Elevator trim during Combat

[eddie_brunette]

Thanks for all the posts!

[buzzard]

I ran across this while looking thru Putnam's "German Aircraft of the Second World War", JR Smith and AL Kay, '72...

"During dogfights between Fw 190s and RAF fighters it was not uncommon for the Luftwaffe aircraft to flick on their backs from a very tight turn and crash at full throttle. The cause of this disastrous behaviour was the pilot making excessive use of the electric tail-trimmer, an ingenious invention of Focke-Wulf, in an attempt to tighten an already very high 'g' turn, the aircraft eventually entering a high speed stall from which there was no recovery".

JL

[drgondog]

Originally Posted by buzzard

I ran across this while looking thru Putnam's "German Aircraft of the Second World War", JR Smith and AL Kay, '72...

"During dogfights between Fw 190s and RAF fighters it was not uncommon for the Luftwaffe aircraft to flick on their backs from a very tight turn and crash at full throttle. The cause of this disastrous behaviour was the pilot making excessive use of the electric tail-trimmer, an ingenious invention of Focke-Wulf, in an attempt to tighten an already very high 'g' turn, the aircraft eventually entering a high speed stall from which there was no recovery".

JL

Buzzard - there is a possible alternate cause. The Germans recognized the problem of high speed stall and eventually figured out one cause (could have been more) for the violent High G Stall.

They figured out that outer wing span area stalled about the same time the as the inboard 80%. The Fw 190 had a fairly unusual application of chord twist, starting at positive 2 degrees at root and ending at 0 at 81.5% of the span - then zero to tip. Most wings will twist all the way to the tip to ensure that the tip will stall last at high angles of attack.

I haven't read the report itself but familiar with the conclusions which state the outboard tip are stalled as a result of elastic deformation under load.

The mode is something I am not sure of just yet.

The aileron on the down wing is 'up' which would make the mean chord, locally in the aileron area, 'negative' in that it would be negative angle from the inboard wing.

The load due to the aileron in that outer region could be torsional (likely) or pure bending if the center of that load was on the main spar.

I suspect torsion, and further suspect that it was to twist the tip area 'up' meaning that the local Angle of Attack reached CLmax faster or at the same time as the inboard region.

The net is that I don't know for sure, but this is more plausible to me than changing horizontal stabilizer incidence to cause the subsequent stall. Changing the tail incidence to me could dangerously increase loads on the fuselage as well as create potential stability issues.

Do you have another source that I could find on the Fw 190 elevator incidence problem?

[kool kitty89]

Though it does seem, by that quote, that the stall was worsened by excessive use of trim, hence the irrecoverable part. (I'd imagine recovery would be possible if it was re-trimmed though, as long as they weren't on the deck)

[drgondog]

Soren briefly mentioned the use of the 109 adjustable stabilizer as a means of accelerating a dive pull out.

Does anybody know one way or the other?

It would be interesting to compare that approach to the opposite approach for the Mustang, P-38 and P-47 in which the use of just the trim tab to assist the pullout was specifically 'discouraged' (I mean if you're gonna do it - what does 'prohibited' mean if you survive it.)

Relatively speaking the trim tabs on all those ships are much smaller in area and should be far less effective at transmitting tail loads to their respective fuselages than an entire 'moving horizontal slab' from an area stanpoint.

[kool kitty89]

Wasn't there some problems with parts of the elevator or the tab gatting ripped off when used at high speeds? (the full span servo tab tested in attempt to improve control in compressibility on the P-38 ripped the whole elevator off when used, or possibly overused)

IIRC most of the structural failures were of the tail itsself due to excessive aerodynamic forces on a relatively small area (also experienced if too much elevator was applied once the a/c reached denser air), not excessive G forces, and not structural failure elsewhere on the a/c. So the use of a large area (the entire tailplane) moved in small increments could have been a safer procedure in that light.

