Why does the Spit VIII roll slower than the IX or XIV?

[Karnak]

I did some quick and dirty tests, which I cannot ensure are acurate, and it seems that the Spit VIII rolls notable slower than either the Spit IX or Spit XIV.

Test method:

Take off with full fuel.
Climb to 500ft.
Reach speed of 310mph.
Trim to neutral.
Roll 360 degrees right without rudder inputs, record time.
Roll 360 degrees left without rudder inputs, record time.

Note: Timing had to be done by counting and so is not precise.

Spitfire Mk IX:

Right: 5 seconds
Left: 5.5 seconds

Spitfire Mk VIII:

Right: 6 seconds
Left: 6.5 seconds

Spitfire Mk XIV:

Right: 5 seconds
Left: 5 seconds

Note: The Spit XIV was harder to test due to it's greater engine power, keeping it at 310mph was problematic.


Now, if I have not grossly mistimed these rolls it seems that the Mk VIII rolls slower than the others.  Why so?

It has shortened ailerons, but the reduced surface area should allow for higher deflection for the same stick force, ending with approximately the same roll force being generate by the ailerons as the larger ailerons on the Mk IX at a lesser degree of deflection.  In addition the Mk XIV also has shortened ailerons.

It also has fuel in the wings unlike the Mk IX.  I don't know how this would affect the roll rate, but the Mk XIV also has fuel in the wings.

I have also not read of any difference in roll rate between the Mk VIII and Mk IX.  The Mk XIV had a similar roll rate to the IX, but favoring the oposite direction due to the Griffon rotating the prop in the oposite direction.


So, are my timings just off or is there something going on here?

[Tails]

Higher aileron deflection is not a good thing. In fact, it can be a very bad thing, with the upward aileron pretending to be a spoiler, and the lower one a drag rudder.

I would think that the travel of the ailerons was not adjusted with the shorter span surfaces, in order to prevent the above mentioned nastyness. But without rigging instructions or mfr's data I can't tell you for certain.

[Karnak]

Originally posted by Tails
Higher aileron deflection is not a good thing. In fact, it can be a very bad thing, with the upward aileron pretending to be a spoiler, and the lower one a drag rudder.

I would think that the travel of the ailerons was not adjusted with the shorter span surfaces, in order to prevent the above mentioned nastyness. But without rigging instructions or mfr's data I can't tell you for certain.

I wasn't thinking that the travel would have been increased.  By the time a Spitfire is going 300mph the pilot can no longer get full deflection out of the ailerons as the pressure on them is too high given the way a Spitfire is layed out.  With less surface area there would be less pressure and thus the pilot would be able to deflect the ailerons closer to full deflection.

I would fully expect the Mk VIII and Mk XIV to have a reduced roll rate at speeds where full deflection of the ailerons can be obtained, but that is below 220mph as I recall.  Maybe below 180mph.  By 300mph the pilot simply is not going to get max aileron deflection.

[Tails]

Originally posted by Karnak
I wasn't thinking that the travel would have been increased.  By the time a Spitfire is going 300mph the pilot can no longer get full deflection out of the ailerons as the pressure on them is too high given the way a Spitfire is layed out.  With less surface area there would be less pressure and thus the pilot would be able to deflect the ailerons closer to full deflection.

I would fully expect the Mk VIII and Mk XIV to have a reduced roll rate at speeds where full deflection of the ailerons can be obtained, but that is below 220mph as I recall.  Maybe below 180mph.  By 300mph the pilot simply is not going to get max aileron deflection.

Ok, I see what you're saying here.

The pilot would be able to get more deflection out of the ailerons if they had shorter span, certainly. The problem is that the shorter span ailerons would have reduced authority compared to the longer ones. So, on a good day, they may even out at high speeds. Or, the short-span surfaces may have LESS authority at higher speeds.

I have no practical experience with Supermarine products, so I'm just going by theory here

[Karnak]

You've got it.  It may be correct as it is, but I've never heard that the roll rate dropped.

I'm asking for an explanation is all.  That could be "It is correct because of X and Y and Z."  It could be "Oops, it should be the same."  It could be "Jeez Karnak, can't you even keep time for less than 7 seconds.  It is the same and you botched the tests."

[Tails]

Depending on the design of the aileron, it could be the short span surface is capable of deflecting too much at high speeds, leading to the problems I described. The aileron acting less as an aileron and more like a drag rudder. Does the Spit8 yaw more than the others in a high speed roll, with neutral rudder? If so, that may be what's happening.

[Karnak]

I'm not sure, but keep in mind these are the exact same ailerons as the Mk XIV has.  They also aren't much shorter, less than a foot off the outer ends.

[Kurfürst]

Odd, really, especially the relative perf of the VIII/XIV.

For what it's worth :

Alex Henshaw, chief test pilot at the Castle Bomwich Spitfire factory.

