Allied tests of captured Bf-109's

Soren said:

So you don't see the difference in windscreen slope ?

NO, it isn't! It's about the difference between a razorback and bubble canopy design!

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Soren - Lednicer discusses the angle of the windscreen in his report. It is the same in the 51D and B. The angle is "35 degrees for the Spitfire, 22 degrees for the fw190's and 31 degrees for ther 51's" pg 87 just above Figure 4 showing the pressure distribution of the P-51D.

I have the Drawing package for the 51A-51K. The angle between the cowling and the windscreen is 31 degrees for ALL of them. Go get a good reference rather that try to use an artist representation. Lednicer used the NAA drawings as well as Ed Horkey's refernces (Chief Aero at NAA and the principle Aero under Eddgar Schmeued).

Are you seriously using an artist representation against those documented references? ROFLMAO.

And NO - once again it is about the your definition of Suction versus Lednicer's (in your confusion) versus Lift, and Suction= Drag (in your confusion) based on the pressure distribution over the P-51D bubble canopy versus the Pressure Distribution over the Malcolm Hood of the Spitfire versus the Pressure Distribution over the P-51B birdcage canopy.

You simply don't know what you are talking about when you use such terms - you just don't have a clue Soren.

At one time I wasn't quite as sure about your academic credentials in Aerodynamics because you talk a fair game about fundamentals of 'rule of thumb' practical calculations.. but you really don't understand either theoretical Fluid Mechanics or Aerodynamics - and you don't connect the challenges in Stability and Control when using those calcs to arrive at pre Flight Test conclusions.

We wouldn't be having this argument if you did.

Soren said:

Hehe, those aren't accurate KK, you can clearly see that by comparing to the real thing, the cockpits on the B C look completly wrong, but it is what it would've looked like if they had used similar front windscreen.

In reality the P-51D's front windscreen is clearly more sloped than the P-51B C's, there's no doubt about it at all.

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Soren - you are dancing and 'spinning' around the issue. Independent of whether Lednicer modelled the windscreen angle correctly or incorrectly, the fundamental question is your knowledge and interpretation of the model results.

You interpreted (incorrectly) the pressure distribution over the P-51D canopy as 'Drag'.. you componded your misunderstanding of the model and Fluid Mechanics by comparing that pressure distribution to wake drag behind a bullet.

I suspect that you now realize what everyone else realizes - that you were wrong. At least the ones that read the report and contemplated the plots and re-read to reflect on what he said... but rather than say "You know, I missed that one - you're right about what Lednicer's model says - even if I disagree with accuracy of model contours" - you spin and dance and talk about lots more stuff you don't know anything about to divert attention.

It is Simple.

You didn't know what you were talking about regarding the theory of the model or interpretation of the results, you called me out on it and I have been relentless about rubbing your nose in it because of your previous snide remarks questioning my knowledge of aerodynamics...

I am willing to let this slide and will be civil in the future - but I ask you to do the same.

Originally from drgondog (sorry I thought it was a duplicate post before I read it):

PS - I just looked at the drawings of the P-51B and D. Soren is correct about the angle of the windscreens. It is steeper for the P-51D than for the P-51B.

The Fuselage station for the canopy/windscreen interface is the same. The bottom forward location for the windscreen is the same... but

the P-51B windscreen is 31 degrees and at the top rounds off and transitions back to the canopy/windscreen interface. The top of the canopy of the P51B at that point is slightly higher than the P-51D.

The P-51D windscreen slopes more, and runs straight to the top of the forward part of the canopy.. in other words there is no transition or round off to go from 'slope to horizontal' as there is in the P-51B. It references the Lines drawings so there is no WL referece to the P-51D canopy. By inspection it (top of windscreen/canopy interface) the P-51D is slightly lowere there.

I'll have to dig more to get the specific angle of the P-51D windscreen but it is more like the Fw 190D than the P-51B. As I re-read Lednicer's report it is possible that when he went from the P-51B model to the D model he assumed the windscreens were the same - as he states in the windscreen slope comparisons.

I suspect the results would be even better for the P-51D than the existing model if a.) he made the error and b.) he pulled the windscreen back more like the Fw 190.

I would love to have access to VSAERO and the compute power to model a Malcom Hodd on the P-51B to see if the possible improvement in flow would yield similar results to P-51D bubble canopy.