I'd immagine such a feature on the P-38 would have helped greatly in dive recovery. (and iirc the trim tabs weren't very effective in helping recovery either) Another thought would have been boosted elevator controls.

[buzzard]

drgondog,

I was simply quoting verbatim from the book. And I was wondering if the use of the variable incidence tailplane (VIT) in such a manner was SOP or not. As for the abrupt stall of the 190, the report I posted from in the 'Germany'44' thread mentioned that in a tight turn to the left near stall speed, the a/c exhibited a tendency to reverse aileron control and then stall without warning. No mention was made of utilizing the VIT in the turning test. The use of the VIT probably just exacerbated the problem.

Here's something that is probably not relevant, but it is interesting...

This is from an interview with Kurt Tank concerning the effects of modifying the 190 prototype to accept the BMW 801.

"Although the extra 50 hp was useful, we found that the extra 160 kg of engine weight plus the additional structure to carry it, and the weight of the armor and the additional equipment the Luftwaffe now wanted, had increased [the fighter's] all-up weight by about a quarter. The wing loading rose from 1.6 kg/m2 [38lb/sq,ft.] of the first prototype to 1.9 kg/m2 [46 lb/sq.ft.], and the turning performance deteriorated accordingly. To restore the aircraft's previously pleasant handling characteristics, we enlarged the wing by extending each tip by just over 50 cm [20"] and reducing the amount of taper so that the outer sections were somewhat wider. In this way, we increased the wing area by just over 3.5 m2 [35 sq.ft.] and lowered the wing loading to a more reasonable 1.5 kg/m2 [35.8 lb/sq.ft.]"

He goes on to say that the wing remained unchanged in all the low-medium alt. models of the 190. (The metric wing loading numbers look to me like a mis-transcription... )I don't have anything else on the VIT 'problem' (I think the 'problem' was pilot-induced...)

KK89,

Lockheed's attempt to solve the compressibility problem by modifying the tail-plane only made it worse...In the compressibility zone, the lift generated by the tail-plane overcame that of the wing, and pulling back on the stick (if you had the strength) only increased the uploading on the tail...and steepened the dive. The compressibility-induced turbulence generated by the wing often tore off the tail. It was by installing small dive-flaps under the wing to increase lift (without increasing the AoA), that the problem was solved.

JL

[Soren]

Bill,

The problem 109 pilots faced at over 750 km/h was that elevator authority reached almost nil, the stick was near solid. The same happened to the P-51 when it redlined.

However the Bf-109 has the advantage of a variable incidence horizontal stabilizer, which was easy to operate. This meant that if entering a high speed dive, wether it was above or below the redline speed the recovery could be significantly speeded up by adjusting the incidence of the tailplane.

This feature was also used to cancel out the 109's habbit of experiencing downwards trim in high speed level flight.

Here is one account of a Bf-109 getting away from a US P-51 pilot by pulling up much quicker than the -51 pilot was able to:

Thomas L. Hayes, Jr., American P-51 ace, 357th Fighter Group, 8 1/2 victories:
"Thomas L. Hayes, Jr. recalled diving after a fleeing Me-109G until both aircraft neared the sound barrier and their controls locked. Both pilots took measures to slow down, but to Hayes' astonishment, the Me-109 was the first to pull out of its dive. As he belatedly regained control of his Mustang, Hayes was grateful that the German pilot chose to quit while he was ahead and fly home instead of taking advantage of Hayes' momentary helplessness. Hayes also stated that while he saw several Fw-190s stall and even crash during dogfights, he never saw an Me-109 go out of control."-


Btw, as mentioned the Fw-190 featured a variable incidence tail plane as-well, which was esp. useful at high speed for trimming. However when entering a dogfight the pilot had to remember to trim back to normal or turn performance seriously suffered. And infact during the US post war flights with a captured Fw-190 Dora the ill turn performance experienced was it seems in big part because of the tail plane being improperly set, making it very difficult for the US test pilot to stall the a/c in turns as he himself explained there not enough force supplied by the elevators. The other reason was ofcourse the low power setting at which the test was performed.

