The relationship between power and downthrust, if any.

[David Davis]

I am familiar with downthrust having flown free flight models in my youth. I will be seventy-six next month! 😄

 

The model in question is a Ben Buckle Radio Queen, powered by an electric motor and built by my protogé Frans Cooremans, a seventy year-old Belgian. I'm very much an i/c engine man myself but having fitted three electric motors to this model I'm beginning to become more knowledgeable! Initially we fitted a motor with a similar power to that recommended by 4-Max. https://www.4-max.co.uk/po-5055-595.html Even with a 3S LiPo this set up proved to be greatly over-powered. if you advanced the throttle the model would climb rapidly and if you didn't correct things quickly enough it would stall. 

 

We installed two less powerful electric motors ending up with an Axi 2820/10 which I had been given a long time ago and while flight characteristics had improved they were not really suitable for a beginner. If I flew it I could anticipate the stall, reduce power and put in down elevator but Franz doesn't have my experience so sometimes he was flying about in a series of barely controlled stalls. We were trying a smaller propeller yesterday when the Axi blew apart on take off. We were unable to find the rotating piece (the can?) so unless we find it we will have to buy another motor. When weighed on my kitchen scales in November the model weighed 5lbs or 2.2 kgs and the centre of gravity is as per plan about 4.5" (11.5 cms) from the leading edge.

 

So some questions for the cognoscenti:

 

1. Would this be a suitable motor for the Radio Queen given that the motors we've used so far have been too powerful? https://www.weymuller.fr/article/moteur_brushless_protronik_dm2615_1050.html  I'm thinking that 50 watts per lb will be adequate for a vintage model like the Radio Queen. If it's not suitable what would you recommend?
2. Should we incorporate some down-thrust, if so how much? We are going to have to rebuild the motor mount anyway.
3. Is down-thrust required only when power is excessive? The original Channel-crossing Radio Queen was powered by an ED Racer and having just inspected the plan, it states that the engine bearer spacing is suitable for a 2.5 cc diesel engine.

 

Incidentally, I had a look at one of the club's four channel ARTF trainers yesterday. It's powered by an OS 46 AX. It had a substantial amount of down-thrust, I would estimate 5 degrees or more.

 

 

 

[Andy Stephenson]

The amount of down-thrust depends on two main things. First, the height difference between the main drag of the model i.e. the wings and the thrust line. Second, how much lifting section the aerofoil has.

The drag of the wing on a high wing model will tend to pull against the thrust of the motor and the faster you try to pull it through the air the more it will rotate the model upwards. Similarly, the more "lifty" the wing section is, the more it will try to lift with increasing speed. With r/c models having variable throttle, more power than originally designed for and particularly with a floaty vintage model that may have been designed for single channel, you can see how the problem arises.

The best trainer I have flown had a symmetrical section which substantially reduced any tendency to "zoom".

 

[Shaun Walsh]

Refit the 4-Max motor with a few degrees of downthrust and try again, repeat until happy. You can always limit the power by throttling down or reducing the throttle range in settings.

I have one model which climbs on full throttle and it wasn't possible to increase the downthrust so I mixed in some down elevator to the throttle which keeps it level as you wind the power up. Mix can be turned on and off as required.

[Don Fry]

If it’s as memory, it has an under camber on the wing. It is what it is, a model design for single channel radio guidance. It has a power setting where it will climb nicely, loose hight in a rudder turn, ad infinitude. A bit of down trim will make it use more power to climb, and it’s very slightly faster. Trainer it is not, unless he is being trained as a single channel pilot. As you say yourself David, you, with your experience live with it and compensate. For a biginner, I suspect something like an Foss Acrowot, modest power would be easier.
BTW, I’m not saying Acrowots are trainers, just easier than a Radio Queen.

[David Davis]

Thank you for the replies.

 

I used to have a Radio Queen initially powered by an OS 52 Surpass. Pictured below with my much younger self. It flew ok well throttled back but it flew better when fitted with an OS 40 Surpass. I used it to give elderly beginners their first experience of wafting around the sky.

