Altered title - Can a low wing model be a good trainer?

[Tony Jones]

There's no reason why your first trainer shouldn't be low wing. Intuition says that high wing models are more stable than low wingers because of the 'pendulum effect'. In fact, there is no such thing as the pendulum effect. The position of the wing does not effect stability. Longitudinal stability depends on the relationship of the C.G to the aerodynamic neutral point and lateral stability depends primarily on dihedral.
Real high wind aircraft like the Cessnas have the wing up there because it provides a better view downwards and makes getting in and out easier.
Best wishes
Tony Jones.

[David Ashby - Moderator]

I guess it's all in the design Tony. Low-wingers are normally faster so they give a learner less time to think and react. Can you think of a slow low-winger that could be used as a trainer by learners?

David.

[Evan Pimm 2]

Best training plane ever designed, get an original (not SIG) drawing of the Astro Hog, leave out the removable top, build it as per, and then with a couple of hours dual you can use it all the way up to advanced pattern, and with a good 0.46 too.
Evan.

[Tony Jones]

David

Absolutely: the Seagull Spacewalker II for a start. Flies as slowly as any high wing trainer on a .46 but CAN fly faster and do decent aeros if you install a bigger engine and increase the control throws above 'trainer' level. With a .62 4 stroke, scale airspeed is attained at half throttle with a delightful burble.
Also, tail draggers teach the correct use of rudder and elevator on landing and don't try to dig their (non-existent) nose wheels in on takeoff.
But then I was trained on a Tiger Moth!
Best wishes
Tony

[Tony Jones]

David
Furthermore, not withstanding and without prejudice, etc, etc....
The Spacewalker is also a semi scale version of a real aircraft and (IMHO) looks pretty - something the average high wing nosewheel trainer ain't. If easy engine access is deemed important, it could be built with the engine upright initially and no cowl, followed by inverting the engine and adding the cowl.

Also... You say low wingers are normally faster but they don't have to be. A high wing doesn't automatically bring good low speed manners either. As far as I know,(Peter Miller may/may like to confirm) stalling speed depends primarily on the shape of the aerofoil and the wing loading. And any low speed 'devices' like flaps, of course.

Another thing I've never understood is the tradition of starting people off on 3-channel models. Why is applying bank through the secondary effect of yaw supposed to be easier than using ailerons? Just a minute: a high winger with rudder only, followed by an 'aileron trainer', followed by 'your first low winger' is three models sold rather than one. Makes perfect sense now I think about it. ;)
Best wishes
Tony

[David Ashby - Moderator]

Looks like a good article there Tony, we could test a few Low-wing models to see how beginners managed with them.

Can you explain how the high-wing configuartion has been adopted as a classic trainer type? Are we all thinking 'Cessna' without reasoning why perhaps?

David.

[Bob Cotsford]

how did we end up with the wing on top? Easy, the simplest way to build it. Since trainers used to have flat bottom wings, it was easier to build a box with a flat top and strap the wing on, low-wingers needed a cutout to suit the wing section. That became the accepted norm for trainers. Possibly the high wing also helped when flying from uncut fields (as ours is at the moment) so high wings were less likely to get puncture on landing. Then there is the fact that low wingers are harder to hand launch as you have to hold them behind the CG.
As for being as stable, possibly, but I seem to remember high wingers being easier to set up on rudder only. Pendulum effect is a fantasy? You live and learn.

[Tony Jones]

Dave and Bob

I reckon Bob has most of the answer to the high wing thing, plus your bit of 'thinking Cessna without reasoning why'. Or rather, thinking a Cessna-like model must be a good trainer 'cos thats they are used for ab initio training in real life.

But Cessnas are not primarily trainers; they are designed as privately owned light aircraft and have high wings due to customer demand for a good view downwards. And in RL, a high winger is much easier to land than a low winger. Again, better view of the runway and less tendency to float in ground effect. That last characteristic is, come to think of it, as valid in RC as in RL.
But don't you just hate those springy, wobbly wire nosewheel legs? I don't mind high wing tail draggers although I still maintain (or rather far more knowledgable people than I such as Alisdair Sutherland maintain) that wing position does not affect stability.
Best wishes
Tony

[Tony Jones]

Please excuse hurried, non proof read posts with typos, extra/missing words etc!

