The sound barrier

[Soren]

Bill,

I haven't called you names or called you stupid ever in this thread, unlike you. As for the "Bolster your own credentials" comment, don't forget who started that little scene, you as always.

Quit the insults Bill, stop saying I said something I didn't and stick with the facts.

 

[Soren]

Guys, I found something interesting in the previous article I mentioned (with the riveted elevator tabs) Gas Turbines Also, on the tabs, it turns out this article is based on US captured examples of Me 262's.

But in any case, (on pg 21 of the article) there's some very interesting info on the controls which largely explains the discrepancies in reports of the high speed characteristics. It seems early models had controls that became very stiff at ~500 mph, an extended stick was proposed to correct this. However, this doesn't apear to have ever been carried out; instead an additional large mass balance was fitted which seems to have solved the problem.

Also, I'll double check, but neither "stiff controls" or directional snaking were experienced in the Wright field test, in fact iirc it was praised for its stability at high speed. (in shallow dives) Snaking wasn't mentioned either iirc.

And Bill, in a straight down 90 degree dive, there will be no pitch down behaivor, so that wouldn't be an issue.

My observations exactly as-well KK.

Soren, on the comparison to the Spitfire, in the dive trials, the Spit was fitted with al fully feathering prop to avoid overspeeding, minimise propeller related compressibility issues, and minimize drag experienced at high speed.

Roger that KK but the prop would still act as nothing more than a brake at those speeds, esp. spinning around.

The Me 262 would indeed still have thrust available at speeds where the Spitfire will have none (though both will be relying mostly on their weight). However, when the air ingested by the engines reaches supersonic speed, they will stall and flame out. (as Mutke experienced iirc) At this point the a/c is relying entirely on its weight alone for thrust. (although the engines had only been providing ~15-18% total thrust in a vertical dive)

Also, I retract my statement about the Spitfire being necessarily terminal at .89 Mach. It would be above its wing's Mcrit (for all sections much greater than ~10%) but given the thin tailplane the elevator may very well be fenctional. Additionally, while "tuck" is usually experience when an a/c's wing's critical mach number is exceeded, this isn't always the case (ie F-84 does the exact opposite) and the extremety of this characteristic veries as well. Assuming there is a tuck, it may be weak enough to counter with the stick or (if controls are too heavy) trim.

The critical Mach of the Spitfire's root section is probably ~.8 and increases as it tapers outward. (at 10% it should be arround .9 Mach) Due to the thicker section, the P-51D probably initially exceeds Mcrit of the wing somewhere arround .7-.75 mach. (iirc the earlier P-51's wing was somewhat thinner in section, as was the P-51H's)

Agreed.

 

[kool kitty89]

In Mutke's case, when did the engines flame out? (if it was after the plane "smoothed out" that wouldn't make sense inless the plane had not yet reached Mach 1)

Would the buffeting stop once airflow over entire wing had gone supersonic? (but before the entire aircraft had reached Mach 1)

Has anyoine found another site with Mutke's full story?

 

[Juha]

KK
Quote: " But in any case, (on pg 21 of the article) there's some very interesting info on the controls which largely explains the discrepancies in reports of the high speed characteristics. It seems early models had controls that became very stiff at ~500 mph, an extended stick was proposed to correct this. However, this doesn't apear to have ever been carried out; instead an additional large mass balance was fitted which seems to have solved the problem."

I doubt that that explain anything, of course it depend on altitude but 500mph (TAS) is too low to be relevant in this discussion. The problem we are talking about began in case of 262 at speeds over Mach .82. At normal speeds for ex Brown noted that flying was pleasant, control harmony good, with stick force per g of 6 lb at mid CG, but still Mach .84 was his max in dive.

Same to Spit, it was famous with its light elevators. Some Germans even thought that its elevators were too light, with some reason. Still 50kg pull needed at Mach .89 to keep the dive angle constant.

This thread is too long, probably most relevant is already said.

Juha

 

[kool kitty89]

I agree on all those accounts, but I was referring to the flight controls prior to entering the compressability region. (where the controls could get heavy stiff resulting from high -subsonic- flow over the control surfaces) Then there was the whole "snaking" issue. Both of which were in the context (as I understood it) of speeds of ~.80-.82 Mach, below the problem speed for the Me 262. (which was a bit off topic)


In any case, the behavior was "nose heavieness" (due to aft movement of the center of lift) at critical Mach, not loss of elevator control. The "heaviness" being too great to counter with the elevator (but the elevator still functioned). Changine the tailplane incedince should have restored proper trim, but as mentioned using the tailplane trim in dives was heavily warned against. (due to possibility of overcontrol)

The tail surfaces (and outer wings) all should be significantly thinner than the inner wing (they all apear to be around 8-9% t/c, though I'm only sure about the wing). So the elevator would still be functional well above the initial critical mach speed. (the ailerons and rudder as well; in the case of the ailerons should still work above .9 mach)


Another point that should be noted is that nowhere in the test flights (allied or Messerschmitt) were vertical dive tests attempted.


