Bf-109K

[Nikademus]

30,000 feet or higher. The P-47 is often described as being suprisingly nimble and agile at very high alts thx to it's high wing loading and supracharged big engine. Nothing specific though. The website I linked a few pages back described it as "suprisingly nimble" for a plane it's weight and size but didn't suggest it was "more" nimble or agile than a FW or Bf at similar altitudes.

Then again, the Bf-109K was described by the flight simm website i linked as also having high speed handling issues due to it having a sup'd up HP engine mated to an aging airframe.

 

[Crumpp]

Then again, the Bf-109K was described by the flight simm website i linked as also having high speed handling issues due to it having a sup'd up HP engine mated to an aging airframe.

That is pretty much true for any of the WWII fighter designs that stuck around the entire war. The handling issues stem from the airframe outgrowing its design stability points.

The exact same thing happened to the Spitfire and P51 series too. It takes a design change to expand these limits. Increasing the weights, increases the moments, and affects the handling. It's just simple physics.

I think this tends to be overemphasized by non-pilots. While it is present and can sometimes be "unpleasant", it does not detract from design performance and none of these changes resulted in a design that was no longer a viable, safe, and controllable aircraft. If it did the design is simply scrapped.

When I say "safe" I am speaking in a relative term as it applies to airplanes!

30,000 feet or higher

I might have some engine charts and as a FTH analysis has been done already on both aircraft we can change the power to see the results.

It's not definitive by any means for specific performance but it will most certainly give us performance trends. It is accurate within the realm of significant digits.

All the best,

Crumpp

 

[davparlr]

You are correct.

I was looking at Knots at a quick glance to an altitude table. Since the basic concepts are not correct, I did not bother to calculate anything out.

However that does not change the fact Mach .91 is well into the transonic realm and compressible theory applies. The drag forces it must over come are nothing on the scale of the P47 or Bf-109 experience in level flight.

It remains a very poor example.

This comment is unsupportable. Zero lift drag coefficient for the F-4 (which I am sure is no cleaner than the F-15) is as follows:
Mach .6 .022
Mach .8 .022
Mach .9 .022
No large increases.

Also, NASA wing drag coefficient for transonic flight for 45 degree wing sweep (F-15) shows very small increase in drag coefficient up to about .94 Mach (see graph).
Of course if that makes you uncomfortable, I could lower the example Mach to .87, which is getting into the realm of commercial jet cruise, with very low transonic effects. Then, at 55kft, we get at TAS of 575 mph and an EAS of 194. Again, from your perspective, the Bf-109k is faster than the F-15 by 100 mph.

It is definitely generating an order of magnitude more thrust than the P47 or Bf-109. This statement indicates you have no idea of the basics concepts of power producers and how they develop thrust.

The F-15 is a thrust producer and follows typical thrust producer characteristics. It has considerably more drag to overcome and needs all of that excess thrust to counter transonic drag rise.

Basic stuff, once again.

Remember EAS for the F-15 is only 209 mph, or 181 knots, probably approach speed. The F-15 is not generating a lot of thrust at approach speed, even in level flight, only enough to counter the low drag at that airspeed. I maybe slighty higher due to some transonic drag, but I have already shown that this would not be significant.

Having flown jet aircraft above 45000 ft I can tell you fuel flow is quite low.

Baloney.

I think it is philosophically correct but is confusing. I won't try to defend any more than to say that speed is a distance, divided by time the distance is traversed. If a Bf-109k is going 299 mph EAS at 24k ft, in one hour it must go 299 miles, right? 299 miles relative to what? It certainly is not over land in a no wind environment, which would be TAS, or 440 miles. it is not in the air mass, that would be TAS also, 440 miles. In the real physical world made up of air mass and earth distance, EAS has no plane of reference. Neither does IAS or CAS. All of these are just comparisons of total pressure and static pressure with many errors. These errors can be correct to get the real velocity through or over air or ground. If you fly IAS or CAS, or try to apply EAS to get any real distance measured for flying an hour, you have to convert it. Now this doesn't mean that IAS, CAS and EAS are not important. They do represent the actual aerodynamic loads on an aircraft and, as a result, is vital for how an aircraft maneuvers and handles, and therefore, either IAS or CAS is mandatory in flying the aircraft. The wings only see and react to, IAS, CAS, EAS, the aircraft as a whole, in relating to the world, either in the air or over the ground, only knows TAS.