 

I have just consulted the plan and have discovered that the engine bearers are spaced for an ED Mark III 2.49cc diesel engine. I believe that the Channel Crossing model was powered by a 3.5cc diesel. I am beginning to think that perhaps a far less powerful electric motor is the way to go. Something which produces about 250 Watts. Less power, less thrust less chance of violent climbing.

[Chris Freeman 3]

If you want to reduce power you can just fit a smaller prop or even better reduce the pitch, that way the CG will be the same

[Peter Jenkins]

David

 

Reduce the wing incidence by packing at the rear as we used to do in the old days.  Keep reducing the wing incidence until you can apply power without the model pitching up immediately.  Down thrust and wing incidence are sort of interchangeable adjustments.  Your problem isn't the power it's the mismatch between the wing incidence and the thrust line.  

 

I know it's very far from an aerobatic model, but that's what we do to reduce the effect of power on the aircraft's attitude.  You will find that it will be fine with the old spec motor if you dial down the incidence till the application of power has no effect on the aircraft's immediate attitude.  As the model speeds up with more power, it will climb as the lift increases but not nearly as violently as you describe.  Equally if you close the throttle it will not dive suddenly.  You will be pleasantly surprised at what reducing the wing incidence will do to overcome your problem.  Indeed, it will just go away!

[PatMc]

Don't bother messing with the downthrust & definitely don't faff with the wing incidence, you only need to reduce the available power to a suitable level. Just try fitting a lower pitch, smaller dia. prop. You will have to experiment with a few to see what works best. For preference try ones designed for ic.  

Some F/F people even used to fit the prop on back to front as a make do temporary field fix.

[Peter Jenkins]

1 hour ago, PatMc said:

Don't bother messing with the downthrust & definitely don't faff with the wing incidence, you only need to reduce the available power to a suitable level. Just try fitting a lower pitch, smaller dia. prop. You will have to experiment with a few to see what works best. For preference try ones designed for ic.  

Some F/F people even used to fit the prop on back to front as a make do temporary field fix.

On what basis do you make your statement about not bothering with adjusting wing incidence please?

[john stones 1 - Moderator]

Laws of Physics are facts, they ain't made up by fussy folk nor can they be ignored with no effect. What you choose to do about them is when the debate starts. "If any" does not apply.

[Simon Chaddock]

If a stable plane is in trim, flying at a constant speed and you apply more thrust (power) it will speed up. Its natural stability will cause it to climb to regain its "trim" flying speed. It will continue to climb as long as the higher power setting is maintained. The more power the steeper will be the climb.

However like any 'balance' system a rapid application of significant power will cause the system to over shoot before the corrective forces have time to make an effect. Stability forces are very small compared to those from control surfaces. Thus applying too much power too quickly can lead to the plane stalling although there is nothing fundamentally wrong with the setup apart from the throttle application.

Models that have free flight heritage will have stronger stability designed in and thus suffer more from throttle effects.

 

The correct down thrust angle can mitigate this effect although technically it only does so at one speed and power setting. Down thrust will not exactly balance the aerodynamic stability effects at all air speeds and power settings. A compromise setting found by trial and error can still be very useful.  

 

[Peter Jenkins]

Hi Simon, sorry, have to take issue with you in your post above.  I agree that applying power from a set trimmed speed will cause the aircraft to accelerate until such time as the the increase in drag force now equals the additional thrust force.  The increased air speed will generate more lift and that's what makes the aircraft climb not natural stability but more lift will also increased induced drag adding to the profile drag caused by increased speed.  The natural stability will act to try to achieve a new balance of forces.  The increase in air speed will also increase the downforce generated by the tail plane (it is always a down force in a stable aircraft) and that will tend to reduce the wing's incidence and hence lift and thus counter the climb until a new state of equilibrium of forces is achieved but in this case with the aircraft climbing.

