Allied tests of captured Bf-109's

[Soren]

Long time no see! Been very busy at work, but now I've got a two week holiday, so nice

Bill,

The DO-335 utilized the exact same radiator design as the P-51, so the gain in thrust would've been similar for both.

[Soren]

Whether Lednicer was correct or not in his interpretation of the difference between the Me 109F and all previous models, he states the different design for better boundary layer control as reported by Brown and Smelt "Aerodynamic Features of German Aircraft", Journal of Royal Aeronautical Society, August 1944. He states from this source that Messerschmidt redesigned the intake specifically to address this drag component, and the new design was incorporated in F and subsequent models.

Your source to the contrary would be?

Contrary ?? Bill you've only confirmed what I said.

The Emil's radiator design is completely different from the F series and beyond, the F series and beyonds radiators benefitting from the meredith effect and actually providing a little thrust.

[drgondog]

Soren sez -Huh ?

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.

In the Lednicer Model there are three basic values of Pressure assigned to colors.

Red is 'suction', is lower than Freestream pressure, to laymen it is called Lift as it is a force PERPENDICULAR to freestream. In a freebody diagram it is opposite direction to Gravity

Blue - at the base of all the canopies and the intersection of horizontal stabilizer and horizontal stabilizer, is Stagnation Pressure, resulting in a force PARALLEL to Freestream and opposing Thrust.

Lednicer spends some time talking about this (Blue) as a flaw in Spitfire Canopy/windscreen design as well as the benefits of 'suction' (Red) to the 51D canopy

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.

You are 100% incorrect in both statements. First, there is essentially zero difference in the calculated Pressure distribution between the two (P-51B and D models) aft of the canopy/cockpit area. This is a FLOW model Soren not a STABILITY AND CONTROL MODEL. There is a difference, and one you frequently do not seem to grasp in this discussion

If you read and comprehend the entire reports - you will note that the entire region of Red includes the top surface of the Wings and Canopy top for the P-51D.

The Red includes the Wings of the P-51B (exactly same distribution as P-51D with same wind) and a small portion on the top of the forward Canopy.

The Red includes the Wings (smaller region due to non-laminar airfoil) and no Red on the Top of the Spitfire Canopy and a LOT more BLUE on the windscreen due to 'steeper' angles which Lednicer discusses in detail.

Correspondingly, the P-51B does NOT have a 'low pressure distribution' over the top of the canopy. Why? you should ask? Because the flow over the P-51B (non Malcolm Hood) windscreen separates (as does the Spitfire but further back) and the pressure distribution from that point aft approaches freestream pressures.

Lednicer goes on to illustrate the separation caused by the Stagnation Pressure Buildup on the Spitfire and the subsequent 're-attaching' the Boundary layer on the top of the Canopy to give it better suction than the P-51B.

NET of Lednicers models. P-51D most aerodynamic canopy, P-51B had less lift over top surface but also less Pressure Drag than the Spitfire windscreen.

Fundamentals for you Soren.

Pressure Drag is in fact a function of separated flow and the resultant force is a.) opposite of thrust, and b.) absent Thrust, in the same direction of Free Air stream.

Induced Drag is a function of lift, wing geometry and is a resultant force in opposite direction to Thrust and 'nearly' same direction as freestream flow.

Friction Drag is a function of surface roughness and is a force in parallel to freestream.

All of these DRAG forces will be aligned in a free body diagram to oppose Thrust axis of the body analyzed.

Lift forces in Lednicer's model are ORTHOGONAL to both induced drag and Pressure Drag (and friction drag).. Suction is the Red 'thingy' area represented on Lednicers Wings and Canopy of the 51D.

Repeat - He didn't have a model that had a force (pressure) distribution in orthogonal directions for same values. He didn't illustrate the Red region over the top of the model wings as "PRESSURE DRAG".. any more than the Red region over the canopies

Repeat - RED is LOWER (attached) PRESSURE DISTRIBUTION region than FREESTREAM PRESSURE. Those that are knowledgeable about Aerodynamics will usually call those forces "LIFT"

BLUE is HIGHER PRESSURE DISTRIBUTION region than FREESTREAM PRESSURE. Those that are knowledgeable about Aerodynamics will usually describe those forces as PRESSURE DRAG..