I'm still trying to get the complete story on Mutke's account though as I haven't gotten to read it in a while. (and the link is dead) According to wikipedia, he reduced power and experienced the flame-out while in the "calm," but the engines should have flamed out on there own as soon as the intake flow went supersonic. (unless the engine's intake and guide vanes somehow lowered the flow to subsonic prior to reaching the compressor)

And reducing power too quickly could have been the cause of the flame-out.

 

[claidemore]

Some quotes I found online, in Mutkes own words:

"The airspeed indicator was stuck in the red danger zone, which is over 1100 km/hr.

I noticed that rivets began popping out of the tops of the wings.

The airplane began vibrating and shaking wildly, banging my head against the sides of the cockpit.

After diving about three miles I again regained control and was able to return to base.

On the runway the mechanics were very surprised by the appearance of the airplane, which looked as though it had been shaken by the hand of a giant."

Mutkes reasoning of how he survived:

"It's like when you pass a finger slowly through a candle flame and your
finger gets burned. When you move it quickly, then nothing happens," said
Mutke. "I went so fast through the buffeting area that it was only heavily
damaged, both engines lost function and the rivets flew out of the wings."

For the last several years, Otto Wagner, a professor at Munich's Technical
University, has done computer simulations to try to verify Mutke's claim.
He has been able to simulate the Me262 at Mach 1.02 -- just above the
speed of sound -- but he says the basic data on the plane's aerodynamics
are not reliable. He's now trying to obtain wind tunnel studies from 1944
at the Messerschmitt factory in Berlin to do a more accurate simulation.

"If I had better data, then I could say it was faster than sound or not,"
Wagner said. "Now I can't say anything."

But the head of the Deutsches Museum air and space collection -- which
houses another Me262 flown by Mutke -- rejects the pilot's claim.

BTW, the Miles M.52 was a straight wing design (started in 1942) which did mach1.5 in October 1948 (unmanned).
In a completely unrelated topic, Hans Mutke also invented the female urinal.

kk- the wiki article says "but he had reduced power", but it is not specific about when. The assumption that he did so while in the 'calm' is because of the order the details are related in that article.

 

[DerAdlerIstGelandet]

Well this thread has gotten out of hand.

I posted my warnings a while back as to what will happen. I do not have time at the moment, as me and my wife have to go to a Birthday party. When I get back effected parties will be notified. You know what I am talking about.

 

[Juha]

OK KK
why they did not try 90 deg dives. IMHO because they had lost pilots in lower angle dives and knew that pilots were lost during near 90 deg dives in other planes. I think test centres try to keep their test pilots alive.

On snaking Brown in his Wings of the LW "The German development program had gone no further than the thickening of the trailing edge of the fin-and-rudder assembly and the provision of strips of sheet metal which were bent outwards along this edge. The overall effect was better than that obtained with Meteor I but inferior to that of the little Heinkel He 162."

Juha

 

[drgondog]

Guys, I found something interesting in the previous article I mentioned (with the riveted elevator tabs) Gas Turbines Also, on the tabs, it turns out this article is based on US captured examples of Me 262's.

But in any case, (on pg 21 of the article) there's some very interesting info on the controls which largely explains the discrepancies in reports of the high speed characteristics. It seems early models had controls that became very stiff at ~500 mph, an extended stick was proposed to correct this. However, this doesn't apear to have ever been carried out; instead an additional large mass balance was fitted which seems to have solved the problem.

Also, I'll double check, but neither "stiff controls" or directional snaking were experienced in the Wright field test, in fact iirc it was praised for its stability at high speed. (in shallow dives) Snaking wasn't mentioned either iirc.

Read the report in Stormbirds Rising. I am too lazy to scan it after I typed much of it out for you and Soren a couple of days ago. Report extracts from F-TR-1133-ND, Feb 1947, 'Evaluation of the Me 262'. It is in Appendix C.

Specific reference to 'hunt at speeds above 350' and 'the handling characteristics were poor at all speeds above 350 mph' are contained in Section 3 Flight Characteristics, paragraph e.

