Performance of the Fw 190A on the Deck?

[Holtzauge]

Originally Posted by drgondog

Let's take one more pass at this.
Assume max TO power, brakes on, zero velocity. I say that is the condition of Max Thrust Available for the system at that altitude.

The system does not accelerate, no drag forces are experienced and the force retarding the thrust is the friction on the wheels with the brakes creating the torque to keep the wheels from rotating.

Release brakes, Same Max Thrust Available.

The Max Thrust available exceeds the thrust required and the system accelerates until the speed is sufficient to create enough lift to overcome weight. Same Max Thrust but the a/c is airborne and continues to accelerate.

Drag forces increase as the system accelerates until the drag forces equal the Max Thrust Available... at that point the system has reached maximum velocity for that system with that Max Thrust available. It is in equilibrium.

At no time did Max Thrust available change from V=0 through V=Vmax for that weight condition.

Add 500 pounds of fuel.

Everything is the same for this System B except that induced drag increases throughout the profile in comparison to the lighter weight system

MAX THRUST available for the System B is the same as it was in the lighter weight system, but the aircraft will achieve equilibrium at a lower speed because the Thrust required to attain an equal velocity of the lighter weight system is insufficient to overcome the increased induced drag due to the heavier weight system.

Summary

Max Thrust for System A = Max Thrust for Sytem B
Max Weight for System A < Max Weight for System B
Max Velocity for System A > Max Velocity for System B
Max Thrust for System A still = Max Thrust for System B

Think of it another way

System A pops a drogue chute at max speed and Max Thrust
Drag increases dramatically, speed drops dramatically, induced drag drops dramatically, parasite drag increases astronomically,

System A achieves equilibrium at lower speed because the Max Thrust Avaialable is lower than Thrust required to maintain the higher speed.

System A makes no changes to internal weight, makes no changes to power settings or altitude, but slows down dramatically...

So, Harri - by your use of the conversion of Jet Thrust to THP conversion equations do you believe that Thrust just increased in the example above by virtue of the reduction in velocity as described in System A above?

The answer is no.

The Max Thrust Available did not change from zero Velocity to Max velocity, nor did the weight of the system change. Only the parasite drag increased and induced drag decreased causing the System to decelerate to a lower velocity. No Thrust change, no Hp change

That equation is all about converting the thrust of a jet to an estimate of work being performed on a system by a propeller/piston engine system, and with modification, to a turbo prop.

I don't have the Hamilton Standard 'Red Book', but it probably has something similar to page 109 of the Blue Book - THRUST-HORSEPOWER CONVERSION in the GAS TURBINE section.

Ask yourself how real Force increases with a reduction in velocity. If you looked at the two states of System A, the first at rest with brakes applied and the second at Vmax - both with same HP/propeller combo/rpm, by your anology the Thrust would be infinite when the system is not moving?

Drgondog, You seem to have problems understanding the propeller thrust formula Harri is using and it's limitations. In the context that has been discussed here (max speed) it is most certainly valid. However, when the plane just starts moving at very low speeds as in your example it is NOT valid. Since you claim to have an Ms in aeronautical engineereing I'm frankly surprised you do not know this and imply the formula is wrong since the thrust would be infinite at zero speed. Look up any old NACA report dealing with propeller efficiency and you will see that the propeller efficiency goes down to zero at zero speed. So we end up multiplying something approaching zero with something approaching infinity. However, at any speed that is interesting for performance analysis, e.g climb, turn and speed performance the formula is perfectly valid.

All the talk in this thread about the meaning and definition of "thrust horsepower" is as meaningful as discussing how many angels can dance on the tip of a pin. You can use different methods to arrive at a correct answer and I would certainly like to be shown why it is not possible to do the analysis according to the method provided by Harri and why one MUST use "thrust horsepower" or else is doomed to failure.

Using the thrust formula Harri posted above in the analysis has a clear advantage over the power method: For a WW2 figther, there is a significant part of the thrust (especially at high speed) that is derived from exhaust thrust. The exhaust thrust is close to constant with speed. It would be interesting to hear how you account for this in the "thrust horsepower" model.

[drgondog]

Originally Posted by Holtzauge

Drgondog, You seem to have problems understanding the propeller thrust formula Harri is using and it's limitations. In the context that has been discussed here (max speed) it is most certainly valid. However, when the plane just starts moving at very low speeds as in your example it is NOT valid. Since you claim to have an Ms in aeronautical engineereing I'm frankly surprised you do not know this and imply the formula is wrong since the thrust would be infinite at zero speed.

The question of Infinite Thrust at zero velocity was the question I posed to help you understand the difference between Net Thrust in the THp equations and Total Thrust (max) available as measured, at rest, for the system. Net Thrust will then decrease as velocity increases from zero up to max V.

So, answer the question - when does the equation for Thp relative to Thrust and Velocity become valid in your workd Holtzauge? Relative to total positive Force in the horizontal plane for level flight?

10kts? 30kts? 100kts? - and why doesn't it apply in deriving a Force vector on the system at zero to low speeds? What law of physics makes it valid at higher speeds? (The correct answer for industry standard practice is 105 kts) but remember it is for Net Thrust.

When do you arrive at a Force value to equate to Force Available - Force Reguired to overcome Drag and accelerate?

