Performance of the Fw 190A on the Deck?

[drgondog]

Originally Posted by Harri Pihl

Ok, at 19680lbs and otherwise the same parameters I got:

V = 523,9 km/h => 42,46 km/h less than at 9680lbs
T = D = 7653,63 N => 485,61 N more than at 9680lbs

Comparing the amount of induced drag at all these weights:

at 9680lbs Di is 382,45 N ie 5,34% of total drag
at 10280lbs Di is 433,60 N ie 6,04% of total drag
at 19680lbs Di is 1847,45 N ie 24,14% of total drag

Hari - carefully
Tmax at 8,000 pounts = Tmax for 10,000 pounds = Tmax for 96,000 pounds gross weight.

Throw Thrust out of the equations when solving for Velocity between the two, rho is not important, live T, it is the same for both conditions. e, AR, engine/prop efficiency is the same for both conditions.

If you wish to do a balance between Induced drag and parasite drag to equal the 'UNADJUSTABLE" Thrust, go for it, but it is simpler to use the parametric rations Crumpp showed you and patiently explained.

They are the right ones to analyze this problem of major increase in fuel weight (5% or more) on real speed performance.

Since you are dealing with the P-51 there is a lot of flight test data out there - go look and compare as long as you use the same engine and boost conditions. The airframe induced and parasite drag are the same between P-51B and D. Only the weights, the engines and the boost/fuels are variables until you get to the P-51H.

In the case of a P-51B it (Velocity) is unbelievable at 8,000lbs, pretty good at 10,000 pounds and hardly moves, if at all, out of the chocks at 96,000 pounds but the Thrust is exactly the same for Max Power for that engine.

[Harri Pihl]

Originally Posted by drgondog

Hari - carefully
If you wish to do a balance between Induced drag and parasite drag to equal the 'UNADJUSTABLE" Thrust, go for it, but it is simpler to use the parametric rations Crumpp showed you and patiently explained.

I can only repeat what I said earlier. Just take a look to the propeller thrust formula:

T = (n*W)/V

It should be clear that at constant power thrust varies with speed ie thrust decreases when speed increases.

Regarding the parametric rations; these can't be used because none of the key parameters is constant (Cl, drag, thrust nor speed). And I have patiently explained that several times above.

[Crumpp]

If you want to see the thrust horsepower in the calculation, just convert the power value back to bhp and multiply with n. However, I don't see any point to do that because, after initial conversions, the entire calculation is done with SI units.

So you are now saying you do in fact calculate thrust horsepower. That is a reversal from your earlier claim.

Harri Pihl says:

I don't actually calculate the thrust hp but directly the thrust from the brake hp; the hp chart I used just gives the bhp.

Which now sounds like a falsehood as you do in fact multiple SHP by propeller efficiency to convert it to thrust horsepower. Conversion of units has absolutely no bearing whatsoever. I can say something is 1 meter or I can say it is 1.09 yards. It is still same. Of course I am sure you have a reason why dimensional analysis is wrong too if you don’t even understand parametric study.

I find it very hard to believe anyone with a formal education in aerodynamics would have made such a statement in the first place as we must figure horsepower to determine propeller aircraft performance.

Why? In a power producer we hold power constant and use it to figure thrust. It is a fundamental principle of power producers.

There is absolutely nothing that changes:

Graham I think it certainly has been quite a while since you worked with aerodynamics. From what I understand, it is like riding a bike and it comes back.

You missed the point on the parametric study. The only way to judge the affects of weight is to hold a constant, adjust weight, and measure the results. Otherwise we miss the forest for the trees.

That is the basic scientific method and is generally accepted for aerodynamics and applied physics.

Not doing it leads to false conclusions on the relationships. We wind up with statements like the affect of weight is insignificant because we only see a small reduction in speed.

All the best,

Crumpp

[Harri Pihl]

Originally Posted by Crumpp

So you are now saying you do in fact calculate thrust horsepower.

First I convert bhp value to SI power unit, watt, and later the power is converted to thrust according to n and V. There is no point where I have THP calculated, you can add such to the calculation if you think that it's important but it's not needed.

[Crumpp]

First I convert bhp value to SI power unit, watt, and later the power is converted to thrust according to n and V. There is no point where I have THP calculated, you can add such to the calculation if you think that it's important but it's not needed.

You really don't understand that you are dealing in Thrust Horspower, do you?

[Harri Pihl]

Originally Posted by Crumpp

You really don't understand that you are dealing in Thrust Horspower, do you?

I'm dealing with power, horsepower being one of the power units but I use SI unit where power unit is watt. There is neither "thrust watts" calculated out; the propeller efficiency formula contain elements for that but I don't actually see it because the formula gives directly the thrust.

And please PM if you are interested about further discussion on this particular subject.

[Crumpp]

I'm dealing with power, horsepower being one of the power unit

Yes....

When you convert that Horsepower to SI units and then mulitply by efficiency...

You have thrust horsepower!

[drgondog]

Originally Posted by Harri Pihl

I'm dealing with power, horsepower being one of the power units but I use SI unit where power unit is watt. There is neither "thrust watts" calculated out; the propeller efficiency formula contain elements for that but I don't actually see it because the formula gives directly the thrust.

And please PM if you are interested about further discussion on this particular subject.

Hari - Thrust is a Force not Power.

The more intersting exercise in aerodynamics is converting Hp to thrust to get the free body equation T=Di+Dp and maintain that equation in balance during steady flight and manuevers. The two most important analytical questions are - what is the real Cdp for that specific airframe, and what is the real Thrust when we only know Bhp at SL, or (rarely, have Bhp at a specific altitude) where the weights and airspeeds and boost are noted

THp and Bhp are not the same as Thrust - and ALL the propeller efficiencies, poweplant states, gearing, torque, rpm, pitch angles, blade geometry, etc are about somehow arriving at equations that give a decent approximation for THRUST.

You throw out simple conversions for an equilibrium state as if they hold true in an integration profile as variables change.

I don't know what you and Crumpp have in past history but I can tell you as and MS Aero Engineering that I listen when we engage in the theoretical because he understands the context of the equations and where they hold true.

I'm not going to bother any more on this thread because I feel that I am talking past you and you probably feel the same.

Go back to the flight tests and compare the 51B-15 with the P51D-15 with the same engines, racjs, etc - just different in weight and explain why the Vmax at Sl is so different when only the weight really changes? you will see it is a LOT more than 1 mph

[Crumpp]

Exactly. It's the grasp of basics and underlying theory that are not consistant.

You see stuff like this:

First thrust at original 9680lbs (4390,85kg) and 352mph (566,368km/h=157,3244 m/s):

1580hp = 1178014 W
exhaust thrust = 120kp = 1176,798N
Propeller Thrust = (0,8*W)/V = 5991,216N
Combined thrust = 7168,014 N

With the expression:

(0,8*W)/V

Force <dimensionally equal to the Horsepower> with a velcoity component multiplied by propeller efficiency.

And you read a statement that says:

I don't actually calculate the thrust hp

Which speaks volumes. Harri you are very smart guy. It's a shame you never pursued this in a university.

All the best,

Crumpp

[Harri Pihl]

Originally Posted by Crumpp

Yes....
When you convert that Horsepower to SI units and then mulitply by efficiency...

You have thrust horsepower!

Nope, in that case I would have thrust watts but I did not calculate that out from the calculation.