F6F, FG-1, P-51, P-47 comparison

[Saxman]

Haven't heard of the elevator authority issue, but I have heard of P-51s swapping ends trying to maneuver with the aft tank full. I think I read or heard Yeager discussing it in an interview.

[Die Hard]

A select few pilots have flown both the P-51 and 109. All of them (that I have read/listened to) say that the two aircraft have similar stick forces; if anything they usually praise the 109 more for its better slow-medium speed control lightness and harmonization. The Merlin powered P-51 had a lot of teething problems, but, for some reason, they are largely overlooked.  It had problems with the canopy frosting over, with jamming guns, with the engine cooling system and the engine itself, and with shedding the tail.  In fact, the plane had so many problems initially that Col. Don Blakeslee, CO of the 4FG, called it "an experimental aircraft" and expressed doubts that it could be successful.

The P-38 had gone through its teething troubles the previous fall and with the introduction into ETO combat of the J model well-pleased its pilots. FGs getting the P-51 were unhappy and pilots grumbled that they would rather have the Lockheed.  It was not uncommon to have almost 30 percent of P-51 sorties aborted for mechanical reasons during the winter and spring of 1944 (typical abort rate for all causes for all USAAF aircraft was 8 percent). When the D model became available in quantity in the summer, cases of the aircraft losing its tail surfaces in flight began to be reported.  Flight restrictions were placed on the aircraft and the tail surfaces were beefed up. 

Wing failures were also reported due to control stick force reversal in high-speed dives.  The bobweight was added to the elevator control system to fix this problem.  But for the aircraft to be even marginally stable, the fuselage fuel tank had to be less than half full. The Mustang still had problems a year later when the 7AF began B-29 escort missions to Japan.  Incidences were reported of tail surface failures in dogfights. In one instance in April, 1945, a P-51D got into a dogfight with a Mitsubishi Raiden.  During the violent manoeuvring, the Mustang first shed its tail control surfaces and then its wings were torn off.  The pilot, 2Lt. James Beattie, did not survive. The Raiden apparently suffered no damage from the severe loads placed on it during the dogfight.

Should all of these vices be modelled in AH? No. Should some of them? Yes.

[Die Hard]

I'd like to see references that describe the exact conditions you mention here.

In addition to what others have provided so far I'll give you these quotes:


Robert C.Curtis, American P-51 pilot:
"My flight chased 12 109s south of Vienna. They climbed and we followed, unable to close on them. At 38,000 feet I fired a long burst at one of them from at least a 1000 yards, and saw some strikes. It rolled over and dived and I followed but soon reached compressibility with severe buffeting of the tail and loss of elevator control. I slowed my plane and regained control, but the 109 got away.
On two other occasions ME 109s got away from me because the P 51d could not stay with them in a high-speed dive. At 525-550 mph the plane would start to porpoise uncontrollably and had to be slowed to regain control. The P 51 was redlined at 505 mph, meaning that this speed should not be exceeded. But when chasing 109s or 190s in a dive from 25-26,000 it often was exceeded, if you wanted to keep up with those enemy planes. The P 51b, and c, could stay with those planes in a dive. The P 51d had a thicker wing and a bubble canopy which changed the airflow and brought on compressibility at lower speeds."



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."

[Stoney]

Nothing like some good old annecdotes to stoke the flames of an argument eh?

Mr. Curtis, with respect to his service and memory, is wrong about the wing thickness issue.

Surely there are some technical references?

[Die Hard]

Surely there are some technical references?

MiloMorai mentioned the P-51's handbook. If you're looking for an official P-51 vs. 109 "suicide dive" test, I don't think you'll find one. I said: "A little known nugget of information is that the 109's were actually known to be able to pull out of dives earlier than Mustangs." And I've now documented that. The technical evidence is that the 109's "flying-tail" elevator trim configuration allowed it to trim out of compressibility dives. The P-51's trim tabs did not, and trying to could destroy the Pony when the elevators regained effect at lower altitudes.

