Aerodynamically speaking F4U, Mustang

[MikeGazdik]

Have always been curious, and since right now I have been on a Corsair kick, I know I'm on the perfect forum to ask.

When looking at the tail / fuselage design of the Corsair, it is much different than most of the contemporary fighters. The rear fuselage keeps its form shape from the cockpit all that way to the end of the tail cone. It doesn't blend into the vertical tail. The vertical tail appears to be "set" onto the structure.

Whereas if you look at the P-51, the fuselage blends into the shape of the vertical tail. And most every other fighter I can think of right now.


Which of these two designs is cleaner and what are any advantages or negatives of either design?

 

[Sweb]

I think this is more of an aesthetic than aerodynamic point of view.

The Mustang originally did not have the flowing lines you mention. Because of yaw stability losses when the bubble canopy was adopted the dorsal section forward of the fin was added. Same holds true for the Thunderbolt canopy transition. Their aesthetics were only a by-product of the need for better yaw stability. The Corsair never had the bubble canopy until much later with the F2G series. Before that it had the raised turtle-deck aft of the canopy which maintained its yaw stability. The F2G vertical was later increased but that was only because an additional rudder section was added to allow the plane the ability to perform a right-hand pattern go-around on a missed approach. Prior to that it could only perform a left hand go-around because of that huge prop's torque affect and its attendant angle of attack (P-Factor) swallowing huge chunks of air to the right of center-line yawing the machine to the left. If you view both aircraft from the frontal elevation you'll see they are both pretty aerodynamically clean, especially considering the Corsair's round engine vs the Mustang's in-line.

 

[Ivan1GFP]

From an aerodynamics standpoint, I believe the P-51 wins, especially in the Razorback A/B/C versions. They need a whole lot less power to go quite a bit faster. I also don't believe the big issue is how blended the tail surfaces are.

- Ivan.

 

[MikeGazdik]

I must clarify. I could only find the view I wanted on a bubble canopy Mustang. I would have liked to compare a B/C model but couldn't find a photo.

Be it Thunderbolt, Mustang, Spitfire, Messerschmitt, all of thier tails blend more into the fuselage. The fuselage appears to narrow more into the same width as the vertical tail.

The Corsair's "cone" shape of the fuselage is kept all the way to the end of the tail cone, and as a visual description, the tail looks to be set on top of this structure, and does not blend into the fuselage or vise versa.

I didn't really intend to compare plane to plane, what I am trying to find out is one design more aerodynamic, or cause less drag. And does one design "grab" the airflow better for increased yaw stability.

For example, what if the tail section of the Mustang was more like the Corsair. How would it change the speed and handling of the aircraft?

 

[Ivan1GFP]

Hello MikeGadzik,
Guess I misunderstood the point of the discussion. I believe the further aft you go on an airframe, the less important things get. (I am sure there is some aerodynamics fellow out there that will blast me for this comment though.) The shape of things very far aft is very dependent on the degree that the things up front have disturbed the air flow. Thus the horizontal tail on the Corsair looks to have a very high positive angle of incidence but is pretty near neutral in the airflow it encounters off the wing.

FWIW, My real answer to your question is that I have no opinion and not enough information to even make a judgement. If you really want to throw a curve into this discussion, look at the tail on the FW 190 series and how wide the fin is.

- Ivan.

 

[mudpuppy]

Certainly not an expert but I had assumed that the reason for the blending of the tail in the mustang was similar to the late P-47 bubbletops. I seem to recall reading once there were some stability issues in the late P-47's corrected by increasing the tail surface with that "blending" onto the rear fuselage.
So; is my middle-aged brain correct in this recall?

 

[renrich]

I believe the Mustang is a cleaner design but the question I have always mulled over was why the vertical stabiliser of the Corsair was set so far forward. The trailing edge of the horizontal stabilizer is well aft of the trailing edge of the VS. I know the huge VS and rudder of the Corsair was needed to offset the torque factor of the big prop and the powerful engine but it seems that if the VS had been set further back, the VS could have been more effective and perhaps smaller. I know there must be a reason for that configuration and I would like to know it.

