# Defining and explaining "Corner Speed"



## drgondog (Dec 21, 2010)

We have recently experienced Troll invasion - one of which is the worst dabblers in the less than arcane arts known as "Aircraft Performance", with subchapters in throttle management, "prop disk loads changing the wingload in non-linear ways", etc.

All interesting topics and equally worth discussing - but this thread is about the huge dissemination of blivet on the subject of CORNER SPEED. 

The first diagram is the Corner Speed Diagram prepared by North American Aviation to insert into the P-51D Pilots Handbook. Note carefully the message at the bottom which states that the Corner Speed calculation is for 8,000 pound Gross Weight and that the G load allowables for each condition must be factored by GWactual/64000 pounds.

Note the 'accelerated stall' area sets one boundary of the lower speed threshold.. so a 3G sustained turn at 160mph at 8000 pounds on the deck is on the threshold for departure but not structural limit and not corner speed. Note that at 6G the sustained turn at ~230mph is at stall but NOT Corner Speed. [email protected] 8G at ~ 260 mph at 8000 pounds at sea level the airframe has reached CORNER SPEED.


More later - but Note for the second (conceptual) diagram that the Corner Speed at ~ 195mph is at the 6 "G" load for Design Limit and that Distortion (non elastic deformation) begins and continues until Ultimate Load Factor (9. =50%) or 13.5G is reached and complete catastrophic failure is predicted. BTW I have no idea why the left value indicators for 6G load has a +9 on the far right

I suspect they meant to either have 6G load factor with a +6 on the right and a +6+50% (3.) on upper right to be consistent.


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## FLYBOYJ (Dec 21, 2010)

From one of my sources - "The lowest air speed at which a fighter can pull structural or aerodynamic limiting" Which tells me either I'm going to stall or break the plane (or both) when I hit this number. Spot on Bill!


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## timshatz (Dec 21, 2010)

So, while the G forces are shoving you head down into your arse, you have to worry about the airplane either falling apart or stalling out? 

Forget the muggers, that is one corner I'll avoid at all costs.


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## davparlr (Dec 21, 2010)

I noticed that the two graphs are slightly different. The one on from the flight manual shows indicated airspeed and the other one shows equivalent airspeed. I would guess the one from the flight manual would have some altitude definition although I don't see one.


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## Matt308 (Dec 21, 2010)

So it defines the horizontal repercussions (distortion/breakup), but what about the vertical post 505mph? Mach tuck? I was under the impression that many ww2 fighter airplanes easily exceeded 550-575mph before recovery was not possible (again altitude dependent).


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## bobbysocks (Dec 21, 2010)

i would worry more about breaking apart than stalling ( unless you dont have enough alt to recover). with stalls you are usually trying to make the plane fly where it doesnt naturally want to....in a steep turn...pulling back too far on take off, etc. usually get a warning ( a shutter if not a blaring horn) and most of the time can recover by simply dropping the nose, slacking off on the stick, adding power, etc. to where the ac rights itself. a lot of the situations where you reach the point of structural failure are more towards the opposite. you are trying to force the ac in a direction different than it naturally wants to go at that point in time. generally it has gotten away from you and is now being flown more by the by the laws of physics than you. you compound the situation by adding higher G forces trying to recover. i.e. you are at 7000 feet and in a steep screaming in a dive going 500+ mph. many a plane has lost its wings in type of senario...


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## Glider (Dec 21, 2010)

A first class description, many thanks


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## cocky pilot (Dec 21, 2010)

have been reading these posts (on the other thread) with amusement.

On one side when a post mentions "thrust in a curve" the poster has moved above Isaac Newton in laws of motion.

On the other side any talk about an aircraft in a curve is complete theory. The weight of a plane is not a constant, unless the oil and fuel tanks of the plane are dirctly on its centre of gravity and centre of lift then its performance is not a constant. The lift of a planes wings are a general value, a plane in a turn has two wings doing two speeds form one extremity to the other and the airflow is not completely straight.


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## drgondog (Dec 21, 2010)

cocky pilot said:


> have been reading these posts (on the other thread) with amusement.
> 
> On the other side any talk about an aircraft in a curve is complete theory. The weight of a plane is not a constant, unless the oil and fuel tanks of the plane are dirctly on its centre of gravity and centre of lift then its performance is not a constant. The lift of a planes wings are a general value, a plane in a turn has two wings doing two speeds form one extremity to the other and the airflow is not completely straight.