 

With model aircraft, we have hugely more power than most light aircraft has so the effect of a sudden power increase is much worse for a model aircraft.  You can make an aircraft very stable such that the stability forces are moderately powerful e.g. a forward CG.  Moving the CG aft, will then reduce the power of the aircraft's stability.

 

For Free Flight aircraft, you want high stability as there is no way of controlling it once it is released so it's stability is all that will keep it flying safely.  These aircraft are generally designed to fly at the lowest sinking speed once power has been lost and will be trimmed to climb relatively steeply under power to gain the required height before the engine cuts or the rubber band has exhausted its power and so have a relatively high wing incidence to allow the fuselage to not be too tail down.  You want the fuselage to produce the least amount of drag from both form drag and skin friction.  Having the fuselage level with the airflow as the aircraft descends, reduces the fuselage's form drag to the minimum.  Using such a model for flying the aircraft as a radio controlled as opposed to guided aircraft means you have to re think the basic aircraft set up. 

 

Stability remains important but response to controls is also important.  Many have flown models with the CG set too far forward and find them unpleasant to fly.  With the CG in the optimum position, while stability will remain positive it will not be as powerful in the case of a free flight model.  The issue David was seeking to fix was that the application of power above a certain minimum caused the aircraft to climb markedly.  The solution is to reduce the wing incidence so that the aircraft can be flown at a greater range of engine power without the controls being overwhelmed by having to apply full down elevator to hold the aircraft level.

 

Positioning the CG is one of the most powerful trimming tools available to us but many pilots are unaware of this fact.  All aircraft will have a range of CG where with a forward CG you could reach the point where on a tricycle undercarriage aircraft you have to apply almost full up elevator to get off the ground.  This makes rounding out for landing very difficult if not impossible and you either need to have more elevator movement or move the CG a bit further aft.  Of course, you get to a point where the CG leaves the aircraft neutrally stable which means it will not correct itself if you disturb it and if the CG is moved further aft then the aircraft becomes unstable - not grossly so just that if the aircraft is pitched up by turbulence it will continue to diverge by pitching more and more until the pilot applies down elevator to stop this and return to level flight at which point the aircraft will have to be flown all the time as it has negative stability.

 

Getting the wing incidence, down thrust, CG and control throws sorted out can make an aircraft a delight to fly from one that is very disappointing.

[Futura57]

5 hours ago, Peter Jenkins said:

...

The increase in air speed will also increase the downforce generated by the tail plane (it is always a down force in a stable aircraft) and that will tend to reduce the wing's incidence and hence lift and thus counter the climb until a new state of equilibrium of forces is achieved ...

 

Hello Peter, please clarify the above extract from your post. Two questions immediately come to my mind. But maybe I misunderstand.

 

1 - If the down force on the tailplane increases, won't this push down the tail and thus raise the nose i.e. pitch up ?

 

2 - You refer to incidence, but do you mean angle of attack? The former is fixed by the geometry of the aircraft design. The latter is a variable of flight?

 

 

 

Edited February 3 by Futura57

[Jonathan M]

Here's a clue from the description on the Ben Buckle website:

 

"The Radio Queen has a flat bottom wing section which means it can manage breezy or windy weather and given enough power and plenty of down elevator it can be flown nose into wind on windy days."

 

I've just had a gander at the plan on Outerzone.  It has a hoofing amount of positive wing incidence, plus some negative tailplane incidence, plus a thick-ish flat-bottomed wing whose drag also acts as a fulcrum above the thrust line.  All of this will always result in dramatic pitching up once power is applied or increased.

 

You can add in some down-thrust which will certainly help, and/or add in an elevator-throttle mix which will also help, and also fit a motor/prop combo which reduces the maximum thrust, etc.  But you're starting with a model that is fundamentally unbalanced compared to a modern RC trainer or casual sports model - where various forces are constantly fighting one another (which pilot-induced or radio-mixed down elevator is just compounding), and this is always going to make life much harder for your mature clubmate learner!

 

Peter is right in his analysis above.  In terms of solutions, once you've fitted a powertrain sufficient for the job (neither too weedy nor too muscular), the most significant taming will come from reducing wing-incidence by packing up the TE (experiment) plus a small amount of down-thrust (experiment), and ignore any complicated thro-elev mix.

