Down Thrust

[Frank Skilbeck]

Just read a letter from Nigel Argall in this months RCME in which he notes that downthrust is added to stop a model from climbing as power is added due to the increase in the models airspeed increasing lift.

I always thought that downthrust was added to compensate for the thrust and center of drag being offset, i.e. on a high wing plane the center of drag is the wing so the engine (or electric motor) being set lower than the wing will produce a turning moment which will cause the plane to nose up as thrust increases, not to compensate for the increase in lift as the model speeds up. Which is correct or is it a combination of both?

[John F]

Applying downthrust is a method of countering the combination of everything that affects the aircraft in flight, but is only one method.

Not all models need it, mind, and not all models need downthrust at all flight speeds.   So instead of downthrust, things like elevator is added to compensate for the increase in speed, or even ailerons or rudder to maintain straight and level, if required.

Edited By John F on 20/12/2015 09:05:39

[J D 8]

It is a bit of both.A high wing with a high lift section just needs more down thrust than a low wing type with a simmetrical section. On some models instead of downthrust [which can look ugly] I use the mix's in the radio to feed in a bit of down elevator as the power is increased,much the same as retrimming for a power setting change on a full size aircraft.

[John Muir]

I agree with you Frank. The use of downthrust as a sort of automatic down elevator substitute is, in my opinion, a hangover from the days of free flight, when the model would be trimmed for best glide and lots of downthrust used to stop the model climbing excessively or even looping under power. In an aerobatic rc model it seems counter productive to me as it turns into upthrust when the plane's inverted and does exactly the opposite of what you want. I can see it might be useful in some cases where the model is always the right way up, maybe vintage or some scale models, and certainly on powered gliders, but there is never any case where down elevator wouldn't do the job just as well, if not better. In fact, the electric gliders I own don't have a lot of downthrust designed in and I tame the climb by simply mixing some down elevator to the throttle.

[Geoff S]

There is an enormous amount of down thrust on the Sonata 'E' electric glider to counteract zooming when the motor is started. I was a bit casual when I sawed off the front to convert it to brushless and didn't allow for enough down thrust - after all it's a very easy plane to fly. Big mistake! It was almost unflyable and I ended up having to put it down (undamaged) several fields away downwind. Every time I started the motor to push it upwind it just looped no matter how much down elevator. So down thrust can be essential in certain circumstances but my Sonata experience was probably an exception.

Geoff

[Martin Harris - Moderator]

Posted by John Muir on 20/12/2015 11:25:04:

I agree with you Frank. The use of downthrust as a sort of automatic down elevator substitute is, in my opinion, a hangover from the days of free flight,

I'm pretty sure that thrustlines are an important factor in full size design as well as models. The high winged Cessna 100 series (152, 172, 180 etc. etc.) all had 2 degrees of downthrust for the very same reasons that a typical high wing model would. As I understand it, the thrust acts about the centres of pressure/gravity resulting in unpleasant trim changes if the offset geometry is excessive. If you look at a high mounted engine on a seaplane, upthrust is provided to compensate for the massive nose-down rotating effect that a non-offset thrustline would provide with the addition of power.

If you look at a typical pattern or aerobatic design, there is little vertical displacement of the thrust line from the aerodynamic surfaces so little or no downthrust will be needed.

Edited By Martin Harris on 20/12/2015 12:57:43

[Don Fry]

John Muir, try your theory at low speed, low altitude, and slam the moter open. You may have a rethink. Take a carrier bag before you try this one.

[PatMc]

Geoff, try moving the cg back incrementally over several flights adjusting the elevator trim & if necessary movement. When you feel the models starting to become too sensitive to elevator return to the last flight's setting. This should improve the glide, increase the model's speed range & reduce the difference in trim between power on & off.

The process could be speeded up by carrying out a few the dive tests & subsequent cg/trim adjustments first.

[PatMc]

Posted by Donald Fry on 20/12/2015 13:06:33:

John Muir, try your theory at low speed, low altitude, and slam the moter open. You may have a rethink. Take a carrier bag before you try this one.

I was nearly caught out like that after doing a fast low pass on the glide. Cured by introducing a delay to the down elevator mix in the Tx.

[PatMc]

I agree with John Muir that downthrust is a hangover from free flight (& rudder only S/C) days. Now that full throttle & elevator control are the norm there's rarely need for any downthrust. As mentioned in my last but one post adjusting the cg back can reduce the trim difference over the throttle range.

Many published plans & kits used to (probably some still do) deliberately advise nose heavy cg positions in order to make the models easier to fly for the lowest common denominator of pilot ability. These models often had/have large downthrust angles shown.

[Frank Skilbeck]

So Patmc, what you are saying is that with throttle-elevator mixing there's no need to have down thrust added on say a high wing trainer, where the center of drag is well above the thrust line?