Very True (although d/dt (m*V*) will take care of the change in mass with respect to time with greater precision than the change in Velocity for all the other reasons mentioned - 

I think I mentioned in another thread that 'perfect circular' motion in an aircraft is an abstraction, due to the effects of turbulence, indicial gusts, unsteady control response to random inputs, total inability of any pilot to maintain a perfect combination of rudder and elevator and aileron responses to the aerodynamic loads, the areoelastic response to the loads changing local angles of attack, the increasing vortex drag due to the high angle of attack required for a banked mid to high G turn, etc, etc , etc.

On the other hand if you wish to assume the same perfect fluid and perfect pilot to deal with the airplane in its own frame of reference, the equations are reasonable when you strip small percent contributions like incremental vortex drag of the fuselage/wing root, epennage in the prop stream tube, the trim drag, etc, etc as well as assume oswald efficiencies, prop efficiencies, tip losses, etc - then the equations derived from F=ma work pretty well

The same may be said regarding structrural analysis of complex materials, complex geometries, and wide range of 'steady', transient and reversible loads.. but we did the best we could. 

So, the combination yields guidelines like "V-n" charts presented above

Your improvements to the foundation equations contained in most "introductions to flight' would be welcome. Pitch in. or not. as you choose... but if you fly, ignore the "V-n" chart which uses those equations and methods at your peril.


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## FLYBOYJ (Dec 21, 2010)

cocky pilot said:


> have been reading these posts (on the other thread) with amusement.
> 
> On one side when a post mentions "thrust in a curve" the poster has moved above Isaac Newton in laws of motion.
> 
> On the other side any talk about an aircraft in a curve is complete theory. The weight of a plane is not a constant, unless the oil and fuel tanks of the plane are dirctly on its centre of gravity and centre of lift then its performance is not a constant. The lift of a planes wings are a general value, a plane in a turn has two wings doing two speeds form one extremity to the other and the airflow is not completely straight.



While you have a point - how else do you show this pictorially so it could be placed in a flight manual? Let alone to be flown by a 21 year old 2Lt?


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## drgondog (Dec 21, 2010)

Matt308 said:


> So it defines the horizontal repercussions (distortion/breakup), but what about the vertical post 505mph? Mach tuck? I was under the impression that many ww2 fighter airplanes easily exceeded 550-575mph before recovery was not possible (again altitude dependent).



First comment on last sentence.. there were many instances of IAS in that range, few even remotely close for TAS. Pitot tube type Instruments didn't work worth a damn in compressibility range 

Matt - the chart is independent of axis of reference per se. Having said that, the 'stall region' defined as 'accelerated stall' is usually associated with a lateral manuever such as a banked turn - but you can stall also in a dive pullout and mush your butt into the ground.

The curve is calculated with the same Vmax equations I presented to Gaston.

The 505mph is the plackard 'do not exceed' velocity and that is all about the Q loads on the airframe.. i.e if you could strap a J-79 on this 8000 pound Mustang - it would go fast but not far.

The Pony started the drag rise around .65 M, the placard was for .75 and the highest achieved in dive was .83-85M depending on the belief system around the instruments. What happens in that regime is that the combination of parasite drag and compressibility drag go way up, very rapidly. At some point between .8 and .9M at 1G things that once were attached are no longer attached to the airframe due to the stresses caused by extremely high dynamic pressure loads.

There was a 1G 'mach tuck' per se in the 51 which resulted in heavy stick forces, but much less than P-47/P-38/F4U, and never resulted in the strong negative pitching moment near Mcr that other WWII fighters encountered.

Theoretically the laminar flow airfoil had serious advantages to delayed drag rise.


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## drgondog (Dec 21, 2010)

FLYBOYJ said:


> From one of my sources - "The lowest air speed at which a fighter can pull structural or aerodynamic limiting" Which tells me either I'm going to stall or break the plane (or both) when I hit this number. Spot on Bill!



Thank you Joe - and we are reminded that the 'limit line' is smooth, non vertical indicial gusting, air...this is not the line you fly in a glider over the Rockies or searching for a thermal around a Texas Killer.

In other words, if we ignore this, we are smited mightily and seek comfort in silk - if available..


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## drgondog (Dec 21, 2010)

davparlr said:


> I noticed that the two graphs are slightly different. The one on from the flight manual shows indicated airspeed and the other one shows equivalent airspeed. I would guess the one from the flight manual would have some altitude definition although I don't see one.



Dave - I pulled from two different sources - one from TF-51D Manual and the other 'generic' - but I liked the generic simply because it helped explain what happens when you exceed Limit load and head toward Ultimate (and kiss your ass goodye) load.