 

Sort out these basic mechanical things (easy to do, even in the field), then you'll have a vintage-style model that is more naturally controllable at various throttle settings (moderate climb at higher revs, moderate descent at lower ones) without stressing your student and frustrating his progress.

[Peter Jenkins]

4 hours ago, Futura57 said:

 

Hello Peter, please clarify the above extract from your post. Two questions immediately come to my mind. But maybe I misunderstand.

 

1 - If the down force on the tailplane increases, won't this push down the tail and thus raise the nose i.e. pitch up ?

 

2 - You refer to incidence, but do you mean angle of attack? The former is fixed by the geometry of the aircraft design. The latter is a variable of flight?

Hi Futura

 

You are quite right for pointing out my error!  I have now drawn myself a diagram, reproduced below that shows what I was trying to say and got round the wrong way when I put it into words!

 

 

In the normal stable position, i.e. the CG is in front of the centre of pressure - the point through which the wing lift acts - that will leave a nose down pitching moment as shown in the diagram.  This is counterbalanced by the tail plane down force.

 

As power is increased and now exceeds the drag force, the wing lift increases and the nose down pitching moment also increases and thus acts as the restoring force to act against the down force provided by the tail plane.  The increase in airspeed will also result in an increase the the tail plane down force but this is not of the same magnitude as the wing lift generated force. Thus the nose down pitching moment increases and tends to restore the equilibrium between that force and the tail plane down force.

 

If you move the CG further and further aft, this nose down moment reduces thus reducing the stability of the aircraft.  If the CG coincides with centre of lift then there will be no nose down pitching moment so the aircraft will be neutral in stability.  Move the CG behind the centre of pressure and the moment acts such that the aircraft is now unstable and requires the pilot to alter the tail plane down force to counter the pitch up or down by the aircraft.

 

I was also loose in using angle of incidence when I should have said the angle of attack.  The angle of incidence, as you point out, is fixed by the relationship between the wing and the fuselage when they are joined together.  The angle of attack refers to the angle between the wing and the oncoming air flow.  The angle of incidence will set the sit of the aircraft at a chosen speed. 

 

As an example, airliners are designed to have the cabin floor level at their designed cruise speed.  When the airlines wanted to save fuel burn they reduced the cruise speed and then the aircraft had to be flown with the wing at a higher angle of attack to generate the same lift but at a lower speed.  Thus, the cabin crew now had a downward slope to the cabin floor when going towards the rear and an upward slope when going towards the front of the aircraft making life a little more difficult for them.  Had the airliner had a wing whose incidence could be adjusted to allow for this then the cabin floor could have been returned to being flat at the lower cruise speed.

 

One of the reasons for using a canard layout for a fighter is that when turning at say 9 g all the lift generated by both the wing and the tail plane is working in the same direction as the canard now provides an upward lift force to balance that nose down pitching moment.

 

Finally,

 

The difference between the thrust line and the centre of drag i,e, the point through which all the drag force components operate through, also will produce a nose up or nose down pitching moment when power is applied or reduced.  If the thrust line passes through the centre of drag then there will be no such moment generated.  That's the aim of up and down thrust adjustment.  You can have zero moment and thus application or reduction of power will have no instantaneous effect or accept the mismatch and have either a pitch up or pitch down with power changes.  Once the power change has resulted in an actual airspeed change then the wing lift is affected and my earlier comments apply.

 

Sorry for not saying what I meant first time round!  Smack on hand adminstered!

[Simon Chaddock]

Peter

Perhaps my explanation of how a plane's stability effects its flight is a bit different to yours. 

[Don Fry]

Or Simon, recreate the original, small number of watts to power it, stick a pound of  lead on the CG, to recreate the radio weight, and it ceases the desire to react like a feisty disco dancer, and it reverts to the far more natural corpulent old age slow waltz.