Personally, based on Wendel's interrogation I believe that what the Wright Pat test pilots encountered was poor rigging on the rudder tabs as contrast with general and universal issues - save the aft cg problem when fully loaded with fuel. See Section 7 Flight, pg 4 in the Wendel Link regarding cg and consequences

Further read Lindner's debriefing contained in Appendix B. The Appendix B references report API (K) No/339/1945. Note that stiff controls only seem to be in effect in the dives as the a/c accelerates into Mcrit. I brought that up because of Soren's comment that 'it was light and responsive in all regimes' or words to that effect. This whole discussion about steadily increasing stick forces is what the Structural and Stab/Control debate has been all about.

Read the POH that Soren thoughtfully provided at the section on Take Off Weights and Balances, as well as the other link Soren posted from KapitanWendel. Both discuss yaw issues due to filling aft fuel tank and moving cg past 30%MAC. That is found in Section 2- Before Entering Pilot's Compartment, paragraph c. Center of Gravity and d. Take Off Weights and Balances for discussion of Yaw issues at aft Cg.

And Bill, in a straight down 90 degree dive, there will be no pitch down behaivor, so that wouldn't be an issue.

That is correct -it will reach a point of zero lift. The challenge is getting to that point KK, then the reverse occurs as you start the pull out.

Whether Mutke did it or not will never be proven.

The problems to be solved in a rigorous analytical study is first the entire aerodynamic load profile as the 262 enters the dive and accelerates past .8, and .86, then whatever speed provides a solution of transition from loaded wing and heavily loaded tail to unload on the wing and tail... then reverse the process. This includes study of the engine nacelle/fuselage shock wave interaction if it occurs as well as the inlet geometry for compressor behavior prediction. I would be looking at the wing spar (due to both aero load and inertia of the Jumo/nacelle combination) during pullout as well as tail attach points (due to severe aero loads to overcome CMac - transmitted to aft fuselage)


The second problem is the stability derivatives as the CMac increases during the transonic migration of the ac to .50 Chord - to point of no load on airfoil and look at this behavior during the transonic, to local supersonic, to complete supersonic - that is the cycle in which the ac moves from stable aircraft as designed to a forward cg condition in which the elevator experiences tremendous forces in very high 'Q'... all the way to Mach 1 - then back again during the pull out when the a/c slows back to transonic with a fully loaded tail structure. Is CMac derivative stable and predictable during the transition? If not - how does that translate to stick forces and stability in general?


The last problem is pure structural analysis, starting with the aero loads on the elevator, then transmitting them to the fuselage, as well as the wing discussion above. If there are no yaw loads during the transonic profile, then torsion transmitted to the aft fuselage stringer/beams will not be severe.

Soren, on the comparison to the Spitfire, in the dive trials, the Spit was fitted with al fully feathering prop to avoid overspeeding, minimise propeller related compressibility issues, and minimize drag experienced at high speed.
The Me 262 would indeed still have thrust available at speeds where the Spitfire will have none (though both will be relying mostly on their weight). However, when the air ingested by the engines reaches supersonic speed, they will stall and flame out. (as Mutke experienced iirc) At this point the a/c is relying entirely on its weight alone for thrust. (although the engines had only been providing ~15-18% total thrust in a vertical dive)

Also, I retract my statement about the Spitfire being necessarily terminal at .89 Mach. It would be above its wing's Mcrit (for all sections much greater than ~10%) but given the thin tailplane the elevator may very well be fenctional. Additionally, while "tuck" is usually experience when an a/c's wing's critical mach number is exceeded, this isn't always the case (ie F-84 does the exact opposite) and the extremety of this characteristic veries as well. Assuming there is a tuck, it may be weak enough to counter with the stick or (if controls are too heavy) trim.

Strictly speaking 'tuck' occurs when the movement from .25 c to .50 c can't be overcome by normal elevator controls. Having said that - moving the ac aft changes the CMac always.

The critical Mach of the Spitfire's root section is probably ~.8 and increases as it tapers outward. (at 10% it should be arround .9 Mach) Due to the thicker section, the P-51D probably initially exceeds Mcrit of the wing somewhere arround .7-.75 mach. (iirc the earlier P-51's wing was somewhat thinner in section, as was the P-51H's)

That would be speculation unless you have the data. My speculation is that Mcrit on the 51BCD wing was closer to .8+. IIRC the MK IV dive trials observed the shock wave charatceristics 'visibly' by the manifestaion of the ammo door deflection at .84 to .85 so the shockwave would have started earlier.

The P-51H wing was the NAA/NACA 66-(1.8) 15.5 and the BCD was a NAA/NACA 45-100

The BCD was a .165 t/c at CL with a straight taper to .115 at tip
The H was a .155 t/c at CL with a straight taper to .120 at tip

the BCD has 1 degree of negative incidence at tip (twist) and the H had 1.8

The differences are slight but lean to H for delay of Mcrit and both relatively fat wings compared to a 262 - at least outboard of nacelles

Further I would think Mcrit for the Spit wing would be at a lower speed than the 262 ~ .82+ Mach. Mcrit is when the local transonic flow first occurs - not the velocity for which the CMac has moved all the way aft

 

[kool kitty89]

Something to also consider (though probably minor), I believe the Me 262's fusalage was designed to have a semi-airfoil shape, and contribute a small portion of lift. So when the fuselage exceeds its critical mach number, this should result in trim changes as well.


