Posted by: ik Jagdgeschwader 77

Message:

Anyone else notice that the Mossie has less roll inertia than the Yak? Let's see, the yak has TINY wings, and the guns are in the nose. The Mossie has a large wingspan, and on top of that has two engines on the wings. This makes no sense to me? Am i too feeble minded to realize the explanation to this paradox?

ik

Posted by: funked =925 CABS=

Message: Ik - I'm not sure what you are observing - does the Yak keep rolling more after you release the stick, or is it slow to initiate the roll?

The following discussion covers either case.

Some definitions:

T (lb-ft) = Roll moment from ALL aerodynamic forces.

I (lb-ft-s^2) = Moment of inertia about roll axis.

alpha (1/s^2) = angular acceleration about roll axis.

Governing Equation:

T = I * alpha

Ik, you are observing alpha with your eyes, and estimating T from your control inputs.

No doubt your alpha estimates are pretty good, but T is hard to do for two reasons:

1. Any two planes have different control systems. A given stick deflection will result in different roll moments on the aircraft

2. Note that T is not just the roll moment due to aileron deflection - it also accounts for opposing forces due to drag, etc. - those forces that tend to slow a plane's rate of roll once the stick is released. These forces oppose the direction of roll and are generally proportional to roll rate.

So my theory is that either/both:

A. The Mosquito has a greater aileron "gain" than the Yak, i.e. more

roll moment for a given stick deflection. Think of the FW190 vs. the Zeke.

B. The Yak has lower aerodynamic resistance to rolling. So once you release the stick, it keeps rolling, giving the impression of large inertia. Kind of like the P51's behavior.

Or,

C. Anticommunist forces have conspired to restrain the dominance of the most badass dogfighting machine of the late-war era.

funked =925 CABS=

(design engineer, aircraft actuation systems)

Posted by: nrts - =4th FG=

Message:

: Anyone else notice that the Mossie has less roll inertia than the Yak? Let's see, the yak has TINY wings, and the guns are in the nose. The Mossie has a large wingspan, and on top of that has two engines on the wings. This makes no sense to me? Am i too feeble minded to realize the explanation to this paradox?

: ik

Well you've got your roll inertia and it is dominated by the polar moment of the wings which depends on the distributed mass of the wings, i.e., guns out on the wings (long moment arm) vs. guns on fuselage (short moment arm). This is the actual roll moment of the vehicle.

But, then you have the "apparent" roll moment which is influenced by control surface area (aileron area). For example, you correctly state that the Mossie should have a hefty roll moment and I agree. The apparent roll moment which manifests itself as roll-rate is determined by the ratio of the torque the ailerons can command about the roll axis to the resistance to roll or roll moment. Yeesh! What I'm saying is that the Mossie must have lots of aileron area for such a low resultant or apparent roll inertia. That should translate to large drag forces when these surfaces are actuated meaning that this airframe should bleed energy in a turn like no other. Hows that for an Electrical Engineer? :)

-nrts-