Name one TAS speed where aircraft performance occurs?

It tells you the actual velocity that the aircraft is going through the air mass. As a result:
1. With wind speed, you can calculate your speed over the ground.
2. With fuel flow and wind speed You can calculate how far you can go with the fuel you have on board.
3. You can calculate how long it will take you to reach a destination.
4. You can compare your aircraft performance to another. For instance, If You are in a P-47M at 32.5k ft and you see a Bf-109K below you at 24k, and you know your max TAS is 467 mph at 32.k, and you know his max TAS at 24k is 440 mph, you know that even though his EAS if higher than yours, it is useless information because, with your higher TAS, you can catch him without losing altitude. On the other hand, if you were in the Bf-109K and saw the P-47M was ahead of you, you would know that, if he saw you, you could never catch him, even if you didn't climb, even with your higher EAS, because you have a lower TAS.

You can do none of the above with EAS.

What is Vs1 at sea level in TAS? What is Vs1 at 20,000 ft?

A lot faster since to maintain the same dynamic pressure at Vs1(sl), Vs1(20k) must be much higher in the low density environment. See discussion on this later.

There are very good reasons why EAS is used for comparing aircraft performance by engineers.

I have no argument now, or ever, with EAS being used for predicting aircraft performance such as stall speeds, maneuverability, climb rates, etc. My argument is that, one, you can't predict aircraft airspeed performance at a higher altitude using EAS alone without hp data, and two, a higher EAS does not mean you are going faster than another aircraft at a higher altitude, but lower EAS.

That is completely different from your original claim that EAS was not used for aircraft performance comparison. Now you are changing your tune.

This is disingenuous. I have never claimed in this post that I thought EAS was NOT used for aircraft performance comparisons other than airspeed comparisons at different altitudes. I have NOT changed my tune.

You said

"EAS is useful for performance comparisons"

I replied "This is true for some aircraft performance parameters, just not for airspeed comparison at different altitudes"

You later said,

"That is why it is industry standard.

TAS assumes density effects at altitude. Any values calculated using the TAS velocity are only appropriate for that specific altitude.

Using EAS eliminates this as a step and there is no need to recalculate all of our values for density effects.

It is then much easier to convert EAS to TAS if we want to account for density effects. The differences in EAS to TAS velocity are solely based on atmospheric properties alone. Two aircraft at the same EAS will have the same TAS if they are at the same altitude.

However if we want to compare aircraft performance, then using EAS puts the aircraft under the same conditions which is essential for a good comparison."

I said "Actually, I don't think I have ever disagreed with any of this except the last statement for comparing airspeed at different altitudes with different aircraft."

Somehow, you have twisted my comments to imply I think EAS is worthless when I have not.

 

[davparlr]

You gloss over this:

Believe me, this misapplication of mathematical logic is difficult to ignore. You ignored my answer so I will restate it. Here is your work.

Well if we compare our DYNAMIC PRESSURE between these two aircraft using TAS:

P47 at FL325 467mph * 1.47 = 686fps

q=.5rV^2 = .5*.000840785 slugs * 686fps^2 = 197.8 psf

Bf-109 at FL24 440mph * 1.47 = 6478fps

q=.5rV^2 = .5*.00110327 slugs * 647fps^2 = 230.9 psf

Note here that there is only two speed values (ft/sec) expressed. No others, and these are 686 fps for the P-47 and 647 fps for the Bf-109. There is density (slugs/ft^3), there is pressure (lbs/ft^2). There is no other reference to speed noted. The speed of the P-47 is 686 fps and the speed of the Bf-109 is 647 fps. No more, no less.

