How much does rear fuselage area contribute to fin area? - RC Groups
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Sep 12, 2011, 11:24 AM
Registered User

How much does rear fuselage area contribute to fin area?

Hello chaps.

Whilst I understand the various calculations for tail volume coefficients and such for calculating vertical stabiliser area, what I am not clear on is the extent to which the fuselage side area contributes to that area.

As an example, some aircraft, such as the Percival Mew Gull, and Pitts S1-11b have what appears to be very little fixed fin area;

But both have a relatively deep rear fuselage, which I assume contributes to directional stability in the same way as a fin.

In comparison to aircraft with a shallow (and I assume more importantly, rounded) rear fuselage such as a Chipmunk;

Seem to have a more conventional amount of fin.

So, when calculating tail volume, how is the side area and profile of the fuselage factored into the equation such that one arrives at a value which gives the right amount of stability?

Any thoughts?

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Sep 12, 2011, 04:51 PM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
I don't know a scientific rule but i think you will find that the general answer is that the fuselage side area contributes to directional stability but not by as much as you might think. There are a few examples of planes where the designer tried to dispense with the vertical stab due to very large fuselage side area and in all cases after test flying a vertical stab was hurriedly added. The Bee Bee R1 being a great example, there are others.
Sep 12, 2011, 07:59 PM
Cognitive dissonance
kcaldwel's Avatar
It wouldn't be that hard to figure out. Most fuselages will act more or less like a wing of an aspect ratio of maybe 0.1 or less. The MAC of the fuselage, where the lift will act, will be at about the 1/4 fuse length point. This will be very close to the CG.

If you crunch the numbers, you will see that the lift curve slope of a wing like that is really shallow: i.e. the lift doesn't increase very quickly with AoA. With very little lift being produced until large slip angles are reached, and the lift vector being very close (maybe even in front of?!) the CG, the directional stability produced by the fuse will be very low, or perhaps even destabilizing.

Sep 12, 2011, 11:28 PM
B for Bruce
BMatthews's Avatar
There's no doubt that the fuselage side area is a factor. Bot the part in front of the CG and that behind it. In one case, can't remember the name, a French airliner from the 'tween wars period had no fin at all. Just a swoopy looking fuselage with a rudder on the end.

And as you've mentioned there were other significant examples that had very small or no fins and yet they flew OK.

Our model and even the full size aircraft equations and charts all seem to ignore the fuselage as it's "assumed" (you know what THAT means I trust ) that the fuselage is "normal" and therefore doesn't contribute. But assumptions of this sort obviously fall fat on their face when confronted with Gee Bee R1's and other craft of that sort.

And if you study the "acceptable" range of vertical tail volume coefficients for models and full size craft you'll find that they cover a rather wide range of values. Much of this range relates to a bunch of other factors also not included in our quick and dirty vertical tail volume coefficient calculations.
Sep 13, 2011, 02:40 AM
rebell's Avatar
Fuselage can be very destabilising, especially if it’s width and depth is close to the same. With a narrower but deeper fuse all the way to the back, you can reduce the fin area, just have a look at the modern pattern models. Flying wings sometimes have no fin or tailplane at all, but a strong wing sweep is evident there. Adding a fuselage type extension to such a wing, depending on its size, placement, form and weight, might have destabilising influence or not. I have a slope model looking like an eagle with tail and all, but no fin whatsoever, and it is flying quite stable in the yaw axis. There are a lot of examples both ways and to many overall design influences that play a role to give a fixed answer.

As for your comment: “very little fixed fin area”. Fin area doesn’t need to be fixed. It can be all moving like an all moving tailplane. The fixed area like on a Pitts is mostly for structural strength on a lightweight frame.
Sep 13, 2011, 11:49 AM
Registered User
Interesting stuff chaps.

What it seems to amount to is that our old friend TLAR is about as good as it gets without getting rather complicated. I'm guessing then that providing that the fuselage shape isn't "extreme", then the normal volume calculations will be close enough.

What I was aiming at was trying to calculate whether a scale model of a model with a rather small fin (I was thinking of the Pitts S1-11b as it happens) would be ok with a scale sized fin, on the basis that the props we use are rather smaller than the scale size would be, and I was concerned that a deep/wide forward fuselage (such as that on the pitts) would leave the fin slightly blanked out.
Sep 13, 2011, 01:39 PM
B for Bruce
BMatthews's Avatar
With the size of our typical radio control models if the full size flies OK then the model will be fine as well.

