WW2 fighter aircraft dive capabilities.

[Ignition Override]

As a transport pilot, I can not understand why the Japanese Zero, being so light and maneuverable, was unable to catch up with an American Grumman Widlcat in a dive, or at least in a diving turn.

Did certain fighter planes have a much lower maximum speed (Vmo), partly due to their lightweight design (Zero) and the materials used in the tail? It can't be just engine horsepower and thrust/weight ratios.

The German Me/Bf-110 was apparently able to turn with or out-turn certain high-performance, single-engine fighter planes (Russian?), at a corner speed of about 290 kph. But if true, this is not well-known.

The P-38 and P-47 were also known to out-dive either the FW-190, Me-109 or both.

How about the Spitfire and Hawker Sea Fury etc? The Me-109's fuel injected BMW engine allowed negative g's, not allowed by a Spitfire's engine.

[John Farley]

Weight can have a big effect in a dive. If two aroplanes have the same drag and the same thrust (however that is generated) and are diving at the same angle then the heavier aeroplane will go faster because in a dive a component of weight acts along the flight path.

But when comparing very differing types as you say structural strength could provide a limit and for types of the WWII era I would suggest high mach contollability and drag rise issues would also come into play.

Complicated stuff aircraft performance!

[stilton]

I have always found it quite amazing just how fast these world war 2 fighters were in a dive, I stand to be corrected but I believe the P51's VMO was 460 KIAS!

Not bad considering the 767 I fly has a VMO of 360KIAS..

[Blacksheep]

WW2 fighter ASIs were calibrated in m.p.h. - at least in the earlier years. The early Spitfires had fabric covered ailerons. Pilots had reported aileron response problems in combat when diving and Vickers Supermarine investigated. They found the aileron fabric 'balooned' at around 425 m.p.h. (I bet it did!) The fix was to fit metal ailerons. Test pilots took them up to 460 m.p.h in a dive and found them satisfactory so they were issued for combat use. 92 Sqn got them first and announced their satisfaction. I guess they were getting well above 460 m.p.h in combat at times, but its easy to undersatnd why. As to turn rates, that's more a matter of wing loading. Seafire pilots found the A6M (Zero) easily outturned them in combat, but then so did the Hurricane. ;)

[stevef]

A 1941 carburettor diaphragm modification called Miss Shilling's Orifice (really!) prevented Merlin engines cutting out in a bunt and was used until Rolls Royce fitted properly-developed negative G carburettors in 1943.

Interestingly, the Royal Jugoslav Air Force fitted a Hurricane with a Daimler Benz DB601 in 1941 and trials showed it to be superior in take-off performance and climb rate compared to that of the Merlin-equipped version or the Me109 in which it was fitted as standard.

[Aaaaaaaaaaaaaaaargh!]

If two aroplanes have the same drag and the same thrust and are diving at the same angle then the heavier aeroplane will go faster because in a dive a component of weight acts along the flight path.
Isn't that the exact opposite of what Galileo proved with his experiment involving the Tower of Pisa, a pebble and a bowling ball? :confused:



...or am I misunderstanding something basic and obvious? :(

[Brewster Buffalo]

Aaaaaaaaaaaaaaaargh!

Yes I thought that but who am I to argue with John Farley :uhoh:..

IIRC isn't that the heavier of two objects, when dropped, accelerates faster to the same terminal velocity?

[k3k3]

Nope, 10m-sec² until terminal velocity, minus air resistance of course otherwise paratroops would have very short careers.

[Conan the Librarian]

Do a quick search for Sqn Ldr Martindale, ex of Farnborough, who was involved in compressibility trials with a Spitfire, in almost terminal dives. I believe he twice came back to Earth with his Spit, but without a prop which had decided enough was enough. Think this was about Mach.75 or thereabouts, but intrinsic idleness prevents me from checking - at least right now...

Conan the slightly inebriated

[Capt Pit Bull]

Back to physics 101 for some of you chaps!

Re: Dropping objects.

Initial acceleration is the same in all cases. More massive objects have a greater downwards force due to gravity (i.e. weight), but then again they also have more mass to be accelerated.

weight = mass * 9.8

acceleration = Weight / Mass

So all objects initial acceleration is 9.8 m/s^2. As shown in the feather/hammer dropping demo on one of the Apollo missions.

