Performance of the Fw 190A on the Deck?

[drgondog]

Originally Posted by Franek Grabowski

Bill
There must have been some differrencies in aerodynamics, because P-51D turned highly unstable at high speeds and had to be modified. That is one thing.

Franek - you are right about yaw issues in dives from .75 to .83
But this discussion is about level deck speeds in the .55 to .6 range.
In a dive you have to feed rudder to keep the nose from wandering but it was not a divergence issue as 'highly unstable' which it wasn't.

Another is accuracy of such calculations. As we know, engineering theories are based on approximates and simplified theories, and quite often we do not know what is actually going on. This is very important in understanding calculations of performance.

Aero engineering is a complex field. Getting precision estimates of performance, pre flight or wind tunnel testing for a new airframe and geometry was a combination of arcane combinations of empirical factors (i.e "e" and "propeller efficiency" and even various components of parasite drag like wheel wells, new antennae, bomb racks, etc.).

Having said this, this discussion and Crumpp's presentation and arguments are about Parametric studies on the same airframes with one variable changed.. so we don't have to screw with "e", AR, predictions about flow separation, etc. That is the point (one of several) he was trying to make

If methods widely used give us 10% accuracy (~40 mph!), and the result must be verified in tests of actual aircraft, which then has some not insignificant margin for quality of production, then we find that those few miles are just unmeasurable.

On the other hand when flight testing the same airframe, with low fuel versus full combat load - same engine, same airframe, same boost, same pilot, same geography and easy to calculate variations for Temp and Altitude it is easy to get accurate results

On the other hand, we know that horizontal speed is just resulting from several factors. The most important is the airfoil used, then wing, then airframe, then engine and prop. Given each factor's share, it was concluded that small changes of weight are just unimportant in overall picture. That is what Graham is trying to show all the time.
BTW
Spitfire IX and Mustang III/IV/IVA were powered by the same Merlin engine. Which one was heavier and which one was faster?

The P-51. For the answer to the obvious question compare the Drag of each airfoil and airframe to get the "aha" for the Speed and range capabilities differences.

Franek - I respect Graham's knowledge but Crumpp in my opinion based on my own academics and practice is entirely correct.

The question for the example discussed is ~ 10mph significant for the 6% increase in weight? It would be for me, particularly if my Mustang was in the lower range of performance in the production series..

[drgondog]

Originally Posted by Harri Pihl

If you care to read the part you quoted, the power unit BHP is allready converted to the Watts (SI unit) at that stage and then directly calculated to the corresponding amount of thrust, Newtons in SI units.

Hari - you increased thrust in your heavier weight calculations. That doesn't happen when the aircraft discussed was at maximum speed, maximum Hp for that specific altitude. At max V the Thrust is Max and a constant.

By your approach you are saying that as weight increases, while thrust is a Constant, velocity Must increase proportionately to Weight? Is that what you believe?

I didn't say much about "e" and engine/propeller system efficiency assumptions by you because the relative values in the comparison won't vary with weight.. but you have to go back to your equations - assume

T= constant for both cases
D= constant for both cases
Solve for V when L=W and CL must change via slightly higher AoA to maintain level flight.

For the first iteration assume that trim drag for example will not alter Cdo, and assume Cdo is constant to arrive at a 'ballpark'

Then after you get Vheavy you can play with all the other factors you wish to see how much they contribute versus W.

[drgondog]

Originally Posted by Franek Grabowski

OK, so what is the point in underlining some of those figures? How about wetted area and the method it was calculated in each case? How about different AoAs or Res? Could you explain to us what is scientific value of the table while discussing aerodynamical characteristics of P-51?

Franek - That is a report that should be very interesting to you and Graham. Gene Lednicer did a detailed analysis to get the geomety correct for both the B and D models as well as the Fw 190A and D , plus the Spit IX. He is a well respected Aerodynamicist that also used his models on a commission by the Strega Mustang Racing team to improve flow characteristics behind the wing on the lower radiator cowl/fairing.

His model is a finite panel, stream tube modelling approach which required several iteration to solve for the Potential Flow balance while introducing Boundary Layer separation for all the airframes investigated.

VSAERO is the model he used and that is why Crumpp used the references showing many of the documented studies and wind tunnel Tests as well as referencing the Reynolds numbers for wind tunnel tests. Look particularly at the Ames test as it was performed with a full scale production P-51. It was later compared with flight test data on a series of dive tests without power on another P-51 as a validation check against the Wind Tunnel and calculation methods - and found to be very close.