 

[drgondog]

Hello MikeGadzik,
Guess I misunderstood the point of the discussion. I believe the further aft you go on an airframe, the less important things get. (I am sure there is some aerodynamics fellow out there that will blast me for this comment though.)

I'm one of 'those' and I agree with you.

The shape of things very far aft is very dependent on the degree that the things up front have disturbed the air flow. Thus the horizontal tail on the Corsair looks to have a very high positive angle of incidence but is pretty near neutral in the airflow it encounters off the wing.

FWIW, My real answer to your question is that I have no opinion and not enough information to even make a judgement. If you really want to throw a curve into this discussion, look at the tail on the FW 190 series and how wide the fin is.

- Ivan.

Tail Design per se is all about stability and control first, then fine tune for trim drag dive recovery characteristics, etc.

I suspect the Corsair had a lot of attention paid to low speed yaw control at high AoA, and while the 51 also had the big torque issues at low speed more of the rudder/horizontal stab design issues were in the higher speed envelopes.

In fact the original horizontal stab and elevator were designed around USAAF specs and found to be deficient when the Merlin was dropped in to replace the Allison. A new world opened up before those attendendant increased torque issues were solved.

IIRC there were more changes/fiddling around with eppenage features ranging from incidence to reverse rudder boost tabs, metal elevators, increases to vertical and horizontal stabilizer areas, beefed up vertical spar and caary through fitting and probably more I don't know about.

The 51 was significantly cleaner than any contemporary fighter with the Ta 152 and Fw 190D series coming close and only the 262 and the P-80 had a lower Cd0.

As to the B/C being cleaner than a D. That subject is still open. The much discussed Lednicer VSAERO studies on this forum stronly implies that the D canopy was superior to the B/C birdcage/turtledeck - but no malcolm hood analysis was performed. The study demonstrated the pile up of positive stagnation pressure distribution on the windscreen of the 51B/C but none on the greater sloped windscreen of the D and virtually no separation over the entire top surface of the teardrop canopy.

 

[renrich]

Bill, I was hoping you would chime in. Do you think the placement of the VS on the Corsair was that it gave better control at high AOA? Not being an engineer that kind of intuitively makes sense.

 

[drgondog]

I believe the Mustang is a cleaner design but the question I have always mulled over was why the vertical stabiliser of the Corsair was set so far forward. The trailing edge of the horizontal stabilizer is well aft of the trailing edge of the VS. I know the huge VS and rudder of the Corsair was needed to offset the torque factor of the big prop and the powerful engine but it seems that if the VS had been set further back, the VS could have been more effective and perhaps smaller. I know there must be a reason for that configuration and I would like to know it.

I can offer some perspective from a structural POV.

If you look at the aft fuselage of the F4U you see a lot going on in the final three feet. Below you have the tail gear and arresting gear stuff, the elevator is set further back than the VS/Rudder and the 'torque' tube capacity to take out the lateral loads of the VS is small because there is little cross sectional area to take out a lateral bending load..

As you look forward of the rudder to the area where presumably the VS spar intersects the fuselage, you see quite a bit more cross sectional area of the fuselage than available near the HS spar intersection.

That gives the airframe structures guy more options to decide how to take the lateral forces from the VS/Rudder, beam it into say a bulkhead via the spar and from there distribute the loads via shear panels and longerons.

Two noticable design features - big ass horizontal stabilizer well set back from VS - implying pitch stability challenges requiring a longer moment arm from the cg, as well as perhaps more area required for high g turn stick forces

-or low speed pitch control during carrier landing.

Carrier qual influenced so many trade offs for the F4U preliminary design that the 51 did not have to fool with.

 

[drgondog]

Bill, I was hoping you would chime in. Do you think the placement of the VS on the Corsair was that it gave better control at high AOA? Not being an engineer that kind of intuitively makes sense.

I said more above but I believe the VS/Very Large Rudder was enough for low speed yaw/roll control and suspect that was the limit design criteria.

The very large Rudder size speaks volumes for the compromise in NOT having the latitude to set a smaller rudder further back on that tapering fuselage tube.