What happens next depends on the drag situation. As the object moves downwards, an upwards force is developed. So the weight is partially counteracted, resulting in a lower acceleration. As the object falls faster and faster, eventually there is enough drag to counter the weight and a terminal velocity is reached. How rapidly this happens depends on the relationship between the draginess of the object and its mass. Hence Feathers etc build up drag very quickly (unless in a vacuum a la apollo demonstration mentioned above) and so their acceleration drops rapidly and a low terminal velocity is reached. Things that are massive and slippery take a long time to build up drag and so attain a higher terminal velocity.


Re: Aeroplanes.

There are 2 issues here:

1. Establishing the dive.
2. Sustaining it at a high rate.

I suspect the initial poster (IO) is more used to thinking about point 2, in the context of a change of weight in the same aircraft type (where in for a given transport aircraft, achievable descent rate decreases as mass increases), hence the comment about the zero being light.

Everything else being equal, a lighter aircraft will have a higher sustained R.O.D. as this is determined by:

Deficit of Power / Mass (i.e. its a climb that happens to be negative!)

The rub of course is that everything else is not equal. Given that power lost to drag = drag X TAS, and given that at that end of the curve drag is proportional to speed squared, this gives aircraft with high maximum speeds huge advantages. especially if they are draggy.

CPB

[Brewster Buffalo]

Do a quick search for Sqn Ldr Martindale, ex of Farnborough, who was involved in compressibility trials with a Spitfire, in almost terminal dives. I believe he twice came back to Earth with his Spit, but without a prop which had decided enough was enough. Think this was about Mach.75 or thereabouts, but intrinsic idleness prevents me from checking - at least right now...

http://en.wikipedia.org/wiki/Supermarine_Spitfire

Under Speed and altitude records - there is a picture of said propless Spit.

and a quote from Jeffrey Quill -

"That any operational aircraft off the production line, cannons sprouting from its wings and warts and all, could readily be controlled at this speed when the early jet aircraft such as Meteors, Vampires, P-80s, etc could not, was certainly extraordinary"

[John Farley]

It may help those who are still not sure about the effect of weight when in a dive to consider the simplest case of a glider that is being held in a vertical dive.

Here only two forces are in play, the weight acting downwards and along the flight path and drag acting upwards and also along the flight path. When the two are in balance (assuming the wings stay on) the glider is diving as fast as possible. If the pilot now jettisons the water ballast he carries (in order to improve his performance in the type of competitions where speed matters) the drag will now exceed the weight and the glider will necessarily slow down until the two forces are again in balance.

If we add a motor to our glider in this dive the only difference is that there will be an additional thrust force in the same direction as the weight. The combined thrust and weight clearly will require a higher speed to generate the extra drag to again bring the upwards and downwards forces into balance. The effect of any weight change remains the same as with the glider case.

[Lowtimer]

If I may take the liberty of dragging the conversation back to the original question, it's basically about the difference between two classes of aircraft which may be broadly similar in shapes and sizes, but have lots of important differences - the Japanese "paper bag" school of fighter design (Zero, Oscar etc), and the "Iron Works" philosophy of the Wildcat, Hellcat etc at the other extreme as the P-51 and the Brits come somewhere in between. There's not a terrific difference in general configuration between a Hellcatand an Oscar. The argument about weight is, I humbly submit, a bit of a red herring in a vertical dive. Arguably, the Brits and Americans, being designed with generally higher speed ranges in mind, will have lower profile drag coefficients, which will help in a steep dive. It certainly makes the difference that the Brits and Americans have about twice the power. While the weight may not be directly helpful in achieving a high dive speed, the extra strength is: because a paper-bag and kitchen-foil fighter will flutter to pieces at speeds far lower than a Yank tank milled out of solid unobtanium. All of this ignores compressibility, of course, which playes by a different set of rules - but at lower or medium levels I'd be surprised if a Zero had enough power to get into serious compressibility trouble before it met terminal velocity anyway.
Incidentally, S/L Martindale's propless Spit PR.XI was not the result of a meagre mach.75, which Spits seem to have encountered quite often without coming to pieces. Martindale hit a peak speed of mach .89, 606 mph TAS. All properly instrumented and verified. It is likely that Ted Powles accidentally got his PR.XIX up to Mach .94 without breaking anything, after having achieved a freakishly high altitude due to oddball weather - though being engages in meteorogical measurement rather than compressibility trials, his aeroplane was less thoroughly festooned with recording devices.