Dive limits

Ad: This forum contains affiliate links to products on Amazon and eBay. More information in Terms and rules

Glider said:
The one that gets my attention is the F4U1-D. It wouldn't take much of a dive to exceed 443 when things get risky
Click to expand...
I was thinking the very same thing. I believe that, like the F6F-5, the quoted speed in the book for the Corsair is for altitudes between 10-15k and at the high end would mean the aircraft would be traveling at a true airspeed of 543 mph or around 0.75 Mach. That's really clipping along!
 
Last edited:
DarrenW said:
Thank you Zipper for the outstanding explanation. So I guess it depends on the situation that you find yourself in. If you just need to make a quick break away during a dogfight and are not being followed too aggressively than a plane like the Lightning would be what one would select. But if you are alone and fighting your way back to safer skies, with the enemy in chase and bent on your ultimate destruction, than a plane with a higher critical Mach number is needed in order to eventually outdistance your pursuer. If you start with enough of a lead of course.
Click to expand...
Things generally go in a different direction, pilots were in the plane they were sat in, it wasn't a choice, from that the best choice of action for a given combat situation evolved. Individual pilots, squadrons and whole air forces developed tactics/gambits to maximise strengths and minimise weaknesses.
 
Zipper730 said:
The aircraft is fairly heavy and, until mach effects appear, fairly streamlined so air has little ability to impose significant resistance to keep the plane from picking up speed very fast with gravity pulling it down combined with the thrust of the aircraft's propellers and exhaust system (which is little as it has a turbocharger).

The propellers produce enough thrust to have allowed speeds of over 400 mph in the prototypes, around 395 in mass production models (extra weight, some drag produced by the bullet proof glass), and up to around 420 mph in later variants, gravity effectively provides (at least in a 90-degree dive) 1g of acceleration, turning every pound of aircraft into a unit of thrust pushing downwards at a little under 9.81 m/s every second, with little resistance.

The aircraft's critical mach number is about 0.65 (airflow hits mach 1 on some part of the wing) drag divergence mach is around 0.67-68 (airflow is now supersonic on some parts of the wing, shockwave strength reaches a point where it can drain enough energy out of the airflow to cause some separation), which causes an increase in drag, as well as a decrease in control effectiveness. The aircraft's loss of control occurs at around 0.74 mach. While gravity still is pulling it down, the drag from the airframe has gone up, and the efficiency of the propellers are affected by airspeed and mach numbers (which is why flying up high helps to a point, but mach effects do start to kick in and, past a certain point, it'll reduce the available net thrust) and by this point the net thrust of the propellers are less than the drag produced by the aircraft. I'm not sure if 0.74 is terminal velocity (a speed at which drag and thrust equal each other, gravity in this case is thrust as well), but going straight down without a means of recovery is quite terminal!

Since the speed of sound is affected by temperature, as the airplane reaches warmer air, some control effectiveness is restored, but propeller thrust also increases for the same reason (not much, but with gravity doing it's thing, it's enough to get you back to 0.74), so once the control is restored one would want to start pulling back, at first with everything you got, then progressively less (as the aircraft comes out of the dive, it will stop accelerating as gravity is being countered by lift to a degree, propeller thrust is fairly low, overall airflow over the aircraft is significant, and lift actually increases drag) to avoid over stressing the airframe (once it slows down enough, you'll regain full control and be well above the maneuvering speed, yanking the stick-all the way back above the maneuvering speed risks snapping off the wings).

Long message short: The plane accelerates very quickly into mach-tuck! The P-38's problems with mach tuck had to do with two things
  • Thick wings: This is largely due to the high aspect-ratio (8.26), which favors a thick structure to avoid excessive aeroelastic flexing
  • The junction between the fuselage and the wings
    • The wings are convex on top, which causes the airflow to accelerate over them
    • The fuselage is convex in shape, causing the airflow to accelerate to either side it
    • The two combine together to produce an unusually high velocity airflow (I'm surprised this isn't thought of as an early example of area rule issues)
Eventually filleting was used to reduce this problem, though the critical mach number was 0.65 after the modification...

An airplane like the Spitfire has less mass, which means it won't pick up speed quite as fast, but it can ultimately tolerate a higher mach number, so it will accelerate a lot longer before it runs into mach tuck or airframe destruction: While this does sort of contradict what we were all taught in school that all objects fall at the same speed, this fails to factor in
  • Air resistance: A heavier object is not affected as much by the mass of the air. If not raindrops would probably kill us.
  • Gravity: This only applies with objects of significant mass, but technically they would both attract each other, and as a result would hit faster than if only one attracted the other.
Click to expand...
Zipper, you obviously know quite a lot but please take more care with units and descriptions. You cannot leap so freely between weight, density and mass especially since gravity is such a major part of this discussion.
 
I came across something interesting the other day. Apparently the do not exceed speed for the late-built F6F-5s was very similar to later model P-47Ds:

F6F Pilot Handbook 1 May 1946:
F6F-5 DNE Speed.png


440 Knots = 506 mph

P-47D (variants 26 thru 35) Pilot's Instructions 25 Jan 1945:
P-47D Dive Restriction.png


Neither correction table in these two manuals had figures for 500 mph, but if they remained fairly constant from 300 mph the P-47D would edge out the F6F-5 by a mere 9 mph.

Now this of course got me thinking. Why would the P-47, possessing such a fearsome reputation in a dive, have a maximum do not exceed speed comparable to other high performance fighters of it's era? What set it apart from the herd?

