Pitch-down in compressibility with the P-47

[elbmc1969]

From Alfred Price, "World War II Fighter Conflict:"

Two American fighters, the P-38 Lightning and the P-47 Thunderbolt, suffered severe compressibility effects at speeds below .7 Mach. In the case of the latter the problem was aggravated by the fact that a sudden reduction in engine power during the dive caused a nose-down trim change, which steepened the dive and increased the speed still further.

Why did the P-47 suffer a nose-down trim change when in the compressibility regime?

 

[FLYBOYJ]

From Alfred Price, "World War II Fighter Conflict:"


Why did the P-47 suffer a nose-down trim change when in the compressibility regime?

I believe this is common on all aircraft when encountering compressibility. Also referred as "Mach Tuck"

"Several classic problems characterize flight in the transonic region:

1) Increase in drag, decrease in lift, and pitch changes occur at the force divergence Mach number, which is approximately 1.05 Mach crit. The pitching moment is due to boundary layer separation in shock wave stall, which causes the center of pressure to shift. See Figures 17.8-17.9 (text pp. 286-287) and Figure 17.11 (text p. 288).

2) Pitch down (tuck under)--shock wave stall decreases downwash, which has been causing a pitch up moment on the horizontal tail. The result is a nose down pitch.

3) Buffet -- due to shock wave stall air disturbances hitting control surfaces and fuselage.

4) Control surface buzz -- also due to shock wave stall disturbances.

5) Diminished control effectiveness (not serious on fighter aircraft) -- due to shock wave stall turbulence, and the fact that control displacement cannot affect airflow forward of a shock wave."

http://pages.erau.edu/~rogers/as309/AS309NotesIII-B.doc

 

[GrauGeist]

In the case of the Me262, pushing foreward on the controls, would break the "tuck" and allow the pilot to regain control.

 

[drgondog]

I believe this is common on all aircraft when encountering compressibility. Also referred as "Mach Tuck"

"Several classic problems characterize flight in the transonic region:

1) Increase in drag, decrease in lift, and pitch changes occur at the force divergence Mach number, which is approximately 1.05 Mach crit. The pitching moment is due to boundary layer separation in shock wave stall, which causes the center of pressure to shift. See Figures 17.8-17.9 (text pp. 286-287) and Figure 17.11 (text p. 288).

2) Pitch down (tuck under)--shock wave stall decreases downwash, which has been causing a pitch up moment on the horizontal tail. The result is a nose down pitch.

3) Buffet -- due to shock wave stall air disturbances hitting control surfaces and fuselage.

4) Control surface buzz -- also due to shock wave stall disturbances.

5) Diminished control effectiveness (not serious on fighter aircraft) -- due to shock wave stall turbulence, and the fact that control displacement cannot affect airflow forward of a shock wave."

http://pages.erau.edu/~rogers/as309/AS309NotesIII-B.doc

Joe - I think he meant to also discuss the shift in aerdynamic Center as a major contributory cause for change Pitching Moment - leading to Tuck under as a factor.

The downwash decrease effect on Elevator authority, while important, is a function of aspect ratio (AR);
For the P-47, was much less (AR=5.6) and P-51 (AR=5.8) was much less than the P-38 (AR=8.2), but the P-38 Tuck issues were worse than P-47 and far worse than P-51.

The P-38 was a perfect storm for blanking out the elevator, as well as increasing negative pitching moment of the airfoil by moving the AC aft. IMO those factors were far more contributory than the reduction in lifting line and tip vortex strength associated with reducing lift in transonic shock movement - or less elevator authority..

The P-38 fuselage/wing filet helped the blanking effect and the addition of dive flaps solved the too rapid transition to movement of AC aft. The P-47 aslo solved the same issue with dive flaps. The P-51 never really had the issue - largely because the AC was further after due to the NAA/NACA 45-100 wing airfoil wit AC futher aft than P-38. The difference in airfoil selection was a.) transonic shock was delayed into higher M range and b.) less movement of AC.

David Lednicer could step in and improve on the points

 

[Timppa]

aspect ratio and AR, for the P-47 was much less (AR=5.6) and P-51 (AR=5.8) were much less than P-38 (AR=8.2)

Aspect ratio is different from AR.?
Or lets say ARR HAR HAR

 

[don4331]

Isn't E elbmc1969 asking why the P-47 had nose down trim accentuated when power is reduced?

Assuming E elbmc1969 are asking why P-47 accentuates, let me see if I can do a "Reader's Digest" version:

When you are flying at cruise, you trim the airplane, so lift equals gravity and you fly at constant altitude.​

If you then increase to fast cruise, your increase speed would increase lift, so you would begin to climb.​

​

To counter act this, one solution is for the designer to mount the engine at a slight downward angle e.g. Griffon Spitfire.​

The result is additional power for fast cruise rotates the air frame to a lower angle of attack (AoA), which cancels the additional lift and plane continues to fly at same altitude.​

This solution results in plane in rotating to higher AoA when throttle is closed, which helps a pilot pull out of compressibility situation​

​

In the P-47, Alexander Kartveli choose not to mount R-2800 at downward angle, but mounted the tail surface at such an angle that the additional speed results in additional lift which rotates plane to lower AoA. And plane continues to fly at same altitude.​

The Thunderbolt has the significant lower fuselage for duct work to the turbocharger/intercooler.​

The lower fuselage drag is minimal when the throttle is open as the air is being sucked into the turbocharger/flows out through the intercooler.​

But when you close the throttle, the air isn't begin pulled into the turbo, and as the turbo isn't heating the air, the vents for the intercooler exhaust close to avoid over cooling.​

The result is significant drag below the wing. The result is the plane tends to rotate downward. Not exactly what you want when fight compressibility.​

Hope I got that right.