"I loved the Spitfire in all of her many versions. But I have to admit that the later Marks, although they were faster than the earlier ones, were also much heavier and so did not handle so well. You did not have such positive control over them. One test of manouverability was to throw the Spitfire into a flick roll and see how many times she rolled. With the Mark II or the Mark V one got two and a half rolls but the Mark IX was heavier and you got only one and a half. With the later and still heavier versions one got even less.The essence of aircraft design is compromise, and an improvement at one end of the performance envelope is rarely achieved without a deterioration somewhere else."

[Bruno]

Manoeuvrability

5......... There was nothing to choose between either aircraft as regards turning circles at any height; whether on offensive or defensive manoeuvres neither could make any impression on the other. In rate of roll, however, the Spitfire IX was considerably better especially at low altitude. A number of full rolls through 360 degrees were timed by the same pilot flying each aircraft in turn and although quanitative tests are difficult to produce, it appeared that there was often more than 1.5 seconds superiority for the Mark IX over the Mark VIII. The Mark VIII feels fairly light on the ailerons but at high speeds it becomes very heavy, and so this new combination of extended wing and small aileron cannot be considered satisfactory.

CONCLUSIONS

7......... There is no difference in performance between the Spitfire VIII and Spitfire IX with Merlin 63 engines, except that with the extended wing tips the Spitifre VIII is performing a little bit better at high altitude.

8......... The smaller span ailerons combined with extended wing tip give the Spitfire VIII an inferior rate of roll.

Spitfire Mk VIII Testing

The above tests were F.IX and F.VIII both with Merlin 63.

[Tails]

There could be other things to factor in that will deteriorate roll rate. Weigth distrobution being one of them, and I'm not talking about just in the wings. Anything outside the axis of rotation will slow down an aircraft's roll rate and other facets of manuverability.

Like I said, I'm not familiar enough with Supermarine products to be able to say for certain what could cause the differences in roll rate. I just know a bit of theory.

EDIT: *Reads Bruno's post* Extended wing tips. That will do it right there.

[Widewing]

Spitfire roll rates in degrees per second, based upon one 360 degree roll.
Air speed: 300 mph
Altitude: 2,000 feet.
Fuel: 50%
Stopwatch used to measure. Average of three tests.

Mk.VIII
Right = 68 degrees
Left = 74 degrees

Mk.IX
Right = 78 degrees
Left = 81 degrees

Mk.XIV
Right = 78 degrees
Left = 82 degrees

Mk.XVI
Right = 112 degrees
Left = 116 degrees

For comparison:

Fw 190A-5
Right = 125 degrees
Left = 131 degrees

F4U-1
Right = 97 degrees
Left = 100 degrees

My regards,

Widewing

[bozon]

I was wondering about this myself.
Not knowing anything about it historically, I'm going by simple physics. In that case one must differentiate between limitation due to "force applied on stick" and limitation due to plane design.

The force needed to be applied on the stick for max deflection really depends not only on aerodynamics of the ailron but also much on the mechanics of the control wires and levers. Simply having a longer stick as a lever will require less force applied by the pilot (although longer deflection of the stick).

If max deflection is achieved, we turn to aerodynamics. Roll is done by uneven lift of the wings. On the rising wing, the aileron increases the angle of attack, but the rolling motion reduces back the effective angle of attack - the longer the wing the more severe this effect since the roll speed is roll rate times the distance from roll axis. The decending wing has the same effect in the other direction. So roll rate will depend both on area and maximum angle of attack of the aileron section and the distance from roll axis - the shorter the wing, the better. Parts of the wing not in the aleron section just hinder roll due to the effective angle of attack. Another consideration is torque of the engine which will make rolling in one direction faster than the other.

clipping the spit wing eliminate the part that hinder roll the most - the most far from the axis that is not part of the aileron section. 8 and 14 have almost the same wing I believe (save the weapons) so I'm not sure why the difference.

Bozon

[Karnak]

Bruno,

The Spit VIII in that test had extended wing tips.  Most, such as the one in AH, had standard wing tips.  Of course the extended wing Spit VIII will roll worse.  It is the oposite of clipping the wings.

Just to clarify, the test Bruno posted does not have anything to do with our Spit VIII.  The extended tips refered to by the test are the long, pointed wings you can sometimes find on Spits.  The rounded tip wings like we have are standard wings and the squared wings as on the Mk XVI are clipped.

So that test is refering to a different wing shape.

[Bruno]

The extended wing tips alone don't account for the IXs better roll rate entirely.

If you read what I quoted:

The smaller span ailerons combined with extended wing tip give the Spitfire VIII an inferior rate of roll.

Your test show a 1 sec roll advantage for the the IX. The test I linked says:

it appeared that there was often more than 1.5 seconds superiority for the Mark IX over the Mark VIII.

[Karnak]

Given how much the clipped wings increased the roll rate of the Spitfire I am not prepared to assume that the extended wings only very slightly reduced the roll rate.

From the Mk XIV testing when compared with the Mk IX:

Rate of Roll
19. Rate of roll is very much the same.

Remember, the Mk XIV has the same shortened ailerons as the Mk VIII.


Spitfire XIV AFDU Tactical Trials