[Bob Cotsford]

I've got a Flair Hooligan, which is high-wing and most definitely has neutral (if any!) stability. I had a low winger some years ago called a Peashooter, from an american kit. I could almost take off, feed in a little rudder trim, put the transmitter down and walk away for the tankful. Only lack of faith stopped me doing so on calm days.

As for typo's, depends how close the boss is watching and thus how much of a hurry I'm in to change screens!

[Tony Jones]

That's a beer I owe you Bob.
Cheers
Tony

[FlyinBrian]

I have had relatively slow flying low wing models and some very fast shoulder and high wingers. Indeed I currently fly (amongst others)a Falcon Aviation "Eurobat", fitted with a 35cc petrol engine this is very docile and would make a great trainer even at an AUW of c13lb It will fly fairly quickly but the 3" deep wing limits the top speed and allows it to fly slowly when required.

Brian

[Peter Miller]

I think that probably to origins of the high wing trainer started with the progression from free flight and the early R/C models. The early trainers were basically free flight designs and so were stable.
I am not too happy with the suggestion that low wing models are as stable as high wing. I base this staement on the fact that a high wing free flight model only needs about 5 degrees of dihedral while a low wing model needs 10 degrees.
I think that these days one good reason for using a high wing model is orientation. It is easier to recognise which way up ahigh wing model is, well, I find that anyway.
On the other had high wing models can be just as aerobatic as low wingers.
I do feel that tricycle undercarriages are a total waste of time unless one is flying from a bowling green or smooth concrete.

[Peter Miller]

Tony.
AS I understand it, stalling speed is governed by the airfoil shape and the angle of attack.
Each airfoil has a stalling angle, the angle at which the airflow breaks away from the top surface.
Wing loading only comes into it because to support more weight (higher wing loading) at a given speed requires a higher angle of attack.
Therefore to maintain level flight with a higher wing loading without increasing the angle of attack the aircraft has to fly faster.
Thus a heavier aircraft has a higher stalling speed.
I think that I have got that right...well, I know what I meant to say.

[Tony Jones]

Peter

I hadn't thought about the derivation of RC trainers from free flight models, but it's obvious now you've pointed it out.

I will put your point about dihedral to my aerodynamics guru (if you don't mind) and see what he says. Why is it that a high winger needs less dihedral to achieve the same stability as a low winger?

Thanks for introducing AoA into the speed equation. As you say, for any aircraft with a fixed aerofoil section, the heavier the aircraft, the higher the stalling speed. And I have seen some very overweight, high wing so-called trainers. Especially after they have been bodged up a couple of times after accidents!
Cheers
Tony

[Bob Cotsford]

"Why is it that a high winger needs less dihedral to achieve the same stability as a low winger?
"
pendulum effect ;)

[Peter Miller]

I think that you will find that the fact that that the high wing model has a centre of lift above the centre of gravity while the low wing model has the centre of lift below the CG has something to do with it.
In other words, the pendulum effect as Bob says.

[Tony Jones]

Peter
Now we come to the nitty gritty! I am not an aerodynamacist but I know people who are. Alisdair Sutherland is one. Here are some extracts (not selected to be in my favour) from his book Aeronautics for Modellers. Chap 9, para 2A (p52): Lateral Stability.
'Fig 9 represents the rear view of an aeroplane which while flying happily along in trim, has been upset and now has a a slight bank to the right'.(I'M SURE WE CAN ALL VISUALISE THIS. TJ).
'I can see no force producing a moment which will tend to correct this bank. I happened to have drawn a high-winged aeroplane with the wing above the CG. It may seem obvious that this in itself will provide stability, but is there a moment about the the CG? The weight cannot have a moment about the CG (by definition)and since the lift acts on the centreline of the aeroplane, it can not provide a moment either. The net moment is therefore zero and there can be no restoring roll effect'.

However, later in the same chapter, while describing the effects of sideslip on yaw and roll, he points out that when an aircraft sideslips, drag now acts rearwards AND sideways. If the centre of drag is above the CG there will be a righting moment. He then says: 'This kind of stability is sometimes referred to as 'pendulum' stablity for a reason which I fail to see, since aerodynamic drag has little effect on pendulums'.

