Performance of the Fw 190A on the Deck?

[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]

drgondog, In these types of discussion I find it irrelevant who has and who does not have a degree. There are a lot of knowledgeable persons out there who do not have degrees but who can hold their own in these types of discussions anyway. I only pursued the matter of education since you bought it up in a previous post and leaned on it for credibility.

For your benefit: I happen to have an Msc in aeronautical engineering myself specialising in aerodynamics and structures. It's from the Royal Institute of Technology in Stockholm, Sweden. I worked in the Swedish aerospace industry for about 14 years for companies like SAAB and Ericsson Radar Electronics on systems for the JA37 Viggen and JAS39 Gripen fighters before leaving the industry to pursue other goals.

So if we now can leave the discussion of degrees behind us, I still think you need to reconsider your position on the usability of Harri's thrust formula and the importance of exhaust thrust.

[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?

[Harri Pihl]

Originally Posted by drgondog

So, at 1kt/hr Thrust is near maximum?, at 2Kts the thrust is coming down and at 300kts it is at minimum?

In my presentation I used constant efficiency but in real life the efficiency decrease to near zero when the speed is reaching zero and in the other end of scale mach effects start to decrease efficiency as well.

In practice highest thrust is reached at speeds where the plane becomes airborne. Above that thrust start to decrease depending on efficiency and speed (and compressibility).

Originally Posted by drgondog

Power is basically constant, Thrust (Force along positive horizontal axis exerted on the stsyem by the engine/propeller combination) is basically constant for this system from V=0 kts and V= 300kts and every velocity in between.

Only the first part about power is true, the rest does not hold water at all.


We know that power is the work divided with time:

W = S/t

Where W is power and S is work and t is time.

And further Work is Force multiplied with distance.

S = T * d

So power can be written:

W = (T*d)/t

And therefore the force can be calculated

T = (W*t)/d

And further

T = W/(d/t)

And d/t, distance per time is same as speed V so

T = W/V

And again Power can be also calculated as

W = T*V

In other words, if we assume constant power, the force must decrease when the speed increase.

[Crumpp]

Franek,

I think the cancer analogy is perfectly valid. One cannot judge the significance of a cancer tumor based on the size ratio to the body. It brings one back to the days of leeching and humors when “doctors” used simple cause and effect.

One cannot deem weight as insignificant simply because we only perceive a small change in level flight velocity. You will find plenty of aerodynamic textbooks which will tell of the importance of weight.

You won’t find any that tell you it is insignificant because we only loose a tiny bit of speed. That is a fact.

Everyone who calculated performances will know.

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

You can't see the forest for the trees.

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.

That is why one must do a parametric study to eliminate all the coupled affects if you want to see just the affect of weight.

All things must be held constant while we change the weight and note the affects.

Of course, what do I know?



All the best,

Crumpp

[Nick Beale]

Do any of you aerodynamicists, aeronautical engineers and allied trades anticipate returning to the ostensible topic of this thread at any point?

[drgondog]

Originally Posted by Nick Beale

Do any of you aerodynamicists, aeronautical engineers and allied trades anticipate returning to the ostensible topic of this thread at any point?

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

Part of the variables in the discussion include not only the engine performance at that time for either ship, whether or not friction drag was reduced by polishing, what the respective weight conditions were, etc.

But I believe if you follow the logic presented by Holtzauge and Harri, you can believe an Fw 190A will not only out run a 51 on the deck but also do it with 5-10,000 pounds of extra weight as the reduction in speed is trifling.

[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

[drgondog]

Originally Posted by Holtzauge

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

No, you asked me to explain the calculation and I did. Next question?

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.

Exhaust thrust is Not dependent on speed. Net Thrust (True Thrust - propeller drag) is dependent on speed

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.

And yet you still don't understand why a ship may have Max Thrust at zero speed on the runway with chocks in place, not can you explain the physics of acceleration from zero to Vmax as it relates to Net Thrust.

So what does that say about your understanding of a free body force diagram?

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.

I will after you answer each question I posed to ensure you are acquainted with F=ma. In particular I would like to see you demonstrate your knowledge for level flight and a vertical dive (to keep you from wandering into Trigonometry).


Obviously my analogy missed the mark in helping you understand all the forces acting on the airframe system

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?

When comparing the System A to System B - different only by weight but same in all other respects, the exhaust Thrust is the same between the two ships and need not be factored in calculating the differences in Velocity at different Gross weights.

"Close" means that the difference in Prop Drag in System A from the Prop Drag in System B is to the inverse ( 1/2*MV1>>2 - 1/2*MV2>>2) which represents the change in mass flow rate of the stream tube. V1 = higher speed, V2 = lower speed due to the extra weight... and I have to do some checking on this relationship - this is strictly from memory and I could be wrong.

Having said that - please offer your own explanation for the difference?

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?