ger56

Hello ,

I am trying to optimize a speed calc on Excel .

Using the standard parameters for pitch and thurst works well for middle sized propeller
When it comes to 18" propeller and so on then the calc is completely wrong .

Example : AT6 CMP about 70" wingspan Mvvs 26cc .
propeller 18x6 - 7600 rpm calc based on pitch and rpm gives a 65.15 km/h ( including fuse + wing drag and add rpm in air ) and 8.25 kgs thrust and 1944 prop charge
Radar gives about 125 km/h in reality ....

Now I need a formula to optimize calc for big propeller :

- Thrust is 8.25 kgs for a 6.5 kgs airplane : Does it needs some additional calc ?
- Diameter is 18" : Does it needs some additional calc compared to s std 11" prop ?

I am also searching parameters for the following :

- Wing profile parameters ( a 8% wing profile will give less drag tna a 16% )
- Wingspan parameters ( 1.4m will give less drag than 1.70 m wingspan )
- Landing gear retract ( 10% ? 15 % ?add drag if no retracts ? )

Thnak you for your answers

Ger56

MJD

My Feedback: (1)

It sounds like you are assuming an airplane will fly at pitch speed. That is hardly ever true.

A static thrust measurement or calculation has no meaning with respect to the airspeed that your aircraft will attain. Thrust is not constant with airspeed. If static thrust had any bearing on the potential airspeed of an aircraft then pylon racers would use 3D props and we'd be bolting helicopter rotors to the front of our sport aircraft.

Wing airfoil parameters? You can find that on the internet by googling "airfoil data".

Wing parameters? You mean how to calculate the aerodynamic drag of a wing?

Not sure what this speed calculator is but it sounds like it is trying to condense complex physics into a few simple numbers and rule of thumb formulae. For example, an 8% wing profile will have less drag than a 16% profile? Is that always true? A 1.7m wing has more drag than a 1.4m wing? Is that always true? No and no.

The problem with simple calculators using broad parameters and assumptions, is that they can be made to work well for a few sets of conditions when the parameters are derived from empirical observation under those conditions. In other words, changing the rules to fit the data. But when you stray too far from those conditions then the reality of how complex the matter really is rears its head and the "calculations" lose meaning.

ger56

Thank you for your answer MJD .

You are perfectly right meanwhile I am sure some Rc modelist already made some calculation or formulas in order to try to
get some speed information .

I believe it is also better trying to have something than having nothing,and this helps to better understand the propeller choice and engine choice too .

So if we do not consider the basic speed pitch based calculation we can get on the web what is the way to go ?

Wing parameters : Yes what can be the drag of a 1.70m against 1.50 m with the same airfoil .



Regards

Ger56.

combatpigg

My Feedback: (3)

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