There's lots of yaw damping issues related to mass moments in the wings to ensure that yaw oscillations are adequitely damped out. But that's where our models with their smaller size and generally lighter than scale structures win out. We almost always have lighter mass inertia moments in our wings and extremeties than the full size prototypes.
Sep 13, 2011, 04:45 PM
rebell's Avatar
...yaw oscillations are adequitely damped out
In a full-size, yaw oscillation is uncomfortable for the passengers, the pilot might get used to it. But in a model you don't really see it so much and it doesn't bother you as much as when you are a passenger. Because of that, if it doesn’t cause problems other that the visual, I will not worry about it too much.
Sep 13, 2011, 06:03 PM
B for Bruce
BMatthews's Avatar
Well, we don't see it a lot in models because the wing weights are a lot lower and the moment arm it operates over a LOT shorter. Which is why I suggested this above.

Hang around some free flight contests though and you'll see a few examples of it here and there. "We" try to keep the vertical tails small on contest duration free flights as it avoids any spiral dive issues if the models fall out of the steep climbs and also a small vertical tail volume for some reason seems to work with the typical assymetrical wing washin twisting to produce a glide which tends to self center itself in thermals.

Smaller catapult and old style javelin style hand launch gliders in particular benifit from keeping the tail as small as possible to where it almost but not quite has some slight amount of dutch roll in the glide.
Sep 14, 2011, 12:59 AM
rebell's Avatar
One can see it sometimes with a heavy(ish ) high rpm prop.
Sep 14, 2011, 02:52 PM
B for Bruce
BMatthews's Avatar
Yep, I've seen that with some aileron and elevator only controlled racer style planes. Or even with some heavier sport pattern style models.

Often if the pilot snaps the model over to a bank or back to level with ailerons only there can be some slight and rapid tail wag that damps out after a moment. That what you're thinking of? I've seen it on models where the tail is pleanty big enough just due to the nature of the sudden trasient forces from the snappy maneuver.

Come to think of it there WAS this one electric ARF racer that had a bit of tail wag at some points in it's flight. But that was years ago that I saw that one.
Sep 14, 2011, 03:42 PM
rebell's Avatar
Yes, that is what I am talking about. It is not so much the destabilising action of the fuselage, it is gyroscopic precession. If you see it with a plane banking rapidly, it is the down and/or side thrust causing it. It is the function of the tailplane and fin to dampen this. If you take a plane that demonstrates this and change nothing else but the fixed blade propeller to a folding propeller, you will not see it anymore. If, by the unlikely chance, you still see it, it was not precession in the first place, but something else.
Sep 14, 2011, 07:51 PM
B for Bruce
BMatthews's Avatar
Well, on such planes the prop is certainly spinning at a crazy rate so I'd not be surprised that the gyroscopic precesion is having an effect. It's not just the prop either. Consider that the motor armature is also part of the system.

Using a folding blade would appear to be a cure but I'm not so sure. It may just cause the effect to translate by 90 degrees and show up as something different.

It wouldn't be the first planes to suffer from such an effect. If you go back and read about flying the old WW1 airplanes that used the rotary engines as I recall they did not use any left, or was it right, rudder at all in flight. For a right turn it required a lot of right rudder. For a left turn you only held a little right for a coordinated turn. It was all because the rotary engine formed a huge flywheel with a strong gyroscopic effect. A flywheel which was something like 50% or more of the weight of the overall airplane. Such a strong effect in a modern lightplane would tax the ability of a lot of today's pilots for sure.
Sep 15, 2011, 01:21 AM
rebell's Avatar
I had a small plane with an oversized prop showing strong precession tendencies. It was tail wagging like a happy dog when coming out of turns or after pitch alterations. When I installed a folding prop it was al gone as far as the eye could see. Of course we don’t see everything.

Those rotary engines were massive. Yes, I think they couldn’t use much left rudder, which is also why pylon racing is mostly anticlockwise, the plane has a natural tendency to turn left, both because of mass forces of motor en prop but also the spiral airflow of the prop over the rudder. (Unless it has the same amount of rudder above and below the centre line of the fuse.) Pylon racers have a saying “Go fast and turn left.” Almost all engines and motors turn clockwise when viewed from the back. Only a few multiengine bombers from yesteryear had engines turning in opposite directions on each wing, and of course multi-motor electric models of today.
Sep 15, 2011, 01:57 AM
Registered User
Unless it has the same amount of rudder above and below the centre line of the fuse
I wonder if that might not be a major factor here, at least in tail wagging.

One model I have seen which seemed to exhibit this tendancy was also and ARTF racer, which had a lowish fin, set on top of the fuselage, with a "T" tail. Is it possible that a T tail would act as an end plate, rather than the spiral airflow spilling spanwise across the fin and creating a vortex at the fin tip, thus aggrevating the issue?

The only other time I have experienced tail wag is with V-tail slope models, some of which seem to wag a bit when coming out of a tight turn or at low speed (such as at the top of a stall turn downline)

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