There was no point I was making in showing one with IAS in mph and the other in EAS. I reality the 'IAS" as near as I can tell is TAS in reality for SL.. when i did the Vmax calc for a [email protected] pounds at SL I got ~250mph at 8g for the corner speed. I suspect the difference is what NAA used for CLmax versus mine based on level flight stall while reducing velocity at 1Kt/sec. If NAA had less than 1.72 that would explain the slight difference between their graph and my calc

EDIT - I tried CLmax=1.6 and it yielded 259 mph at 8G @8000 pounds on the deck... which seems very close to the manual chart..


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## davparlr (Dec 23, 2010)

drgondog said:


> Dave - I pulled from two different sources - one from TF-51D Manual and the other 'generic' - but I liked the generic simply because it helped explain what happens when you exceed Limit load and head toward Ultimate (and kiss your ass goodye) load.
> 
> There was no point I was making in showing one with IAS in mph and the other in EAS. I reality the 'IAS" as near as I can tell is TAS in reality for SL.. when i did the Vmax calc for a [email protected] pounds at SL I got ~250mph at 8g for the corner speed. I suspect the difference is what NAA used for CLmax versus mine based on level flight stall while reducing velocity at 1Kt/sec. If NAA had less than 1.72 that would explain the slight difference between their graph and my calc
> 
> EDIT - I tried CLmax=1.6 and it yielded 259 mph at 8G @8000 pounds on the deck... which seems very close to the manual chart..



I didn't think you were trying to make some point. I was curious and just pondering the difference between IAS and EAS in the chart. No pilot uses EAS for anything. It is primarily a design tool to estimate vehicle performance. Calibrated airspeed, equivalent airspeed, and true airspeed, and, if no wind, ground speed, are all equal at SL standard day. Indicated airspeed will vary slightly due to installation and position error.


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## drgondog (Dec 23, 2010)

davparlr said:


> I didn't think you were trying to make some point. I was curious and just pondering the difference between IAS and EAS in the chart. No pilot uses EAS for anything. It is primarily a design tool to estimate vehicle performance. Calibrated airspeed, equivalent airspeed, and true airspeed, and, if no wind, ground speed, are all equal at SL standard day. Indicated airspeed will vary slightly due to installation and position error.



All true - the only aspect of it that I cared about was the nice representation of the bad things that happened when moving from limit to ultimate load - that would have been the point.

As an aside the EAS=CAS*Sqrt{rho/[email protected]} to reduce all calibrated airspeeds at different altitudes to a standard of values that people (engineers) can use and know exactly what everyone means.


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## billswagger (Dec 23, 2010)

> ww2 fighter airplanes easily exceeded 550-575mph before recovery was not possible (again altitude dependent)



I would add, how many aircraft could exceed those speeds if not from a steep dive from high altitude?
a shallow dive?
In most test dive charts, it's revealed that most fighters had to be dove from 35-40K feet and 550-575 TAS was probably a more realistic top speed than the presumed 600+ mph touted in previously published assessments. 
maybe the late Spitfire from used in the mid 50s, but nothing from WW2. 


> There was a 1G 'mach tuck' per se in the 51 which resulted in heavy stick forces, but much less than P-47/P-38/F4U, and never resulted in the strong negative pitching moment near Mcr that other WWII fighters encountered.


The stick force of the P-51 was actually heavier according to some tests between the FG-1 (8lbs), P-47(12lbs) and P-51(20lbs). 
That has more to do with the context of the stick force described.
A P-47 in compressibility would obviously have more issues, though other than descriptions of the stick having the feeling of being set in cement, their were also the tests where it was not an issue of force, but an issue of response, because the pilot could pull full deflection but there was a disruption of the airflow over the elevators.
It has been argued that the heavy stick force was worked into the design of the P-51 to keep pilots from over stressing the aircraft, and in some ways the heavier stick force allowed pilots to better gauge aircraft limitations. 
The P-51 was initially quite flawed when it came to mach tuck vs control force, where the P-47 was generally considered better suited for similar types of dives, being more structurally sound for the uncoordinated effects of mach tuck. It was eventually learned that a primary cause for the tuck was the pilots tendency to throttle back in the dive, increasing the drag therefore increasing the tuck. 

They did manage to better engineer the later P-51Ds to manage the tuck and trim effects encountered in a dive. 
In the dive, there is a nose up tendency, which the pilot would normally counter with trim. If the pilot encountered tuck he would let out trim to hold the angle of dive, but in the absence of tuck the nose would want to pop back up suddenly. The retooling of the elevator throw involved easing the stick force so the pilot was less reliant on trim and could simply use a moderate amount of stick forward to hold the dive.