"Dynamic pressure is a function of speed:"

This is not totally true, is it?

Dynamic pressure is a function of speed (ft/sec) AND density (slugs/ft^3).

In other words, mathematically speaking, dynamic pressure can go up by increasing density and not changing speed. Increased dynamic pressure can reflect an increase if only density is increased and with NO increase in speed! This is basically what is reflected in these two equations.

In fact you regurgitate the very reasons we use EAS to compare performance. Using EAS eliminates the density effects of our altitude!

The equivalent airspeed is a direct measure of the incompressible free stream dynamic pressure. It is defined as the true airspeed multiplied by the square root of the density ratio (air density at some flight altitude over density at sea level). Physically the equivalent airspeed is the speed which the aircraft must fly at some altitude other than sea level to produce a dynamic pressure equal to a dynamic pressure at sea level.

Okay, let's use your formula
q=.5r(sl)V(sl)^2

Lets use an aircraft going 300 mph EAS at SL and we want to fly at 300 mph at 10k. So, since EAS(SL) = EAS(10k), and since equal EAS means equal dynamic pressure, we have q=q or .5*r(sl)*V(sl)^2 =.5*r(10k)*V(10k)^2

Plugging in the known values, we get,

.5*.0023769 (SLugs/ft^3)*441 (ft/sec)^2 =.5*.001756 (slugs/ft^3)*V(10k)^2

Then {(.0023769*441^2)/.001756}^.5 = V(10k)

Or V(10k) = 513 ft/sec or 349 mph

So, in order to have a dynamic pressure at 10k that you have at SL, V(10k) must be 513 fps. This is greater than V(sl) at 441 ft/sec. In other words, to keep your dynamic pressure constant as you get into a thinner atmosphere (less dense), you must increase V(10). This is mathematically correct and is horse sense. As resistance gets less due to thinner air, speed must increase to maintain the same dynamic pressure.

Of course we can ignore the science and just go ahead with a silly comparison of airplanes under very different conditions using TAS.

Or, we could ignore mathematics and use EAS for airspeed comparison.

And how about this example for horse sense. The Curtis P-6A, a biplane, has a top speed of 198 mph TAS at sea level, which equals to an EAS of 198mph. The Ta-154H, has a top speed of 426 mph TAS at 41k ft, which equates to an EAS of 172 mph. Now, according to you, the P-6A, a biplane, is flying faster than one of the fastest aircraft of WWII by over 25 mph! I am sure even Soren doesn't know this. The answer is that the V(42k) necessary have the same dynamic pressure as the P-6A at SL can not be obtained by the Ta-154 at 42k ft., but it is still really going fast anyway, a lot faster than the P-6A and will leave it far behind in no time flat.

Read questions 1 and 2. Notice what they want the answer's in when comparing aircraft performance!

I never argued these points. Check my posts. EAS is what you want to work issues of climb, turn rate etc. What I have argued and only the items I have argued is as follows:

1. While EAS is very helpful in comparing aerodynamic performance, like stall, turn rates, etc., at various altitudes, it cannot predict an aircraft's airspeed performance at a different altitude without knowledge of horsepower/thrust available.

2. For aircraft at different altitudes, a larger EAS does not necessarily mean a higher airspeed. Something I think is intuitively obvious and something I have proved mathematically, and demonstrated by multiple examples!

That's because we cannot navigate without TAS speeds.

Huh? And what does this have to do with the price of tea in China? It is because the air mass is passing by at TAS.

 

[magnocain]

wow davparlr, you did your homework.

 

[Soren]

Ok, let me straighten things out here;

First of all the reason Crumpp wants to use EAS for comparison reason is like he says "using EAS puts the aircraft under the same conditions", which is very true when you want to compare the maneuverability of a/c. AFAIK Crumpp never claimed that because two aircraft feel the same EAS then they're going just as fast, he never claimed that.

