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Engine off-set angle?


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I think it depends on the overall design and type of airframe. Vintage types tend (in my limited experience) to have quite extreme angles of down thrust built in. I guess it's to overcome the wing design, angles of incidence and what I'll call the pendulum effect of the motor thrust line being below the centre of aero thrust.

Side thrust - now that's a different matter - I've got some vague recollection that it's to do with counteracting the tendency of the prop wash to turn the model, but I'm willing to be corrected on that one.

hth

Kim

eta I guess it's down to experience and/or trial and error to ascertain the correct thrust line for a given 'plane. Someone like Peter Miller on here will no doubt be able to give better explanations than I possibly could.

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Posted (edited)

Kim is right Graham, for a Free Flight had to have some predisposed help, a 4 channel proportional radio control model has an on-board pilot.

With own designs where the thrust line is above or below, or the motor at the front of at the rear of the wing, then some alternate thrust may be required, but I must admit that all my 4 channel+ models have zero side of up or down thrust set up.

Edited by Denis Watkins
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Graham, it doesn't matter whether the motive force is glow or electric the engine thrust shown on the plan is what's required. The vintage high wing planes with a larger turning moment due the vertical distance between the engine thrust, higher wing drag and short nose need a bit more down thrust to compensate for this. If you don't have enough the plane will nose up on applying power, you can compensate by adding down elevator, but go with what the plan shows.

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Peter Miller is a long time proponent of no thrust line offset at all so I doubt he’ll give you any reasons why it’s a good idea. 
 

Down thrust counteracts pitching up with power and compensates for large differences between the centre of lift and centre of gravity - in practice, high wings benefit from downthrust and low/mid wings much less so. 
 

Side thrust gives a proportional correction for torque/propeller effects so can reduce workload - very beneficial for single channel or free flight where the practice originated. 
 

Nowadays, we have a left and right thumb to use on the majority of models and purists will just use them. Some mixing will reduce workload at the expense of developing expertise. 

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Just dug-out the plan. It says 2 deg Rt and 2 deg Down but adjust on 'flights'

I have (I guess) zero down, and 1 deg right now I have adjusted it! (before thinking the plan might say something. 

 

An experienced flyer flew the plane early last year in 4 mph breeze and had difficulty getting it straight, it seemed trick to fly compared to his Junior 60 he flew before hand on the same day.

Strikes me up/down thrust is more sensitive to a calm flight than Rt/Lt, but I have ability to trim of course on the Tx which should have enough to get around these angles?

 

My flying skills are still crude, still knocking the UC off my Kingfisher at times, but i do hope to fly this in the middle of the year and need it to be docile.

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I flew a large Buccaneer (probably one of the 90 inch versions) for a club mate some years ago and it was extremely sensitive to power changes but very unresponsive without power applied - I’ve no idea whether it was built accurately to plan but it made for an interesting challenge using blips of power to make heading changes on the approach  without overshooting. 
 

Perhaps you can find an experienced flyer who could help you with some mixes or trimming the thrust lines as if it behaves in the same way it won’t be easy to fly despite the apparently benign looking design. 

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1 hour ago, 911hillclimber said:

Just dug-out the plan. It says 2 deg Rt and 2 deg Down but adjust on 'flights'

I have (I guess) zero down, and 1 deg right now I have adjusted it! (before thinking the plan might say something. 

 

An experienced flyer flew the plane early last year in 4 mph breeze and had difficulty getting it straight, it seemed trick to fly compared to his Junior 60 he flew before hand on the same day.

Strikes me up/down thrust is more sensitive to a calm flight than Rt/Lt, but I have ability to trim of course on the Tx which should have enough to get around these angles?

 

My flying skills are still crude, still knocking the UC off my Kingfisher at times, but i do hope to fly this in the middle of the year and need it to be docile.

A mix is an easy way to compensate for insufficient down or side thrust IF the problem is not too great.  However, the mix is only good for one speed as the aerodynamic forces change according to the square of the speed.  So, if you have adjusted for a set speed as you slow down or speed up the problem will recur as either too much or too little compensation.  For that reason, it's best to adjust the side and down thrust to match what's shown on the plan or included in any instructions.  That is not the end of the game though!  After you fly it you may need to make some adjustments if you want to get the aircraft to be relatively insensitive to pitch change when you apply and reduce power.  For side thrust, if you pull the aircraft into the vertical and apply full power if it pulls to the left, increase right thrust and vice versa.   For a sport model, getting the adjustment spot on is not needed as it would be for an aerobatic model.  What you want to avoid is having large changes in aircraft attitude with changes in power.

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Bear in mind a lot of these models were designed for free flight or maybe just single channel (rudder control) so were designed for gentle climb under power transitioning to a gentle glide when the engine cut, so the model had to be trimmed for both power on and power off.