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## drgondog (Dec 23, 2010)

billswagger said:


> I would add, how many aircraft could exceed those speeds if not from a steep dive from high altitude?
> a shallow dive?
> In most test dive charts, it's revealed that most fighters had to be dove from 35-40K feet and 550-575 TAS was probably a more realistic top speed than the presumed 600+ mph touted in previously published assessments.
> maybe the late Spitfire from used in the mid 50s, but nothing from WW2.
> ...



If the Mustang had a 'tuck' issue it certainly did not experience a 'nose up' issue. Nose up (pitch forces) would normally be a result of CM changes due to a near complete loss of airflow over the wing. In that mode the horizontal stab would have a negative lift component causing the nose to pitch up - but that wasn't an issue in the 51 (to my knowledge).

Near Mcrit the issue was a rearward movement of the aerodynamic center aft, causing a nose down moment coefficient CM to dominate (i'e. P-47 and P-38).

The Mustang Manual had specific warnings to NOT use trim to recover form a dive


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## billswagger (Dec 24, 2010)

> That would be incorrect relative to the 51. What sources led you to conclude that?


In reading up on compressibility dives of the P-51 and P-47.
It was the thickness of the wing and structural strength, not so much the drag profile. You are also aware of that Mustangs had varying wing thicknesses and that also had much to do with the structural integrity.

The behavior is also outlined in manuals for both aircraft, where in the late P-51 manual it describes the adjustments made for better high speed control under critical mach conditions. Now that you mention it, it may have been the changed incidence to allow for better stick force during high mach speeds. I dont remember the specific parts only that it is specified in the later D manual. 



> Nose up (pitch forces) would normally be a result of CM changes due to a near complete loss of airflow over the wing


In the context of my description, mach tuck requires the pilot to counter the increased nosed down force of the aircraft. He does this with stick, or with trim. In mach tuck, he has to pull up to hold the angle of dive. 
You can imagine that shifting tendencies of the aircraft can become quite cumbersome. 

Again there is a better description in the late mustang manual, that describes both the improvement and behavior of the aircraft. 


Bill


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## drgondog (Dec 24, 2010)

Bill - do you know what causes a "mach tuck"? It is caused when the shock wave forms and moves aft, changing the aerodynamic center of the wing. 

Most conventional wings with an aft tail have a wing airfoil with negative value for Moment Coefficient (counterclockwise) about the aerodynamic center. This is normally balanced by a conventional horizontal stabilizer/elevator combination which produces lift in down direction to offset the CMac. When that wing lift is disrupted, it contributes to the pitch down moment about the 1/4 point and tends to overcome the stab/elevator contribution until significant additional tail down force is created by up elevator or trim

Additionally when a shock wave initiates, the flow separates earlier and the resulting reduced lift also reduces positive moment about the Cg, as well as reduces the relative angle of attack of freestream hitting the stab/elevator. This reduces negative lift at the tail and further pitches the nose down. In fact it is the latter condition which occurs first and was first discovered on the P-38 (at least for US).

This phenomena is referred to as a Mach Tuck. Different airfoils have varying magnitudes of CMac, all generally negative, and generally independent of AoA for normal flight profiles.

What made the Mustang modified laminar flow wing 'different' is that it's max thickness is around 40% chord where the Jug and P-38 (and Me-109 and Fw 190) had max T near the 1/4 chord point. 

The Aerodynamic center for all of these airfoils (and the Mustang)were near 1/4 point.

As the flow over the top of the wing reaches M=1, it happens first at max T of the airfoil. When that happens to a conventional 1/4 Chord maximum T, it also happens to be close to the a/c... for a Mustang however, M=1 around 40% chord and the onset, despite the fact that the 51 had a medium 'thick' airfoil, was delayed to a higher Mcrit than would exist for a conventional airfoil of the day.

Another interesting feature of the shape of the Laminar flow designs is that they very much resemble the supercrit airfoils which are designed with max thickness even closer to the trailing edge.

I'll dig up the NACA report that outlines the delay to critical separation and movement of shockwave that was inherent in the Mustang's modified laminar flow wing

As to 'varying wing thickness' - true but for both the Mustang and Jug (and virtually all WWII and beyond fighters) ALL varied 'wing thickness' spanwise but maintained the ratio of thickness to chord spanwise by virtue of wing taper out to wing tip area. Most wings were designed with a 'thinner t/c" at the tip to try to reduce induced drag/tip losses.


