Re: Propeller parameters

----- Original Message ----- 
From: "bass pumped" <[email protected]>
To: <[email protected]>
Sent: Monday, April 04, 2005 7:27 PM
Subject: [Flightgear-flightmodel] Propeller parameters


> I'm new to flight gear and I was looking over JBsim's Aeromatic 0.7
> aircraft configuration file generator.  I generated the propellor
> profile file and I was presented with this:-
>
>
> <xml version="1.0"?>
> <!-- Generated by Aero-Matic v 0.7
>
>     Inputs:
>       horsepower:   270
>       pitch:        fixed
>     Outputs:
>       linear-blade-inches: 87.087886643321
> -->
>
> <FG_PROPELLER NAME="prop">
>  IXX        1.8955295471191
>  DIAMETER   75
>  NUMBLADES  2
>  MINPITCH   30
>  MAXPITCH   30
>  MINRPM    700
>  MAXRPM   2700
>  C_THRUST   11 1
>  0.0  0.0936
..................
>  1.0  0.0040
>  C_POWER   11 1
>  0.0  0.0594
..............
>  1.0  0.0090
> </FG_PROPELLER>
>
>
> I have an understanding of the IXX, diameter, numblades etc...   What
> I do not understand is what follows
>
> C_THRUST   11 1
>  C_POWER   11 1

> To me it looks like it might be the power profile of the engine, but I
> would like a better clarfication about it if its possible.

    First, the values ranging from 0.0 to 1.0 at the left must be Advance 
Ratio, J.  Paired with Ct and Cp.

    Similar tables are in MSFS AIR files.  I put some prop formulas in the 
AIR file editor:

    Power absorbed, P = Cp*rho*[N^3]*[d^5]
    Thrust, T = Ct*rho*[N^2]*[d^4]
    Torque, Q = Cq*rho*[N^2]*[d^5]
    P =2Pi*N*Q
    J = V/Nd = TAS(mph)*[88/RPM] *d(ft)
    Eff = J*Ct/Cp    (= Ceta)
    N=RPM/60
    rho = air density (sl/ft^3)
    d = Prop Diameter (ft)

    Note Power absorbed varies with N^3 (assuming J is constant).  While 
Thrust and Torque vary with N^2.

    Prop MoI varies with D^5, and can be estimated for typical props as:
    MoI ~ 0.5 * Prop_Weight(lb) (less hub) * [D^2]/386

    Working backwards from the Aeromatic Ixx = 1.9 sl)ft^2 I'd figured that 
prop weighs about:

    1.9 ~ Weight * 0.5 * 0.1;  Weight ~ 1.9/0.05 = 38 lb.  Seems a bit low 
for handling 270 HP.

    Ron

 


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