[Crumpp]

And without continous personal remarks and harsh language, you have presented.

Not really. I think that is your personal perception.

I think you have been very rude.

Well, you just calculated how much additional speed was needed to maintain the Cl. And that is impossible at constant power, the top speed situation ie aircraft has no such ability.

You are still confused on parametric study, huh?

Harri Pihil, I calculated the affect of weight on the aircraft. That affect does not change simply because the aircraft no longer has the ability to compensate for it.

This is not the Mulberry bush for us to go round and round.

You claimed:

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Quote:

Originally Posted by Crumpp
Ok then. All I am asking you to do is just explain what is different.

Let's demonstrate the differences using the values of the P-51B I allready calculated for constant power above for 9680lbs and 10280lbs. We allready know that, for these given parameters, my calculation is correct. Because your calculation is made for calculating required speed for constant Cl, we can solve the very same problem with my calculation as well and see the principal differences:

The lift coefficient at 9680lbs (4390,85kg) and 352mph (566,37km/h=157,324 m/s) at sea level is, as allready demonstrated:

Speed V =157,324m/s
density r = 1,225kg/m3
wing area A = 21,83m2
Lift L = 4390,85*9,81 = 43059,51 N

Cl = L / (A * 0,5 * r * V^2) = 0,13011

At 10280lbs required lift is:

L = 4663,01*9,81 = 45728,487 N

So we can solve the required speed at constant Cl at higher weight using these:

V = SQRT( L / (A * 0,5 * r * Cl)) = 162,17m/s = 583,66 km/h

And we can check this with your formula as well:

V2/V1 = SQRT(W2/W1)

583,66km/h/566,37km/h = SQRT(4663,01kg/ 4390,85kg)

1,031 = 1,031

So we can see that both calculations methods give the same result for that specific problem.

However, the result for this specific problem, the speed change required for constant Cl at higher weight, is over ten times higher than the result for the problem in our hand. Note that we calculate steady conditions here ie D=T at level flight.

Speed change due to weight change if power is constant and the other parameters are adjusted accordingly as demonstrated earlier is:

delta V = -1,48 km/h

while the speed change to keep Cl constant when the weight change is:

delta V = +17,3 km/h

We can also calculate the power required using the similar drag and thrust calculations as demonstrated above and we find that this higher speed at constant Cl requires 1748,98hp ie 169hp more than with original values (assuming otherwise the same parameters, including exhaust thrust).

What we see here is:

1. Calculating speed change to keep Cl constant at varying weight is completely different problem than calculating speed change at constant power when the weight changes (ie our problem in hand)

2. The magnitude of the results is completely different, over ten times difference in this particular case.

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I responded:

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Now we are getting somewhere!

Quote:
Speed change due to weight change if power is constant and the other parameters are adjusted accordingly as demonstrated earlier is:
Quote:

delta V = -1,48 km/h

Certainly! Why is that change so small? What the differences?

Because the aircraft does not have the Power available to meet the new power required! The Angle of attack must increase and the velocity slow down.

Of course in a propeller aircraft as our velocity decreases we know our thrust increases!

The affect of weight is very much present and the aircraft's entire envelope is still reduced

There is change in the aircraft's ability to compensate for that affect but the affect of weight is exactly the same.

No change in the significance of weight.


So this result:

Quote:
Speed change due to weight change if power is constant and the other parameters are adjusted accordingly as demonstrated earlier is:
Quote:

delta V = -1,48 km/h

Is really exactly the same as this result:

Quote:
while the speed change to keep Cl constant when the weight change is:
Quote:

delta V = +17,3 km/h

As demonstrated by your first set of calculations:

Quote:
And we can check this with your formula as well:
Quote:

V2/V1 = SQRT(W2/W1)

583,66km/h/566,37km/h = SQRT(4663,01kg/ 4390,85kg)

1,031 = 1,031

So we can see that both calculations methods give the same result for that specific problem.

So when we isolate the affects of weights in a parametric study to see the true affect, we can only conclude that weight has very significant affects upon an aircraft, even at high speeds.


All the best,

Crumpp