As to the effect of wing position on aircraft with NO DIHEDRAL, he says: 'On a low-winged aeroplane you find that a sideslip to the right causes a clockwise rolling moment which is destabilising', because the centre of drag is usually below the CG. But he points out that this can easily be overcome with dihedral - which agrees with Peter's comment on the 10th that high wing models need less dihedral than low wingers.

So I'll modify my original point and say that low wingers CAN BE (not ARE) as stable as high wingers with the right amount of dihedral. But I stick to the opinion that pendulum stability is a myth. It's all down to the righting effect of sideslip.
Best wishes
Tony

[Peter Miller]

I would never argue with Alasdair on the matter of aerodynamics.
There are an awful lot of myths and wierd theories that used to go about in the days when aeromodellers were into that sort of thing in a big way.
For example, I remember one very strange one.
Lift acts verticaly, it has to because it counteracts gravity.
Dihedral works because in a bank the low wing presents more area because it is jnow flat while the other wing is at a greater andgle and so presents less projected area.
Don't laugh, I remember seeing that in a magazine. I think I may have even believed it for a time.

[Bob Cotsford]

Ok - lets take the cofg as being along the fuselage centreline and the centre of lift along the wings chord line and span centreline. I'm visualising the model in a bank as requested. With a high wing model I think I see the centre of gravity now held out to the outside of the turn and the centre of lift on the inside. Whether you take the centre of lift or the centre of gravity as the fulcrum point, there has to be a self righting moment, which is reversed for a low winger with cog on the inside and cofl on the outside of the turn.
Of course all this ignores things like drag and sideslip, but these ideas were bred when models generally flew slowly and drag was less ofg a concern. I don't claim to be an aerodynamacist, but it seems to make sense to me. Of course the faster your model flies, the more drag factors will have an effect on the handling, but I'd have thought that the pendulum effect has to exist and influence stability to a degree.
I wonder if this is why a lot of aerobatic machines seem to get the CofG, CofL and CofD close together? Mid wing should result in perfect neutral stability and handling.

[Tony Jones]

Bob
What you say seems intuitively correct. But with the model in a balanced, banked turn the 'gravity' it experiences isn't directly downwards, it is on a line through the vertical plane of the fuselage. Thats why the pendulum effect is a myth - or so I'm told, (and convinced by) far more knowledgeable people than me.
Alisdair accepts that for any given dihedral angle a high wing aircraft has more lateral stability than a low winger but it has nothing to do with pendulums and everything to do with aerodynamics - primarily the self righting effect of side slip.

However, I have come across instances of conventional wisdom being proved wrong so many times before, my mind is always open - I hope.

I mean, for an observer on the surface of this planet, what evidence is there that the earth goes round the sun?
Best wishes
Tony

[Peter Miller]

I have said that I would never argue with Alasdair Sutherland BUT and it is a big BUT, I would argue even less with A.C. Kermode.
I have beend doing a little reading and Kermode says.
"Aeroplanes in which wings are high and the centre of gravity low may be stable without any dihedral angle. Many high wing monoplanes are examples of this type. The weight being low acts like a pendulum while the aeroplane sideslips, the resistance of the wings to their motion through the air holding them back and thus supporting the pendulum. As might be imagined, this method of obtaining stability tends rather to a rolling motion from side to side."

From "Flight without Formulae" Which has been in continuous publication from the 30s right through until at least the 90s.

[Tony Jones]

Peter

Excellent! And it makes sense - at least at the moment of lateral disturbance and during the sideslip. Are you going to tell Alisdair or shall I? I bet he says he was talking about balanced turns.

Are we at least agreed that the so-called pendulum effect does NOT happen in balanced turns?

Best wished
Tony

[Tony Jones]

Peter

I've just ordered Flight without Formulae from Amazon (£24.99. I lost my original many years ago. They also have The Mechanics of Flight by Kermode, Barnard and Philpot. Any idea how this differs from FwF?
Cheers
Tony

[Bob Cotsford]

as I said in a previous post, my Flair Hooligan's flight characteristics argue that pendulum effect doesn't exist, and riding a motorcycle also supports what you say about centripetal force changing the effects of gravity - until you slow down! I still think that pendulum effect exists, but that it's influence on model behaviour varies with airspeed and model weight, hence it was of more concern in the days of tissue covered freeflight and single chanel models with comparatively low power.