Hello all,

starting with the visual examination of a wing section, proceeding toward characteristics like thickness, camber amount and position, ending in the study of the polar curves, how can I estimate if a certain wing section is suitable for slats and flaps, and what is its practical wing loading range?

Many thanks.

Enrico

I'll start out with a simple answer based on what I have observed in both modeling and actual full scale practice.

The basic airfoil is selected to work in the cruise or whatever the desired flight regime of the airlane might be designed for (speed, heavy lifting, gliders, etc.). My on approach is to look at what the fanatics in the hobby are using and copy them. They usually have been willing to put in the research and flight testing to determine the best section for their needs. Not being that fanitical copying seems a good approach.

Flaps or slats are added as needed to that basic airfoil to try to get the landing speeds down (if needed). All wing sections can have flaps and slats put on them (well most of them anyway). Since you are trying to get drag and slow down it doesn't matter much if you mess up a good section characteristic.

Workable wing loadings are a function of airplane size for the most part. Indoor flyers need a few ounces per square ft, big .60 to 2.00 cu.in. powered machines can be 20 to 30 ounces per square ft., giant scale gets higher and so on until you get to full scale modern jets with really big wing loadings.

Thanks Ben,

I was aware that these are subjective (and why not, common sense) choices rather than scientific processes.
My designing practice is several years old, and I was wandering if some more technical rule arised in the meanwhile.
Anyway a'm happy learning from really expert people.

Greetings.
Enrico

Pocopagni,

"Practical" wing loading has a lot to do with two things: How fast do you want it to land. How fast do you want it to takeoff and climb(how much power do you have available). Heres a quick rough example:

6 pound plane, Aspect ratio of 5, Wing Area of 6 sqft, lets say the maxCl is 1.

Velocity=sqrt(52.6/Cl)*sqrt(wingload_oz)
This come from the lift equation: Lift=Cl*.5*q*V^2*area
Velocity=sqrt(52.6/1)*sqrt(16)
Velocity=29 fps This would be the slowest landing speed.

Say we added flaps and now the maxCl is 2.
Velocity=sqrt(52.6/2)*sqrt(16)
velocity=20.5 fps So we doubled the Cl but it only reduced the landing speed by a 1/3.

Now if you look at the power requirements.

Sinkrate=Velocity/glideslope (There is a strong correlation between Aspect ratio
and glideslope at Clmax so I will say glideslope=Aspect ratio)

EX 1: Sinkrate=29/5 or about 6 fps

Power for level flight=sinkrate * weight
Pow level flight= 6*6 or 36 fp/s

Climbrate=(available power-power needed for level flight)/weight.

Lets say I have a .40 bushing type motor that delievers about 400 fp/s the actual power available at takeoff speed will be between .20 to .30 of this number. Let's say .25 so we have 100 fp/s of power available.

Climb rate= (100-36)/6
Climbrate=10.7 fps pretty mild by RC plane standards, and we are probably approaching the practicle wingloading for this engine wing combo because if it much heaver it will never get off the ground and will have poor flight performance even if it does. If we put a .61 generating 1000 fp/s then we could up the wingloading quit a bit and still have good climb performance but the landings might get pretty hot.

Anyway I know this is a pretty rough way of looking at it but it does give some design numbers to start with.

Hello DipStick,

Thanks a lot for your precious indications.
These are a very good starting point to forecast and to plan the model behaviour.
I've read other wing loading related posts and I find your inteventions always clarifying.
I think I'll need your knowledge again in the future.

Greetings.

Enrico