat plate to their advantage when flying in their intended manner. At the size and speed they fly at there actually are still a lot of advantages to be had by using a proper airfoil that works well at that size instead of a flat plate. The airfoils used on smaller RC gliders show this as they are often flying at around the same wing loading and flying speed as the 3D flat foamies.

But it doesn't take much of a reduction in size to get to where a flat plate IS the right choice. By the time we get down to smaller and lighter rubber models flat plates or flat plates with a slight camber curve work better than built up airfoils. Free flight models have shown this for years now where it really doesn't matter a whole lot what shape we use for the smaller and slower models as long as they can be thin and have a sharp leading edge.... sounds like a cambered flat plate "all sheet" sort of wing, doesn't it?

Even the more recent airfoils expressly designed for DLG RC gliders and even up to 2 meter size stuff is coming out VERY thin by the standards of yesteryear. I point to the airfoils designed by Mark Drela as examples of this. The airfoils used on his Bubble Dancer molded wings are very thin in comparison to similar size designs from 15 to 20 years back.
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Old Mar 18, 2012, 03:16 PM
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Gliders are gliders and IF I were flying them , I would likely try shaping the wings - mainly for strength to weight but for a fully aerobatic ( the bipe pictured is a terrific slow speed performer indoors having 80 sq inches of wing and weighs all up -- 29 grams) the advantage of any curve or plate streamling appears to be whimsey at best.
I adopted thin airfoils long ago in the 1980s-on my FAI pattern planes and the Buckers (TOC models) The big bipes CHyde had at the same time had even thinner wings but they required rigging. I got away with 10% thickness and no rigging because the structure used permitted it

As many have found, thin foils and low wing loadings go hand in hand -even in very violent aerobatics - the only caveat is that strength to weight really has to be watched. really good structural approaches are paramount.
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Old Mar 18, 2012, 07:01 PM
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Richard, I agree that it would likely be a waste of time trying to put a smooth airfoil on something very small and light. I also agree that thin is the way to go at those sizes. I think this is more because the thickness isn't needed---a thin balsa plate can withstand the bending. Also, you're argument is based on size, and I think I generally agree your argument based on practicallity.
So which "rules" were you talking about bending or ignoring?

Also, my question was regarding Dave's statement that specifically mentioned Reynolds number.
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Old Mar 18, 2012, 10:07 PM
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I find (as have others) - balsa is not a good weight/strength medium for these ultra small light models
the new foamed materials and carbon fiber work better
anyway- the "rules" which can be bent easily are those regard stalled flying surfaces and desireable /safe CG's.
These are models and can do many aerodynamic tricks simply impossible with full scale

Why? because the power to weight and size can be far in excess of what is possible in full size.
I am not referring to small hand launch or indoor microfilm
these are full bore aerobatic types.
What types small models have you worked with?
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Old Mar 18, 2012, 10:42 PM
Grad student in aeronautics
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I've stayed in the one to two pound range, other than some balsa gliders (and I agree balsa is not ideal).

I wouldn't be surprised if the ability for these small aircraft to do the very fast tricks is due to the very low moment of inertia. One thing that doesn't depend on moment of inertia is the steady state roll rate. What have you noticed on the ability of these small aircraft in the way of a steady roll?
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Old Mar 19, 2012, 06:33 AM
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Steady roll rate- absence of a roll couple?
These models -not the little sporty bipe I showed in the pic, have extraordinary roll characteristics
example - some of the designs fly as easily -in a straight line- on edge as in level or inverted flight .
the side areas of the fuselage and wing dams are such that lift against gravity, is pretty close to the same in any portion of the roll.
rolls are very constant with a given aileron input and also turns from in level flight require no aileron inputs or corrections -just rudder nput - very odd ato see at first .
The models are just about perfectly neutral.
The little bipe has sweep in both panels and a small amount of dihedral
It does extremely sharp radius, flat turns, will also hover, torque roll,-a real torque roll, loop inside or outside with no "roll out" and is extremely docile in very high alpha flight
but - it simply will not roll. It is easy for even novice flyers -in a small gym
two completely different setups -both tons o fun
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Old Mar 19, 2012, 07:31 AM
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Yeah I've seen some interesting youtube videos of them. I forgot they have enormous control surfaces as well.
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Old Mar 19, 2012, 08:10 AM
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Yes - there is a bunch of videos of indoor competition from Germany which show how these perform - the accurate flight paths they can follow are amazing

We worked for years to get accuracy in outdoor pattern plane (and huge IMAC /TOC planes) but these indoor models are simply beyond anything we had for outdoor flying.
Extremely low ineria is obviously at work here .
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Old Mar 19, 2012, 09:47 AM
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Richard,
To use a blanket statement for slow flying models based on pattern planes and small foam aerobatic planes is absurd. The planes you are talking about need inverted performance (symmetrical airfoils) and use high drag to help control speed on the down leg of maneuvers, correct?

If the airfoil shape made no difference other than structural there would be no need for glider classes that do not allow camber change.

My first model airplanes were control line. A friend and I went to a hobby store and bought the same kit, .049 engine, prop, etc. We would fly in the same circle and my friend’s plane was sooooo slow. As you can imagine he was not very happy so we took his plane to the hobby store for help. After they stopped laughing… they pointed out that my friend skipped the “sand the balsa wing to an airfoil.” Yep, it had a square leading and trailing edge wing, just like a Depron plane.

His plane probably would have been far superior to mine for control line stunt though, wouldn’t it?
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