Hi guys,
This ought to get a good discussion going.
Is down thrust on a trainer a good thing?
Or does it slow ones progress in learning how to fly, by damping changes of the pitch due to changes in power levels?

anything that evens out the trim differences at differing speeds -is good.
A high wing model -- has all of the lift/drag above the thrust line - which makes power applications tend to INCREASE the upward angle of the craft .

by pointing the propeller downward quite a bit -- the forces can be balanced out --somewhat .
basically the crummy semi symmetrical wings on trainers add to the problem.
a nice symmetrical wing on a very low wing loading model -again a high winged thing - makes for much easier trim relative to speed
the plane will fly just as slowly as the supposed trainer airfoil.if wings are the same size/shape.
Learning the nuances of speed trimming whilst learning to guide the model around -is a bit much for some .
I find some so called aerobatic types easier for even the raw recruit.
Thes models are quite neutral inall axis and if wing loading is quite low they are a piece of cake to learn with.
The Diablotin -the larger one is a good example
this model flies very slowly and yet has solid control resonse at almost walking speeds.

Thanks Dick,
The reason I asked is I am in the midst of setting up a trainer I just built for my wife. I was going to start on the cowl which I will be using to mount the motor, and while I was looking at the plan noticed that they called for 6 degrees of down thrust, relative to the centerline. In my experience 1 or 2 degrees is not all that uncommon for up or down thrust, but was taken aback by such a large number called for with this trainer.
That got me thinking about the issue in general as far as the aerodynamic efficiency of such a compromise. I was thinking along of the lines of large down thrust would call for a lot of down force on the tail, which would cause a lot of drag. Granted this may keep the top speed down so that may be a good thing, but were this is a glow to electric conversion of a popular trainer, I am a bit concerned about the drag cause by this type of set up. Given that increased drag would raise the power consumption and shorten the flight time. The second part of your reply just reinforced my basic belief that what makes a good trainer is a plane with light wing loading and good control, not one where the design is compromised in order to idiot proof the plane.

Eagle, it's generally a good idea in a trainer as the CG is usually further forward then you would normally put it for a typical sport model. Because of that with added power that produces some added speed you get a much stronger nose up reaction. Downthrust helps control that extra strong nose up reaction.

As you move the CG back for more sporty flying and re-trim the elevator to compensate then you'll find you need less downthrust to provide the same response to power.

Bruce,
Thanks for the information, I will make sure to allow for the thrust angle to be changed. I will have to incorporate that in the design of the cowl. As I had drawn the plan out last night, for the cowl assembly, there is no provision to change that value. I will have to come up with a way to do that, as it would not be possible to do so as I have it now. An how I have the cowl mounted within the front fuse ears it was to be bolted from the sides, instead of the firewall as is typical for the engine to be mounted to the plane.

Actually, dealing with pitch changes at different throttle settings is a skill you shouldn't need, except maybe when helping beginners flight-trim their trainers. The more advanced models (racers, aerobatic airplanes) won't change pitch at all with changes in throttle setting. If they do, there's something wrong with them.

Six degrees is a lot! The most I have ever seen was about four, and that was on an electric boost-glider. A normal R/C trainer would want two, maybe three degrees ... usually.

Duane Gall
RCPRO

Bruce,
The more I thought about what you said earlier, the more I began to consider moving the set up COG and not using as much down thrust. The reason I am thinking about that is this is her second plane, she has an Easy Star and flew that some last year. She had also put in time on Next Star and an lt-40 and was doing pretty good (much better than I did as a kid). She had progressed very well, and had gotten to the point of doing some basic aerobatics. Now keep in mind she had done so either doing hand off of the or using a buddy box. In other words she has not done a solo takeoff and landing yet but has a feel for it already. She uses the G2 FS at least once or twice a week and has done many takeoffs, and landings with it.
She even flew the cap without a crunch (which surprised me).
Given that what would you think of my idea of a more perky high wing trainer?
Would you care to make a recomendation on this, as far as number I should use?
Thanks,
Paul


BTW
Thanks for giving me a better understanding of this issue.

There's no way to make a recomendation of how much downthrust to use as it depends a lot on the airfoil as well as the CG and the tail moment arm length and the stabilizer area. At least not for a specific amount. The general rule of thumb for anything that is intended for general sporty flying would be that the thrust line should be about 4 degrees negative to the wing's chord line. Note that this sounds like a lot but if the wing is set at +2 to the arbitrary centerline then the downthrust is only -2 degrees. If it needs adjusting from that point then we are only talking about adding shims to produce an additional 1 or 2 degrees at most.

From the sounds of it she would do OK with a slightly more sporty model and just reduce the control throws until she's ready for it. The big issue for most students at this point is to get something light enough to keep the speed down. With less speed comes more time to be able to keep up with the model and a general softening of the control responses. Also something draggy so in a dive the speed doesn't suddenly jump up too fast.

In taking a quick look at the Tower Hobby online catalog I see that Goldberg has an ARF version of the old classic Falcon for a 40 to 46 size engine. With a milder 40 I think you'd have a great model that you could both play with. Assuming that it comes out as light as the originals used to. The old Goldberg kits used to build up fairly light. Just use the dual rates to slow things down for her until she gets a few more hours under her belt. There was a design that could be set up for anything from mild to wild just with a movement of the pushrods on the control horns. Just put the CG back but not too far back. I'm sure there's lots of other options for this sort of thing.

