Down Thrust

[John Muir]

Stuart, absolutely. So on a high wing plane with the engine low down, downthrust is good. It helps prevent pitch up due to rotational force when the throttle is opened. On a low wing plane with a high thrustline such as some flying boats and gliders with power pods, upthrust might be required to help prevent pitch down with power. And the nearer the thrustline is to going through the CG the better I suppose. That's the aerodynamicist's view of it according to what I've read.

On the other hand there is still a school of thought that says downthrust is good in all sorts of models as it stops or reduces the model's tendency to climb when power is applied. I just don't get that. On most of my models I trim them to fly straight and level at cruise and when I open the taps the plane accelerates, generates more lift and goes up. When I close the throttle it comes down. If I want go faster I put in some down elevator. I'd be confused if I opened the throttle and the plane didn't climb TBH.

[Cabbage Man]

Every one of my models, from a 3.5m competition glider, vintage model and scale ww1, has downthrust from 1 -5 degress depending on performance. I test fly with the motor in a neutral setting and adjust from their to suit personal preference. I like models to have a modest climb under power , say 20 degrees.

I have tried without and found you needed to keep constant and varied pressure of down elevator. If you didnt, you risked a stall.

How much is dependant on many factors, but are all unique to the model and your flying preference. On the test flight, I get the model into wind and at level flight . I then open the throttle to max and depending on what it does in terms of climb, determines the downthrust I want and prefer. As stated, a gentle climb is my preference. Unless your into F3A or 3d, that suits most models.

[Frank Skilbeck]

I must admit my gliders don't have any thrust built in , I have some on my motor gliders though 

But interestingly if you go on some of the fullsize forums the debate on downthrust is very active and Piper at various times included downthrust (PA11 have 4 degrees), but their discussions were more on reducing downthrust and using elevator trim in the cruise to give a higher speed, but they did note the payoff was trickier handling. Also it looks like on some of the planes although no downthrust relative to the center line was added, the wing incidences is such that the plane flies slightly nose down for better visibility.

Each plane is unique and I find some benefit from downthrust to stop then plane rearing up as power is added and others, mainly low and mid wing aerobatic planes need none at all.

Edited By Frank Skilbeck on 21/12/2015 11:37:22

[Biggles' Elder Brother - Moderator]

Hi,

Mr Argall's letter which is what prompted the thread took me and Tim H too task for comments in our respective articles in the mag, so I thought I'd chip in my halfpenny's worth!

The first comment I would make is that the article is part of a series, and in an earlier installment I had spent some time explaining how to set your elevator trim so that at approximetely 1/2 to 2/3 throttle you got level flight - the cruise state. So my comments on the behaviour of the model are premised on the fact that this is how your model is trimmed.

Secondly, I think Mr Ardall is confusing two different phenomina. If when you open the throttle the nose of the model pitches upward markely, then yes you have a problem with regard to the relative directions of the thrust and drag lines and their position with respect to the CoG. This is very undesirable behaviour, as I am sure we would all agree, and the solution is indeed to incorporate some downthrust to counter this. As others have pointed out this trait is far more likely to occur on high wing monoplanes where the centre of drag is high compared to the path of the thrust line. So adding downthrust lifts the direction of the trust line along the longitudinal axes and so helps to alleviate - or ideally eliminate - the problem.

But this behaviour is quite different from what I was refering to in the article. If the model is trimmed to fly level at 1/2 throttle, and has the correct downthrust (if needed) to counteract the "nose up" effect refered to above, then opening the throttle will result in the aircraft climbing. Note "climbling" - as in increasing altitude, not "nosing up" with is something quite different. This is perfectly normal - indeed desirable - behaviour. To try to add so much downthrust as to eliminate this normal climb behaviour is not at all desirable!

Mr Argall goes on to say that when he reduces throttle he does not get descent behaviour - he says his model just slows until it eventually stalls. Well, I'm afraid he's just answered his own question there! If the aircraft stalls, as he says, then he must be holding the elevator in to create the higher AoA needed for the stall! In other words what he would seem to be doing is reducing throttle, but adding elevator to maintain altitude - well if you do that then yes you will just see a speed reduction followed by a stall! But what I was talking about is that if, again, the model is trimmed to fly level on 1/2 throttle and then you reduce the throttle (without adding up elevator!) then the model will descend, usually in a slightly nose down attitude in order to maintain airspeed. Think about it; if what Mr Argall says was correct then if you simply chopped the throttle in mid-flight (and do nothing else) what would happen? Does your model just slow down then stall? No of course it doesn't! It slows, drops its nose slightly and takes up its natural glide angle. Descending, just like I said! Well "no throttle" is of course an extreme - all we are doing when we drop the throttle to below cruise is a milder form of that.

For me, and I suspect the majority of pilots, this is normal desirable behaviour! Its what we want to happen. Establish a cruise condition - throttle up from that produces moderate climb, throtle down produces a rate of descent that is controllable with the throttle.

Finally there is one bit of common sense that we can bring into play here - the Law of Conservation of Energy! If the model climbs then it is increasing its potential energy, that increased energy has to come from somewhere. It either comes from the model's store of kinetic energy (in which case the model slows down runs out of puff and stops climbing) or it comes via the engine pumping in more energy to the system - ie we open the throttle a bit, it climbs! Simples!

OK - you could put so much downtrust in (well beyond that needed to correct any misalignment of the thrust line relative to the centre of drag) so as to eliminate climbing of opening the throttle - but then my question would be "Why on earth would you want to do that?" !!

BEB

[Steve Hargreaves - Moderator]

Posted by Biggles' Elder Brother - Moderator on 21/12/2015 12:49:33:

Mr Argall goes on to say that when he reduces throttle he does not get descent behaviour - he says his model just slows until it eventually stalls. Well, I'm afraid he's just answered his own question there! If the aircraft stalls, as he says, then he must be holding the elevator in to create the higher AoA needed for the stall!

I had wondered about this too BEB & it occurred to me that he might have so much downthrust that he was having to compensate with "up" elevator trim.....as the power is reduced therefore the up trim simply holds the nose up with the inevitable result...

[Colin Anderson]

I've just read this post with much interest. I can't say that I've understood everything here, but it does seem to have answered some questions for me - particularly BEB's answer.

Recently I completed the build of RCM&E's Chapter One - with the aileron wing and tail dragger set up. I got one of our club instructors to maiden it for me (I'm still a novice) and he had it flying perfectly, but it did, to my eyes, have quite a bit of down trim on the elevators. Heading into wind and opening the throttle slightly showed a tendency to "baloon" or pitch up under power. As I recall, the take off was textbook.

As soon as conditions were favourable I took myself up to the strip to fly my model myself. In calm conditions I found the plane a joy to fly, but take offs were another matter entirely. It was only the fact that the ground slopes away on one side of our strip that I was able to get the plane aloft, at full throttle! Regardless, I made 4 beautiful flights which, as a novice I was proud of; on the fifth flight I got things too low, too slow caught a gust under one wing and cartwheeled into the ground. I've since repaired the plane and it flies well again - my flying session only being cut short when a collet came loose and the wheel fell off! However, the plane was still exhibiting a reluctance to "unstick".

I've since given this some thought. Could the elevator trim be causing the problem? To my way of thinking, at the point where I needed a neutral elevator , with the amount of trim on the elevator I was probably trying to take off with down elevator or insufficient up elevator. Thinking this way, I've now zeroed the elevator trim and increased the motor's down thrust. I've yet to try it out, but it does seem in reading all your comments that this might help to alleviate the problem.