The Model is a Potential Flow singularity distribution model with subroutines and Iterative calculation solution methods to intruduce boundary layer separation. By definition Potential Flow is a perfect inviscous flow so the other modelling capabilities have to be introduced to approach realistic/wind tunnel results.

The areas where there are little diiferences in freestream pressure and the boundary layer pressures are represented by the collage of color different from RED and BLUE.



Hope this clears up your confusion a little.

[Soren]

You are 100% incorrect in both statements.

No I'm not Bill. The sudden drop WILL and DOES create turbulence to the rear which means extra drag, and it's EXACTLY the same with bullets, hence the transition from flat based spitzers to boattailed ones. So I'm 100% correct.

The problem is you're relying on data from computer generated models of the -51 in this discussion, and obviously something isn't entirely right about these seeing the -51D is shown to have a higher wetted area than the -51B which wetted area should be larger because that huge hunk of extra fuselage.

It is interesting to note the slightly lower pressure on the -51D's aft fuselage though, again confirming the reason to the stability issues.

What I did notice just now though, from looking closely at the two a/c's profiles, and this DOES alter the argument, is the -51D's higher windscreen slope compared to the -51B's which is much more straight, something Lednicer oddly doesn't mentioned. This increased slope will help maintain the boundary layer while the -51B's windscreen will cause seperation right away, which kinda offsets the advantage of the razorback design. This difference in windscreen design will have a very direct effect on how the pressure distribution is over the rest of the canopy, and this explains Lednicers illustration. In short had the front windscreen been the same (Which I thought it was) Lednicer's results would've been very different.

So like I said, a razorback design is less draggy than a bubble canopy one IF (and this is normally the case) the front windscreen remains atleast similar.

[drgondog]

Originally Posted by Soren

No I'm not Bill. The sudden drop WILL and DOES create turbulence to the rear which means extra drag, and it's EXACTLY the same with bullets, hence the transition from flat based spitzers to boattailed ones. So I'm 100% correct.

His model shows exactly the opposite conclusion from yours, namely the flow remains attached and is less pressure for the P-51D all the way to the aft fuselage deck - than the freestream around it. The 51B separates almost immediately off the top of the canopy

The problem is you're relying on data from computer generated models of the -51 in this discussion, and obviously something isn't entirely right about these seeing the -51D is shown to have a higher wetted area than the -51B which wetted area should be larger because that huge hunk of extra fuselage.

Er, this ISN'T my PROBLEM. This DISCUSSION is ABOUT the results of the computer generated Model and the data extracted from it and the conclusions drawn about the results.

Independent of what it should be for Wetted area, if the wetted area in the model is more for the P-51D than the P-51B despite losing the turtledeck, then the comparisons between the P-51D wetted drag in real life will be to the advantage of the P-51D. Less wetted are in real life would mean less total friction drag

It is interesting to note the slightly lower pressure on the -51D's aft fuselage though, again confirming the reason to the stability issues.

The Lednicer Model has nothing to do stability, nor does it show 'slightly less' pressure on the 51D's aft fuselage. In fact the pressure distribution at near freestream pressures are the same aft of the canopy for the D as aft of the windscreen for the P-51B.

Further, if you knew what you were talking about the only time the 'pressure distribution' on the side of the turtleback would show up asymetrically (i.e Rudder Effect) would be in a YAW condition - this model is Symetrical Soren.

What I did notice just now though, from looking closely at the two a/c's profiles, and this DOES alter the argument, is the -51D's higher windscreen slope compared to the -51B's which is much more straight, something Lednicer oddly doesn't mentioned.

He doesn't mention it because it isn't so. Perhaps you are comparing the Fw 190 to the P-51B/D.. The 190 has the least angle, the Spitfire the greatest and the 51B/C/D exactly the same

This increased slope will help maintain the boundary layer while the -51B's windscreen will cause seperation right away, which kinda offsets the advantage of the razorback design.

You are dabbling in a.) faulty conclusions and b.) a subject you haven't demonstrated much grasp of. The 51B flat windscreen to forward rounded top of windscreen shows an initial attachment then immediate separation. The Spitfire shows more stagnation pressure on the (flat plate like windscreen in comparison with other 3 models) frontal area but it re-attaches and then maintains attachment about 2/3 of the way back and on both sides of the Malcolm Hood... despite the B having more slope than the Spitfire.