[MiloMorai]

Robert C.Curtis, American P-51 pilot:
"My flight chased 12 109s south of Vienna. They climbed and we followed, unable to close on them. At 38,000 feet I fired a long burst at one of them from at least a 1000 yards, and saw some strikes. It rolled over and dived and I followed but soon reached compressibility with severe buffeting of the tail and loss of elevator control. I slowed my plane and regained control, but the 109 got away.
On two other occasions ME 109s got away from me because the P 51d could not stay with them in a high-speed dive. At 525-550 mph the plane would start to porpoise uncontrollably and had to be slowed to regain control. The P 51 was redlined at 505 mph, meaning that this speed should not be exceeded. But when chasing 109s or 190s in a dive from 25-26,000 it often was exceeded, if you wanted to keep up with those enemy planes. The P 51b, and c, could stay with those planes in a dive. The P 51d had a thicker wing and a bubble canopy which changed the airflow and brought on compressibility at lower speeds."

The P-51D had a wider wing in the area of the leading edge extension at the root.

NAA/NACA 45-100 was used on models up to the H. If a thicker wing was used on the P-51D then the plan form of the wing would give a much greater wing area. That is the chord and wingspan would have to increase.

[Die Hard]

I'm not saying Robert C. Curtis is right with regard to the P-51D's wing thickness, only that I believe him when he says he couldn't catch 109's in fast dives.

[Cthulhu]

The P-51D had a wider wing in the area of the leading edge extension at the root.

Just for clarity:
IIRC the extension at the leading edge of the wing root you're talking about was not done for aerodynamic reasons, it was done to accomodate the wheels needed to handle the larger diameter brakes needed on the earlier model ponies. Subsequent improvements in brake performance allowed smaller diameter brakes => smaller diameter wheels and tires => no need for LE root extension on P-51H's. 

[MiloMorai]

I'm not saying Robert C. Curtis is right with regard to the P-51D's wing thickness, only that I believe him when he says he couldn't catch 109's in fast dives.

Oh, I know Die Hard but it is one of those myths that won't die.

Yes Cthulhu.

[Stoney]

The technical evidence is that the 109's "flying-tail" elevator trim configuration allowed it to trim out of compressibility dives. The P-51's trim tabs did not...

There's no doubt that the Mustang POH states to not use elevator trim to pull out of dives.  Obviously the 109 could do so.  What has not been established is any technical reference that supports your statement that a 109 could pull out of dive faster than a P-51 because of excess stick force in the P-51.  There's an anecdote from a pilot that on an occasion had a 109 outdive him.  My statement regarding Mr. Curtis's statement was not to impune his credibility as a Mustang pilot, but rather to call into question his aerodynamic expertise with respect to the reason why the 109 out dove him.

A trim tab may be a helpful assist to the 109 pilot, but may not even be necessary for the P-51.  I'm not a Pony fanboi, just trying to provoke some actual tactile evidence to back up your claims.  Especially when you say that a P-51 with a full fuselage tank should suffer a deep stall and spin when stalled.  From what evidence do you make that claim?  Everyone knows that the full fuselage tank was a destabilizing influence on the plane, but to say it creates conditions from which the Pony departs controlled flight is something else.  For example, if the Pony stalls but maintains coordinated flight, it cannot spin--no aircraft can.  Aft CG conditions do not make aircraft more prone to spin.  They can create instability in the pitch axis and make the aircraft difficult to trim.  They can make the aircraft more difficult to recover from spins, but in and of themselves create no spin bias whatsoever.

Stories are helpful for context and I enjoy a good tale as much as any other, but they should always be considered as offered--rememberances and not hard truths necessarily.

[Stoney]

NAA/NACA 45-100 was used on models up to the H. If a thicker wing was used on the P-51D then the plan form of the wing would give a much greater wing area. That is the chord and wingspan would have to increase.

The wing chord was greater at the root as you say, although the profile of the root fillet wasn't a NACA 45-100.  It created a negligible increase (1 or 2%?) in wing area.  The P-51/A/D all shared the same planform, whereas the B/C didn't have the root fillet.  However, with respect to conpressibility, the profile thickness for the wing did not change.  Therefore, the wing's compressibility properties would have remained the same.  And, I think you got your terminology mixed up about the wingspan increasing.   

[bozon]

...
Aft CG conditions do not make aircraft more prone to spin.  They can create instability in the pitch axis and make the aircraft difficult to trim.  They can make the aircraft more difficult to recover from spins, but in and of themselves create no spin bias whatsoever.
...

I am not sure this is true. When you move the CG, you change all the moment acting on the plane in all axes - the arms length is different. In the yaw, this would make the stabilizing arms (tail) shorter and others (asymmetrical wing drag, uncoordinated flight on the fusalage, prop etc...) longer or shorter. Remember our old mosquito FM with the aft CG? it would start swinging the tail whenever speeds got slow and the tail became less effective in damping them.