Like Professor Dave Esser puts it:
"It is the EAS that the aircraft feels. EAS is a measure of the dynamic pressure exerted on the aircraft. This dynamic pressure plays a key role in the lift and drag created by the aircraft. For a given EAS the aircraft feels the same dynamic pressure, and therefore lift and drag, regardless of altitude. The higher the density altitude, the thinner the air, and the faster an aircraft must travel through the air mass to obtain the same EAS."

So lets say two identical are aircraft flying at the same EAS but at very different altitudes, one at SL and the other at 40kft. The a/c at SL needs much less TAS to reach the same EAS as the a/c at 40kft, this is because the air is thinner the higher you go, and therefore you need more speed to achieve the same dynamic pressure around the a/c as compared to at lower altitudes. So if you keep the EAS constant then the higher you go the faster you're going to go as-well. The reason EAS is used for comparing a/c is that at similar EAS similar a/c will have similar turn roll rates regardless of altitude, cause like has been said EAS is what the a/c feels, its the dynamic pressure exerted on the a/c.

Neither Crumpp nor Davparlr seem to be unaware of the above, they both do however seem to have completely misunderstood each other.

So my Conclusion is that Davparlr Crumpp are talking past each other.


For the newcomers:

EAS = Estimated Air Speed
TAS = True Air Speed
IAS = Indicated Air Speed
CAS = Calibrated Air speed


PS: Davparlr, its the Ta-152H, not the Ta-154, the Ta-154 never got to see service.

 

[HoHun]

Hi Davparlr,

>Also, NASA wing drag coefficient for transonic flight for 45 degree wing sweep (F-15) shows very small increase in drag coefficient up to about .94 Mach (see graph).

Is this graph from a NACA/NASA report? In that case, I'd very much appreciate a pointer to the report because it is interesting for me in a completely different context.

Thanks in advance!

Henning (HoHun)

 

[Nikademus]

yea! bump to the front.

 

[Soren]

Nik, do you understand better now or are you still confused ?

 

[davparlr]

Ok, let me straighten things out here;

AFAIK Crumpp never claimed that because two aircraft feel the same EAS then they're going just as fast, he never claimed that.

He may have not made that statement but he did say the equivalent.

Crumpp stated

The effect of altitude is to increase velocity by the SMOE. An aircraft traveling a velocity of 200KEAS at sea level is traveling 200KTAS.

An aircraft traveling 200KEAS at 35,000 ft is traveling 326KTAS.

Both aircraft however are traveling at exactly the same velocity of 200KEAS and neither aircraft has any speed advantage. One just benefits from the effects of altitude.

A quick SWAG of altitude effects using the data provided in this thread shows the Bf-109K4 to be traveling at an equivalent airspeed of 299mph while the P-47N is traveling at 272mph.

440mph / 1.4678<SMOE FL24> = 299 mph EAS
467mph / 1.71295<SMOE FL325 = 272mph EAS

The Bf-109K4 is the faster of the two aircraft according to this data.

So, I understand that you agree with both of these statements? Am I right?

How about this statement
Crumpp said

Well if we compare our DYNAMIC PRESSURE between these two aircraft using TAS:

P47 at FL325 467mph * 1.47 = 686fps

q=.5rV^2 = .5*.000840785 slugs * 686fps^2 = 197.8 psf

Bf-109 at FL24 440mph * 1.47 = 6478fps

q=.5rV^2 = .5*.00110327 slugs * 647fps^2 = 230.9 psf

Wow! Check that out. The slower aircraft has higher dynamic pressure! That can't be if our planes are under the same conditions.

Dynamic pressure is a function of speed:

q=.5rV^2

If our planes are under the same conditions, then the faster one MUST have the higher dynamic pressure. It's our frame of reference and I am sure you know how important that is in physics.

Do you agree here also? Or do you think maybe dynamic pressure is also a function of air density?

Like Professor Dave Esser puts it:
"It is the EAS that the aircraft feels. EAS is a measure of the dynamic pressure exerted on the aircraft. This dynamic pressure plays a key role in the lift and drag created by the aircraft. For a given EAS the aircraft feels the same dynamic pressure, and therefore lift and drag, regardless of altitude. The higher the density altitude, the thinner the air, and the faster an aircraft must travel through the air mass to obtain the same EAS."