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## billswagger (Dec 24, 2010)

drgondog said:


> Bill - do you know what causes a "mach tuck"? It is caused when the shock wave forms and moves aft, changing the aerodynamic center of the wing.



exactly.
I don't see how anyone could be a fan of aviation, particularly WW2 fighters, and not have an understanding of mach tuck.
There are also some other contributing factors to the actual tuck, which may not actually be related to compressibility but instead the actual drag profile of the aircraft and how the air screw distributes air over the wing. 
If the pilot throttles back the slowing airscrew and this causes more drag. It was found that pilots could recover from tuck by increasing throttle in the dive, where decreases would cause a more severe tuck. Again, this is also outlined in the manuals, particularly in later manuals where they may have had a better understanding of emergency procedures and compressibility. 
In reviewing a Wright Field test for the P-47 dive flaps, the amount of tuck was from 65 to 67 degrees,(4 seconds of dive) a mere 2 degrees. It may have steepened more if the pilot had not deployed dive flaps, but also test pilots may have also had a better understanding in how to combat mach tuck or to minimize its effects. 
Pilots were advised to start dives with medium amounts of throttle so that increases could be made in the dive. 



> As to 'varying wing thickness' - true but for both the Mustang and Jug (and virtually all WWII and beyond fighters) ALL varied 'wing thickness' spanwise but maintained the ratio of thickness to chord spanwise by virtue of wing taper out to wing tip area.



I know the Mustang had specific variants with thinner wings and lower load limits. Its apples to oranges when comparing them to the wing thickness of the P-47, i only mention it because perhaps my reading was not in the context of the P-51D which had higher load limits and wing thickness than previous variants. So saying the P-47 was better suited to handle mach tuck, may not have included the P-51D. 
It doesn't take long to find several examples of P-51 pilots diving from 20k ft in vertical dives while chasing or eluding enemy aircraft. It was certainly a very capable fighter in that capacity.


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## drgondog (Dec 24, 2010)

billswagger said:


> exactly.
> I don't see how anyone could be a fan of aviation, particularly WW2 fighters, and not have an understanding of mach tuck.
> 
> *Knowing that 'mach tuck' results in a pitch down is different from understanding why it happens.*
> ...



The Jug with the dive flap (-30? and above) would slow down faster than the 51 which could be an advantage when chasing a 109.

In various dive tests to compare the two fighters, they were extremely close on maxximum sustained dives - each around .85 which is ~ .8 to .9 above the placard for each in their respective manuals.

The Jug gets the edge but its not enough to make a difference in a chase.


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## renrich (Dec 25, 2010)

I am not an engineer like youall but one thing that I don't hear mentioned in this most interesting dialogue you have posted about mach tuck is that I understand that because the speed of sound varies according to air temperature, and since air temp rises as the airplane gets lower, what might be mach .80 at 25000 feet is something less than that at 20000 feet and so on. Therefore it seems to me that if mach tuck occurs, for instance at mach .80, as soon as the airplane gets lower, where the air is warmer, even if his TAS is the same, his mach number is lower and the mach tuck goes away. 

An example of this is in 
"The Great Book of World War Two Airplanes". During the Korean War, Ensign Dan Bryla, launched from USS Valley Forge, was detailed to bomb a hydro electric plant in his Corsair. He nosed over into a steep dive from 17000 feet. Soon he noticed intense buffeting and vibration which he interpreted as the onset of compressibility. He jettisoned his bombs and brought back the throttle to reduce speed while pulling back on the stick. The Corsair rolled inverted and at the speed he was traveling the ailerons could not be deflected enough to roll back upright. Still in a dive and inverted Bryla tried to pull through a half loop but this steepened the dive, rendering the elevators almost as useless as the ailerons. Finally, at about 4000 feet, with both hands pulling back on the stick and one foot on the left rudder pedal, he managed to bring the AC through a partial loop and into level flight and subsequently landed on his carrier. This account is from a Navy report published in 1953 and the mission must have been over North Korea in the winter with very cold air temps.

Incidently, Bryla was in a great deal of discomfort flying back to the carrier. It was found that he had broken his left hip and strained back and shoulder muscles in his attempts to pull out of the dive. The Corsair returned to duty the next day.


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## drgondog (Dec 25, 2010)

renrich said:


> I am not an engineer like youall but one thing that I don't hear mentioned in this most interesting dialogue you have posted about mach tuck is that I understand that because the speed of sound varies according to air temperature, and since air temp rises as the airplane gets lower, what might be mach .80 at 25000 feet is something less than that at 20000 feet and so on. Therefore it seems to me that if mach tuck occurs, for instance at mach .80, as soon as the airplane gets lower, where the air is warmer, even if his TAS is the same, his mach number is lower and the mach tuck goes away.
> 
> *Ren - true, during the course of the dive the air is denser and as a result drag is higher, which causes a steady reduction of velocity to below Mcrit (for WWII aircraft).. at the point where the airflow over the top of the wing goes below Mcrit and the wake turbulence reduces, the pressure distribution moves toward 'high speed' normal.
> 
> ...



