The sound barrier

[Soren]

It has been solidly confirmed by studies done in Germany that the Me-262 was perfectly capable of breaking the soundbarrier.

The problem which could occur was as KK describes if the pilot failed to stop the negative G the a/c automatically started to generate above .86+ Mach by trimming the tailplane.

 

[kool kitty89]

Delcyros refrenced the article on the first 2 pages of this thread.

Besides the -G loading issue, there are a couple other factors that have to be right for the Me 262 to survive the event.

The load has to be right (too allow recovery in time), the dive angle has to be steep (both to accelerate fast enough and to minimise pitch-down stability problems), and finally the starting altitude has to be right. (near the ceiling for medium load)
See the graph on pg.1 as well. (the study was for vertical dives)

If Mach 1 is reached the engines will stall and with the reduction in thrust as well as the rapidly increasing air density as well as the increasing local speed, the a/c will quickly drop to below critical Mach and regain elevator authority for recovery. (though if the tailplane is already retrimmed to counter the pitch-down, when speed drops below ~.86 mach the a/c will already want to pich up with neutral elevator)

 

[Soren]

Regarding the discussion link you provided earlier: It is completely incorrect that the Me-262's airframe couldn't sustain the stress of a Mach 1 dive, as proven in studies done in Germany in 1999. That needs to be made clear.

That it would be a problem to reach Mach 1 is another issue, one which I agree with.

 

[Waynos]

You guys are way ahead of me on the techspec side of things so I'm not going to argue but I just wanted to add that according to Glancy's book the Spitfires wing was not designed to any naca aerofoil section but was thinner than the recommended ratio, which led to doubts about its strength in the early days but was obviously later found to be perfectly ok and this is what gave it its advantage in a dive. I never said that 0,94 was achieved in a dive did I? Hopefully that part was obvious. sorry.

Also the P-84 was not part of the trials so I can't comment on that one but the P-80 was, as were the Vampire, Meteor, GA2 and Attacker (laminar wing) as also was the Spiteful (same wing) and it was a surprise that the Spitfire was still the fastest in the Dive. This was far from bragging for Supermarine, it was actually a bit embarrassing.

I'm not sure how accurate the 1999 tests for the Me 262 were (I don't know anything about them) but the 1945-46 tests were carried out with actual airframes not mathematical extrapolations and the fact that Messerschmitt supported the findings kind of convinces me, no offense as you clearly know your stuff.

Regarding the Messerschmitts 'looking different', who knows? Maybe he was thinking along the lines of the HG.III and P.1110? (just guessing there)

 

[kool kitty89]

What the Messerschmitt tests showed (similer to the British tests) was that elevator control was lost at Mach ~.86 and a strong pitch down behavior devloped. (trim changes were not mentioned in corelation with this in anything I've read)

The lower the dive angle the greater the chance of -G damage from pitching down. (and controling the pitch down with trim may be more difficult as well)

Vertical dives give the best chances, as the greatest net trust is avialable (the a/c's total weight) and the faster the a/c can accelerate, the shorter time spent in the region between Mach .86 and 1.0. (the longer time spent in the "conmpressibility region" the greater chance of structural damage or failure from buffetting)


The key factor in Mutke's story is that while still accelerating durring strong buffetting, there was a breif period of calm where control was apparently restored. (which is characteristic of exceeding Mach 1, though elevator control may or may not work in this region of flight -I don't know the specifics behind this)


In any case, buffeting damage on Mutke's a/c was quite severe, with considerable warping of the wings and missing rivets. (resulting in a difficult landing and the a/c subsequently being scrapped iirc)



Everything I've seeen on data of the Spitfire's airfoil is that it's a conventional NACA 2200 series airfoil (in fairly common use at the time) with a root thickness of 13%, this would still have made it the thinnest wing on any production a/c (let alone fighter) up until the introduction of the first jets. And it very well could have had the highest critical mach number (and terminal diving speed) of any piston engined fighter.

But I'd think that the .89 dive speed figure would be a terminal dive (well past critical mach) and elevator control would be lost. (though that doesn't mean recovery would be impossible)



Another note, is that the flight manual from US testing of the Me 262 at Wright Field in 1946 mentiones similar behavior in terminal dives as in Mutke's case.

 

[davparlr]

If Mach 1 is reached the engines will stall and with the reduction in thrust as well as the rapidly increasing air density as well as the increasing local speed, the a/c will quickly drop to below critical Mach and regain elevator authority for recovery. (though if the tailplane is already retrimmed to counter the pitch-down, when speed drops below ~.86 mach the a/c will already want to pich up with neutral elevator)

Good point. On looking at drawing of the Jumo engine, it appears that the inlet is a convergent inlet. If so, the airflow will go supersonic before the aircaft, especially at the compressor blades, causing shock waves and airflow disruption. In order to determine whether the Me-262 could penetrate the sound barrier, an inlet analysis must be made to determine airflow characteristics.

