Question about wing loading...

[glockengockel]

:confused:

Hello,

I am building a Dornier with 41"/103cm span and 13.9sqdm/1.5sqft wing area.

My question is how much wing-loading I may accept to stay within practicable and scale speed.

Advice appreciated!

Regards Hans

[ChrisP]

Hallo Hans

Depends where you want to fly it (eg indoors vs outdoors).

If you are thinking about something like an outdoor park flyer, you should be somewhere in the region of 14 - 15 ozs/sq ft = ca 42 - 45 gms/sq dm.
Shoot for something like 600 - 650 gms AUW which means you are in geared 400 territory for the power unit.

mfg

ChrisP

[Ollie]

To calculate what the wing loading has to be for scale speed, you must give us the full scale span, the full scale stall speed and the model airfoil so that we can calculate the lift at the scale speed. Lift equals weight. With the weight and wing area it is easy to calculate the wing loading

[MikeSt]

Originally posted by Ollie
To calculate what the wing loading has to be for scale speed, you must give us the full scale span, the full scale stall speed and the model airfoil so that we can calculate the lift at the scale speed. Lift equals weight. With the weight and wing area it is easy to calculate the wing loading

Somethin I'm interested in also ;)

Ok. The "uibiquitous" C172 - F.S. span - 36', stall 44 knots, model scale, span 3.6', i.e. 1/10 scale. Model weight 2lbs.

I know how to calc wing loading, for the particular wing size / weight I've given.

What I don't know is how the "scale" speeds relate to the full scale speeds.

Mike

[Ollie]

The scale speed is easy. In the example of 1/10 scale the scale speed is a tenth of the full scale speed. This is based on the fact that a model flying at some distance, from the observer will appear to have the same size as the full scale plane at ten times the model's distance from the observer. If the model is flying parallel to the full scale plane, it will appear to be flying in formation with the full scale when the model is flying at a tenth the full scale plane's speed. Therefore, the model of the example should have a stall speed 0.1 x 44 = 4.4 knots or 5.7 feet per second.

The reason I asked for the model's airfoil is that model airfoils tent to stall at lower lift coefficients than full scale airfoils. The wing loading varies inversely as the coefficient of lift and as the square of the airspeed. the formula is:

W/S = 0.0012 x Cl x V^2

where W is in pounds, S is in square feet, Cl is dimensionless and V is in feet per second. The number 0.0012 represents the density of the standard atmosphere (atmospheric pressure, temperature, humidity, etc.)
Assuming the model's wing stalls at Cl = 1.0, The wing loading for the example should be:

W/S = 0.0012 x 1 x 5.7 x 5.7 = 0.39 pounds per square foot or 0.62 ounces per square foot.

The example illustrates that models almost always appear to have a higher speed than their full scale counterparts because it is difficult to get the model's wing loading low enough or the wing's coefficient of lift high enough. Also, the smaller the model the more difficult it is to achieve scale speed.

[ChrisP]

Originally posted by Ollie
.........because it is difficult to get the model's wing loading low enough or the wing's coefficient of lift high enough.
I took the 'scale flying speed' bit with a pinch of salt. I didn't think it would be helpful to suggest a wing loading around 3 ozs/sq ft (my rough calculation) as a target assuming this is an OUTDOOR model.
OK - back to reality ;)
My Bloody Mary is at 7.3 ozs/sq ft. It has the lowest wing loading of my fleet and I fly it outdoors on nice days. The structure is minimal, but it could be entirely feasible for a scale model. (A Dornier what ? Something like a Merkur would be easy).
I have 3 styro/Depron models just under 10 ozs/sq ft. I have flown all of them in quite windy conditions. Would also be a valid target.
Most of my 400 powered models are around 13 - 17 ozs/sq ft and not at all 'critical'. That's why I suggested going for 14 - 15. However if scale-like speed is high on the wish list, I think
something around 7 would give a model which flies in a scale-like manner like my Bloody Mary without being too delicate.

[Ollie]

Like almost all design decisions, wing loading of scale models is a matter of desiding where the balance should fall between conflicting objectives. I am not much of an advocate for using scale speed as a high priority because it so adversely affects practical flyability in any wind and results in a very fragile structure.

However, the question of scale speed and associated wing loading was asked and I answered it accurately from an aerodynamic point of view. I only hinted at how the answer should be used. After all, exactly how to use the information depends on a lot of things that the questioners did not specify. If, for example, the questioners intended to use the information for an indoor rubber powered scale model, the information could be applied with fewer compromizes.

[glockengockel]

Hello all,

thank you for all your answers. You convinced me that scale speed is less important than the practical flying abilities. My current design is:

wing span 40.6"
cord 4.3"
wing area 215.3 sq-in
weight 340 g
wing loading 1.58 g/sq-in
Re 47000

Well, do you think this is ok for save and practical flying abilities?

Regards Hans:rolleyes:

[MAC & MEYER]

A good place to start.!
http://www.astroflight.com/scalespeed.html

[Ollie]

Mr. Bob Boucher's introduction of the "scale time" definition destroys the direct visual experience of scale flight of a model. Using Mr. Boucher's definition of scale time and speed requires that the model's flight be photographed and and then played back in slow motion to regain the visual experience of full scale flight. Mr.Boucher's definition of scale speed is self serving in that the weight of an electric motor and battery make it virtually impossible for an electric powered model to present a direct visual experience of scale flight. After all, he is in the business of selling electric motors. There is nothing illogical in Mr. Boucher's point of view except that it destroys the appearance of scale speed when the model's flight is viewed directly instead of in slow motion. In fact his argument is brilliant.