 

[drgondog]

Aspect ratio is different from AR.?
Or lets say ARR HAR HAR

Hi Timppa - thanks for highlighting the perils of not proofreading before hitting 'send/post'

 

[FLYBOYJ]

Isn't E elbmc1969 asking why the P-47 had nose down trim accentuated when power is reduced?

No - he was asking why the nose pitches down when the aircraft reaches compressibility speeds.

From Alfred Price, "World War II Fighter Conflict:"

Two American fighters, the P-38 Lightning and the P-47 Thunderbolt, suffered severe compressibility effects at speeds below .7 Mach. In the case of the latter the problem was aggravated by the fact that a sudden reduction in engine power during the dive caused a nose-down trim change, which steepened the dive and increased the speed still further.

Why did the P-47 suffer a nose-down trim change when in the compressibility regime?

 

[don4331]

No - he was asking why the nose pitches down when the aircraft reaches compressibility speeds.

From Alfred Price, "World War II Fighter Conflict:"


Why did the P-47 suffer a nose-down trim change when in the compressibility regime?

OP statement was in response to the quote: In the case of the latter (the P-47) the problem was aggravated by the fact that a sudden reduction in engine power during the dive caused a nose-down trim change, which steepened the dive and increased the speed still further.

 

[FLYBOYJ]

OP statement was in response to the quote: In the case of the latter (the P-47) the problem was aggravated by the fact that a sudden reduction in engine power during the dive caused a nose-down trim change, which steepened the dive and increased the speed still further.

And if you read the link I posted the "sudden reduction in engine power during the dive" is one of the occurrences encountered when you reach compressibility.

 

[elbmc1969]

And if you read the link I posted the "sudden reduction in engine power during the dive" is one of the occurrences encountered when you reach compressibility.

No, Price is referring to the problem that if you reduced engine power in a P-47 (to control your dive speed), the nose would trim down. I thing it's important that he used the term "trim," not "tuck" or "pitch." He also specifically notes that he's talking about a peculiarity of the P-47, not a general effect on all aircraft, such as Mach tuck.

 

[FLYBOYJ]

No, Price is referring to the problem that if you reduced engine power in a P-47 (to control your dive speed), the nose would trim down. I thing it's important that he used the term "trim," not "tuck" or "pitch." He also specifically notes that he's talking about a peculiarity of the P-47, not a general effect on all aircraft, such as Mach tuck.

OK, but the same characteristics were found on other piston engine aircraft when they encountered compressibility. I don't see any recip maintaining power when they have a wall of compressed air in front of them, even in a dive!

 

[FLYBOYJ]

"a sudden reduction in engine power during the dive caused a nose-down trim change, which steepened the dive and increased the speed still further.

Which means the aircraft is pitching down and to correct this you would have to TRIM nose up.

I'll stand to be corrected - drgondog Bill M - what's your take?

 

[Dimlee]

In the case of the Me262, pushing foreward on the controls, would break the "tuck" and allow the pilot to regain control.

Counter intuitive, isn't it... Reminds me of the story about B-47:
"At 440 knots on the airspeed indicator, compression became so great that it prevented the ailerons (which bank the aircraft) from moving. At speeds above 440, instead of moving the wings' aileron controls up or down, right or left deflection of the control wheel actually warped the outboard section of the wing. Warping produced an opposite from expected effect. Above 440 KIAS, the pilot had to steer right to turn left."
(McGill, Earl. Jet Age Man (p. 30). Helion and Company. Kindle Edition. )

 

[thunderbird]

Nose pitch down is mostly caused by the differences in thickness ratios between the wing and the horizontal tail. The thicker wing would see transonic effects, including a reduction of lift curve slope. before the tail. But at some point. the shockwave on the horizontal tail positions itself close to the elevator, reducing (or in some cases. eliminating) pitch authority.

I wrote a program that calculated all of these effects on static longitudinal stability when I was working on a Masters. In BASIC, of all things, knowing full well that the empirical methods available (DATCOM, et. al.) provided ballpark answer, as at the time, it took flight testing to see how the aircraft really flew.

 

[drgondog]

"a sudden reduction in engine power during the dive caused a nose-down trim change, which steepened the dive and increased the speed still further.

Which means the aircraft is pitching down and to correct this you would have to TRIM nose up.

I'll stand to be corrected - drgondog Bill M - what's your take?A couple of comments. First, I know Price served in RAF, as a electronics officer but have no idea what his aero/flying training was to consider the causes of 'nose down/tuck' as a Trim phenomena in transonic region. I would have said 'Nose Down Pitch Change' requiring elevator Trim until the dive speed enters shock wave formation and movement.