[PatMc]

Frank, do you mean a set up akin to a full size Piper Cub ?

BTW when you say throttle-elevator mixing with reference to my post, you do realise I make no mention of programing the Tx for this purpose ?

[Mike Freeman]

I find elevator compensation doesn't work particularly well on electric models. It might be ok at the start of the flight but as the power falls off towards the end of the flight the compensation becomes overpowering. Even on sports aerobatic models with semi-symmetrical sections and/or models with high wings I think down thrust is beneficial and I always add some to my designs.

When I designed the TABOO (electric sports aerobatic model - free plan Feb 2012 RCM&E) the semi-symmetrical wing and tailplane were set at zero-zero and I built a profile fuz with an adjustable front former so the thrust angles could be adjusted. I then did lots of flying trying different CofG positions and thrust angles (down and right) until I found a set up I liked. The end result was 2degs down and no right thrust. The original is still flying a treat, when the taps are opened there is no change in pitch right from the launch until the end of the flight.

These photos should give you the idea. 1/64" ply shims were added/taken away until I was happy - then I built the proper fuz to these angles.

[Martin Harris - Moderator]

I've certainly added elevator to power mixes on various models to compensate for thrustline deficiencies, but to my mind, a thrust line which compensates for power change pitch changes is part of a proper design philosophy. When designers of full sized aircraft take pains to establish the best thrustline, it seems logical to follow their lead and design in an appropriate angle from the start!

Mike - your model's layout probably doesn't need any significant thrustline adjustment but that doesn't mean that it applies to all designs. I don't follow your logic though, as downthrust (where appropriate) is largely self compensating.

Edited By Martin Harris on 20/12/2015 16:17:32

[Mike Freeman]

Yes Martin, that is my point - no matter what throttle I use there is no change in pitch. The 2degs downthrust I arrived at via experimenting achieves the self compensation you mention.

I don't like models that balloon up when the power is applied and the above post was my way of making sure it didn't happen by adding downthrust. Obviously, the time honoured way of fitting washers behind the motor would have achieved the same effect but the nose of the TABOO wouldn't have worked with washers!

[J D 8]

Hi Frank, Yes you can have throttle elevator mix instead of down thrust but you do need to be able to set up the mix on your TX and then experiment to find the the right ratio of mix.

I assign an on/off switch for the mix on the TX and on first Take off's just hold in some down. Experiments with the amount of mix and its effects are done at hight, landing to adjust then try again. Much like putting washers under the motor mounting to adjust the downthrust. However a bit of downthrust on a high wing trainer may still be needed to prevent excessive amounts of mix being needed. John.

[Martin Harris - Moderator]

Posted by Mike Freeman 1 on 20/12/2015 16:27:15:

Yes Martin, that is my point - no matter what throttle I use there is no change in pitch. The 2degs downthrust I arrived at via experimenting achieves the self compensation you mention.

I don't like models that balloon up when the power is applied and the above post was my way of making sure it didn't happen by adding downthrust. Obviously, the time honoured way of fitting washers behind the motor would have achieved the same effect but the nose of the TABOO wouldn't have worked with washers!

Sorry Mike - somehow, I misread your post to think you were saying that no downthrust was required! On re-reading, I'm not sure how, though - total brainfade!

Edited By Martin Harris on 20/12/2015 16:48:03

[Frank Skilbeck]

Posted by PatMc on 20/12/2015 15:57:53:

Frank, do you mean a set up akin to a full size Piper Cub ?

BTW when you say throttle-elevator mixing with reference to my post, you do realise I make no mention of programing the Tx for this purpose ?

Yes, I've got a Clipped Wing Cub that has some downthrust and it flies just fine regardless of the power setting, but your post implied that you could remove the downthrust and then trim with the elevator, so I assumed you meant with a throttle - elevator mix. We've also had models which were not tail heavy but would pitch up when power was applied, which we resolved by adding downthrust.

BTW I agree an aerobatic model where the wing is either below or on the thrust line rarely needs any downsthrust and in my Parkmaster Pro instructions they tell you how to adjust the thrust by watching which way the model pulls in a vertical climb.

But then original point I was making was that downthrust was used to compensate for the center of thrust being offset from the center of drag and causing an overturning moment, hence we add some downthrust in to compensate for this rotation by the offset forces, not to compensate for the fact that an aircraft will generate more lift as it flies faster (at a given angle of attack).

[Mike Freeman]

No worries Martin. Going back to Franks OP I think downthrust is required to compensate for both the drag of a high wing model and the increase in airspeed. The TABOO has a mid position wing with the wing and motor almost on the same longitudinal line - no significant thrust/drag moment. My downthrust is mainly required because of the extra lift generated from the semi symmetrical wing as the speed increases. Had I built the model with a high mounted wing and done the same experiment I'd bet the downthrust would have been slightly more.