 

[kool kitty89]

Delcyros already mention the compressor stall issues on pg.2 of this thread. (this penomenon was unavoidable in this configuration -fitting a lengthened intake with a shock cone may have prevented it though)

But, in a vertical dive and at full power the engines will only be generating ~20% of the total thrust. (the rest coming from the a/c's weight) And the engines wouldn't stall until a shock wave develops at the intakes. (at or very very close to Mach 1)


I also forgot to add that, while Mutke's discription is a fairly good indicator that he may have indeed reached Mach 1, unfortunately the best proof of breaking the soud barrier would probably not have been noted in this case, a sonic boom. If he did indeed exceed Mach 1, the boom could have simply been contributed to a crash or an explosion as it was an unknown phenomenon. (likewise when George Welch first dove through the Barrier, nearby civilians who heard the boom expected it to be a crash. But it's this that gives the best proof for Welch's claim -allong with corisponding flight notes from Welch)

 

[Soren]

Waynos,

Let's get something clear here, the 1945-46 tests didn't establish that airframe couldn't take the stresses of a Mach 1 dive, all they did was conclude that at above Mach 0.86 elevator control was lost. Remember these weren't straight down vertical dives.

 

[drgondog]

Waynos,

Let's get something clear here, the 1945-46 tests didn't establish that airframe couldn't take the stresses of a Mach 1 dive, all they did was conclude that at above Mach 0.86 elevator control was lost. Remember these weren't straight down vertical dives.

Right or wrong, all the reports I have seen regarding the 262 in dives - discusses the pitch down characteristics around .82-.86 M, apparently due to the elevator being blanked out due to shock wave as wing went transonic.

If that was the true cause and the pilot could not achieve nose up trim another way, there would be a large potential for continuation in the 'nose down tuck' - which could easily lead to structural failure - in any airplane - at those kinds of speeds.

That would not be a condemnation of the 262 structural integrity as all WWII types were at the edge (or beyond) structurally at .85 M, as they sure as hell weren't designed for 'negative G' at those aerodynamic load conditions.

 

[Waynos]

quote ; " Waynos,

Let's get something clear here,"

Thats a rather arrogant turn of phrase, if I may say so soren. I don't appreciate being talked down to, thank you.

Especially as you backed it up with nothing more than a statement that contradicts what I have read in more than one source. Who am I to believe, and why?

For example, the exact quote from 'Spitfire' by Glancey on page 160 is;

"Tests demonstrated that the 262 would fly out of control at a speed in excess of mach 0.86 and that its airframe was in danger of breaking up at a marginally higher speed, the Spitfire had bettered this in 1943".

Regarding the mach 0.94 dive I mentioned earlier I have also found this quote, from the same book, page 115, attributed to Jeff Quill when talking of the event;

"That any operational aircraft off the production line, cannon sprouting from its wings and warts and all could readily be controlled at this speed when jets such as the Meteor, Vampire, P-80 etc cannot is certainly extraordinary"

 

[kool kitty89]

The comment about the Spitfire being "readialy controlled at this speed" doesn't make sense.

If the spitfire genuinely could dive to such speeds, it they would be terminal dives, not controlled ones. (as would the .89 Mach dive) It would be well above critical mach and the elevator would be ineffective. (rudder and ailerons may still be functional)

Now, the Spitfire may have gotten luck, and was inherantly stable above critical Mach, but that's a different issue. Also, to avoid structural damage to the airframe at such speeds, buffeting must have been relatively minimal, or the speeds were only maintained for very short periods of time. (the vibrations and flexing from buffeting can cause damage independantly of -G or stress directly related to the high speed flow)

 

[Soren]

I aint talking down to you Waynos, I'm just telling you that the tests didn't conclude anything about the Me-262's airframe not being able to take the stresses of a Mach 1 dive. And the airframe could take it as established in studies done in 1999. The problem was the pitching down above 0.86 Mach, something which could only be corrected by use of horizontal stabilizer trim, which I have heard the British weren't aware of, I think Hohun knows about this.

There are no miracles in physics mate, all things in aero can be measured, just like your sig lets us know.

 

[Soren]

The comment about the Spitfire being "readialy controlled at this speed" doesn't make sense.

If the spitfire genuinely could dive to such speeds, it they would be terminal dives, not controlled ones. (as would the .89 Mach dive) It would be well above critical mach and the elevator would be ineffective. (rudder and ailerons may still be functional)

Now, the Spitfire may have gotten luck, and was inherantly stable above critical Mach, but that's a different issue. Also, to avoid structural damage to the airframe at such speeds, buffeting must have been relatively minimal, or the speeds were only maintained for very short periods of time. (the vibrations and flexing from buffeting can cause damage independantly of -G or stress directly related to the high speed flow)

Agreed.

It is a case a false speed readings that's all, a known issue at very high subsonic speeds. The Spitfire couldn't go that fast without falling apart.

 

[drgondog]

I aint talking down to you Waynos, I'm just telling you that the tests didn't conclude anything about the Me-262's airframe not being able to take the stresses of a Mach 1 dive. And the airframe could take it as established in studies done in 1999. The problem was the pitching down above 0.86 Mach, something which could only be corrected by use of horizontal stabilizer trim, which I have heard the British weren't aware of, I think Hohun knows about this.

