flap/elevator setting

[Den Moran]

Hi, everybody,

I came upon a dilemna for myself the other day - I always thought that when flaps go down , the elevator does as well, to counteract the lift the flaps give.

Looking at the FMS 8s Phantom f4 video on Motion RC, I noticed that the elevator was set to up, i.e. climb. So that left me with the dilemna!

Hence the appeal for some expertise here - thanks, keep safe, Den, Llangyfelach, Cymru.

[Frank Skilbeck]

It's to counteract any pitch introduced by the flaps changing the centre of lift, plus if they introduce additional drag below the planes thrust line they could cause a pitching down motion.

[Jon H]

I currently fly 5 models with flaps and 4 of them need up elevator trim with flaps down. The 5th needs no trim adjustment at all.

Generally, if a model pitches up wildly you either have too much flap, are flying too fast, the model is so light you dont need flaps, or a combination of the above. That isnt to say you cant use them, as models like the MPX funcub use them to do even more ludicrous things, its just that is why they behave that way.

Larger/heavier models, especially warbirds, need the flaps as much for drag as anything else and so behave differently.

Most warbirds also have retractable gear so if you trim the model for normal cruise speed, then halve it, then add the drag of the gear dangling out the bottom, then add flap which is yet more drag down low and often gives a down pitch of its own you end up with a model wanting to bury its nose in the dirt. As a result substantial (sometimes enough to loop the model at normal flying speed) up elevator trim is required for this landing configuration so the ele/flap mix is set as required.

[Steve Houghton 1]

Someone once told me that high wing aircraft require down elevator compensation and low wing aircraft require up elevator compensation. How true that is I don't know as I don't have any low wingers with flaps.

[The Wright Stuff]

Posted by Steve Houghton 1 on 14/08/2020 13:54:52:

Someone once told me that high wing aircraft require down elevator compensation and low wing aircraft require up elevator compensation. How true that is I don't know as I don't have any low wingers with flaps.

That would certainly make sense for models where the drag force is dominant over additional lift. If the additional drag caused by the flaps is above the CG, then it would tend to pitch up, requiring down elevator. And vice versa...

[Jonathan M]

To slow and kill lift on landing my Ahi in tight spaces I've programmed (up-moving) spoilerons, but the ailerons are so wide (in themselves and as a percentage of the already very wide root-chord) and the glider so light that this results in a dramatic pitching up (the ailerons acting almost as an up-elevator on a flying wing!), so down-elevator compensation is in fact needed.

The same need to kill lift on my (traditional narrow aileron) Middle Phase with up-moving spoilerons needs a smidge of up-elevator compensation, i.e. moving in the same direction, as normally expected.

The downward-moving flapperons on DLGs go far down to act as an almost instantaneous brake, and (again as normally expected) down-elevator compensation is needed, but this has to be on a curve (rather than proportional) to ensure pitch remains constant through the flapperon range.

[Den Moran]

Thanks, everyone.

Jon, that makes it very clear, and I can see the sense in that. Diolch eto, Den, Llangyfelach.

[Jon H]

No worries Den.

Flaps are a bit of a misunderstood control surface as the general idea if 'they make you fly/land slower' is not the full story. I am sure many of my warbirds would fly just as slowly without the flaps, but getting them to that speed and keeping them under control would be another story all together.

Your super slippery jet will be similar although as a foamy its drag/weight ratio will mean it slows better than a heavy turbine model for example.

If you are not sure about how the elevator flap mix needs to go enable it, set it a tiny amount in the direction you think you might need and then leave the tx on that menu for the maiden flight. Once the model is trimmed and all sorted you can get into a landing configuration and see what happens. If you need up or down, all you need to do is adjust the tx on the fly until the model is behaving as you want. On my radio i can do this well enough on my own with the dial wheel but if your tx is not as easy, or you dont feel confident, you can always as someone else you twiddle the dials while you keep the model on an even keel and offer feedback on what needs to be done.

[Tim Kearsley]

Posted by Jon - Laser Engines on 15/08/2020 10:37:22:

If you are not sure about how the elevator flap mix needs to go enable it, set it a tiny amount in the direction you think you might need and then leave the tx on that menu for the maiden flight. Once the model is trimmed and all sorted you can get into a landing configuration and see what happens. If you need up or down, all you need to do is adjust the tx on the fly until the model is behaving as you want. On my radio i can do this well enough on my own with the dial wheel but if your tx is not as easy, or you dont feel confident, you can always as someone else you twiddle the dials while you keep the model on an even keel and offer feedback on what needs to be done.

Could be tricky in these times of social distancing!

Tim.

[Richard Wills 2]

I set each flap/gear combination as a separate flight mode and the elevator trim flight mode specific and then trim each mode as needed.

