Aircraft 'nods' between narrow AoA range
Lee Elliott <leee <at> spatial.freeserve.co.uk>
2004-07-27 21:23:45 GMT
To keep my hand in at modelling from scratch (as opposed to updating existing
models or working on FDM/AP stuff), I've started on an English Electric
Canberra B(I)8 - this is very similar to the early B-57A.
If I use a high altitude cruise data, I can get it to solve and fly reasonably
well but there's a narrow range of speed/AoA where the a/c starts to 'nod'.
Where the speed is low enough that the AoA is higher than the upper critical
value the a/c is stable and it's also stable once it's going fast enough that
the AoA is below the lower critical value.
I haven't given specific values for the critical speeds/AoAs because I can
vary them by tweaking various parameters but the range is always about 0.6-7
If I use low-altitude data there's no problem, but then I'm pretty sure that
the a/c wouldn't be able to get anywhere near it's operational ceiling -
46500ft. By todays standards the Canberra is a pretty low powered a/c - 2 x
7500 lb thrust turbojets but it was nevertheless a good performer at
altitude, and quite fast too - 620 mph (538 kt) at sea-level, I assume - the
high-alt values I found, and which I'm using are 470 kt at 40000 ft.
Can anyone shed any light on what might be causing this 'nodding'?
For those who're interested, http://www.b-57canberra.org/labs_detail.htm gives
some info on the Low Altitude Bombing System (LABS) technique, which was used
with Canberras and B-57s.
Basically, it involved approaching the target at 425 kt and 1000 ft agl,
initiating a 3.5 G pull-up and then releasing the bomb when the gyro tumbled
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