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Posted (edited)
4 minutes ago, Ron Gray said:

GPS, telemetry and logical switches? 🤔

Or fit a flight stablised Rx/flight controller that keeps the model level, personally I just treat engine thrust adjustment as another trimming exercise, much like fine tuning the centre of gravity. Although I do have some powered gliders that exhibit a strong upward rotation when applying power, and as it's not possible to easily adjust the motor thrust some down elevator is mixed in with throttle, but as the purpose of the motor on a glider is just to get to thermal hunting height it's no big problem.

Edited by Frank Skilbeck
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There's actually no such thing as a specific fixed thrust angle unless if it's taken with respect to some other horizontal or near horizontal straight line (e.g the wing's angle of incidence). No matter what is taken as the reference the thrust angle will be changing with every change in fore & aft trim.

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13 minutes ago, 911hillclimber said:

I think I need to find a way to measure reasonably the 2 degree off-sets first to the plane/fuselage?

Is a Junior 60 as sensitive?

The Junior 60 doesn't need any side or down thrust. Any tendency to turn is easily catered for by rudder trim, throttle control takes care of rate of climb & speed when combined with elevator trim.

 

Mine has no side thrust but does have several degrees of built in down thrust only because I didn't know any better when I built it around 1987.

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Posted (edited)

The way I see it any aerodynamic trim set to fly 'hands off' with power off will not give the same hands off when power is applied and vice versa.

You can get pretty close using motor down & side thrust but it requires quite a bit of trial and error experimentation to gat an acceptable, to you, setting.

.

It is of course possible to mix both aileron (to counter motor torque) and elevator (to prevent throttle climb) with the throttle but it too requires the same, if not more, experimentation. Even then what mix works well in level flight will not be exactly correct in a steep climb where the airspeed is low yet the throttle is at full power. The effect of giving a burst of throttle on a low slow approach can be "interesting" with such a mix in place. 😲

I have set such a mix on one of my powerful Depron slow light weights but eventually took it off again and went back to just modest down thrust as I felt it gave a better compromise.

 

There is no substitute for "flying the plane" unless of course you install a full 6 channel flight director with GPS but then who/what is actually flying the plane! 😉 

 

 

   

Edited by Simon Chaddock
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Posted (edited)
13 minutes ago, Simon Chaddock said:

The way I see it any aerodynamic trim set to fly 'hands off' with power off will not give the same hands off when power is applied and vice versa.

You can get pretty close using motor down & side thrust but it requires quite a bit of trial and error experimentation to gat an acceptable, to you, setting.

.

It is of course possible to mix both aileron (to counter motor torque) and elevator (to prevent throttle climb) with the throttle but it too requires the same, if not more, experimentation. Even then what mix works well in level flight will not be exactly correct in a steep climb where the airspeed is low yet the throttle is at full power. The effect of giving a burst of throttle on a low slow approach can be "interesting" with such a mix in place. 😲

I have set such a mix on one of my powerful Depron slow light weights but eventually took it off again and went back to just modest down thrust as I felt it gave a better compromise.

 

There is no substitute for "flying the plane" unless of course you install a full 6 channel flight director with GPS but then who/shat is actually flying the plane! 😉 

 

 

   

Simon, the idea is to mix rudder to throttle using a curve not just a fixed point.  So, as power is increased so is the amount of rudder so that the two work in tandem - in theory.  However, as aerodynamic force is proportional to speed squared, one can only really be absolutely right at one airspeed.  That's why adjusting engine side and up/down thrust is the best way forward but again this also generally is only perfect around a set airspeed.  For most sport aircraft you could probably get away with just mixing whereas in the world of precision aerobatics you would want to go to the absolute maximum that gave you no adverse effects with changes to the level of engine/motor power without any mixes.  As you say, it requires experimentation to achieve the optimum results.

Agree that there is no substitute for flying the plane but in some disciplines e.g precision aerobatics, you want an aircraft that is the easiest to fly so that you can fly the manoeuvre without needing to compensate for quirks in the model's handling.  If you fly a well set up precision aerobatic model, particularly a 2 mtr one, you will find that it is so easy to fly because of the need to fly large complex manoeuvres.  However, this is a long way from the OP so it's really a matter of horses for courses.

Edited by Peter Jenkins
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31 minutes ago, john stones 1 - Moderator said:

It's deflection, as throttle comes off it'll pull the model that way, a bent model.

It's not uncommon on full size aircraft whereas side thrust is very uncommon. 

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Update:

2 deg right now confirmed, and zero degrees up or down, that is the motor shaft axis is parallel to the engine bearers.

 

On this design of frame, the wing is not mounted parallel to the engine bearer 'line', the leading edge is about 3 deg higher, the tail plane is parallel to the engine bearer 'line'.

There are no ailerons on this plane, essentially a 3 channel design.

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13 hours ago, PatMc said:

It's not uncommon on full size aircraft whereas side thrust is very uncommon. 

 

We are talking about a model aeroplane, it's not uncommon for many designers to give a starting point re side/downthrust on their plans, in fact it is quite common,  it is also common that deflections on your models have consequences through the speed range and differing maneuovres.

And the oft said "Well I can use my rudder" is another deflection.

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