Performance of the Fw 190A on the Deck?

[Juha]

Ah, long debate
now according to Dornier's tests max speed of Do 17Z-2 was 425 km/h at 5000m at 8600kg and 433km/h at 7400 kg. To me the effect of 1200kg more mass to max speed wasn't very big.

Juha

[Crumpp]

Wonder why it is such a hard concept to grasp that although the speed difference is small, the weight affects are extremely significant because it means every performance parameter of the design has been reduced?

[Harri Pihl]

Originally Posted by drgondog

Given that it doesn't work at zero velocity, what do YOU use to yield the Thrust/Total Thrust/Total Force (all interchangeable) at max power on the ground, chocks or brakes in place? There IS a Force at work but no acceleration as it is in equilibrium. Demonstrate your knowledge of Physics.

In other words, given your vast knowledge of Aero Engineering, based on the easy to understand explanations by Harri, could you calculate take off run, distance to clear`50 ft obstacle and what would your velocity be over the 50 ft obstacle if;
a.) you knew Bhp, or in case of jet - static thrust
b.) wing Aspect ratio,
c.) gross weight,
d.) CL and CDp,
e.) wing area.

Before you start what do you need to know about Cdp that we have not yet discussed?

The specific problem to be solved was the effect of the weight to the max speed at constant power. What I did was that I calculated Cd0 ie Cdp backwards from the flight test data, using estimated but good enough values of n, e and exhaust thrust. Because the assumed weight change was rather small and speed rather high, the small errors in these estimated parameters can't cause large error; less than 1km/h in that specific case.

Note that accuracy of the Cd0 is not that critical for this specific problem because we are dealing with the same plane at different weights; using Cd0 from say Lednicer's works would have resulted very similar change of the speed due to weight change.

Other point of the analysis was to show why the parametric analysis can't be used to solve this specific problem. There is no constant key parameter needed for the parametric study ie the speed, Cl, drag and thrust are not constant. Therefore polar approach using iteration process is the right way.

However, calculating static thrust, take of distances etc. are other kind of problems which need different approach and indeed out of topic of this thread.

[Harri Pihl]

Originally Posted by Crumpp

Actually to illustrate my previous point, if you just calculate the direct effects such as being done here, you completely miss the importance.

No one has denied the effects of the weight nor importance of them, Graham's original point was just that effect of the fuel state is small at high speeds near sea level in this kind of planes. And that has been proven true.

Originally Posted by Crumpp

Why? There are quite a few coupled affects we are actually witnessing which are hidden behind the scenes.

At full power in sustained level flight:

When we add weight our drag increases.

When we add weight we need more power to increase our dynamic pressure. If we don't have more power, the wing must increase AoA to a higher coefficient of lift, and coefficient of drag.

This higher CL occurs at a lower maximum velocity.

As the velocity slows, the thrust increases but so does our Power available. It increases at the rate of velocity squared for drag other than lift and to the first power for induced drag. So we have more power available the slower we go until we get to Dmin.

Thrust is increases too as we slow down. We are getting the "double whammy" as our thrust required is reducing and our thrust available is increasing.

The coupled affects manifest themselves in the direct affect of a small change in velocity. It becomes easy to make a false assumption if all you do is parrot formulas to draw a conclusion like Pihl.

All these changes are accounted in my demonstration ie higher Cl, therefore higher Cdi and Cd and further slower speed and therefore even higher Cl and higher thrust and so on.

At higher weight there is new balance point of drag and thrust at lower speed and at this new balance point all the key parameters (V, Cl, Cd, T and D) are different than at original weight.

This is also why the parametric study can't be used because we don't have a constant key parameter. This was demonstrated well when you tried to use constant Cl for this particular problem; the analysis gave completely wrong picture of the effects in this specific case. Same problem was with drgondog's analysis when he tried to use constant thrust.

[Crumpp]

Graham's original point was just that effect of the fuel state is small at high speeds near sea level in this kind of planes.

No Grahams original point was in response to my statement:

Crumpp says:

Weight very much affects our aircrafts envelope. It has a direct relationship with velocity.

Graham then says:

No. Weight affects the envelope, indeed, but the direct relationship is wrong.