Bill,

You're being unduly unfair and harsh at the moment, which I don't appreciate. So I misread the color chart, so what ?? Is that a crime ? I'm not familiar with VSAREO, never used it, so can I be blamed of being clueless ?? Hardly.

Fact still is that a razorback configuration is more drag efficient than a bubble canopy (Assuming the same front windscreen ofcourse), you can dance around this fact all you want Bill, it's really common knowledge within the aero industry. It was certainly clear to the guys who designed the P-51 as they obviously altered front windscreen angle to offset this disadvantage.

Had the front windscreen angle been the same in Lednicer's model we would've seen quite different results than we did, cause the boundary layer would've then like on the B C series started to seperate right near the base of the windscreen, and the flow over the entire canopy and rear fuselage would've then been a lot different and turbulent. If the B C series had the same front windscreen as the D series then the flow distribution would've been completely different and more drag efficient than on the D series.

Anyway I'm looking forward to your start at being civil.

Hmm... thread title says:

Allied tests of captured Bf-109's

Soren said:

Bill,

You're being unduly unfair and harsh at the moment, which I don't appreciate. So I misread the color chart, so what ?? Is that a crime ? I'm not familiar with VSAREO, never used it, so can I be blamed of being clueless ?? Hardly.

No, failing to comprehend graphic displays is not a crime. Not being familiar with VSAERO is not a crime. Making statements that the presentation of the P-51D canopy as shown meant it was Drag isn't a crime but it WAS ignorant. Being convinced you were right without the requisite background in the 'physics' of Aerodynamics led you into the assumptions you made.. that was ignorance of the worst sort because you were and are still passing yourself off as a 'practitioner' with these lates rounds of comments.

Fact still is that a razorback configuration is more drag efficient than a bubble canopy (Assuming the same front windscreen ofcourse), you can dance around this fact all you want Bill, it's really common knowledge within the aero industry. It was certainly clear to the guys who designed the P-51 as they obviously altered front windscreen angle to offset this disadvantage.

Facts are the Model shows otherwise. I actually agreed with you that the angle should have been swept more by Lednicer for his P-51D. But even here you are missing the point... and falling back to snide remarks about 'dancing'

HE USED THE SAME ANGLE on the windscreen for both and GUESS WHAT - the RAZORBACK configuration was NOT more drag efficient, The BUBBLE TOP WAS. See me dancing??

Facts are that if you were correct in your blanket assumption of razorbacks being more Drag Efficient, ALL fighters would have some form of Razorback to squeeze every bit of performance out of them.

Facts are - you didn't have any facts to make that claim - and then compounded it by (and still do) by making the same claim - unsubstantiated by any data or tests or theory and unsubstantiated by an analytical model that you admit you don't understand..?? Why do you do this?

What is 'common knowledge' in the Aero Industry? Where does one go to uncover this priceless fount of knowledge? What are your references to make such a blanket, unfounded, unsubstantiated claim?

Had the front windscreen angle been the same in Lednicer's model we would've seen quite different results than we did,

Repeat - the front windscreen angle is the SAME in the model, so where do you go from here?

cause the boundary layer would've then like on the B C series started to seperate right near the base of the windscreen, and the flow over the entire canopy and rear fuselage would've then been a lot different and turbulent.

If the B C series had the same front windscreen as the D series then the flow distribution would've been completely different and more drag efficient than on the D series.

In real life if the windscreen slope of the B was as acute as the D and Malcolm Hoods were used in comparison there is room to speculate, based on these models, that the P-51B may have come close to being as efficient as the P-51D.. but it wasn't, and the model reflected the results. Under those circumstances one might (no proof) think that the P-51B would be nearly as, or same as, efficient as the Spifire - but still fall short of the P-51D

Soren - I am truly beginning to believe you have an English language reading comprehension problem. Read the Lednicer report carefully if you wish to debate on the facts of his report and what he says versus what you think. He SAYS the angles of the "51's are 31 degrees. That means the angle of the P-51B and the angle used for the P-51D are the same - 31 degrees. He specifically says (correctly) that the fast buildup of stagnation pressure when the flow hits the windscreens (all of the), the "Boundary layer Separates, then re-attaches at the top of the windscreen.. for ALL of them.

That region of high Stagnation pressure is the pretty 'blue' thingy on the windscreen of all models- the Spitfire seems to show the highest values and coverage of the stagnation pressure region

Anyway I'm looking forward to your start at being civil.

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I think after this post that deal is off. Check back with me some other time on this.

