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Downthrust and Sidethrust.


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My first powered model aeroplane was a free flight Veron Cardinal. If I remember correctly it had considerable sidethrust and downthrust. Can somebody please explain why they were required and are downthrust and sidethrust nercessary or desirable on a modern r/c model? Or does it vary from one model to another?

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The Cardinal was rigged for a gentle glide (plenty of positive angle between the wing and tail surfaces). As the speed increases, this gives a strong "up elevator" effect. Under power, the speed will be greater than on the glide, and can lead to excessive climb angles and stalls under power. The downthrust prevents this from happening.

 

A zero-zero rigged aerobatic model does not need downthrust - indeed in is a disadvantage as it will lead to different handling characteristics between upright and inverted flight.

 

Sidethrust is an attempt to overcome two problems with lightweight models like the Cardinal. Under power, the torque reaction from the engine will tend to make the aircraft roll to the left. Also the "corkscrew" slipstream behind the propeller can hit the fin and rudder, causing a further left yaw effect. Offsetting the engine to the right slightly can counteract these tendencies, but you are trying to counter a roll effect (from engine torque) with a yaw function (sidethrust). On models like the Cardinal, the dihedral angle of the wings will help convert this yaw into roll.

 

Again, on an aerobatic model with little dihedral, side thrust will have little effect and is best avoided.

 

All these effects are far more noticeable on very lightweight models like the Cardinal.

 

I'm not an aerodynamicist, but this is my understanding of the situation, and its stood me in good stead for several decades! 😀

 

--

Pete

 

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All the models i have built in the last decade have 0-0 thrust lines and they all fly fine. In loops most of my models need a fair shove of right rudder, but that is no hardship so i just get on with it. 

 

When i comes to free flight i would assume you need the right thrust to counter engine torque and down thrust to minimise the difference in trim power on vs power off. The whole idea being to make the model do what you want with its little trim tabs. With r/c we have the ability to adjust the model every second so its far less of a worry for us. 

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In my short career I can answer the last question in that it does vary from one model to another.

Yes, they can be necessary and desirable on modern r/c models.

Side thrust counteracts the model swinging to the left as power is applied.

Downthrust counteracts the model tendency to climb again as power is applied.

 

Steve

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IMO its whatever the designer chooses to do and its part of the overall design and performance of said model. Perhaps it to cure an issue they have designed in and its an easy fix, then again it might be a trait of the original aircraft that the model is based on...Unless you can discuss with the designer what their intent and rational is hen its just a guess (could well be an informed one).

 

I know of one designer who does not specify side thrust and just says that's what the rudder is for! I have two of his designs and that approach seems to work fine.

 

EB is correct with his summery 👍

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It is interesting that some "classic pattern" models, for instance, Terry Westrop's "Loaded Dice" use quite a lot of side thrust. I assume that in this case, it is there to try to keep some purity of line in vertical manoeuvres? Especially prior to the advent of rudder/throttle mixing etc. 

 

Screenshot_20220714_112628.jpg

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All single-engined RC models I've built have some down- and side-thrust specified.  Is it not the case that, generally speaking, more airspeed creates more lift, and a small amount of downthrust is intended to counter this?  Side thrust, as already mentioned, is to counter the torque and/or vortex effect.

 

Of course, both effects can be countered by the pilot's use of elevator and rudder if they're aware of the issue.  But why not build the model to fly straight without the need for pilot correction?

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Thank you for some very interesting responses gentlemen. I am currently building a DB Sport & Scale "Skyrider" as a surprise for my partner who wanted a model finished in purple and pink! The model is described as an aerobatic trainer but the engine on the plan is shown with both side and down thrust. I think I'll build everything straight and see what happens. https://forums.modelflying.co.uk/index.php?/topic/50509-the-cost-of-building-recording-the-financial-cost-of-building-a-db-sport-scale-skyrider-a-surprise-for-miss-blue-eyes/page/2/&tab=comments#comment-925510

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I would build it as specified, DB knew what he was doing! 😄

 

I recently built a "Ghost Rider" for my Galloping Ghost equipment, and it is similar to the SkyRider. It has needed a LOT of downthrust to keep it flying straight under power. In fact, I haven't got it right yet, and at full throttle, full down elevator is insufficient to control the climb! On the glide, it is perfect with neutral trim.

 

I should add that it really needs bigger elevators, as the elevator control is marginal. Nonetheless, there is a very marked trim change under power!

 

--

Pete

 

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7 minutes ago, Peter Christy said:

I would build it as specified; DB knew what he was doing!

I agree, as a relative novice I have tried 'everything straight' and found it hard to control with my un-coordinated thumbs. Building in the designed thrusts will make it easier for a novice. With no downthrust the model will climb rapidly if this is corrected with down trim then it will dive when the power is cut, for me it was impossible to fly holding in down elevator. No side thrust means using rudder to control the left yaw combined with aileron to control the induced roll, again uncomfortable for a novice.

 

Steve

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  I agree with Peter above with this one. Without down thrust you will be adjusting the trim [ as has to be done in many full size types ] between power on/off or at least having to hold in some down stick while at full power, it is just the type of high wing aircraft that need some. Pic is of my radio assist Cardinal, downthrust/sidethrust as plan.

SAM_1616 (2).JPG

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2 hours ago, J D 8 said:

as has to be done in many full size types

I recently got a better radio with sliders. I put a mix on one slider to allow for adjustable elevator trim. Full travel is a few % correction and does seem helpful on a model that I sometimes want to pootle about with and sometimes want to go faster. Flight modes could be made to do the same thing of course.

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Whether any downthrust and/or sidethrust is employed depends entirely on the model, the amount of power, the size of the prop, and the type of flying it (model) is intended to do. 

 

Some models need it and some fly okay without it. . Most will benefit from a touch of right thrust. 

 

It's a journey of discovery, revolving around trial and error. . . It's called "gaining experience."

 

Have fun. 

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11 hours ago, David Davis said:

Thank you for some very interesting responses gentlemen. I am currently building a DB Sport & Scale "Skyrider" as a surprise for my partner who wanted a model finished in purple and pink! The model is described as an aerobatic trainer but the engine on the plan is shown with both side and down thrust. I think I'll build everything straight and see what happens. https://forums.modelflying.co.uk/index.php?/topic/50509-the-cost-of-building-recording-the-financial-cost-of-building-a-db-sport-scale-skyrider-a-surprise-for-miss-blue-eyes/page/2/&tab=comments#comment-925510

David

Build it as the plan, I was given a Skyrider many years ago by a club mate and this was a really nice model to fly, I am in the process of digging out the part kit I have to build one, my first build of this model, it makes a really good hack and is a good old style club model without any nasties to bite you.

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David

Just to note that full size do not normally have side or down thrust regardless of the plane configuration. The pilot just accepts that for any power setting or CofG position there will be a specific trim setting for 'hands off' straight and level flight.

In RC you are not in the cockpit so we try to achieve straight and level independent of the power setting by using a degree of motor side and down thrust.      

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Relating to full size is a little misleading. With the exception of aerobatics, the flight profiles are typically very different with much longer intervals between making power/trim changes on full size plus there’s continuous feed back through the controls.  Therefore it’s hardly an imposition to re-trim for different phases of flight. 
 

Because of the above, I’m happy to accept a little assistance from well designed or trimmed aerodynamic tweaks and/or transmitter mixes - maybe not completely nullifying the effects but reducing the workload. 

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