I agree with Martin's comment above, trying to fudge the design by introducing elevator compensation to do the job of the downthrust isn't the right way to go about it.  It's far better to balance the aerodynamics and thrust lines to achieve self compensation.... especially on an electric powered model!

Edited By Mike Freeman 1 on 20/12/2015 17:15:58

[PatMc]

Mike, the thrustline is only relevant when refered against the angle of attack. In level flight your model will have some AoA depending on how fast it's going. The thrustline when refered to level flight will vary by the same amount.

I prefer to control the climb rate by manual adjustment of throttle & elevator in most models though I'm not averse to using some Tx mixing with, for example, some E-gliders.

I don't follow your comment "I find elevator compensation doesn't work particularly well on electric models. It might be ok at the start of the flight but as the power falls off towards the end of the flight the compensation becomes overpowering."

[Mike Freeman]

Posted by PatMc on 20/12/2015 17:45:14:

Mike, the thrustline is only relevant when refered against the angle of attack. In level flight your model will have some AoA depending on how fast it's going. The thrustline when refered to level flight will vary by the same amount.

I prefer to control the climb rate by manual adjustment of throttle & elevator in most models though I'm not averse to using some Tx mixing with, for example, some E-gliders.

I don't follow your comment "I find elevator compensation doesn't work particularly well on electric models. It might be ok at the start of the flight but as the power falls off towards the end of the flight the compensation becomes overpowerin

Hi PatMc, my experience of elevator comp on an electric model was with an electric glider kit - no names! but it had a terrible tendancy to balloon up when the throttle was opened. Wing and thrustline on the same longitudinal line but no down thrust. I programmed in some elevator comp (lets say 3mm of down elevator with full throttle). This worked fine and stopped the ballooning at the start of the flight when the battery/motor were producing maximum thrust but then, as the battery wore down and the thrust reduced, the 3mm of down elevator was too much for the lower thrust and, as a result the model dived down. Elevator comp will work on an oily lump with constant thrust (until the tank empties!) but even then, introducing down thrust is the "proper" solution.

Edited By Mike Freeman 1 on 20/12/2015 18:00:24

[PatMc]

Mike, I virtually always use full or zero throttle on E-gliders & either hunt thermals or do any aerobatics on the glide. I never run the battery down to the power is noticeably waning before landing. On the Tx I program in some elevator/throttle mix with timed delays, mainly to try and ensure a smooth transition at the top of a climb which is usually around 200m making it difficult to judge from the ground.

With sports/aerobatic models I adjust elevator & throttle manualy which means that even if the battery is sagging my elevator adjustment is in response to the actual power not the throttle stick position. That's why I was puzzled by your comment.

I rarely see downthrust as the "proper" solution, IMO it's an outmoded bodge.

I guess we just fly using different techniques.

[Frank Skilbeck]

So Patmc what you are saying is that if you have a plane that's trimmed in the glide and pitches up when throttle is applied you just hold in some down elevator, so for a power model you'd be holding in down elevator when say flying level at half throttle?

BTW I do have some powered gliders that pitch up and climb under power and manually hold in down as the climb is usually pretty short, but on powered models like the Clipped Wing Cub, WOT 4 etc, I prefer them to fly at a level attitude under all power settings without having to manually hold in down and find adjusting the engine thrust line the most convenient method for me, but maybe I'm old school.

[PatMc]

Posted by Frank Skilbeck on 20/12/2015 19:12:28:

So Patmc what you are saying is that if you have a plane that's trimmed in the glide and pitches up when throttle is applied you just hold in some down elevator, so for a power model you'd be holding in down elevator when say flying level at half throttle?

No, that's not what I'm saying.

I establish the cg position together with the elevator trim combination that suits me. After that aerobatic models don't need much, if any, elevator adjustment during throttle changes. Vintage & sedate sports models etc can be flown so that throttle controls the climb & elevators the speed as per most full size light aircraft.

If you take a look at full size single engined, high wing aircraft such as Aeronca Champion, Piper J-3 & PA-12, Cessna 120 & 140 etc they don't have any built in downthrust. OTOH many single engined, low wing fighters such as the Griffon Spitfires, Spitefuls etc, P51, YAK-9, Curtiss Kittyhawk etc, Grumman Hellcat & Bearcat do have some downthrust.
Now I'm not saying no high wing aircraft & all low wing aircraft do have downthrust or that the ones I've listed are any more than a sample. But I do think there are fewer high wing light planes than there are low wing fighter types with downthrust.
To me this suggests that using downthrust is more useful to contend with very high power than it is to autotrim any nose up tendencies due to any high centre of drag.

[Stuart C]

Is it not best to have the thrust line pass through the c of g, thereby eliminating power pitching effects over the full flight speed range?