So, let's summarize.

a. Model has same angle 31 degrees for both the P-51B and P-51D.
b. The Model assumptions for the panel size, the contours and the singularity distributions over both models were the same - Except for cutting the turtledeck and building the bubble canopy for the P-51D
c. The Plots were given Red for Pressure < Freestream, Blue for Pressure > Freestream.
d. The results were presented and compared to wind tunnel results.
e. Conclusions were made regarding the model data and differences between Wind Tunnel and Model data were discussed and explained.
f. Significant time was spent discussing the results of the windscreen and canopy results including the 'interesting' distribution over the entire P-51D canopy versus the P-51B

So, The P51B RAZORBACK was less Drag efficient than the P51D Bubble Top

The Spitfire with Malcolm Hood was more Drag efficent in canopy region than the P-51B.

Lednicer concluded that had the Spitfire design team seen these results they would have changed the slope of the windscreen to improve it more.

You continue to argue points with no fact basis ("common knowledge in areo industry that razorbacks are more drag efficient", etc - then you say that I am 'dancing'??

Everyone lets tone it down a bit allright!

I think the testers missed a really important item in their rating of the 109 which gives me pause for thought on their insights , there isn't anywhere for the pilot to take an over night bag

pbfoot said:

I think the testers missed a really important item in their rating of the 109 which gives me pause for thought on their insights , there isn't anywhere for the pilot to take an over night bag

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You had to go there didn't you?

Get ready pb!

pbfoot said:

I think the testers missed a really important item in their rating of the 109 which gives me pause for thought on their insights , there isn't anywhere for the pilot to take an over night bag

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German engineers very pragmatic - why waste space for overnight kit when you can't fly far enough to need one... hell there was no space in that airframe for a condom kit, much less an over night bag.

drgondog said:

German engineers very pragmatic - why waste space for overnight kit when you can't fly far enough to need one... hell there was no space in that airframe for a condom kit, much less an over night bag.

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But even here you are missing the point... and falling back to snide remarks about 'dancing'

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Ha! You perhaps didn't say this in the post just before mine?:
"Soren - you are dancing and 'spinning' around the issue"

Or what ???

Sure I'm the one making the snide remarks around here

He SAYS the angles of the "51's are 31 degrees. That means the angle of the P-51B and the angle used for the P-51D are the same - 31 degrees

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What ?! No it doesn't Bill, that's just something you're wishing to be the case!

What Lednicer says is:
"The Spitfire's windscreen is at a 35 degree angle to the forward deck, while the FW-190's is at a 22 degree angle and the P-51's is at a 31 degree angle"

The question that then arises is: Which P-51 ??

If you look at the 3D models you can clearly see that they arent the same, furthermore they couldn't have been the same cause as Lednicer himself says:


So the models in the flow simulation obviously didn't have the same slope windscreen cause they're based on North American's own drawings, also if they did have the same windscreen they would've looked like the profiles KK presented and Lednicer would've definitely known that wasn't right!

Or don't you trust that Lednicer was thurough enough to ensure that the a/c were accurately recreated in the flow simulation ?? If he wasn't the whole article is worth nothing and a waste of time.

Soren said:

Ha! You perhaps didn't say this in the post just before mine?:
"Soren - you are dancing and 'spinning' around the issue"

Or what ???

Sure I'm the one making the snide remarks around here



What ?! No it doesn't Bill, that's just something you're wishing to be the case!

What Lednicer says is:
"The Spitfire's windscreen is at a 35 degree angle to the forward deck, while the FW-190's is at a 22 degree angle and the P-51's is at a 31 degree angle"

The question that then arises is: Which P-51 ??

So, let me understand your thought process. He models 5 ships - a Spitfire IX, a Fw 190A and D and a P-51B and a P-51D? We ok with that?

If he says "the P-51s at 31 degrees" does he mean two P-51B at 31 degrees, two P-51D's at 31 degrees or a P-51D and P-51B at 31 degrees? You tell me.

If you look at the 3D models you can clearly see that they arent the same, furthermore they couldn't have been the same cause as Lednicer himself says:

So the models in the flow simulation obviously didn't have the same slope windscreen cause they're based on North American's own drawings, also if they did have the same windscreen they would've looked like the profiles KK presented and Lednicer would've definitely known that wasn't right!

Did the Bubble top or the razorback in his model have the better pressure distribution profile? At the end of the day I agreed with you that Lednicer probably failed to account for (or model) correctly) the more stearmlined slope for the P-51D canopy. Do you feel that he a.) got it right with both at 31 degrees or got it wrong and mistakenly got the P-51D at the same windscreen angle?

