Spits in a dive

[Dragon Tamer]

This is just a quick question (I say that but it will probably turn into a 5 page thread anyway). A spit 8 in a dive, at what speed does it compress? I chased a spit down yesterday and my jug compressed but the spit was still pulling away from me and actually pulled out of the dive without any noticeable dampening on it's performance.

[Karnak]

It doesn't.  The Spit was capable of diving to higher speeds than any other WWII aircraft, even the Me262.  It has aileron control problems at speed, but the elevators work just fine, too well in some cases.

[Brooke]

Several planes have higher critical Mach numbers than the P-47 (Spitfires, Bf 109's, FW 190's, P-51's do -- not sure if there are some others).

Critical Mach (where compressibility starts) depends (to sort of a first-order approximation) on t/c where t is thickness of airfoil at thickest part, and c is chord.  That smaller the t/c, generally, the higher the critical Mach.

Not all planes with small t/c will be able to get to critical Mach before failure of some other part of the airplane, though.  For example, a Ki-84 or Zero might have a low t/c compared to a P-47, but they break apart at very high speeds.

[Tank-Ace]

It doesn't.  The Spit was capable of diving to higher speeds than any other WWII aircraft, even the Me262.  It has aileron control problems at speed, but the elevators work just fine, too well in some cases.

I question the control authority the Spitfires have at high speeds. It just seems like the controls should be quite heavy at some of the speeds you see Spitfires diving to.

[BaldEagl]

As a long time Spit pilot the practical top speed in a dive is ~480-500 mph IAS but you have to pull out gently so as not to break a wing.  Spits don't compress.  You can push the numbers I stated higher but at the risk of losing a wing.  A steady gentle pilot might see 550. 

When the dive is over Spits hold E just as they do in a turn.  Where some other planes (P-47's for example) use weight to create E for a run or a zoom climb Spits do so through aerodynamics (i.e. low drag).

Full span spits get quite heavy in roll authority at those speeds, particularily the Spit I but also the XIII and XIV.  The V and the XI don't seem to lose as much, most likely due to the lower power (i.e torque) vs the VIII or the XIV.  The I's fabric covered wings flex hampering it's roll ability at speed.

There's planes that are both faster and slower in a dive than a Spit but, like every other flight charachteristic, the Spit is among the top of the line; not the best but right up there.

[Bruv119]

the spitfire wing was amazing.  

   R.J Mitchell

[Karnak]

I question the control authority the Spitfires have at high speeds. It just seems like the controls should be quite heavy at some of the speeds you see Spitfires diving to.

After about 450-500 you cannot roll in a Spit VIII or Spit XIV because the ailerons are so heavy.  The elevators did not get heavy, and that imbalance of control forces caused more than one Spitfire pilot to pull back to fast and lose his wings.  The light elevator control to extremely high speeds is historical.

In reality the longer aileron Spitfires, like the IX and XVI, sometimes also suffered aileron reversal at high speeds, where pushing the stick to the right caused the Spit to roll to the left and pushing left caused it to roll to the right.  The shorter ailerons in the Mk VIII and Mk XIV were a fix for that. The root cause was the force of the air at high speeds causing the thin wings to twist a bit.

[BaldEagl]

The shorter ailerons in the Mk VIII and Mk XIV were a fix for that.

I thought about the ailerons being a more likely culprit in roll authority in those two varients than engine torque last night after I posted but I'd already shut down for the night.  Glad someone interjected.

[Tank-Ace]

Wasn't the cause of heavy control surfaces just the airflow over it (trying to push it to the most aerodynamic position)?

Now please correct me if I'm wrong on the above, but it seems like the above would make heavy controls pretty universal. I'm not saying that it should be as heavy as a 109,but the near full authority they have just instinctively seems quite unlikely.

[Karnak]

Wasn't the cause of heavy control surfaces just the airflow over it (trying to push it to the most aerodynamic position)?

Now please correct me if I'm wrong on the above, but it seems like the above would make heavy controls pretty universal. I'm not saying that it should be as heavy as a 109,but the near full authority they have just instinctively seems quite unlikely.

Well, Supermarine had to put counterweights on the elevators to make them heavier after several Spit Vs were lost when pilots pulled out of a dive too rapidly due to overloading the wings.  At some point it will enter full compression, but until then the elevators remained light.

The Spit XIV comparative notes mention that its elevators were noticeably heavier, but still easy to move.

[Tank-Ace]

Any idea of the aerodynamics behind this? Or will I have to wait for my physics class this fall?

[Karnak]

Any idea of the aerodynamics behind this? Or will I have to wait for my physics class this fall?

No, I don't know the cause.  I doubt it was intended as when the Spit was designed aircraft engineers knew very little, if anything, of the effects of those speeds.

I know the US dinged the Spitfire for imbalanced controls when we evaluated it.  Ideally for smooth operation you want elevator and aileron forces to be similar so that the pilot isn't having to change the force on the stick, pulling hard to either side and gentle to the rear when rolling into a turn for example.

[earl1937]

Any idea of the aerodynamics behind this? Or will I have to wait for my physics class this fall?

I don't know the engineering behind this, but I do know the shape of the fuselage has a lot to do with the forces acting on elevator and rudder. Some fuselages were either shorten or length added, depending on how the aircraft was acting and what the design engineer was trying to accomplish.

[Brooke]

Any idea of the aerodynamics behind this? Or will I have to wait for my physics class this fall?

Basic physics won't cover it, but it will start to give you the fundamentals to understand more about aerodynamics.

There are some aspects of performance that are easier to estimate based on generally available measurements of the airplane (things like drag based on coefficient of lift, wing aspect ratio, etc.)

Then there are things that are not at all easy to estimate because they depend on subtle, detailed aspects of design that are not easy to model.  Control force and effect vs. speed is in that category.  Not all designs result in higher force per g as speed increases.  Some designs started to get lower force per g at high speeds and had to address that as a problem if the effect was too significant lest pilots inadvertently overstress the airframe to the point of failure.

Here are some books on such things if you are interested in the math details:

[PH] Airplane Performance Stability and Control, by Courtland D. Perkins and Robert E. Hage.
[VM] Theory of Flight, by Richard Von Mises.
[RS] Fundamentals of Flight, Second Edition, by Richard S. Shevell.
[BM] Aerodynamics Aeronautics and Flight Mechanics, Second Edition, by Barnes W. McCormick.
[JA] Introduction to Flight, by John D. Anderson, Jr.

[GScholz]

The Spitfire was unmatched in dive speed in WWII.