So lets say two identical are aircraft flying at the same EAS but at very different altitudes, one at SL and the other at 40kft. The a/c at SL needs much less TAS to reach the same EAS as the a/c at 40kft, this is because the air is thinner the higher you go, and therefore you need more speed to achieve the same dynamic pressure around the a/c as compared to at lower altitudes. So if you keep the EAS constant then the higher you go the faster you're going to go as-well. The reason EAS is used for comparing a/c is that at similar EAS similar a/c will have similar turn roll rates regardless of altitude, cause like has been said EAS is what the a/c feels, its the dynamic pressure exerted on the a/c.

This exactly what I have been stating, maybe not a fluent, but the same and I have not varied from it.

"Now this doesn't mean that IAS, CAS and EAS are not important. They do represent the actual aerodynamic loads on an aircraft and, as a result, is vital for how an aircraft maneuvers and handles, and therefore, either IAS or CAS is mandatory in flying the aircraft. The wings only see and react to, IAS, CAS, EAS, the aircraft as a whole, in relating to the world, either in the air or over the ground, only knows TAS."

and, "So, in order to have a dynamic pressure at 10k that you have at SL, V(10k) must be 513 fps. This is greater than V(sl) at 441 ft/sec. In other words, to keep your dynamic pressure constant as you get into a thinner atmosphere (less dense), you must increase V(10). This is mathematically correct and is horse sense. As resistance gets less due to thinner air, speed must increase to maintain the same dynamic pressure."

Neither Crumpp nor Davparlr seem to be unaware of the above, they both do however seem to have completely misunderstood each other.


So my Conclusion is that Davparlr Crumpp are talking past each other.

So, tell me, what did he say that passed me by? Remember I was never arguing the benefits of EAS on comparisons of maneuvering performance, only his comments in the above quotes relative to airspeed comparison.

PS: Davparlr, its the Ta-152H, not the Ta-154, the Ta-154 never got to see service.

Sorry, late night typo. I know better.

 

[Soren]

Davparlr,

With all due respect you're nitpicking right now.

Do you agree here also? Or do you think maybe dynamic pressure is also a function of air density?

Notice the 'r' in the equation Crumpp presented:

q=.5rV^2

Speed can be different at the same altitude, density can not, density is stuck to alt - hence Crumpp's comment. This seems to have went passed your nose.

And now I have a question Davparlr;

AFAIK you're an engineer, but I've never seen you claim that you were an aeronautical engineer, so are you ? Just to make it clear.

Finally don't take the above as a blow to your nose davparlr, I'd like to point out that I can't see where I disagree with both of you on the subject at hand, so I'm not taking any sides here, my perception of both of you is that you're both unbiased honest individuals.

Lets keep the debate friendly people.

 

[Crumpp]

This comment is unsupportable. Zero lift drag coefficient for the F-4 (which I am sure is no cleaner than the F-15) is as follows:
Mach .6 .022
Mach .8 .022
Mach .9 .022
No large increases.

Once again you have no formal education in Aerodynamics and that is obvious.



The exact onset of Drag rise due to compressibility is not a set standard in Aerodynamics. However it universally agreed upon that by Mach .8 you will be encountering the effects. Commonly it is applied at Mach .3 and above.

Here is the one method of estimating that drag rise due to compressibility:

This is not totally true, is it?

Dynamic pressure is a function of speed (ft/sec) AND density (slugs/ft^3).

One more time, removing the density effects is the entire reason why we use EAS. Think about that statement for a moment.

It is very hard to compare an airplane to another airplane with removing it.

Quote:
A quick SWAG of altitude effects using the data provided in this thread shows the Bf-109K4 to be traveling at an equivalent airspeed of 299mph while the P-47N is traveling at 272mph.

440mph / 1.4678<SMOE FL24> = 299 mph EAS
467mph / 1.71295<SMOE FL325 = 272mph EAS

The Bf-109K4 is the faster of the two aircraft according to this data.

