Performance of the Fw 190A on the Deck?

[Harri Pihl]

Originally Posted by 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.

Hello Juha,
I took values from the "Lentäjän Näkökulma IV" and got 423,66km/h at 5000m for 8600kg. Assuming e=0,8, n=80% and 120kp exhaust thrust (Do 17Z seem to have ejector outlets). So while not exactly accurate, the ballpark appears to be correct.

Edit: And for the Bf 109G-2 I got 3,22km/h speed reduction for 3000=>3500kg at 1,30ata 2600rpm. e=0,8 n=80% exhaust thrust 70kp (based on MT-215 data)

[Holtzauge]

Originally Posted by drgondog

Holtzauge - you have a somewhat poisonous debating style.

You have already called into question my academic credentials, you ague formulae without demonstrating context knowledge of when and why these formulae work (and don't work).. you keep challenging my questions with sarcasm - but don't answer the questions.

One more time - why doesn't the THp equation yield Total Thrust in the context of Velocity, and why doesn't it work at zero velocity.

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?

After take off, what is the Total thrust vesus the Net Thrust over the 50 foot obstacle?

If you can answer these questions I will know you know the difference between Net Thrust and MaxThrust available - (for each case use the same power setting to simplify).

Out of curiosity hat is your academic and industry background to denigrate mine, or Crumpp's?

I'm sorry if you feel offended by my replies. It has not been my intention to denigrate anyone but if you take a step back and look at your own post above you will see that it's not entirely lacking material of a personal nature with little bearing to the context of the discussion.

I have no interest in derailing this thread with discussions about 50' obstacle clearance, Cdp or some other issue which has nothing to do with the issue at hand. I entered this debate to discuss the impact of weight on top speed at S/L and I have now contributed my input on that subject. It's your privilage discount my view on the matter it if you want to and in that case we can agree that we disagree and leave it at that.

[Crumpp]

Graham's original point:

Go back and re-read. My posting speaks to the fact we cannot characterize the affects of weight as insignificant despite the fact we only see a small reduction in velocity at top level speed.

You have invented your own issue.

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.

Certainly. The reason it is higher is because we do not have the coupled affects to deal with and cloud the issue.

We see the magnitude of the affect of weight in our parametric study.

Why do you think the forumla is there under the portion explaining the affects of weight in the aerodynamic textbooks?



All the best,

Crumpp

[Crumpp]

I have no interest in derailing this thread with discussions about 50' obstacle clearance, Cdp or some other issue which has nothing to do with the issue at hand.

Well start another one....

[Holtzauge]

Originally Posted by 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

That is my understanding to. But that is for a weight increase of roughly 16% (from 3000 to 3500 Kg) If we use this info to make a ballpark estimate of a 6% increase as was the case for the Mustang (from 9680 to 10280 lb) we discussed earlier, we get around 1 Km/h for the Me109 from Kurfurst diagram.

That tabs pretty well with the 1.4 Km/h that Harri and I calculated for the Mustang previously and shows that weight within reasonable bounds is not an issue at low level high speed flight.

Q.E.D

[Harri Pihl]

Originally Posted by Crumpp

Go back and re-read.

Graham's original point was clearly not a reply to your statement. period.

Originally Posted by Crumpp

We see the magnitude of the affect of weight in our parametric study.

Nope, it gives too high magnitude for this specific problem.

Originally Posted by Crumpp

Why do you think the forumla is there under the portion explaining the affects of weight in the aerodynamic textbooks?

The formula is mainly for calculating cruise speeds at L/D max, just like Graham pointed out earlier. It assumes constant Cl so it is not a good method for our problem here.

[Harri Pihl]

Originally Posted by Holtzauge

That tabs pretty well with the 1.4 Km/h that Harri and I calculated for the Mustang previously and shows that weight within reasonable bounds is not an issue at low level high speed flight.

I got pretty much exactly same values as the chart at sea level varying the weight. At FTH I got slightly lower reduction than chart but difference is not big, at 3500kg a bit over 1km/h. Given that at clean configuration the weight was never that high, the estimation appear to work pretty well

[drgondog]

Originally Posted by Harri Pihl

Yes, and that is what I have been saying all the time.

What I do is that I convert the power (W) to the force (T) according to propeller efficiency (n) and speed (V):

T=(n*W)/V

And your failure is that you don't understand that this means that the force (thrust) varies with speed at constant power. Lets quote the Hamilton Standard Red book:



As I have explained my knowledge, and noted that the Hamilton Standard Blue Book has the same equation on page 109, as well as explained what it really calculates, perhaps you can answer my questions now?

Harri - you have been very evasive about zero Velocity Thrust as well as explaining why the 'practical' approach to calculating high Hp/low velocity thrust equation doesn't work... or more to the point what is missing from the Trust discussion.

Could you please answer the questions?

Why the Thp equation breaks down at low speeds

What the Real Thrust of the Engine/Propeller system is at zero Velocity and max power is comprised of.

Is there a propeller drag at zero velocity?

Does propeller drag increase as freestream flow increases

Does exhaust Thrust increase or decrease or remain the same, with increases to velocity of the freestream?

If exhaust thrust remains the same at all speeds, where did you incorporate that in your Force balance equations for Cdi and Cdo = Thrust

Is Thrust Net Thrust (Total Thrust - Propeller Drag) or Total Thrust in the Thp equation.

What is your approach to calculate Thrust, or Net Thrust, or??, if different, and how does your 'new' approach work at both high speed, low speed and zero velocity?

[Crumpp]

Nope, it gives too high magnitude for this specific problem.

So your claim is the affect of weight is different at top level speed than the affect of weight at L/Dmax?

Have I got that right?

Can you explain, in words, what happens at top level speed with a change in weight one step at time for us?

Can you explain in words as well what happens to an aircraft trimmed for level flight at best range cruise that suddenly increases in weight? Sort of a reverse of the albatross effect.

All the best,

Crumpp

[Juha]

Hello Harri
Quote: “I took values from the "Lentäjän Näkökulma IV" and got 423,66km/h at 5000m for 8600kg. Assuming e=0,8, n=80% and 120kp exhaust thrust (Do 17Z seem to have ejector outlets). So while not exactly accurate, the ballpark appears to be correct.”
You are too modest, your result is well inside normal variation between individual a/c.

I thought that result of 500 lb weight increase should be fairy insignificant and even if I remembered Do 17Z tests I tried to look some fighter results and got the idea to look Spit Vc, which was some 400 lb heavier than Vb when armed with 4 Hispano cannon but found out that the max speed of the Vc tested was between the speeds of the first 2 Vbs I checked. So I thought that I must look something more substantial weight increase and checked Spit IX, 900 lb heavier and 30 mph faster at was that 2000 ft. Now I knew that that was more complicated because of different nose contours and because of bigger (and dragger) radiator in IX, I found out power curve for Merlin 61 and was looking one for Merlin 45 when I checked this thread and found out Kurfürst’s message and saw that there was no need for more effort.

Quote: “Do 17Z seem to have ejector outlets”
Most seemed to have but not all, at least DN-54 and one other, was that DN-64, anyway one Z-5, ex Z-3 a/c, seemed to have older system, ie 2 exhaust pipes which came through upper part of NACA ring.

Juha