Which is not correct.

It certainly does have a direct relationship with velocity.

You have tried to morph my position into something of your own invention.

My contention is:

While weight effects are smaller at high velocity, they are far from insignificant. Weight certainly makes a difference not matter what portion of the envelope we examine.

Does velocity change when we add weight? YES

Does this change in velocity signal a reduction in the entire envelope of the aircraft no matter how small the specific change in velocity? YES.

All these changes are accounted in my demonstration ie higher Cl, therefore higher Cdi and Cd and further slower speed and therefore even higher Cl and higher thrust and so on.

Wow. You don’t get it. Of course they are “accounted for” when determining the specific performance.

What they do not account for is the magnitude of the affect of weight. To do that, we need a parametric study.

Your contention:

This is also why the parametric study can't be used because we don't have a constant key parameter.

The idea we need a steady parameter to hold the parameters steady is laughable.

It is complete horsepucky of your own invention. The whole point of a parametric study is to isolate affects in a complex system of changing parameters.

The airfoils key relationships of L/D ratio to AoA are held constant to determine the affect of weight in isolation:

V2/V1 = SQRT(W2/W1)

V1 = 300KEAS

V2 = ?

W1 = 10000lbs
W2 = 10500lbs

V2 = {SQRT(W2/W1)}*V1

V2 = {SQRT(W10500lbs/10000lbs)}*300KEAS
V2= 307.4 KEAS

If we hold angle of attack constant, we must increase our speed 7.4KEAS or 8.5 mph if we add 500lbs weight. Well one just might think that is insignificant. It certainly isn't gong to make much difference in what we can we catch or run from.

Now lets look at it from the Power required relationship in our fictional aircraft. Using standard BGS formulation for a power producer:

Pr1 = 2000thp

Pr2/Pr1 = (W2/W1)^3/2
Pr2 = {(W2/W1)^3/2}*Pr1
Pr2 = {(10500/10000)^3/2}*2000thp

Pr2 = 2680thp

Or a 34% increase in the amount of power required! That 4-5 KEAS in reality represents a very significant reduction in the designs power available.

All the best,

Crumpp

[Kurfürst]

Effect of weight on Bf 109G maximum speed:
http://kurfurst.allaboutwarfare.com/...fect_speed.jpg

[Juha]

Thanks a lot Kurfürst!
if I understood right, 500kg (appr. 1100lb), from 3000kg to 3500kg, increase dropped 109G speed at SL 3 km/h.

Thanks
Juha

[Juha]

I almost forgot
one more theory on seemingly inconsistend performance at deck. FW 190A had air cooled radial but for ex Typhoon, Spit and P-51 had liquid cooled engines. According to Finnish tests on Bf 109G-2 fully open radiator flaps could decrease speed up to 50 km/h at low level, in LaGG-3 case the effect of the fully open radiator flap was appr. 15km/h.

Juha

[Jukka Juutinen]

Good point. Would be interesting to know the effect on especially the Typhoon. Also would be interesting if there is info on which one of the above was affected most by the ambient temperature. After all, didn´t Finnish test show that e.g. the SB had to pull back on power even on relatively mild conditions (Finnish summer vs. e.g. tropics or desert) as even fully opened rad flaps didn´t provide adequate cooling at FT.

[Harri Pihl]

Originally Posted by Crumpp

No Grahams original point was in response to my statement...

Graham's original point:

"Actually, fuel state is not that important with regard to maximum speed."

Is in the first page and posted before you entered to this discussion. And at page 3 of this thread you state that:

"My issue has always been that Graham's original post:"

and then you quote that same part which Graham posted before you appeared.

Originally Posted by Crumpp

Wow. You don’t get it. Of course they are “accounted for” when determining the specific performance.

What they do not account for is the magnitude of the affect of weight.

It's all there, the changed Cl, Changed D, Changed T and changed V. All at new balance point due to increased weight.

Originally Posted by Crumpp

The idea we need a steady parameter to hold the parameters steady is laughable.

The point is that you need at least one constant key parameter, and in your calculation you keep Cl/AoA constant. Therefore you also estimate speed reduction to be much higher than it actually is.