I once had a sort of control line combat model set up for RC. At 1/4 throttle and with the dual rates turned down I handed off the Tx to a near beginner. He did fine despite the model being set up very close to neutral pitch stability just because the speed slowed down the control input reactions and because the model WAS so nuetral that it didn't do any funny after effects. When he corrected it stayed corrected instead of nosing up or doing other funnies. But like your wife he had some previous stick time and didn't tend to over control like so many beginners do at first.

Thanks Bruce,
I think I will go with 4 deg down with this one too given the airfoil. The wing has the +2deg AOI and like you said this will give it an actual down thrust of around -2 as you said. I will leave it just a tad heavy in the nose when I do the maiden, it should not be all that bad and the COG won't change in flight, it is an electric. Wing loading looks like it will be in the range of 14 to 15 oz per ft so I would say it is fairly light. I am going to keep the controls at factory for the trainer setup to start then adj with time as she wants more.
A falcon type would be a good one for her, and the motor that she has would be more than enough for it (axi 4120/18), it will not be overworked. lol She understands how to use the left stick and to her credit usualy does very well at not wasting power/fuel. She could go to a 60 size with her power set up with out any problem, she has a 70 amp esc that will take 24C nimh or 6 C lipo. Although to start she is going to get 16-20C ni-ca they take impact better, not to mention a bit less pricey.
Myself I got several planes ready to break, fly this summer think I leave hers be, for now.
I got a pair of .2 and .4 size electrics if I want to do the e-fly thing, plus some glows and gaser to spread some balsa with. lol
Thank Again to all...

Up/down thrustline positioning is used to reduce or eliminate pitch changes with throttle change. Depending upon the layout of the wings and tail, airfoils used, sizes, etc., the model will have more or less tendency to change pitch with throttle change. Here's how you can test to determine which way you have to tilt the engine.

1) Fly the model in level flight at full throttle.
2) Abruptly chop the throttle and watch the model's behavior.
3) Fly the model in level flight at a low throttle settng.
4) Abruptly open the throttle to full and watch the model's behavior.

If the model sharply pitched down when you chopped the throttle, it means you were holding down elevator to keep it level. You need to add some down thrust. This model would also likely pitch up sharply when you added throttle in step #3.

If the model pitches up when you chop the throttle, then you need to remove down thrust. The same is true if the model tends to pitch down when you add throttle.

The ideal behavior would be that when you abruptly chop the throttle, the model will tend to coast forward and then smoothly pitch down. There should be no abrupt pitch change. When you add throttle, the model should tend to accelerate and smoothly pitch up into a climb. An airplane with positive pitch stability will tend to maintain an airspeed that it's trimmed for, and will pitch up as it accelerates, and pitch down as it decelerates.

You want to adjust the thrust angle to give you the least amount of change when you change power. This is the same reason you'd adjust the horizontal thrust line of the model. To reduce the yaw tendencies introduced with change in power. Many models will use a bit of right thrust.

For Aerobatic models, you'll also adjust the thrust line to get the model with a neutral a feel as you want. Different fliers will want different 'feels', but thrustline is one of the things to adjust, along with balance point, control throw amounts, and so on. The Great Planes Giles ARF is a model with thrustlines set up to account for the force arrangements of the model's components. The starting point is 2 degrees of right thrust, and one degree of up thrust.

upthrust on a Giles?
sounds like an underpowered setup or the measuring stick is off.
The only time we found low wing-high thrustline setups to need up thrust -- was on models which lost speed on uplines
lose of speed = need for upthrust
same situation as as in level flight .
when we wre testing this -it was on a TOC design - increasing thrust (more power)-solved the problem
If speed remains the same -- trim remains the same - made sense to me

what do ya think?

Thanks guys,
Your advice really helped. I am adding provisions to the motor mount/cowl so I can adjust the down thrust rather then having it at a fixed value as I had originaly though of doing.
(Probably was not a good idea to make a mount/cowl asembly that way anyhow.)
Besides the motor will probably be changed out later and put in a sport model for her, just hope that this one will keep her busy for the season, need some time to fly/repair my own...

As far as that one model is concerned, the incidence and thrust angles were determined in testing by one of the design team members who was an active IMAC competitor at the time. He'd competed at the highest levels. I just reported what the initial recommended setup was.

Of course, to get the best performance out of any design, a competitor will fly, adjust, fly, and adjust until the model's flying just the way they like and need it to.

Thanks Bax this will make it easy to trim the beast out.

Sig Kadet LT40 needed indeed about 6 degrees downthrust, with less it was gliding perfectly and as you add power it went looping, not enough (down)elevator to keep it level!

But here is what classic books says:

"The resultant of total lift and total drag must pass through point of intersection of propeller thrust and gravity force. If propeller thrust passes through CG, then the resultant of total lift and total drag must also pass CG."

Thanks for the reply....
This plane may require less down thrust, due to the washout on the wingtips which was added to the stock model. There were also a few other changes added over the stock configuration such as: pull pull controls for the rudder and elevator, and aerodynamic cowling over the electric motor.