And it turns out that the only mention of control properties at high speed (20-30 degree dives) in the Wrigh Field Me 262 handbook was that there was no elevator flutter, no mention of directional stability characteristics was made.


And, excuse my ignoracnce, but what does CMac refer to?

 

[drgondog]

Something to also consider (though probably minor), I believe the Me 262's fusalage was designed to have a semi-airfoil shape, and contribute a small portion of lift. So when the fuselage exceeds its critical mach number, this should result in trim changes as well.


And it turns out that the only mention of control properties at high speed (20-30 degree dives) in the Wrigh Field Me 262 handbook was that there was no elevator flutter, no mention of directional stability characteristics was made.

Read the excerpts from the Report. There are no discussions of flutter. There is a distinct comment about unsatisfactory flight characteristics above 350 kts.


And, excuse my ignoracnce, but what does CMac refer to?

Moment Coefficient about the aerodynamic center - usually 'located' around 1/4 c at the Mean Aerodynamic Chord. Thin symmetrical wings tend to mitigate the CMac changes at Mcrit

 

[kool kitty89]

Thanks.

What I meant by my statement about the handbook was that it provided no pertinent information regarding the snaking issue. (I had remembered differently, but then I read through it again)

And that's 350 kts TAS?

 

[davparlr]

Thanks.

What I meant by my statement about the handbook was that it provided no pertinent information regarding the snaking issue. (I had remembered differently, but then I read through it again)

And that's 350 kts TAS?

I don't think TAS is ever referenced in performance limits for the pilot. TAS is primarily used in navigation. Indicated/Calibrated reflects the actual dynamic airflow over the aircraft (what the wings see). At high altitudes and airspeed, Mach becomes a crtitical factor also.

I would guess that, up until the 60's, very few aircraft had TAS indicators, although the Me-262 apparently had one.

 

[drgondog]

Thanks.

What I meant by my statement about the handbook was that it provided no pertinent information regarding the snaking issue. (I had remembered differently, but then I read through it again)

And that's 350 kts TAS?

Dave - you are right of course, and particularly to 99% of WWII fighters. From what I have read and heard the Germans built an airspeed indicator, some what successful, that indicated TAS.

My inference from the POH developed at Wright Psat was that the pilot was referring to TAS whether or not a post flight calculated number or IAS/TAS per the 262 airspeed indicator - in other words I have no real clue

 

[kool kitty89]

And the Me 262 had 2 needles on the indicator, a white one for normal IAS, and a yellow one for CAS so it's unclear which reading is sited sometimes. (I'm not sure how sophisticated the system was determining CAS though)


Slightly different, but the post war Meteor F.4 encorporated a Mach meter, which was important due to the ease at which it exceeded critical mach, with violent results in some circumstances. And though still somewhat primitive it gave pilots a good idea of when they were approaching the limiting .8 Mach. (which was increased slightly, along with much improved characteristics at critical mach, with the adoption of the new tail on the F 8 )

 

[Soren]

The manual says the second needle lists TAS.

 

[Soren]

Read the excerpts from the Report. There are no discussions of flutter. There is a distinct comment about unsatisfactory flight characteristics above 350 kts.

Only if the CG was too far back. Otherwise the flight characteristics of the a/c were good at all speeds, which Hans Fey, Fritz Wendel and the Allied manuals also mention.

 

[kool kitty89]

Soren, you do not include critical Mach and above in "all speeds," right?

You may disagree on the snaking issue, but I'm sure you'll agree that once you get into that realm, control is not "good." (certainly not in pitch)

 

[drgondog]

Only if the CG was too far back. Otherwise the flight characteristics of the a/c were good at all speeds, which Hans Fey, Fritz Wendel and the Allied manuals also mention.

Earlier - you did not admit to any cause except 'poor quality'. You still have difficulty in acknowledging that 'control was NOT good' in the transonic.

Maybe you will come around to that position based on the flight test and intelligence debriefings of the Messerschmidt test pilots?

 

[Soren]

Soren, you do not include critical Mach and above in "all speeds," right?

No, up to the top speed of the a/c = 870 to 900 km/h, where the handling of the a/c was excellent.

You may disagree on the snaking issue, but I'm sure you'll agree that once you get into that realm, control is not "good." (certainly not in pitch)

Agreed.