Re: Roll Rate on MiG-29 and Su-27

On the assumption that moment of inertia being a primary fact in
limiting roll rate I did do a thought experiment where I compared the
"moment of inertia" or "I" of three diameter 1 cylinders arranged in
two different ways and also one large cylinder of equivalent area.
"I" = sum of masses x radial distance from axis. The three cylinders
represent two engines and fuel. Three cylinders in a "row"
represents an F-14 tomcat or similarly widely spaced aircraft with the
center cylinder representing fuel and aircraft systems. Three
cylinders in(triangle) arrangement represents something like an F4,
F18, Tornado. One large cylinder represents a single engined
aircraft like and F-16 or F-35.

these are relative figures I got:

I(row) = 2.5 (1 x 1 x 2 + 1 x 0.5)
I(triangle) = 1.55 (3 x 0.55 x 1)
I(single) = 1.5 (0.5 x sqrt(3) x 3/sqrt(3)) (single engine only
better due to savings in material not distribution)

(PS you may want to go over my figures but best to use graph paper)

Aircraft with widely spaced engines thus have a significantly higher
moment of rotational inertia for the fuselage. Having a single engine
versus two closely spaced ones makes little difference and actually
probably makes things worse. A single engine only helps by reducing
whetted area and therefor material required to enclose the same
volume. Having said that the widely spaced engines provide practical
fuel volume and also fuselage area that provides a lot of lift. And
Israeli F-15's returned from an exercise with an entire wing ripped of
by mid air collision with the pilot not even noticing that the whole
wing was missing, In addition the relative moments of inertia would
be less since the I didn't take into account the wings (which should
be the same) and the fact that the bulk of the weight ahead of the
engine. Just by halving the differences as a rough guess I think
would tend to produce results as follows:

I(row) = 1.9
I(triangle) = 1.55
I(single) = 1.52

The limitation on roll rate may not be aerodynamic or purely inertia
but structural. The F-14 probably has ailerons or differential
spoilers for roll control when its wings are forward but must rely on
differential opperation of the all moving tailplane when swept past a
certain point. Aircraft with fixed wings can also use their wing
controls wheras the F-14 may have been limited to using only its slab.

Nice analysis.

For the F-14: Spoilers, not ailerons + differential tail. But, given the
stab's area (same size as the entire wing of an A-4) and range of
deflection, 33 degrees leading edge down IIRC, not really a limitation. The
F-14's roll stab reduced roll rate at high speed, I think to avoid
overtorquing the fuselage.

SNIP All the WW2 stuff, though it's excellent.

R / John



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