Then I read this from an article called Flight Test Comparison...Ending the Argument, found in the June 1990 edition of the EAA's Sport Aviation magazine, and it seems that how an airplane behaves in a dive may be just as crucial as it's maximum safe dive speed and overall acceleration:

Dive Comparison.jpg


In essence there was far less pilot input required with the P-47 than the other types during these tests which, besides instilling the pilot with extra confidence to push his machine even harder, allowed for a faster build up of speed (hence acceleration) as it was easier to keep the airplane under control as it dove in a straight line. I would think this is a huge factor why it is the first airplane people think of when discussing the diving ability of WWII airplanes.....
 
Last edited:
1. I should have really adjusted my numbers to factor errors in speed gauge with early calculations in dive performance
2. I have a mach/airspeed calculator that I'm printing into a table
 
DarrenW said:
I came across something interesting the other day. Apparently the do not exceed speed for the late-built F6F-5s was very similar to later model P-47Ds:

F6F Pilot Handbook 1 May 1946:
View attachment 487105

440 Knots = 506 mph

P-47D (variants 26 thru 35) Pilot's Instructions 25 Jan 1945:
View attachment 487104

Neither correction table in these two manuals had figures for 500 mph, but if they remained fairly constant from 300 mph the P-47D would edge out the F6F-5 by a mere 9 mph.

Now this of course got me thinking. Why would the P-47, possessing such a fearsome reputation in a dive, have a maximum do not exceed speed comparable to other high performance fighters of it's era? What set it apart from the herd?

Then I read this from an article called Flight Test Comparison...Ending the Argument, found in the June 1990 edition of the EAA's Sport Aviation magazine, and it seems that how an airplane behaves in a dive may be just as crucial as it's maximum safe dive speed and overall acceleration:

View attachment 487111

In essence there was far less pilot input required with the P-47 than the other types during these tests which, besides instilling the pilot with extra confidence to push his machine even harder, allowed for a faster build up of speed (hence acceleration) as it was easier to keep the airplane under control as it dove in a straight line. I would think this is a huge factor why it is the first airplane people think of when discussing the diving ability of WWII airplanes.....
Click to expand...
I just saw this. Prior to dive flap installation in both P-47D (-30) and P-38J (25) both ships experienced pitch changes (Delta Cm) due to traveling CP in transonic range. The P-47C/D experienced nearly the same pitch down/long recovery difficulty as the P-38 until the dive flap was installed.

The Mustang experienced light 'pitch down' Cm changes due to the NAA/NACA 45-100 airfoil upon which the velocity gradient was lower (than more conventional P-47 and P-38 airfoils with T/C max @~ 26% from LE to T/C max =37%. The airfoil design for the Mustang was a dominant reason for delayed Transonic Parasite Drag increases @ .55M to 0.75M.
 
drgondog said:
I just saw this. Prior to dive flap installation in both P-47D (-30) and P-38J (25) both ships experienced pitch changes (Delta Cm) due to traveling CP in transonic range. The P-47C/D experienced nearly the same pitch down/long recovery difficulty as the P-38 until the dive flap was installed.

The Mustang experienced light 'pitch down' Cm changes due to the NAA/NACA 45-100 airfoil upon which the velocity gradient was lower (than more conventional P-47 and P-38 airfoils with T/C max @~ 26% from LE to T/C max =37%. The airfoil design for the Mustang was a dominant reason for delayed Transonic Parasite Drag increases @ .55M to 0.75M.
Click to expand...
From what I remember, the P-47 could dive somewhat faster off the bat: I'd almost swear they were able to get to 0.78 in the P-47B/C variants (it was outside the placard limit, but they could recover).
 
Zipper730 said:
From what I remember, the P-47 could dive somewhat faster off the bat: I'd almost swear they were able to get to 0.78 in the P-47B/C variants (it was outside the placard limit, but they could recover).
Click to expand...
Both the 51 and 47 were near equal in terminal dive. Yeager and Hoover made many such comparisons post War while stationed at Wright Pat.

The top recorded dive for 51D was .85M with significant structural damage that caused it to be salvaged after the flight.
 
drgondog said:
Both the 51 and 47 were near equal in terminal dive. Yeager and Hoover made many such comparisons post War while stationed at Wright Pat.

The top recorded dive for 51D was .85M with significant structural damage that caused it to be salvaged after the flight.
Click to expand...
Yeah, Greg's Airplanes & Automobiles listed a document (I think it was for the P-47, but I could be wrong) that specified 0.745 as a mach number. That's very very close to the P-51's placard limit of 0.75. I think that's 3-1/2 miles an hour difference at altitude
 
mig-31bm said:
Does the later F4U-5 has the same top speed limit for diving as F4U-1?
Click to expand...
The F4U-5 was the first Corsair with all metal wings (no fabric panels) and this obviously would affect the dive limit in a positive way. I'm sure if we could locate a F4U-5 pilot's manual there were be G limit charts that we could examine.
 
DarrenW said:
The F4U-5 was the first Corsair with all metal wings (no fabric panels) and this obviously would affect the dive limit in a positive way. I'm sure if we could locate a F4U-5 pilot's manual there were be G limit charts that we could examine.
Click to expand...
I feel like F4U-5 manual doesn't exist , i cant locate it anywhere though. Is there a photo comparison of F4U-5 and F4U-1 side to side
 
You must log in or register to reply here.

Users who are viewing this thread

Total: 1 (members: 0, guests: 1)

Similar threads

B
WW II airframe G limits
Replies
7
Views
3K
Barrett
B
R
SBD dive bombing procedures
2 3
Replies
52
Views
6K
Retattack
R
JoblinTheGoblin
Upper Limits of Imperial Japanese Aerospace Technology
2
Replies
22
Views
2K
JoblinTheGoblin
JoblinTheGoblin
pinehilljoe
British Dive Bombers or lack thereof
8 9 10
Replies
189
Views
14K
SaparotRob
SaparotRob
T
Hurricane Dive Speeds
Replies
2
Views
1K
ThomasP
T
Back