EAS

So, I understand that you agree with both of these statements? Am I right?

How about this statement
Crumpp said

Well if we compare our DYNAMIC PRESSURE between these two aircraft using TAS:

P47 at FL325 467mph * 1.47 = 686fps

q=.5rV^2 = .5*.000840785 slugs * 686fps^2 = 197.8 psf

Bf-109 at FL24 440mph * 1.47 = 6478fps

q=.5rV^2 = .5*.00110327 slugs * 647fps^2 = 230.9 psf

Wow! Check that out. The slower aircraft has higher dynamic pressure! That can't be if our planes are under the same conditions.

Dynamic pressure is a function of speed:

q=.5rV^2

If our planes are under the same conditions, then the faster one MUST have the higher dynamic pressure. It's our frame of reference and I am sure you know how important that is in physics.

TAS

TAS is EAS with density effects.

It's just basic aerodynamics. I did not point this out as you claimed to be an engineer working in the aerospace industry. There would be no need if you did have the background you claim. Anyone trained in aerodynamics would recognize the density differences and understand we must be using TAS.

It would also be quite obvious to you as EAS is corrected for density effects by definition. That is why it is used for comparison of all aircraft performance!

Therefore in our dynamic pressure equation, only Velocity can change as our density is fixed when using EAS.

Once again, this is a given by the very definition of EAS. Where did you go to school and take Aerodynamics again?

And how about this example for horse sense. The Curtis P-6A, a biplane, has a top speed of 198 mph TAS at sea level, which equals to an EAS of 198mph. The Ta-154H, has a top speed of 426 mph TAS at 41k ft, which equates to an EAS of 172 mph. Now, according to you, the P-6A, a biplane, is flying faster than one of the fastest aircraft of WWII by over 25 mph! I am sure even Soren doesn't know this. The answer is that the V(42k) necessary have the same dynamic pressure as the P-6A at SL can not be obtained by the Ta-154 at 42k ft., but it is still really going fast anyway, a lot faster than the P-6A and will leave it far behind in no time flat.

The Ta-152 can travel at some 405KTAS at FTH at 29,527ft. This means it is doing 251 KEAS and is a much faster airplane than the P6A.

Now if we do go to FL41 then our Ta-152H is only producing ~700PS at 12.5KM. Using your numbers, 172mph EAS is not bad considering we are using only 1/3 the Ta 152H's power production capability at an altitude the P6A is incapable of producing any power at all.

There is no misunderstanding nor are we talking past each other. You do not understand aerodynamics. Your education is incomplete and has failed to link key concepts together for you.

All the best,

Crumpp

 

[evangilder]

Enough of the personal attacks, Crumpp. Any more of them and I will lock this thread.

 

[DerAdlerIstGelandet]

You beat me to the punch Eric.

Seriously guys everyone can learn from one another in a peaceful manner.

 

[Crumpp]

 

[Crumpp]

Enough of the personal attacks, Crumpp. Any more of them and I will lock this thread.

It's not a personal attack, Evan. It is just facts. Saying someone is ignorant is not the same as claiming they are stupid. Ignorance is just a lack of correct information. We are all ignorant depending on the circumstances!

If someone comes along and claims that the moon is made of green cheese are you attacking them when you say, "No, you are ignorant of the facts. While some fairy tales my claim the Moon is made of green cheese, we know from science it is not".

It also becomes a fact and not a personal attack when we say such claims do not come from a formal education in astronomy.

All the best,

Crumpp

 

[evangilder]

Once again you have no formal education in Aerodynamics and that is obvious.

I did not point this out as you claimed to be an engineer working in the aerospace industry. There would be no need if you did have the background you claim.

Where did you go to school and take Aerodynamics again?

You do not understand aerodynamics. Your education is incomplete and has failed to link key concepts together for you.

Since you clearly cannot talk rationally, and think that those statements are NOT a personal attack, I am locking this thread. You clearly cannot be civil.