 

[renrich]

Thanks Bill. I do know there were some issues with the early Corsairs fish tailing or tail wiggling right after touch down. I believe it was thought that the wing was blanking out the air flow over the empannage. Anyway, they were solved by extending the tail wheel which also helped pilot visibility over the nose. An interesting anecdote, perhaps, from Linnekin, "80 Knots to Mach Two." Linnekin was a graduate aero eng., test pilot and career Navy pilot. Flew everything from Stearmans to F4s and this is from his experience in Corsairs. "Before one of our training flights, I was number three in the runup area waiting for takeoff clearance. There was an 18 to 20 knot wind coming right down the runway. We were sitting in a row in echelon, cocked about 45 degrees out of the wind toward the runway. The wind component perpendicular to my airplane's big rudder was about 14 knots. I found that if I held the rudder neutral against the force, my leg fatigued and started to shake, more than in my description of coping with the Hellcat's right rudder forces. I took the path of least resistance. I let the rudder go to the end of it's travel, my right foot of course moving with the rudder pedal. That drove my right knee up so that it poked above the cockpit sill. It felt like it was right under my chin. I could find no position that was comfortable, and the wait for takeoff clearance seemed interminable. I wondered, if the force caused by 14 knots of wind against that barn door of a rudder was that hard to handle on the ground, what would it be like with substantially higher winds? Or their aerodynamic eqivalent-in a spin, for instance? In a spin, the rudder would be stalled. Full rudder applied against the spin would stop the auto rotation, but not by "flying" it to a stop. The rudder initially would be acting as a speed brake, creating drag to yaw the airplane against the spin. I wondered if the inability of a pilot, simply not strong enough, to get the rudder against the stops and hold it there explained some of the Corsair stall spin fatalities."

 

[drgondog]

Thanks Bill. I do know there were some issues with the early Corsairs fish tailing or tail wiggling right after touch down. I believe it was thought that the wing was blanking out the air flow over the empannage. Anyway, they were solved by extending the tail wheel which also helped pilot visibility over the nose. An interesting anecdote, perhaps, from Linnekin, "80 Knots to Mach Two." Linnekin was a graduate aero eng., test pilot and career Navy pilot. Flew everything from Stearmans to F4s and this is from his experience in Corsairs. "Before one of our training flights, I was number three in the runup area waiting for takeoff clearance. There was an 18 to 20 knot wind coming right down the runway. We were sitting in a row in echelon, cocked about 45 degrees out of the wind toward the runway. The wind component perpendicular to my airplane's big rudder was about 14 knots. I found that if I held the rudder neutral against the force, my leg fatigued and started to shake, more than in my description of coping with the Hellcat's right rudder forces. I took the path of least resistance. I let the rudder go to the end of it's travel, my right foot of course moving with the rudder pedal. That drove my right knee up so that it poked above the cockpit sill. It felt like it was right under my chin. I could find no position that was comfortable, and the wait for takeoff clearance seemed interminable. I wondered, if the force caused by 14 knots of wind against that barn door of a rudder was that hard to handle on the ground, what would it be like with sustantially higher winds? Or their aerodynamic eqivalent-in a spin, for instance? In a spin, the rudder would be stalled. Full rudder applied against the spin would stop the auto rotation, but not by "flying" it to a stop. The rudder initially would be acting as a speed brake, creating drag to yaw the airplane against the spin. I wondered if the inability of a pilot, simply not strong enough, to get the rudder against the stops and hold it there explained some of the Corsair stall spin fatalities."

that makes sense. If the VS/Rudder was further back it would have been smaller and require less rudder pressure in the cockpit.

 

[MikeGazdik]

Thanks to all, these were the replies I was hoping for. Could the placement of the VS, have been dictated by the "valleys" in the gull wing of the Corsair. Perhaps high angle of attack required the VS to be brought farther forward on the airframe?

Also, what affect would the horozontal tail have towards the vertical tail? In high AOA could the HS blank out or minimize the VS, or vise versa? ( if the VS was in a more traditional location above the HS)

 

[renrich]

Mike, those are good questions and perhaps very relevant. I read that one aero engineer stated that the inverted gull wing created aerodynamic and production problems such that, if Vought had known that large scale production was in the offing, they would have changed the design. May or may not have been true.