[bert baker]

Can’t say I’ve ever come across needing up elevator trim with flaps deployed,
My models with flaps are low wing, I have flown in full size high wing plane and that also needed down trim with flaps deployed.

The only way to truly know is to fly the model and find out what the individual model requires

[Martin Harris - Moderator]

I can set my mixes on an adjustable knob for fine tuning in the air before programming them permanently (+1 for Richard's flight mode method which I use extensively but not all transmitters have this flexibility) but I do quite a lot of maiden flights for other members with a wide variety of transmitters and what I do is make a mental note of the amount of elevator deflection required with full flap (I'll sometimes put a random estimate on a switched mix beforehand) and reprogram once on the ground.

[Bob Cotsford]

I've always used a down elevator mix of between 10-15%, I usually start at 10% and adjust for the individual model. I'd rather need to be adding a little back stick on approach to limit speed rather than pushing forward to keep the speed up. I can't remember one model where I've needed to mix in up, though none have really been that high of a wing loading. Certainly my gliders have needed down elevator for flap or crow function.

Then again, I don't see the point in small amounts of flap (other than when thermaling) on models as they rarely need extra lift, usually I want to add drag to get a steeper approach and to stop them floating on.

[Jon H]

Posted by Bob Cotsford on 15/08/2020 12:26:37:

I'd rather need to be adding a little back stick on approach to limit speed rather than pushing forward to keep the speed up.

The idea is to set the mix to give you the correct trim for level flight at just over landing speed. If you need to descend you cut power, if you need less rate of descent you increase power.

This removes the elevator from consideration during the approach and helps guard against tipstall as you leave the elevator alone and so dont start massively changing the angle of attack. If you get slow, the model naturally drops its nose as its trimmed for a slightly higher speed. Elevator is then only used for the final flare before touchdown.

[Bob Cotsford]

Jon, with the greatest of respect you can't do that at my local club field very often, unless I've forgotten how to fly and need to hang up my transmitter. Approaching from one end you have a tree either side on the crosswind leg then turn onto finals passing between two trees and low over the hedge. From the other end you are passing over hedges then heading towards an uphill runway through the turbulence generated by the hedges and the fold in the land. Behind us we have trees. The air is nearly always so turbulent you fly right down to the runway through rapidly changing wind gradients.

It is nice when you can just set up an approach and leave the model to it but it doesn't happen often. Maybe we just need a bigger field without the trees, hedges and slopes? I do vaguely remember other past fields where we had open approaches and at those your way does work.

edit - it occurred to me that in the past when I flew larger models from this field the air always seemed smoother during take-off and landing.  Is it the case that size matters, or just down to climate change?

Edited By Bob Cotsford on 15/08/2020 16:44:26

[Keith Simmons]

That's very true that size does matter as smaller models has lower wing loading. Models tend to fly faster to compensate. You can scale models but not air itself so to achieve scale speed, you need lighter loading on the wings. Full size has greater penetrating power due to the mass of the aircraft.

[Martin Harris - Moderator]

I'm very much with Jon in principal and try to instill the basic principle of power controlling descent and elevator controlling airspeed in anyone I teach. Draggy flaps just allow you to accentuate these effects even more. It is important to realise that there's interaction between the two controls which will result in fine adjustments being needed when one is altered for any reason.

A model set up on the back of the drag curve seems to have an instantaneous connection with the throttle and glide path corrections using it are so rewarding. However, as Bob has observed, the real world usually throws in turbulent air and so elevator is needed to correct pitch disturbances and restore the right angle of attack. See my last point in the paragraph above...

I find it frustrating to watch a model's attitude going uncorrected - worst case is when you can see the nose high and the airspeed bleeding off - and the pilot then seems surprised to find his model dropping a wing or mushing into the ground!

Edited By Martin Harris on 15/08/2020 18:03:23

[GrumpyGnome]

So.... my Wot 4 needs down; my T28 needs down: my umx ss cub needs huge amounts of down; my typhoon needs up; my explorer needs nothing; my spitfire needs nothing......

Prevailing wind where i fly requires me to land by flying over/between treees and a large pile of cow manure so tends to needs a fair bit of juggling. Or i have to land downhill, or over a ditch and hedge, so ditto. Fourth direction is great! Nice and clear.... so adjustable on a knob would be tricky. Just have to set a mix by trial and error. (makes it sound like a poor field but it is really lovely and a really really good patch - best club field i have ever had

GG

[Tim Flyer]

On my small Acrowot ARTF powered with a 4s laser 70 I use upward moving flaperons for landing . That is mixed with a very small bit of up elevator. It makes landing considerably shorter and removes the long floaty tendency and slows the plane nicely. As it has more than ample power I do find the upward flaperons a great benefit on that model.