[kool kitty89]

Yep (on the P-38 ) and there seems to be some misinformation about the dive flaps, the biggest (which I had assumed as correct) claiming that the flaps reduced the shockwave and allowd elevaor operation. Others state them as simple airbreaks, but as mentioned they act to quickly increase lift w/out changing AoA and thus pitching up.

Similar flaps were installed on late model P-47's and on P-80's (in addition to the airbrake) and in all cases facilitated recoveries from dives exceeding limiting Mach. (a totaly different problem was found on the P-84 with the violent pitch-up stall when ~.8 mach was exceeded at low level)


The variable incedence tail also helped the Me 262 maintain control at limiting mach, and facilitated recovery from dives exceeding this. (and kept the a/c from pitching down and failing under high -G loads)
(similar to the advantages on the 109, albeit at a considerably higher Mach number)

[buzzard]

This is from a test report of the Fw-190 at Wright Field, spring '44.

" The outstanding maneuverability feature of this airplane is it extremely high rate of roll. The radius of turn, however, is poor and it is only slightly improved by using the maneuvering flap position of 15 degrees. If pulled fast, the airplane tends to stall out abruptly with little warning. Elevator control forces are very heavy in a tight turn, requiring constant use of the elevator trim control."

It looks like the use of the VIT trim was likely commonly used in turning fights.

The VIT in WWII fighter design seems to be an exclusively German phenomena. Did any Allied fighters have this feature?

JL

[Soren]

Improper aileron adjustment plus a faulty engine is what caused the poor turn performance at wright field buzzard. The high elevator forces are suspect as-well as the 190 featured perhaps some the lightest control forces of any a/c of WW2.

A good example of when the ailerons were properly adjusted are the British tests with a heavy Jabo version, this a/c despite being heavier & less aerodynamically clean and running at low power was able to turn just as well the P-51 Mustang Mk.III. This a/c did also, unlike the others with improper aileron adjustment, give the pilot warning of the approaching stall with a slight buffet. (Just like vet Fw-190 pilots note it did)

[drgondog]

Originally Posted by buzzard

drgondog,

I was simply quoting verbatim from the book. And I was wondering if the use of the variable incidence tailplane (VIT) in such a manner was SOP or not. As for the abrupt stall of the 190, the report I posted from in the 'Germany'44' thread mentioned that in a tight turn to the left near stall speed, the a/c exhibited a tendency to reverse aileron control and then stall without warning. No mention was made of utilizing the VIT in the turning test. The use of the VIT probably just exacerbated the problem.

Here's something that is probably not relevant, but it is interesting...

This is from an interview with Kurt Tank concerning the effects of modifying the 190 prototype to accept the BMW 801.

"Although the extra 50 hp was useful, we found that the extra 160 kg of engine weight plus the additional structure to carry it, and the weight of the armor and the additional equipment the Luftwaffe now wanted, had increased [the fighter's] all-up weight by about a quarter. The wing loading rose from 1.6 kg/m2 [38lb/sq,ft.] of the first prototype to 1.9 kg/m2 [46 lb/sq.ft.], and the turning performance deteriorated accordingly. To restore the aircraft's previously pleasant handling characteristics, we enlarged the wing by extending each tip by just over 50 cm [20"] and reducing the amount of taper so that the outer sections were somewhat wider. In this way, we increased the wing area by just over 3.5 m2 [35 sq.ft.] and lowered the wing loading to a more reasonable 1.5 kg/m2 [35.8 lb/sq.ft.]"

It is EXTREMELY relevant - you found a jewel. This explains why the Fw 190 did not undergo an entire wing re-design. What they did was increase the span (and aspect ratio) and increase the tip/chord ratio to improve induced drag characteristics... what they didn't do is maintain the same twist ratio on the extension - resulting in the unanticipated bending problems combined with torsion at the tip - thereby losing tip control in a High G turn -

causing all the trauma Soren and I have been duking it out on.. Great Catch!

