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Why did the P-47 suffer a nose-down trim change when in the compressibility regime?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.
I believe this is common on all aircraft when encountering compressibility. Also referred as "Mach Tuck"From Alfred Price, "World War II Fighter Conflict:"
Why did the P-47 suffer a nose-down trim change when in the compressibility regime?
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.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."
Aspect ratio is different from AR.?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)
Hi Timppa - thanks for highlighting the perils of not proofreading before hitting 'send/post'Aspect ratio is different from AR.?
Or lets say ARR HAR HAR
No - he was asking why the nose pitches down when the aircraft reaches compressibility speeds.
Why did the P-47 suffer a nose-down trim change when in the compressibility regime?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.
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.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?
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.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.
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.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.
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!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.
Counter intuitive, isn't it... Reminds me of the story about B-47:In the case of the Me262, pushing foreward on the controls, would break the "tuck" and allow the pilot to regain control.
"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.
I believe that is a manifestation of aileron reversal aileron reversal - SearchCounter 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. )
Thanks Bill - love your explanations and participation!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.
Dave hereJoe - 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
Very interesting, also at the windshield at 0.75 like the Spit.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.
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