There are no miracles in physics mate, all things in aero can be measured, just like your sig lets us know.

If true regarding the slab elevator being blanked out in turbulent flow, how did elevator trim work to solve the problem. If there is no laminar flow over the elevator, there is nothing for the trim tab to work on.

If the issue was not turbulent flow but compressibility initiation over the elevator, the movable and boosted 262 elevator would have worked fine.

If it was the shock wave moving the Cp aft to ~ 50% Chord over the wing, causing a pitch down moment but elevator still in flow (i.e not blanked) then the elevator would still work and no trim required.

Which case for the 262?

 

[Waynos]

I can well believe that the speed reading was not entirely accurate but there is little doubt that at least m0.9 was reached and no doubt at all that it was the fastest diving aircraft of the 2nd world war (er, except the V-2 I suppose). This much was proven with real aircraft on actual flight tests.

If mathematical theory 'proved' the idea that the 262 could withstand mach 1 in 1999, why did Willy Messerschmitt support the RAE findings? wouldn't he have published the opposite view, at some point if not immediately? but he never did and even the intended 'high speed' HG.III model was not tested beyond mach 0.86 in any wind tunnel.


The controllability aspect is a good one however, I suppose that the fact the Spitfire recovered and landed with no damage is evidence of controllability, perhaps?

In a further quote on the subject, and going back to the wings being thinner than the accepted norm the aerodynamiscist Sir Morien Morgan who specialised in the transonic region during the 1950's wrote with hindsight;

"Considering that he could have had little knowledge of Mach effects, Mitchells decision to use such a thin wing was not only bold, it was inspired. We know know it was a close run thing. Had he made it just a little thinner the wing would have been too weak and aileron reversal would have been encounterd lower down the speed scale and if that had happened the Spitfire would have been just one more aircraft that didn't quite make the grade"

Again, this points to a wing that was not just thin, but unusually so.

 

[Soren]

If true regarding the slab elevator being blanked out in turbulent flow, how did elevator trim work to solve the problem. If there is no laminar flow over the elevator, there is nothing for the trim tab to work on.

If the issue was not turbulent flow but compressibility initiation over the elevator, the movable and boosted 262 elevator would have worked fine.

If it was the shock wave moving the Cp aft to ~ 50% Chord over the wing, causing a pitch down moment but elevator still in flow (i.e not blanked) then the elevator would still work and no trim required.

Which case for the 262?

Bill,

You're missing the fact that the entire horizontal stabilizer was movable on the Me-262 and was used for trim.

 

[Soren]

I can well believe that the speed reading was not entirely accurate but there is little doubt that at least m0.9 was reached and no doubt at all that it was the fastest diving aircraft of the 2nd world war (er, except the V-2 I suppose). This much was proven with real aircraft on actual flight tests.

Well I disagree. Also the a/c which did reach Mach .9 doesn't seem like it didn't take any damage, it looked rather terrible afterwards IMO, and thats when it probably didn't even reach Mach .9.

If mathematical theory 'proved' the idea that the 262 could withstand mach 1 in 1999, why did Willy Messerschmitt support the RAE findings? wouldn't he have published the opposite view, at some point if not immediately? but he never did and even the intended 'high speed' HG.III model was not tested beyond mach 0.86 in any wind tunnel

.

Willy wasn't the man behind the Me-262 Waynos, Dr. Waldemar Voigt designed the a/c and had his own team leading the research surrounding it.

Furthermore no WW2 a/c designer knew all there was to know about the transonic region much less the supersonic one, so how do you expect them to correctly answer a question regarding it ?

 

[Waynos]

But he didn't say 'I don't know'. He agreed with the RAE. And he DID design a supersonic jet that flew in the HA 300 so he would have been qualified to contradict his earlier position by the mid 1960's at the very latest. But, as far as I know at least, he didn't, for whatever reason.

Speaking purely subjectively I think the p1101 could have gone supersonic in a dive relatively safely but I am happy to accept the RAE/Messerschmitt view that the 262 could not.

Yes, Martindales Spit did take a lot of damage, due to the catastrophic failure of the reduction gear, but it does not follow that this failure would automatically occur in every engine when diving at this speed (even if it is quite likely to happen), so the fact that the 81 Sqn aircraft did not suffer this failure cannot be taken as proof that it was travelling more slowly, a sample of only 2 cannot be indicative.

The IAS of Martindales dive was 0.92, this was subsequently revised downwards to 0.89 and is considered the accurate speed.

 

[drgondog]

Bill,

You're missing the fact that the entire horizontal stabilizer was movable on the Me-262 and was used for trim.

I didn't miss the point - I said slab elevator. And then I asked you how 'trim' would solve the problem if the entire elevator was immersed in turbulent flow.

 

[Graeme]

Jeff Quill

Hi Wayne.

Quill wrote an extensive article for AI (September '76) on the 40th anniversary of the Spitfire and he discussed the high speed dive tests, as you described. I've tossed in a 'graph' for good measure...