Quote:
Originally Posted by Juha
Dear Crumpp
the tread began with a question on the speeds of certain aircraft at deck, not how weight affects on the aircraft.
You claimed we this and that, who are "we"? Or are you maybe a Royal person?
Just asking
Juha
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That might be me, although 'we' are not royal.
I jumped in earlier to bring up two or three points as Harri moved into precision calculations to 'calculate' the differences.
One is that he calculated a Cd0 for the Mustang rather than use one or more of the much lower referenced values.
Two he used the Hp to Thrust conversion as if it was a law of physics and seemed to not fully understand the full context of all the forces acting on the Mustang, including Thrust of the airplane at rest.
I never got an idea that he fully understands that calculating propeller thrust in context of force must take into account propeller drag as the velocity changes.
The approach used is a good ballpark. The reason I started nitpicking is that the 'true' equation for the solution sets are
Tp+Te = Dprop + Di + Dparasite at V1
and
Tp+Te = Drpop + Di + Dparasite at V2
or
Tp1/q1S + Te1/q1S= Dprop1/q1S + CL1**2/(pi*AR*e) + Cd0
and
Tp1 + Te1 = Dprop2/q2S + CL2**2/(pi*AR*e) + Cd0
Where q1S = 1/2*rho*V1**2 and q2S=1/2*rho*V2**2
Cd0 = Cd0 for both states, but I wanted Harri to walk through this to show that the relationship between (Tprop1- Dprop1)=Thrustnet holds true for both velocites (and weights) in order to use the equation Thp=thrust x V/375 in mph or Thp=thrust x V/326 in kts and hold THp constant
Last but not least, both of these aircraft are pushing Mach numbers in .55 range where profile drag based on activity factors and compressibility become increasingly important and in my GUESS different, for both of these ships, as the propeller designs are both different in diameter, activity and tip speed.
My own experience (many years ago) was to use the Crumpp's parametric approach as a guesstimate in preliminary design was the better way to figure out impact to performance profiles due to weight alone.
It has also been the 'easy way out' to convert Bhp from charts to THp for Sea Level parametric studies on such things as Turn or climb performance and fall back on Thp equation for thrust and Velocity when calculating Thrust available versus thrust required. Both approaches are flawed if precision is what you want.
But it is not adequate, by and of itself, at M> .55, or at increasing altitudes bwhere density changes are increasingly important.
Harri - I respect your approach - but don't think it is adequate for precision for the reasons I have stated.
Nuff said - I am bowing out of this conversation