Lucky guy. The 'do not' exceed velocity for an F4U in a dive was much lower than for a Jug or Mustang.


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## renrich (Dec 25, 2010)

Bill, unless I am mistaken though, since the air gets progressively warmer at lower altitudes then the effects of compressibility go away if the same speed is maintained since the speed of sound varies only as the air temp.

An exaggerated example might be: a supersonic airplane can reach Mach one at 40000 feet AGL where the air temperature is minus 50 degrees F if he is making 500 mph TAS. That airplane at 500 mph is well into compressibility. 

Another example is that I have read that the F80 at very high altitudes, around 40000 feet, had a very small envelope to fly in, because, since the speed of sound was so low at that altitude, if flown too fast it would feel the effects of compressibility. Yet, if flown to slow, it's wings could not maintain lift in that thin air and it would stall out. It did not have that problem lower down because the speed of sound was a lot higher.

That same airplane doing 500 mph TAS at 10000 feet over Texas in the summer where the air temperature is around 75 degrees F is not close to Mach one and therefore feels no effects of compressibility. I have seen unlimited class warbirds at Sherman, Texas( why would we name a town for that SOB?) doing around 500 MPH in a shallow dive beginning the pylon races and they obviously were not into compressibility.

What I am trying to say, (poorly) is that as an airplane into compressibility problems at high altitudes in a dive will automatically gradually get out of those problems even if he maintains his speed at lower altitudes because the air is warmer and the speed of sound is higher.

The Corsair, according to Dean was limited to Mach .72 at higher altitudes and at 10000 feet was limited to .70 Mach or less if a high G pullout was attempted.


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## billswagger (Dec 26, 2010)

> The Mustang from XP-51 through P-51A and B/C/D/K all had the same airfoil with the same thickness



In that case, then the P-47 was considered to be more structurally sound for compressibility dives than the Mustang, keeping in mind that the actual speed attained by the Mustang would be slightly higher before it entered compressibility. 
They did make thinner winged mustangs with lower load limits but i don't have information on where they served. It was the thought of British designers to make a design that did not meet the requirements of the US load limits to see how the sleeker design benefited in speed. 
That may have been where i got that idea from. 



> since the air gets progressively warmer at lower altitudes then the effects of compressibility go away if the same speed is maintained since the speed of sound varies only as the air temp.


True. Pilots usually reported compressibility problems above 25,000ft. Compressibility usually keeps the aircraft from falling faster because of the high increase in drag. In some ways its rare to see a plane gain enough speed to maintain compressibility through out the dive.


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## drgondog (Dec 26, 2010)

billswagger said:


> In that case, then the P-47 was considered to be more structurally sound for compressibility dives than the Mustang, keeping in mind that the actual speed attained by the Mustang would be slightly higher before it entered compressibility.
> 
> *Bill - recall that while the airfoil section of the P-47 was ~11% versus 14.8% for all versions of production P-51s (except 8) the P-47 had a 87.5 inch mean chord while the 51 had a 79.6" mean chord.
> 
> ...



Practical compressibility occurs when the velocity equals M=1 at a local point on the airfoil ~ 25% chord for conventional wings and closer to 40% for Mustang. The Mustang Wing, because of the more gradual pressure gradient along the chord, reached M=1 and initiated separated later than say an equal t/c for a conventional wing of the day. 

Having said this, the M=1 varies from 1116 fps (760mph) at SL, to 1077 fps at 10,000 to 1036 fps (706 mph) at 20,000 to 1016fps (692mph). That physical existance is the same for all the fighters. What is different is how the airfoil/airframe influences the local velocities, and in particular, over the wing. The P-38 experienced local M=1 at a much lower TAS than the Mustang (or the P-47).

If you go back to the NAA/Mustang Corner Speed/G Limit diagram on the first post - you will see the 'vertical line' out aroung 505mph. That is where the Dynamic Pressure Loads are so high that the airframe has reached 8g applied loading and begins to bend the airplane. At SL, 505mph is about .65 M


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## billswagger (Dec 27, 2010)

> There is no way to conclude that a P-47 was more structurally sound based on the wing dimensions.


Certainly, there is much more to structural stability than the wings. There is the tail section, the nose and canopy, etc. 
It was the general thought of pilots that the P-47 could be dove more aggressively, where P-51s were found to fold a wing when attempting to dive like a P-47.


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## drgondog (Dec 27, 2010)

billswagger said:


> Certainly, there is much more to structural stability than the wings. There is the tail section, the nose and canopy, etc.
> It was the general thought of pilots that the P-47 could be dove more aggressively, where P-51s were found to fold a wing when attempting to dive like a P-47.



