Lengthened fuselage to improve longitudinal stability

[TD&H]

I know this is about old aeroplanes, but I wondered if a test pilot could explain the aerodynamics in benefits and deficits of lengthening the fuselage to improve longitudinal stability?

In particular I have read how the FW190 had an insert of a parallel section of fuslage to extend the fuselage just forward of the tailplane. This was added to the later marks, presumeably to compensate for the longer and more powerful engines in the -D and Ta152 versions.

Now I wonder whether the same could have been done for the Spitfire? With increasing power from the two-stage Merlins and then the Griffins, there were longitudinal stability problems. Now I'm aware of the difficulties Joe Smith and his team had, so this is not a dig at them at all, and by getting some answers it would help explain why they didn't go down this route. I know the later marks had larger tailfeathers, which were used for just this problem.

My thoughts are that a longer fuselage, especially if this is done by using a parallel section insert just in front of the tailplane (for ease of manufacture and modification), would have given greater rear keel surface to offset the effect of more prop blades and longer engine. It would have provided some rearward CG shift to compensate for the heavier engine and prop without using ballast.

Now the Spitfire is reputed to be very light on the elevators, would the lengthend fuselage distributed the weight over a greater length to dampen the lightness, but also would it have made it lighter still by putting the control surfaces further aft with greater moment arm? Would the longer fuselage provide damping in pitch against the instability caused by the rear fuel tanks? Any general comments or thoughts on what this 'mod' would have done, not only to a Spitfire, but to any other aeroplane similarly increased in power and weight?

Many thanks for any responses.

Cheers, H

[Mad (Flt) Scientist]

The additional surface area added by extending the rear fuselage is essentially valueless for helping longitudinal stability. In terms of the longitudinal axis, you can almost disregard the aft fuselage - simplistically, you have a forward fuselage (longitudinally destabillising, but it doesn't lift too effectively), a wing (obviously a good source of lift!) and a tailplane (nice and stabilising). If you didn't need something to fasten the tail to, you wouldn't really need the aft fuselage.

Adding to the "tail arm" by increasing the distance from wing to tail makes the tail generally more effective in stabilising the aircraft and also more powerful as a control surface. Simplistically, a 10% increase in "tail arm" is worth the same as a 10% increase in tail area. Since it's usually easier to lengthen a bit of fuselage than redesign the whole tail, that's what people usually do.

As to what would it have done to the Spitfire. Made it a bit more stiff in pitch (higher frequency short period) and given a bit more pitch control power. All depending exactly where the cg was. Would have made the ground angle less nose up - might have been good or not, no clue about taildragger behaviour. They didn't do it, and it's a relatively simple mod, so they clearly didn't want to do it, and probably didn't see enough of a need to do it.

[safetypee]

The BAe146 / Avro RJ series aircraft came in three lengths; most other things being unchanged. The longer variants had or appeared to have increased stability; the doubt that I introduce comes from some difficulties with control cable friction due to a longer fuselage run, cg range, and the balance between tail arm and higher mass – any inertia effects. I don’t have the details to clarify these points.
Thus the longer aircraft were more sluggish than the shorter variants. Relating this to the fighter world then the shorter BAe 146–100 (RJ70) was for more ‘fighter’ like, with much quicker pitch response than the longer variants. Thus this appears to support the theory that fighters should trade stability, be less stable, in order to have greater manoeuvrability.
If the FW190 fuselage extension was in conjunction with more a powerful engine, could the mod be associated with directional stability?
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Airspeed and Upwardness

[411A]

Can't say about fighter aeroplanes, but the DC-6B's handling was improved considerably over that of the original DC-6.
Even the TAS (same weight and cruise BMEP) was increased slightly by the longer fuselage.

And then we come to the original B707's.

The original short body machines were a real handful, especially during takeoff and landing. Higher cruise altitudes were a problem as well.

With the later long body machines, handling was vastly improved, not just by the longer fuselage, but by the taller vertical fin...and much improved yaw damper.

[TD&H]

Thanks for the replies, gentlemen.

I should, of course, have titled it as a question about directional stability as well, for the lengthened nose and increased number of propeller blades meant the fin and rudder had more work to do. Thus my comment about increased keel area. The comment about ground angle would certainly have been of consideration for not reducing the limited prop/ground clearance.

Cheers, H

[TD&H]

Five Star Rating?

Just curious as to who (and why) my question, and more importantly the answers have been awarded Five Stars.

I rarely got that many at skule!

[Genghis the Engineer]

Not me !!!!!!!!!!!

G