Or don't you trust that Lednicer was thurough enough to ensure that the a/c were accurately recreated in the flow simulation ?? If he wasn't the whole article is worth nothing and a waste of time.

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It was an excellent presentation independent of whether he was right or wrong about the P-51D windscreen slope.

It led you to expose the fact that you a.) know very little about Potential Flow/Singularity Distribution models, b.) do not know the difference between Suction, Lift and Drag, and c.) do not know how to interpret either his model, his results or his conclusions.

In short I found it excellent!!

Have a good day.

Yeah Lednicer definitely used the same windscreen for both

Soren - carefully. There are three discussions going on about the Lednicer Report.

Terminology (i.e. Suction, Drag, Stagnation, etc)
Accuracy (i.e. The angles for the windscreen, canopy model, describing the angles)
Interpretation (Razorback drag efficiency versus Bubble top drag efficiency in this model)

Accuracy
If Lednicer said in his report that he used 31 degrees for the 51's - as you pointed out earlier, he should have used something closer to 26 degrees for the P-51D.

But, carefully, as you have noted by reproducing what he said, he didn't say they were different angles, and then specify the difference. He said he 'used 31 Degrees for the 51's" Correct?

Then he said he built the model from the North American Drawings from Arthur Bently in the UK. They are the same drawings I have from the Microfilm Collection I got from NASM. The entire package is contained in T.O. 01-60J-13 and contains 24 rolls of microfilm for P-51A, B, C, D, K, and H, including Tooling, parts, assy, standards, and methods.

The windscreen Assy and Inst'l drawings are 83-31826 for A-C, 106-318226 for D/K and 117-318131 for the P-51H. A little later I will dig into them and give you the answer for the Windshield angle referenced to the forward Cowl line at the bottom of the windshield.

Having said that it looks like the P-51D windshield slopes more acutely to the cowl line than the 51B.

In your blow up of the plots it seems to me that Lednicer did in fact model the two windshields correctly with the 51D showing the angle closer to 26 degrees. Correct?

So, he either misspoke when he said he used "31 degrees for the 51's" because he din't realize that there was a difference, but in fact modelled it correctly with the WL/FS reference points for both ships. Correct?

Or, conversely the model did in fact use 31 degrees incorrectly and we are seeing an optical illusion on the plot. Correct? Any other possibility?

So, either his model used an incorrect and greater angle for the P-51D than he should have but the model, even under such adverse assumption, still shows attached flow and lower pressure distribution (i.e Suction) over the bubble top..

or he used the correct lesser angle for the 51D and the model shows attached flow and lower pressure distribution over the bubble top than over the Razorback canupy/windscreen combo.

Which way do you want to go? Less efficient Razorback in case 1 or less efficient Razorback in Case2?

I think we have dissected this enough. I think we have rounded up all the tangential excursions away from the prime questions.

Is the Razorback in this model a.) more efficent with respect to Drag than the P-51D based on your interpretation now that we have gone through this exercise?

Is Suction as Lednicer used the phrase to describe the pressure distribution over the Bubble canopy of the P-51, the same meaning as Drag as you claimed, and further illustrated with wake drag behind boat tail/spitzer bullets?

You have claimed and repeated YES to both of these questions. Is that still your claim?

Oh when it comes to the issue of pressure distribution and boundary layer seperation I see things quite clearly Bill, and nothing of what I explained is wrong, nothing.



And what's with the sudden paranoia Bill? You seriously think I've been away from the forum because of a discussion we had ? Bill I could care less, besides the discussion was over in my eyes.

And as to suction, well I thought we had settled this already, and yes suction equals drag. A razorback design has less drag than a bubble canopy one, the simple reason being that there's not the turbulent area right behind the canopy creating extra drag. I thought you understood this.

The sudden drop over the top of the canopy is what causes the boundary layer to seperate, causing turbulence to the rear(Hence the stability issue), and therefore extra drag. It's the same with bullets Bill, if you say cut way the boattail you'll get sooner seperation and more turbulence which means more drag, hence why spitzer bullets aren't as drag efficient as boattailed ones.

If you do remember my original question that sparked this debate (on the best pison engine fighter ever thread iirc) was in context to the P-47's bubbletop/razorback canopy...

And also a question (not by me) why the 109 wasn't adapted to a bubble canopy, on an earlier thread.


