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#7
Oct15-06, 07:43 PM
P: 8
Rainman thankyou very much for all your help so far. Ironically another site has just given me a formula also. Literally within the last 5min after trying to locate this for over a month I have two! lol

Anyway, I need to calculate the values for the object to rise vertically from a stationary position at ground level. Not in flight with lift generated by lift surfaces.

1) In your formula above Treq = Drag = CD*0.5*rho*Airspeed^2*(Aircraft Wing Reference Area) what does the value 'CD' represent please?

2) The prop pitch varies from hub to tip, but when purchasing they have a given pitch value, can this value not be used to give a relatively accurate approximation for the prop?

The other formula I have come across is:

Ct = T / p . n^2 . D^4
Where:
Ct = Thrust Co-Efficient
T = Thrust
p = rho
n = rev per sec
D = Diameter

3) I think this formula is more appropriate however I dont know what 'Ct' or 'T' are measured in. T would undoubtedly be Newtons but 'Ct' ?

4) And whats the relationship between them so far as the difference between T and Ct? ie: How do I calculate Ct?

I also found another formula that I think is not what I need, but it could be relevant. Its:

F = .5 x r x A x [Ve^2 - Vo^2]
Where:
F = Thrust
r = Air Density
A = Prop Disc Area
Ve = Exit Velocity
Vo = Aircraft Velocity

5) My major problem with this formula was that I couldn't figure out the value for Ve. Any ideas?

6) Last but not least, Im going to be using either a 4 or a 5 bladed propeller. How does this change my formulas if at all? Im thinking that for example with a four bladed prop I would just double the resultant thrust. Or would I have to increse my powers in the formulae by a factor of two? (ie: ^4)

Thanks for any help. Its really great to have someone to ask this of.

Esentially what Im trying to establish in the end is what hopsepower engine I need to lift my object when I know its weight, my prop diameter, pitch, and rpm.