[David Davis]

This afternoon Frans and I installed a Roxxy C28-34 motor which apparently produces 320 Watts with a 10x7 propeller. This equates to 66 watts per lb. With a 5000 mAh 3S lipo installed we went to the club's flying field at about 16.00 local time. Having checked everything, with the model on the runway I advanced the throttle progressively and the model rolled backwards! Frans swapped two of the cables and this time we had forward motion. The model took off, climbed adequately and I continued to fly it about for several minutes before I brought it into land on the club's tarmac runway.

 

As I thought, we had been using motors which were much too powerful resulting in unpleasant flying characteristics. The cg was more rearward too today. Not a problemwith a design like the Radio Queen.

[Frank Skilbeck]

Almost a canard eh 😉 

But I had similar with a Fokker D8, changing from a 4s to a 3s battery, same prop, made the model more pleasant to fly.

[John Stainforth]

Personally, I think downthrust is completely unnecessary in RC models. It's a hangover from the days of free-flight and single-channel RC models where we were could not adjust the trim in flight. All one has to do to stop a plane climbing under power is reduce the angle of attack by elevating the model down with either the elevator or the elevator trim. This reduces the lift and the drag; no need to reduce power at all. The way to fly a model as fast as possible in level flight is to have the thrust line straight through the C.G. with the plane trimmed to fly level on full power. 

[Martin Harris - Moderator]

I must admit that I thought lift acted around the centre of pressure and stability depended on its relationship with the centre of gravity. 
 

With a typical low or mid wing model, little downthrust is required to compensate for power changes but in a typical vintage high wing model, total lack of downthrust results in noticeable pitch changes with power so its use will make for a far nicer mannered model - particularly for an inexperienced pilot.  
 

I’ve certainly managed to loop models by application of power and recommended the owners to add downthrust.  None have complained after doing so…

 

For basic training purposes, I want a model that will pitch up and climb positively under power and adopt a gliding attitude at idle.  How else do you instill a power controls height, elevator controls speed awareness in a beginner?

[Christopher Wolfe]

1 hour ago, John Stainforth said:

Personally, I think downthrust is completely unnecessary in RC models. It's a hangover from the days of free-flight and single-channel RC models where we were could not adjust the trim in flight. All one has to do to stop a plane climbing under power is reduce the angle of attack by elevating the model down with either the elevator or the elevator trim. This reduces the lift and the drag; no need to reduce power at all. The way to fly a model as fast as possible in level flight is to have the thrust line straight through the C.G. with the plane trimmed to fly level on full power. 

 Sorry John I must beg to disagree with this statement.

 

Sure, it is easy nowadays to dial in a bit of trim when engine thrust varies but is it efficient?

 

As a free flight tragic I have found that to trim an r/c model (aerobatic or otherwise), fine tuning the thrust line is as critical as fine tuning the c/g and the control surface.

 

To fly fast without significant trim changes, it is also important to also have not only the thrust line through the c/g but also have the wing and the horizontal stabiliser along the same thrust line and ideally the fin/rudder equally distributed.

 

As an extreme example, I fly a few model floatplanes with the engine mounted above the (high) wing; they often require quite a bit of upthrust for stable flying characteristics. The same applies to full size aircraft of similar layout.

 

* Chris *

[David Davis]

I'm just pleased that we've achieved a model which Frans will be able to fly easily. This will raise his confidence and enable him to go on to fly more demanding aircraft.

[David Davis]

PS. On consulting the Ben Buckle plan I noticed that the engine bearers were spaced for a 2.5 cc engine.

 

Nuff said!

[Andy Stephenson]

A club mate had a Paper Aviation Easypeze with a fair excess of power and insufficient down-thrust. Without being able to adjust the down-thrust on the field, we mixed in some down elevator with increasing throttle. It was a very unsatisfactory fix because it probably needed a nonlinear mix to match the throttle response. The other problem we noticed was that when a dead-stick landing is mandated, the throttle stick position affected the elevator trim during the glide.

The down-thrust was increased for the next outing and it transformed the flying characteristics.