Yes - but if one a/c is designed to 8G Limit and compared to another a/c also designed to 8g Limit load - and both are at design Gross weight, or both are the same percentage overload - then both should behave similarly to the same applied loads. Every flight safety component/sub component was designed to 8G Limit with 1.5 safety factor for Ultimate.

The wing is the usual suspect for symmetrical loads and the tail is the usual suspect for asymmetric loads - there are examples of both ships failing in a dive. AFAIK there was no tendency of one over the other in number of incidents. 

I know for example that the 355th FG had two fatal accidents (non-combat) due to eppenage loss of a P-47 in dives in Sept and Dec 1943. There were no such accidents for the Mustang in the 355th.

Having said that, the 8th AF P-51B/C had several structural failures traced to uplocks failing to restrain main gear in a high G pullout, popping the gear down and causing a catastrophic failure of the wing, as well as losing the tail in a high speed slow roll or snap roll. There were also suspected failures of the ammo doors popping open in a high speed dive causing the wing to fail.


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## bobbysocks (Dec 27, 2010)

drgondog said:


> I know for example that the 355th FG had two fatal accidents (non-combat) due to eppenage loss of a P-47 in dives in Sept and Dec 1943. There were no such accidents for the Mustang in the 355th.



what was the reason the eppenage bent or broke from the 51s prior to the mod which re-inforced of the tail section on the later D models? was it what year did they add this? I do not know of fatalities but do know they had several instances of bending or breaking off....hence the mod.


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## drgondog (Dec 27, 2010)

renrich said:


> Bill, unless I am mistaken though, since the air gets progressively warmer at lower altitudes then the effects of compressibility go away if the same speed is maintained since the speed of sound varies only as the air temp.
> 
> *Essentially yes *
> 
> ...



The Mustang and Jug had limit dive speeds about 60mph (IIRC) above the F4U and more than that re: P-38 - but both the Jug and Mustang have been pushed close to .85M.


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## renrich (Dec 28, 2010)

I was only using the the speed of 500 mph TAS at 40K feet as an example of the speed of sound being lower at high altitude where the air is colder. I knew it was probably inaccurate. My recollection is that at SL, it is about 750 mph.

Dago Red was one of the AC in that race and if I am not mistaken, it was the winner. I have photos somewhere. There was a Yak in the race but he did not get off the ground. The Super Corsair was also in the race as well as a number of Mustang types. We were told by the announcer that when the race was declared on and the planes were in this shallow dive toward the pylons that they were doing 500 mph. That may have been an exaggeration. This was about 1990-92. I think the pace plane might have been an L39.

I learned all about Mach tuck from the British film, "Breaking the Sound Barrier" The airplane was called "Prometheus" and it was a Supermarine "something" The pilot on the radio was screaming, "Nose heavy, trimming back!"


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## billswagger (Dec 28, 2010)

drgondog said:


> Yes - but if one a/c is designed to 8G Limit and compared to another a/c also designed to 8g Limit load - and both are at design Gross weight, or both are the same percentage overload - then both should behave similarly to the same applied loads.



However, both did not behave similarly to the same applied loads, nor did either behave similarly in dives nearing or exceeding Mcrit other than tuck.

For that reason its safe to say that loads exerted on either airframe is disproportionate despite having similar limiting factors. 
Yaw was found to be a dangerous factor in dives, even with the P-47 because it exerts tremendous pressure on the tail frame. Pilots were advised to center the ball during the dive otherwise there was eminent danger of breaking the tail frame. The p-51 experienced excessive yaw while entering Mcrit, and that could be the difference if such yaw could not be trimmed or countered with rudder force. Such yaw may have meant that the load amount was not symmetrical and one wing would be handling an excessive load amount compared to the other. 
This is supported by more specific reports saying that the Mustang was found to fold a wing while matching dives with a P-47, while most pilots felt that exceeding or entering compressibility in a P-47 was not as dangerous and somewhat more predictable upon recovery. 

To bring the thread back on topic, i would expect similar cornering speeds between the P-47 and P-51 given similar load factors.


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## drgondog (Dec 30, 2010)

billswagger said:


> However, both did not behave similarly to the same applied loads, nor did either behave similarly in dives nearing or exceeding Mcrit other than tuck.
> 
> *Oh, how were they different?
> 
> ...




Only if CLmax is the same (which they are not) - 

Given equal CLmax, then equivalent GW to Design Weight ratios and given the same design philosophy of 8G Limit/12G ultimate - then the corner speed profile betwen flight and accelerated stall is the same. The end state design limit as shown in forst post at 505 mph is based on Q load and presumably near the same for both ships.