The .0053 wetted drag number is the one to be used in contrast to .0040-42 values as they are 'lower cowl/radiator fairing - less". The .0053 CDw is comprehensive in reducing all the factors associated with the surface geometry to one value, but assumes all the surfaces (including the Spit and FW) to be the same surface, but it does predict different stagnation flow regions contributing to drag for both the canopy areas as well as boundary layer separation. The model IIRC was compared to other profiles at 300+kts but I could be wrong about that.

These modelling techniques, with which I experimented with in the late 60's, have gone light years beyond state of art in those days and as a result yield much better performance predictions for a V-22 than for an F-102 in design stage.

[Harri Pihl]

Originally Posted by drgondog

Hari - you increased thrust in your heavier weight calculations. That doesn't happen when the aircraft discussed was at maximum speed, maximum Hp for that specific altitude. At max V the Thrust is Max and a constant.

Please, check again my calculation:

9680lbs => V = 566,368km/h
10280lbs => V = 564,887km/h

So the speed at higher weight is lower, therefore thrust is higher, despite constant power, because:

T =(n*W)/V


9680lbs => T=7168,014 N

10280lbs => T = 7183,721 N

Note that in both cases T = D.

Originally Posted by drgondog

By your approach you are saying that as weight increases, while thrust is a Constant, velocity Must increase proportionately to Weight? Is that what you believe?

Please check the calculation again:

At 9680lbs:
drag lower
thrust lower
speed higher

at 10280lbs:
drag higher
thrust higher
speed lower

[drgondog]

Originally Posted by Harri Pihl

Please, check again my calculation:

9680lbs => V = 566,368km/h
10280lbs => V = 564,887km/h

So the speed at higher weight is lower, therefore thrust is higher, despite constant power, because:

T =(n*W)/V


9680lbs => T=7168,014 N

10280lbs => T = 7183,721 N

Note that in both cases T = D.



Please check the calculation again:

At 9680lbs:
drag lower
thrust lower
speed higher

at 10280lbs:
drag higher
thrust higher
speed lower

So, Hari - plug in 10,000 extra pounds and see what your calcs tell you about Thp and velocity.

[Holtzauge]

Originally Posted by Harri Pihl

Please, check again my calculation:

9680lbs => V = 566,368km/h
10280lbs => V = 564,887km/h

So the speed at higher weight is lower, therefore thrust is higher, despite constant power, because:

T =(n*W)/V


9680lbs => T=7168,014 N

10280lbs => T = 7183,721 N

Note that in both cases T = D.



Please check the calculation again:

At 9680lbs:
drag lower
thrust lower
speed higher

at 10280lbs:
drag higher
thrust higher
speed lower

Hi Harri,

I do not have the P51B model up and running in my C++ simulator but I just ran the P51D with the weight numbers you use and the 67" boost engine I got modelled. The numbers I got at S/L were:

9680 lb > 552.88 Km/h
10280 lb > 551.45 Km/h

This gives a staggering speed difference of 1.43 Km/h which compares rather well with the 1.48 Km/h speed difference calculation you made above

BR/Holtzauge

BTW: Try not to be offended by Crumpps personal attacks since that's his signum when losing the factual battle

[Harri Pihl]

Originally Posted by drgondog

So, Hari - plug in 10,000 extra pounds and see what your calcs tell you about Thp and velocity.

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

[drgondog]

Holtzauge - you would care to share your C++ model equations?

If you are approaching the solution, as Hari does, re-think adjusting Thrust (increase) to accomodate the increase in Drag. Thrust remains constant, Drag remains constant but induced drag goes up while Parasite drag goes down as the a/c maintains max power available in level flight.

Simply - no more Thrust available. Go to the equations and solve for velocity by replacing Lift with Weight.

I don't quite understand whay Hari insists THp will Increase beyond maximum available to accomodate his solution - are you saying the same?

[Harri Pihl]

Originally Posted by Holtzauge

Hi Harri,

I do not have the P51B model up and running in my C++ simulator but I just ran the P51D with the weight numbers you use and the 67" boost engine I got modelled. The numbers I got at S/L were:

9680 lb > 552.88 Km/h
10280 lb > 551.45 Km/h

This gives a staggering speed difference of 1.43 Km/h which compares rather well with the 1.48 Km/h speed difference calculation you made above

What the ####... almost 10% difference

[Harri Pihl]

Originally Posted by drgondog

I don't quite understand whay Hari insists THp will Increase beyond maximum available to accomodate his solution - are you saying the same?

The Problem here is that you don't seem to understand that at constant power the thrust decrease when the speed increase.