First, for level flight - as you know, All changes in power necessitated a trim change for all high performance fighters. That is all about increase/same/decrease of lift at higher to lower Lift to offset Weight. CL is a function of angle of attack (pitch angle)... Also accompanied by rudder trim requirement. The P-47 and P-51 both tended to hunt during highspeed dives requiring rudder feed to dive in straight line. Dive tests were conducted at 1G.

Trim was only used when stick/elevator authority was lost in compressibility regime - but as discussed above, for the P-47 and P-38 the dominant reason for pitch down was a combination of loss of elevator authority AND a change in the aerodynamic center of the wing which caused the increased pitch down Moment coefficient. When elevator authority was lost during dives in a P-38 it was a combination of buffeting and loss of stable airflow over the elevator, but also movement of AC aft causing the pitch down. The theory and application of the dive flap was to delay the acceleration of airflow velocity over the wing into transonic shock wave and thereby increase resultant lift and delay movement of AC.

 

[pbehn]

Counter intuitive, isn't it... Reminds me of the story about B-47:
"At 440 knots on the airspeed indicator, compression became so great that it prevented the ailerons (which bank the aircraft) from moving. At speeds above 440, instead of moving the wings' aileron controls up or down, right or left deflection of the control wheel actually warped the outboard section of the wing. Warping produced an opposite from expected effect. Above 440 KIAS, the pilot had to steer right to turn left."
(McGill, Earl. Jet Age Man (p. 30). Helion and Company. Kindle Edition. )

I believe that is a manifestation of aileron reversal aileron reversal - Search

 

[FLYBOYJ]

First, for level flight - as you know, All changes in power necessitated a trim change for all high performance fighters. That is all about increase/same/decrease of lift at higher to lower Lift to offset Weight. CL is a function of angle of attack (pitch angle)... Also accompanied by rudder trim requirement. The P-47 and P-51 both tended to hunt during highspeed dives requiring rudder feed to dive in straight line. Dive tests were conducted at 1G.

Trim was only used when stick/elevator authority was lost in compressibility regime - but as discussed above, for the P-47 and P-38 the dominant reason for pitch down was a combination of loss of elevator authority AND a change in the aerodynamic center of the wing which caused the increased pitch down Moment coefficient. When elevator authority was lost during dives in a P-38 it was a combination of buffeting and loss of stable airflow over the elevator, but also movement of AC aft causing the pitch down. The theory and application of the dive flap was to delay the acceleration of airflow velocity over the wing into transonic shock wave and thereby increase resultant lift and delay movement of AC.

Thanks Bill - love your explanations and participation!

 

[Aeroweanie]

Joe - I think he meant to also discuss the shift in aerdynamic Center as a major contributory cause for change Pitching Moment - leading to Tuck under as a factor.

The downwash decrease effect on Elevator authority, while important is a function of aspect ratio (AR,) for the P-47 was much less (AR=5.6) and P-51 (AR=5.8) were much less than P-38 (AR=8.2), but the P-38 Tuck issues were worse than P-47 and far worse than P-51.

The P-38 was a perfect storm for blanking out the elevator, as well as increasing negative pitching moment of the airfoil by moving the AC aft. IMO those factors were far more contributory than the reduction in lifting line and tip vortex strength associated with reducing lift in transonic shock movement - or less elevator authority..

The P-38 fuselage/wing filet helped the blanking effect and the addition of dive flaps solved the too rapid transition to movement of AC aft. The P-47 aslo solved the same issue with dive flaps. The P-51 never really had the issue - largely because the AC was further after due to the NAA/NACA 45-100 wing airfoil wit AC futher aft than P-38. The difference in airfoil selection was a.) transonic shock was delayed into higher M range and b.) less movement of AC.

David Lednicer could step in and improve on the points

Dave here

So far, regarding this issue, I've only looked at the F8F in CFD. You can see in these images the shock on the wing (at the end of the blue bubble in the first image) and that the shock-induced separation immerses the horizontal tail, producing the pitch down (in the second image). We ran again with dive flaps, but didn't really see the effect experienced in flight test. I haven't run the P-51 yet (but will) and I just got P-38 geometry from Hugh. I'm really looking forward to analyzing the P-38 at these conditions.

 

[drgondog]

Dave here

So far, regarding this issue, I've only looked at the F8F in CFD. You can see in these images the shock on the wing (at the end of the blue bubble in the first image) and that the shock-induced separation immerses the horizontal tail, producing the pitch down (in the second image). We ran again with dive flaps, but didn't really see the effect experienced in flight test. I haven't run the P-51 yet (but will) and I just got P-38 geometry from Hugh. I'm really looking forward to analyzing the P-38 at these conditions.

View attachment 675977
View attachment 675978

Very interesting, also at the windshield at 0.75 like the Spit.

Dave, recall that when you model the P-38 that the YP-38 (IIRC) that was tested at Langley did not have wing/fuselage filets. Addng the wing filets during the test greatly reduced the flutter issues encountered in the full scale wind tunnel.

Are the shock separations at 30% C for the 23018?