Originally posted by Simaril
Does RELATIVE turn data change with altittude? In other words, would the ratio between two planes' minimum circle diameter, or degrees per second, stay fairly stable as altitude changes?
Generally speaking no, the relative performance between two aircraft won't stay the same with altitude, sometimes the difference will increase, and sometimes it will decrease and even reverse. It all depends on the particular aircraft involved. I can illustrate that better with an example. Some time ago I was looking for an aircraft that could be used to fight the P51B, I wanted something that the P-51B couldn't run away from, and couldn't out turn and so I made some likely comparisons. First up, was the F4U1. Here is an EM diagram for both aircraft at Sea Level, at full War Emergency Power (WEP). The diagram also shows the situation with full flaps.
The key features were that both aircraft had a similar top speed, and although the F4U1 has slightly lower Ps performance across the range, which means that it would bleed speed more quickly in the turns, it had a similar sustained turn rate and a slightly smaller turn radius. However, you can also see that with full flaps the F4U1 is dominant with more than 3dps turn rate advantage and almost 200ft smaller turn radius. So this looked like a good candidate P51B killer.
However, the that situation doesn't stay constant as the altitude increases. Take a look at the EM diagram for the other altitude extreme, 30,000ft, shown below.
Notice now that the situation has reversed, at this altitude the P51B is not only faster, it has almost a 2dps better sustained turn rate and almost a 400ft better turn radius.
In that example, the advantage reverses, and so there will be an altitude above which the P-51B has the advantage and below which the F4U1 has the advantage.
However, that isn't always the case for every pair of aircraft. For example, I did a comparison for the last Battle of Britain Scenario and provided a comparison for all the fighters from Sea Level up to 30,000ft. The EM diagrams are in Annex A at the back of this file: (right click and save target as)
http://www.badz.pwp.blueyonder.co.uk/Files/pdf/AH-BOB.pdfIf you look at the EM overlays you will notice for example that the Spitfire Mk1 is superior to the Bf109E-4 at sea level, and all the way up to 30,000ft where the difference between the two decreases as the altitude increases, but remains advantageous to the Spitfire.
One other interesting point you may notice from those diagrams is that while the Bf109E4 is the superior German fighter at lower altitudes, at 30,000ft, the Bf110C matches it in the turn, and is faster.
Anyway, that should give you an idea of how things can vary with altitude.
ANd in a related question, does the optimum turning INDICATED speed stay stable as altitude changes?
Yes, but with one caveat. Things like the stall speed and corner velocity should occur at the same indicated airspeed, however, at higher altitudes it may not be possible for the aircraft to reach those speed. For example, if you check the corner velocity of the Battle of Britain aircraft again, you will notice that while all of the aircraft can generate 6g at 20,000ft, none of them can generate enough lift to reach 6g at 30,000ft and the amount they can reach is limited by their top speed and possibly Mach effects if they can go fast enough, so at that altitude their corner velocities will vary.
Hope that helps.
Badboy
