Quote:
Originally Posted by Kurfürst
Indeed the roll chart you posted for another Spitfire Mark V tested in the UK shows far higher roll rate than any other test result or source, UK or US. Its a pity we do not know anything about this Spitfire, but it seems this aircraft was either extraordinary good in this regard, or it had some non-standard aileron setup. We can only guess.
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Read the chart. The pilot used 50 lbf rather than 30 lbf. It is therefore hardly surprising that a higher roll rate was achieved because constant helix angle could be maintained to a higher TAS. Agreement between the charts is actually extremely good if you take this into account.
I already pointed this out in my last post, had you but taken the time to read it...
BTW:
- Roll rate is just a function of helix angle and TAS.
- Helix angle is a function of wing design and control deflection.
- Stick force to a first order approximation is proportional to Q.
- Deflection limited roll rate is therefore just proportional to TAS
- Force limited roll rate is inversely proportional to Q
- Roll rate achievable at altitude will vary in proportion to (TAS/EAS)
However, altitude itself doesn't "cause" this; the aeroplane knows nothing about its position relative to the geoid or to MSL. The relationship is simply caused by air density variation, and therefore it's quite possible for roll rate to be affected by the weather.
On a related note, the reduced helix angle associated with any given absolute roll rate as TAS increases is responsible for the reduction in roll damping at high altitude; again, the underlying mechanism is the TAS/EAS relationship, which means that roll damping is also a function of the weather conditions.
NB, the simplistic relationships explained above deliberately ignore the effects of compressibility upon stability & control. In particular, shockwave formation can dramatically reduce the effectiveness of control deflections by reducing the ability of control deflections to influence the flowfield upstream of the shock; stick forces are not reduced in proportion to the reduction in control effectiveness, and therefore pilot reports tend to read as though the controls have suddenly become heavier, when in fact they've just become less effective. But I digress...