He goes on to say that the wing remained unchanged in all the low-medium alt. models of the 190. (The metric wing loading numbers look to me like a mis-transcription... )I don't have anything else on the VIT 'problem' (I think the 'problem' was pilot-induced...)

KK89,

Lockheed's attempt to solve the compressibility problem by modifying the tail-plane only made it worse...In the compressibility zone, the lift generated by the tail-plane overcame that of the wing, and pulling back on the stick (if you had the strength) only increased the uploading on the tail...and steepened the dive. The compressibility-induced turbulence generated by the wing often tore off the tail. It was by installing small dive-flaps under the wing to increase lift (without increasing the AoA), that the problem was solved.


JL

If I could restate their problem (P-3. The transonic issues created a lot of turbulent flow between the engines/fuse wing body -----> leading to masking of the horizontal tail. That alone would tend to an 'pitch down' problem.

From my perspective the wing dive brakes created enough drag that the P-38 that it kept the airspeed in a controllable flight regime ~ .65-70 Mach in a dive and below the compressibility effect - in effect a 'governor' .

That's my story and I'm sticking to it - I have been wrong before.

[kool kitty89]

I think the dive flaps did both, obviously adding a lot of drag (35 degree deployment), but they also caused a pitch up (with the elevator neutral or inoperable). (the shock wave dispersion thing was totally wrong though)

Hence the reason the P-80 was fitted with BOTH a belly mounted air brake and wing root mounted dive recovery flaps.

[kool kitty89]

Wikipedia seems to have gotten it right:

After months of pushing NACA to provide Mach 0.75 wind tunnel speeds (and finally succeeding), the compressibility problem was revealed to be the center of lift moving back toward the tail when in high-speed airflow. The compressibility problem was solved by changing the geometry of the wing's underside when diving so as to keep lift within bounds of the top of the wing. In February 1943, quick-acting dive flaps were tried and proven by Lockheed test pilots. The dive flaps were installed outboard of the engine nacelles and in action they extended downward 35° in 1½ seconds. The flaps did not act as a speed brake, they affected the center of pressure distribution so that the wing would not lose its lift.

Also buffetting was entirely separate problem which, unlike the compressibility problems, was completely solved:

Buffeting was another early aerodynamic problem, difficult to sort out from compressibility as both were reported by test pilots as "tail shake". Buffeting came about from airflow disturbances ahead of the tail; the airplane would shake at high speed. Leading edge wing slots were tried as were combinations of filleting between the wing, cockpit and engine nacelles. Air tunnel test number 15 solved the buffeting completely and its fillet solution was fitted to every subsequent P-38 airframe. Fillet kits were sent out to every squadron flying Lightnings. The problem was traced to a 40% increase in air speed at the wing-fuselage junction where the chord/thickness ratio was highest. An airspeed of 500 mph (800 km/h) at 25,000 ft (7,600 m) could push airflow at the wing-fuselage junction close to the speed of sound. Filleting forever solved the buffeting problem for the P-38E and later models.

[buzzard]

drgondog,

I'm glad you like the 'jewel' It did seem that it might have some possible relevance to the point of contention, altho' with my rudimentary knowledge of aerodynamics, I wasn't sure. I should also mention that Tank said that the tail surfaces were also increased in proportion to the enlarged wing.

Tank's goal was to build a 'Dienstpferd'; a cavalry horse, not a racehorse, and he over-designed the '190 for future weight gain. Perhaps the basic wing design was considered strong enough to handle the extra area without subtantial modification of the basic structure, and this contributed to higher than expected deformation under the increased tip load. Understand that this is pure conjecture on my part. I know little beyond the basics of aero theory.

As to the P-38...As a big fan of the P-38, I've read numerous accounts of the compressibility problems, its causes, and the rationale behind the solution. They consistantly mirror what I and KK89 have posted.

JL