And another thing is that the ballistics example of boat-tailed vs spitzer isn't completely comparable as a spitzer has a sheer drop off compared to a slight taper of a boat-tailed bulled. But in our comparison we're comparing a tear drop shaped canopy design to one with it fared int the fuselage. (like comparing an extremely tapered bulled on the trailing edge to a very long bulled with a gentler trailing edge taper)

Nothing to do with planes, but a friend of mine made himself a little fishing boat out of aluminum. It was flat bottomed, 6 feet long, and 3 feet wide, and was designed to fit on a custom made frame to be pulled behind a 4 wheel drive ATV. He wanted it to ride low, as the trail to the lake was 40 miles long, and extremely rugged, so he cut 'wheel wells' into the sides so the boat could sit down over the tires on the trailer. Basically it looked like a pickup truck box with the front end bent up, a design and engineering marvel to say the least!
Unfortunately, he didn't test it before we headed out into the bush. Spent 2 days on the trail, 3rd day he sticks the boat into the water, puts a 3.5 hp motor behind it, fires it up, and roars away..... at about 1 1/2 mph.
The boat held 2 guys, with plenty of freeboard, but those two cutouts for the wheels created an enormous amount of turbulance. The water literally foamed along the sides, even at that painfully slow speed. It was pretty funny, motor screaming, water foaming, and I could walk along the shore faster than they were going.

Not quite the same as in the models being discussed, but a graphic illustration of how an interruption in air/water flow creates problems.

O yeah, they did catch some nice lakers, but they were always late gettin back for supper.

Bill,

It is quite obvious that Lednicer used the right dimensions for both a/c in his flow simulation, hence the flow models. And this directly affects my argument as it from the start was assuming a similar windscreen, ofcourse. The P-51D, C B however don't share a similar windscreen, something which I hadn't noticed to begin with, so I was in disbelief when I examined Lednicers results really, until I noticed the further slope of the D series windscreen which explains it. This further sloping of the windscreen decreases drag and helps the flow over the canopy stay non turbulent a longer way.

So if I had to choose between your questions it would be nr.2.

But I still stand by what I said from the beginning, razorbacks are more drag efficient than bubble canopies. (Assuming similar windscreen)

Oh and suction does create drag, I stand by that as-well. But I admit Lednicer wasn't refering to this.

Soren said:

Bill,

It is quite obvious that Lednicer used the right dimensions for both a/c in his flow simulation, hence the flow models. And this directly affects my argument as it from the start was assuming a similar windscreen, ofcourse. The P-51D, C B however don't share a similar windscreen, something which I hadn't noticed to begin with, so I was in disbelief when I examined Lednicers results really, until I noticed the further slope of the D series windscreen which explains it. This further sloping of the windscreen decreases drag and helps the flow over the canopy stay non turbulent a longer way.

So if I had to choose between your questions it would be nr.2.

But I still stand by what I said from the beginning, razorbacks are more drag efficient than bubble canopies. (Assuming similar windscreen)

You have no data, wind tunnel results or model results to support this conclusion. You do have a Lednicer model which shows a.) a better pressure distribution of the P-51D over the P-51B, and b.) a better pressure distribution over the Spit Malcolm Hood than over a P-51B.

The Spitfire shows a greater separation and IMPLIED stagnation pressure over the 35% windshield than the P-51B with the 31 degree windshield.

So you can make some conclusion that the more vertical windscreen on the Spit may have greater Stagnation pressure and turbulence on the windscreen, as well as make a statement that the turbulence and stagnation pressure for both the P-51B and Spitfire are greater than the P-51D.

Then you could make a statement that the razorbacks in BOTH cases were more 'draggy' than the bubble top...but the Spit with a 'bubble top' malcolm hood looked better than the birdcage P-51B

But you continue to state that razor backs are more efficient with respect to drag... why? Zero proof points to support your thesis, your statement directly contradicts a very sophisticated model, but you say otherwise.. why??

Oh and suction does create drag, I stand by that as-well. But I admit Lednicer wasn't refering to this.

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Suction in a blow job will create minor drag in the direction you are looking at - but not the 'suction on the wing/body (canopy)' that Lednicer presented, which was orthogonal to the freestream flow

Lednicer wasn't referring to Suction as Drag, but you equated 'lower pressure distribution' over the bubble canopy as Drag. You were simply wrong but you can't simply say that. You can't admit you didn't know what you were talking about.