 

[drgondog]

Thanks to all, these were the replies I was hoping for. Could the placement of the VS, have been dictated by the "valleys" in the gull wing of the Corsair. Perhaps high angle of attack required the VS to be brought farther forward on the airframe?

Also, what affect would the horozontal tail have towards the vertical tail? In high AOA could the HS blank out or minimize the VS, or vise versa? ( if the VS was in a more traditional location above the HS)

The VS should not be affected by high AoA flight nor the Gull wing. Nor should the VS be affected in any way by the HS.

The HS will not be affected by, nor will it affect, the VS. It is downstream of the VS in any case.

The only conceivable scenario for any form of interaction between VS and HS would be in a high g, high control surface deflection, (hard rudder, stick back) turn with the wing in slight asymmetric loading (i.e one aileron up, other down) in which the flow over the HS is also slightly asymmetric due to flow past VS with large rudder deflected.

I believe the placement of the VS was all about airframe structures requirements due to the tapering fuselage and tail wheel and arresting gear - a lot of stuff aft of what looks to be the bulkhead area supporting the VS. I don't have a cutaway but it looks to me that it all comes together structurally at that point (look at the line of the VS/Rusdder intersecting just forward of the front of the tail wheel door. I'll bet you there is a substantial bulkhead structure from which the tail gear, and the VS spar come together in same location.

 

[Timppa]

The VS should not be affected by high AoA flight nor the Gull wing. Nor should the VS be affected in any way by the HS.

One factor affecting the spin recovery characteristics is the shielding of the rudder by the tailplane. The rough rule is that at least 1/3 of the rudder area should be outside the wake of the stabilizer. The F4U seems to be excellent in this regard. In some aerobatics planes the stabilizer is way forward of the rudder for this reason. In F4U it is well behind.

 

[renrich]

The Corsair, I believe, was not to be intentionally spun. In one test flight Boone Guyton got it into a spin and, if memory serves did 13 revolutions before being reminded by radio that a chute was available in the tail section to get him out of the spin.

 

[MikeGazdik]

I don't have a cutaway but it looks to me that it all comes together structurally at that point (look at the line of the VS/Rusdder intersecting just forward of the front of the tail wheel door. I'll bet you there is a substantial bulkhead structure from which the tail gear, and the VS spar come together in same location.

I believe you are right. I do have a nice cutaway and have been looking at it during this conversation. There is definately a bulkhead at the forward section of the tail gear, even with the front of the tail gear doors. It also appears to have another substantial rib at the rudder post for the VS. This is just ahead of the leading edge of the HS. But in my diagram this area is only partially drawn, so the artist could show the tail gear retracted and down. Also, FWIW, the Corsair seems to have alot if stringers, more than even the Hellcat. I have cutaways of both. The Hellcat has 14 stringers / bulkheads between the rudderpost going forward to the pilots seat. The Corsair has 21. My book also shows the Mustang with about 14, and the Thunderbolt with 13, but those are probabley made of cast iron.

 

[drgondog]

I believe you are right. I do have a nice cutaway and have been looking at it during this conversation. There is definately a bulkhead at the forward section of the tail gear, even with the front of the tail gear doors. It also appears to have another substantial rib at the rudder post for the VS. This is just ahead of the leading edge of the HS. But in my diagram this area is only partially drawn, so the artist could show the tail gear retracted and down. Also, FWIW, the Corsair seems to have alot if stringers, more than even the Hellcat. I have cutaways of both. The Hellcat has 14 stringers / bulkheads between the rudderpost going forward to the pilots seat. The Corsair has 21. My book also shows the Mustang with about 14, and the Thunderbolt with 13, but those are probabley made of cast iron.

The deeper the fuselage - the deeper (and all things equal) the stiffer the 'beam' from which to transfer bending and torsion moments.

The stringer/longerons are there to take compression and tension loads, while the skin(and fasterners) transmit shear and re-distribute the loads from one longeron to the next.

The bulkheads stabilize the longerons and provide first re-distribution of torsion loads (twist about CL)