What is your value for Force at equilibrium with brakes on, zero velocity

Using the thrust formula Harri posted above in the analysis has a clear advantage over the power method: For a WW2 figther, there is a significant part of the thrust (especially at high speed) that is derived from exhaust thrust. The exhaust thrust is close to constant with speed. It would be interesting to hear how you account for this in the "thrust horsepower" model.

It is more interesting for you and Hari to answer the repeated and specific questions I Posed -

However since you asked, for a turbo prop in equilibrium at maximum Hp in level flight - Equivalent Shaft Hp =

ESHp = Direct SHP + T/2.5 noting that 2.5 is Strictly empirical, and T is the Net Trust (real) from the exhaust gas.. and BTW this equation along with Shp = (Net Thrust x V)/325x.8 is an approximation due to the uncertainty of calculating true thrust for any propeller/engine combo.

The exhaust Thrust is a real Force to be added to Total max Thrust available at rest - minus the prop drag, the induced drag and the parasite drag

You guys are treating Net Thrust as The Total positive Force acting in the Horizontal system when in fact it is the Force (max) at rest minus the drag on the propeller system at any specific velocity... the drag on the propeller system does decrease as velocity decreases bringing the Net Thrust value higher at the lower speed until it approaches Maximum Thrust.

Having said that, the Maximum Thrust, and the Total Force acting in the positive direction are one and the same.

BTW, Crumpp and I have been back and forth over modelling the manuever performance of various Fighters at different altitudes and Bhp profiles.

One of the reasons for this debate is that solving a free body equation when parasite drag in NOT known or clearly accessible for many ships REQUIRES that one assume that Thp is converted to true thrust so that Dparasite can be solved when the velocity at Vmax is known.

But when True Thrust(Max) is offset by an unknown Propeller drag to achieve Net Thrust which is calculated by the methods we have been debating, one more drag component (unknown) is intruduced to the (often) unknow Parasite Drag

At different speeds this positive Total Force is offset by the increased drag on the prop, the induced drag and the parasite drag. As the speeds increase to max V, the Net Thrust is sufficient to overcome the Induced and Parasite Drag - but it is Not the Total Thrust Force available from that engine/prop combo..

At the end of the day, however there is no other easy way to get 'close' other than to assume that Net Trust does indeed cancel out the Prop drag and exhaust thrust and will yield a 'close enough' approximation for parasite drag

The reason that an a/c can dive faster than horizontal speed is that the Weight vector is added to the Max Force of Prop Vector until the increased drag on the prop, plus the induced drag and parasite drag are again in balance - resulting in terminal velocity. It can't go any faster for that flight profile.

[drgondog]

For your benefit Holtzauge

[Holtzauge]

Originally Posted by drgondog

The exhaust Thrust is a real Force to be added to Total max Thrust available at rest - minus the prop drag, the induced drag and the parasite drag

Of course exhaust thrust is a real force. Have either I or Harri said otherwize?

The exhaust thrust is added to the thrust formula according Harri's post which is SPEED dependant and should not just be added to the thrust available at rest as you claim.

I find Harri's way of calculating dividing the problem up in thrust and drag parts perfectly understandable and have seen many calculations done the same way in both technical litterature and technical papers before.

You now make a mix above of thrust, exhaust thrust, propeller drag, induced drag and parasite drag in the same sentence with no clear indications how they relate. How do we take these into account by your method? Please, show us mathematically how you intend to calculate this in a practical manner.

Originally Posted by drgondog

At the end of the day, however there is no other easy way to get 'close' other than to assume that Net Trust does indeed cancel out the Prop drag and exhaust thrust and will yield a 'close enough' approximation for parasite drag

Now on what grounds is it reasonable to assume that "Net Trust does indeed cancel out the Prop drag and exhaust thrust"? Sources please? What do you mean by "close"? Close to what and in which context?

[Franek Grabowski]

Originally Posted by Crumpp

That theory is very much like saying a small cancer tumor is not important.

The comparison is not valid. You can refer to cancer discussing corrosion, but not speed. Simply if you have few pounds more or less on your body it will not affect your performance. If you are 100 lbs overweight, then it will. I consider that due to low speed we do not discuss aerodynamics of human body.

Not one aerodynamic text can be found that states weight is unimportant because one of the affects is a relatively small reduction in top level speed.

What for? Everyone who calculated performances will know.

Obviously references are available. One has to wonder why there are not used.

Use your references and show us on an equation a full formula for P-51 speed. Please explain us what percentage of speed is affected by what percentage of factor.

[Harri Pihl]

Originally Posted by Crumpp

I said you wouldn't understand a dimensional analysis.

The dimensional part of the calculation is, after initial conversions, entirely at SI units. There is no THP calculated, nor anything equivalent like "thrust watts". The power is directly converted to thrust according to n and V.

If someone clearly has problems with dimensional units here, he is the one who wrote that:

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

You have thrust horsepower!"

You wrote that.

Originally Posted by Crumpp

Bill was refering to the muliple times you have confused them over the course of the thread Harri. Not a carefully worded response your Kabuki theater.
...
Obviously references are available. One has to wonder why there are not used.

Well, then we have an interesting situation here.

On one side we have drgondog, supported by you, who claim that thrust remains constant.

On other side we have Hamilton Standard (designer and builder of the propeller for the Mustang) who claims that thrust varies with speed just like I did in the calculation.

[Crumpp]

Sure Nick - sometimes the Fw 190A could out run a P-51D or B on the deck and sometimes it couldn't.

I think that perfectly sums up the performance of the FW190A on the deck.

All the best,

Crumpp

[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

[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)

[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