Airplane Design Questions

[FredN]

I have been doodling planes since I was a little kid.
I started making them into models. I have read some
posts about "fine tuning" the design. I have some
basic questions:

1) What happens if the fin/rudder is to small?
2) What happens if the fin/rudder is to big?
3) What happens if the stab/elevator is to small?
4) What happens if the stab/elevator is to big?
5) What happens if you raise the stab?
6) What happens if you lower the stab?

Are they any pointers to sites/books that discuss these
questions?

thanks for the help,
Fred

[Ollie]

These questions should be considered in the context of the purpose of the model and and the priorities of the designer.

In general, if the vertical tail is too small for the wing span, CG location and tail moment arm, the plane will have insufficient yaw stability and damping. it may also have insufficient yaw control power.

If the vertical tail is too big, there will be excessive drag and the handling characteristics in maneuvers requiring small or neutral yaw stability may be un desirable. Too large a vertical tail is rare indeed.

If the stab area is too small and the tail moment arm is too shart in relation to the wing area and wing chord for a given CG location, then the plane will lack sufficient pitch stability. In extreme cases the plane with too little pitch stability will be uncontreollable.

If the horizontal tail is too big there will be excessive drag and there may be un desirable handling qualities for aerobatics.

If you raise or lower the horizontal tail the effects will be minor in most cases. There may be exceptions for specific types of aircraft.

Model Aircraft Aerodynamics by Martin Simons and R/C Model Airplane Design by Andy Lennon are a couple of good reference books.

[Peter D Rieden]

<1) What happens if the fin/rudder is to small?>

The aeroplane will lack directional stablity - it will not tend to point in the right direction while flying. In extreme cases the aeroplane may turn around and fly backwards, in mild cases it may be "vague" and prope to draggy side-slips. There could be a tendency to dutch-roll if the aircraft has dihederal or sweep on the wings. It might also lack rudder authority and (for completeness) it could also suffer directional stability problems when going transonic due to CP shift, but I doubt that would be a significant issue in this case.

2) What happens if the fin/rudder is to big?

The aeroplane could be spirally unstable - it could be prone to dropping into spiral dives.

3) What happens if the stab/elevator is to small?
4) What happens if the stab/elevator is to big?

These two aren't a simple question, but essentially the size and moment arm of the tailplane determine the stable CG range and the degree of control available. As the tail area gets smaller the CG position becomes more critical, although this can be offset (to a degree) by reducing the aspect ratio of the wing and/or sweeping the wings. I've flown models with tailplanes as small as 5% of the wing area and as large as 100% of the wing area. All can be made to work, but not all work well. The 5% example (a thermal soarer) was difficult to fly in turbulent air.

5) What happens if you raise the stab?
6) What happens if you lower the stab?

The vertical location as such isn't important, but ensuring that there's roughly the same amount of "keel area" above and below the tailplane reduces the probability of undesirable secondard effects from the rudder. If you look at a Miles Magister you'll see the tailplane is mounted at the top of the fuselage and in front of the fin, so there's no keel area above the tailplane. This aeroplane would pitch nose-down when the rudder was used - almost to the point of uncontrollability. The reasons for this are too lengthy to go into here - just keep it in mind.

In general the best way to determine whether a particular design will be flyable is to knock up an 18" span chuck-glider version in 1/16" sheet. Throw this into unusual attitudes and see if it does anything nasty. Remember you're noit looking for perfect, stable flight (because a strongly stable R/C model is boring to fly) you're only looking to see if it flies sideways, drops into spirals or spins, or has divergent characteristics.

<Are they any pointers to sites/books that discuss these
questions?>

The martin simmonds book isn't bad, but a better source for this kind of thing is "The Design of the Aeroplane" by Daryl Stinton. This is actuallu about building full-size aircraft rather than models, but it is an excellent source and a throroughly interesting read anyway!

PDR

[Ed_Moorman]

Let me add my 2 cents to the great answers you have already gotten.

With regard to vertical tail too large, I can relate from a personal experience. I had a small. .61 powered, original design acro plane several years ago that, unfortunately, had some reverse roll with rudder-not enough dihedral. I had hear about adding fin/rudder area below the center line of the aircraft to correct this, so I gave it a try. Doesn't work.

As I added more and more area, I got into spiral instability. This is recognized by the bank increasing throughout a turn. You roll in, neutralize the ailerons and add back stick. The plane turns, but the bank slowly increases as you go around the turn. In a rolling dive like we all do for a low pass, for example, when you pull, the bank increases. This can be disconserting near the ground. My plane had strong ailerons so it wasn't a problem, but on a plane with lesser control authority, it could have been. I finally cut off the excess fin and changed the dihedral. This, by the way, was only painful this first time. I regularly chop wings and fix dihedral now, especially to add anhedral to high wing planes.

As for horizontal tail size, for my flying, powered acro planes, I have yet to find a down side to a large tail. I tend to play with a rearward CG and from my experience, I have found that a large stab gives you much more room to play with. If you are up around 25% of the wing area or more, you can really move the CG. I have gone as far as 50% back and still been flyable for me. It was uncomfortable, but I flew a giant, original acro design this way after an engine change from gas to glow for 6 flights before I fixed the CG. You could blip in a little up, or down, and release and the plane would loop on its own. On landing, you "propped" the tail up with down elevator on final.

[Dick Huang]

Fred,
All the advice you've received are very sound and you should follow it in your design efforts. I am going to enclose two figures that are based on over 100 FF models as to the tail size (vertical and horizontal) this will allow you to put some numbers to your design.

Dick Huang:)

[Dick Huang]

Fred,
The second plot