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## bobbysocks (Dec 31, 2010)

The 'fold a wing' experience was specifically caused during pullout when the early B/C models main gear dropped under the high g pullout and when forcing the gear door to open caused a huge spike in the Q load.. this was fixed with wheel uplock kits. 

dr, what were the circumstances they dropped gear? were they trying to create drag in a high speed dive to slow down?


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## FLYBOYJ (Dec 31, 2010)

bobbysocks said:


> , what were the circumstances they dropped gear? were they trying to create drag in a high speed dive to slow down?



The landing gear can be deployed if the G limit of the aircraft is exceeded, basically the wing structure twists to the point where the uplocks disengage and there is enough force to bring down the gear. If this happens at high speed, other things are quickly going to disintegrate.


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## bobbysocks (Dec 31, 2010)

Oh... so this was an unintentional deployment?? structure flexing basically unlatched the gear from the up position? i thought hydraulics would have kept them in place..


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## FLYBOYJ (Dec 31, 2010)

bobbysocks said:


> Oh... so this was an unintentional deployment?? structure flexing basically unlatched the gear from the up position?


Yes


bobbysocks said:


> i thought hydraulics would have kept them in place..



No - after the retraction line pressue in the up side of the hydraulic lines go to zero and the uplocks usually hold the gear in place. I don't think you want to fly around with 1500 psi inside the wheel well.


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## Zipper730 (Sep 16, 2016)

drgondog

*Post #1: 12/21/10*



> All interesting topics and equally worth discussing - but this thread is about the huge dissemination of blivet on the subject of CORNER SPEED.
> 
> The first diagram is the Corner Speed Diagram prepared by North American Aviation to insert into the P-51D Pilots Handbook. Note carefully the message at the bottom which states that the Corner Speed calculation is for 8,000 pound Gross Weight and that the G load allowables for each condition must be factored by GWactual/64000 pounds.


And this determines the maximum g-load allowable?



> Note the 'accelerated stall' area sets one boundary of the lower speed threshold.. so a 3G sustained turn at 160mph at 8000 pounds on the deck is on the threshold for departure but not structural limit and not corner speed.


Just to be clear, when you say sustained turn: Do you mean the ability to hold the g-load in a level-turn without a loss of speed, or the ability to hold it for more than a fraction of a second before immediately stalling?

I say this not to be pedantic, but some terms change a little over time: Modern day at least, it means the maximum g-load that could be pulled level without loss of speed.



> Note that at 6G the sustained turn at ~230mph is at stall but NOT Corner Speed. [email protected] 8G at ~ 260 mph at 8000 pounds at sea level the airframe has reached CORNER SPEED.


Is corner velocity the same as maneuvering speed, and isn't there a mathematical formula based on computing this by using the square root of the g-load times the stall speed?



> More later - but Note for the second (conceptual) diagram that the Corner Speed at ~ 195mph is at the 6 "G" load for Design Limit and that Distortion (non elastic deformation) begins


When you say non-elastic I assume you mean permanent bending to the structure that does not contribute to failure?



> BTW I have no idea why the left value indicators for 6G load has a +9 on the far right


I would have thought a different weight would have explained it.
*
Post #9: 12/21/10*



> Very True (although d/dt (m*V*) will take care of the change in mass with respect to time with greater precision than the change in Velocity for all the other reasons mentioned


d/dt (mV): Isn't that something like f=ma? I vaguely remember this formula, but I honestly don't know what units I'm supposed to put in.

*Post #11: 12/21/10*



> The Pony started the drag rise around .65 M, the placard was for .75 and the highest achieved in dive was .83-85M depending on the belief system around the instruments.


Was the aircraft fitted with normal instrumentation used by the USAAF, the RAF, or specialized instrumentation?



> The Jug with the dive flap (-30? and above) would slow down faster than the 51 which could be an advantage when chasing a 109.


Just do be clear

When you say -30, you mean P-47D-30 right?
Why would you want to slow down while in the dive? I thought the purpose of a dive was to speed up until the pullout at least.



> In various dive tests to compare the two fighters, they were extremely close on maxximum sustained dives - each around .85 which is ~ .8 to .9 above the placard for each in their respective manuals.


I assume you're talking a P-51B/D normal and P-47D-30 with dive-flaps?


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## fubar57 (Sep 17, 2016)

Time to get more popcorn...

Reactions: Like Like:
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## Venturi (Dec 18, 2016)

Excellent info from drgndog


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## GregP (Jun 27, 2017)

I am assuming all these airspeeds are IAS.

At Reno, some P-51s have lapped at 512 mph or so (maybe 514?), but the the IAS was down around 485 at the time. So they didn't exceed the 505 mph placard, even with 3,800+ HP.


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