[Stoney]

"If the CG is too far aft, it will be too near the center of lift and the airplane will be unstable, and difficult to recover from a stall. [Figure 1-2] If the unstable airplane should ever enter a spin, the spin could become flat and recovery would be difficult or impossible."

Page 1-3 http://www.faa.gov/library/manuals/aircraft/media/FAA-H-8083-1A.pdf

"By maintaining directional control and not allowing the nose to yaw toward the low wing, before stall recovery is initiated, a spin will be averted. If the nose is allowed to yaw during the stall, the airplane will begin to slip in the direction of the lowered wing, and will enter a spin. An airplane must be stalled in order to enter a spin..."

Page 4-13 http://www.faa.gov/library/manuals/aircraft/airplane_handbook/media/faa-h-8083-3a-3of7.pdf

[Cthulhu]

"If the CG is too far aft, it will be too near the center of lift and the airplane will be unstable, and difficult to recover from a stall. [Figure 1-2] If the unstable airplane should ever enter a spin, the spin could become flat and recovery would be difficult or impossible."

Page 1-3 http://www.faa.gov/library/manuals/aircraft/media/FAA-H-8083-1A.pdf

"By maintaining directional control and not allowing the nose to yaw toward the low wing, before stall recovery is initiated, a spin will be averted. If the nose is allowed to yaw during the stall, the airplane will begin to slip in the direction of the lowered wing, and will enter a spin. An airplane must be stalled in order to enter a spin..."

Page 4-13 http://www.faa.gov/library/manuals/aircraft/airplane_handbook/media/faa-h-8083-3a-3of7.pdf

Yep  , what that guy said.

[Die Hard]

What has not been established is any technical reference that supports your statement that a 109 could pull out of dive faster than a P-51 because of excess stick force in the P-51. 

No, that has not been established, and I don't see why it needs to be since I've never made that claim. If you go back and read my post again you'll see that I think the 109 and P-51 should both have limited elevator control at high speed, not that the P-51 should be worse. The nugget about 109's pulling out earlier is only due to the flying-tail trim system, not to any superior elevator control on the 109.

The fact remains that the 109 and P-51 both suffered from heavy stick forces at high speed. Pilots of both planes attest to having to use both hands to control their aircraft at high speed. This comes as no surprise with regard to the 109; it is commonly known to have high stick forces, but it may come as a surprise to many that the P-51 suffered from the same ailment. Like I said earlier, for some reason the P-51's many vices seems to have been overlooked in popular postwar publications.



"The P-51 Mustang has always been the machine of dreams for pilots-gorgeous, fast and with a Merlin engine. Certainly anything that looks that good must fly most wonderfully, but pilots who have owned them often say that they fly like a Peterbilt with wings, that it takes both hands on the stick to pull through a loop and the most fun of flying one is taxiing out in front of your friends with the canopy open.

Now comes scientific proof. A 1991 study by John M. Ellis and Christopher A. Wheal published by the Society of Experimental Test Pilots compared four leading U.S. World War II fighters-the P-51D Mustang, P-47D Thunderbolt, F6F-5 Hellcat and FG-1D Corsair-concludes that the P-51 was the best of them, overall, but that it had such a high stick forces that it often required two hands and that it would snap and spin absolutely unpredictably, often so violently that it would jerk the stick from the pilot's hands.

Said the report, "[The P-51] scored high in performance, was well-suited to long-range escort missions and would do well intercepting non-maneuvering targets. However, its extraordinarily high stick forces, totally inadequate stall warning and vicious departures make it quite unsuited to the air combat maneuvering environment. It is a tribute to the adapability of the pilots who flew them that Mustangs scored so many kills against the opposition.""



"Like the P-51B, the P-51D was difficult to handle with a full fuselage tank, and it had high stick forces under combat maneuvers. Worst of all, under such maneuvers it gave no warning of stall and could fly abruptly and wildly out of control. It was also difficult to bail out of, since air pressure tended to trap the pilot in his seat, and Mustang pilots had to learn the Luftwaffe trick of simply turning the aircraft over and falling out. However, the aircraft's virtues were such that these vices were forgiveable, though not forgettable."



"The allison Mustangs were noted for their lack of need for trimming changes, the Merlin Mustangs found more directional trim changes with speed and power changes.  It is noted some pilots trimmed their P-51s "almost continuously" to wash out high stick or pedal forces in combat."