This difference in windscreen design will have a very direct effect on how the pressure distribution is over the rest of the canopy, and this explains Lednicers illustration. In short had the front windscreen been the same (Which I thought it was) Lednicer's results would've been very different.

Lednicer a.) said that, b.) illustrated it in the Spifire/Malcolm Hood discussion and c.) contrasted it with the less effective transition for a top surface that was not a.) a Malcolm Hood or b.) a bubble canopy. You may have missed it the first couple of times through the article but I didn't.

The Primary difference is Not the slope of the windscreen between A/B/C an D/K models - it is the top surface of the windscreen in cross section. All of the birdcage and Malcolm Hoods were 'curved surface' when viewed from front, whereas the D was flat where the top of the canopy interfaced with the top of the windscreen.

The slope of the H (and J and G) was 'steeper - in terms of less angle between WL reference and slope of windscreen... more like the 190D

Where the D/K became superior to the A/B/C was exactly in that transition. In a profile for the B you went from slope (Windscreen) to flat (top surface of birdcage canopy) instead of continued slope of forward part of P-51D canopy transitioning to an airfoil like cross section on the D versus 'flat plate' for the B. He showed how the Malcolm Hood nicely enabled flow to re-attach on the tops and sides of the Spitfire - but NOT the P-51B. The 51D did not separate at all on the canopy top.

Your argument about boundary layer on the windscreen is interesting since for all of his models the windscreen piles up stagnation pressure with the Spitfire being the worst - coming closer to 'flat plate angle relative to 51 and the Fw 190 had the steepest angel - even more than the H.

What you miss is that there is NO Boundary Layer attachment on the windscreen in that high stagnation pressure area. The airflow bypasses it entirely. The Fw 190 will have the lowest stag pressure component and Spifire the highest.. This will manifest as Pressure or Form Drag )or equivalent Flat Plate) - use your preferred term

So like I said, a razorback design is less draggy than a bubble canopy one IF (and this is normally the case) the front windscreen remains atleast similar.

Like you said - you are flat wrong. The angle of the flat windscreen between the A-K model Mustangs is same. The top surfaces differ for the Malcolm Hood and D/K in that there is a continued angle at the top of the windscreen transitioning gradually to the top of the canopy about 18 inches behind windscreen rail, then on aft and downward very much like a fat airfoil.

The Malcolm hood on the Spitfire loses the nice lower pressure distribution it regained as it nears the aft part of the malcolm Hood to the (parallel) transition of the fuselage.. On the P-51D it loses the lower pressure transition when it reaches aft fuselage deck behind the canopy... same reason but the low pressure laminar flow region is greater on the P-51D than the Spitfire (and the Fw 190D) and far greater than the P-51B.

The P-51B never really has reattachment except for the very small portion as it leaves flat windscreen top.

Your conclusions are exactly opposite Lednicer's and exactly opposite the grahically presented data from the VS Aero Model, and exactly wrong relative to Razorback vesrus Bubbltop drag.

And your definition of what Lednicer meant by Suction is?? You still think it is Drag or the equivalent to wake drag on a bullet?

[Soren]

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




More ??







Is it not VERY apparent to you ???

Er, this ISN'T my PROBLEM. This DISCUSSION is ABOUT the results of the computer generated Model and the data extracted from it and the conclusions drawn about the results.

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

And as to the lower pressure area I observed, what's a matter with it ? Did I not read it correctly ? Perhaps I got it the opposite way around ?

[kool kitty89]

Soren, I think the slope difference might be an optical illusion, look at these:

[Soren]

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.

[Soren]

There's clearly a difference in slope, somehitng which will have a big difference on the pressure distribution over and behind the canopy.

[Graeme]

Originally Posted by Soren

those aren't accurate

These aren't either, but I am leaning towards your assertion that the P-51D had a more acute angle for its front windscreen. Problem is resolution on such a small aspect of an aircrafts frame. Soren, the photos don't illustrate the angle, to my eyes anyway. Clave would be the man to illustrate the differences better.

[drgondog]

Originally Posted by Soren

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!

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.

[drgondog]

Originally Posted by Soren

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.

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.

[DerAdlerIstGelandet]

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.

[Soren]

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.

[Kurfürst]

Hmm... thread title says:

Allied tests of captured Bf-109's