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Flaperon differential


David Ramsden
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David, if you had dedicated flaps then flaps down would be the best way as the outer board ailerons will still work, but with full span flapperons when you put them down and then try to use ailerons to bank the plane, you'll increase the drag on the aileron that is going down and trying to raise the wing causing the model to yaw in the opposite direction.

So for landing set the ailerons to go up and just use any small downward motion, less than 5 degrees, as a method of adjusting the camber for slightly increased lift. Just be aware that if you have set any aileron differential then when you raise both ailerons then when you apply aileron control the upgoing ailerons may reach it's maximum travel and if the differential is still applied the down going one won't go down very much, so you'll have much reduced roll control (but no adverse yaw).

You could put the camber control (down flaps) on a switch but I'd recommend that the upgoing spoilerons for landing are on a progressive control. I have a Staufenbiel Vegas warmliner which doesn't have flaps so I raise the ailerons for landing, the main effect is to dump lift, on this model they also cause the nose to come up slightly but not enough to need any elevator compensation.

PS the picture in my avatar is my old 1/4 scale HP 18 which has flaps which are around 2/3rds span and ailerons a 1/3rd span, in the photo it's on a landing approach with the flaps down around 60 degrees and ailerons raised around 30 degrees. 

Edited By Frank Skilbeck on 07/10/2020 21:59:42

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Posted by Frank Skilbeck on 07/10/2020 21:56:29:

David, if you had dedicated flaps then flaps down would be the best way as the outer board ailerons will still work, but with full span flapperons when you put them down and then try to use ailerons to bank the plane, you'll increase the drag on the aileron that is going down and trying to raise the wing causing the model to yaw in the opposite direction.

So for landing set the ailerons to go up and just use any small downward motion, less than 5 degrees, as a method of adjusting the camber for slightly increased lift. Just be aware that if you have set any aileron differential then when you raise both ailerons then when you apply aileron control the upgoing ailerons may reach it's maximum travel and if the differential is still applied the down going one won't go down very much, so you'll have much reduced roll control (but no adverse yaw).

You could put the camber control (down flaps) on a switch but I'd recommend that the upgoing spoilerons for landing are on a progressive control. I have a Staufenbiel Vegas warmliner which doesn't have flaps so I raise the ailerons for landing, the main effect is to dump lift, on this model they also cause the nose to come up slightly but not enough to need any elevator compensation.

PS the picture in my avatar is my old 1/4 scale HP 18 which has flaps which are around 2/3rds span and ailerons a 1/3rd span, in the photo it's on a landing approach with the flaps down around 60 degrees and ailerons raised around 30 degrees.

Edited By Frank Skilbeck on 07/10/2020 21:59:42

Thanks Frank. So, once I get a bit of height, lower the flaps a tiny bit and see if the ailerons are still effective. If they are, then lower the flaps a bit more and try again Okay. I'll do that.

With a modest amount of dihedral and a big rudder I'm hoping that rudder use might result in some banking too.

The upgoing flap only reaches maximum travel when the flaps are fully up AND with full aileron input so the ailerons should not lose any authority when the flaps are up.

Also, following earlier comments, I have set it up so that the maximum aileron movement exceeds the maximum flap movement (in both directions) and both the aileron movement and the flap movement are 'more up than down'.
I am using the left stick (throttle (ratchet) control) for the flaps.

Most useful Frank. Thank you.

Edited By David Ramsden on 07/10/2020 22:30:19

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That's a lovely looking model thumbs up

For me I would prefer to use spoilerons for landing, (ailerons going up) then flaperons to kill the lift and reduce the height. I would however prefer to build in separate flaps and make it a full house model with CROW braking and the usual thermal & speed modes. CROW will give you more accurate landings if you are flying on particularly rough grould, to help you avoid rocks etc. I'd happily fly it as it is now though it looks great!

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Posted by David Ramsden on 07/10/2020 20:54:30:

...I've never flown a model with flaps (let alone flaperons) however I am familiar with how down flaps are used on a standard powered approach and finals. I'm also aware that up flaps (particularly full length flaps) will reduce lift tremendously. I'm guessing that up flaps will be best for losing altitude and down flaps best on finals. Am I right?
Do you think that down flaps or up flaps will be most useful for landings? And what would you expect the required elevator inputs to be?

If I was flying this model in it's current 2 servo wing guise I would setup spoilerons in the landing mode activated off the throttle stick. Full deployment would need to give 40-45 degrees of reflex on the surface to get useful braking performance. That will normally need up elevator compensation on a mix (ideally on a custom curve), probably a fair bit.

I would also have the aileron differential reversed (i.e. more down than up) once the spolierons are deployed if your TX can do it; this will prevent you running out of aileron authority under braking, though with such big surfaces I doubt that will be an issue anyway. This is the setup I have on my Durafly Excalibur; I have the differential change on a curve as I deploy the spoilerons so it goes smoothly from slightly positive diff at zero braking to very negative at full deployment. As the brakes come in I also add in an aileron to rudder mix too. An admittedly complex TX setup, but it makes landings very simple!

Edited By MattyB on 08/10/2020 09:23:50

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Posted by David Ramsden on 07/10/2020 22:28:25:

Thanks Frank. So, once I get a bit of height, lower the flaps a tiny bit and see if the ailerons are still effective. If they are, then lower the flaps a bit more and try again Okay. I'll do that.

With a modest amount of dihedral and a big rudder I'm hoping that rudder use might result in some banking too.

The upgoing flap only reaches maximum travel when the flaps are fully up AND with full aileron input so the ailerons should not lose any authority when the flaps are up.

Be aware that the biggest danger you have with here is not a loss of aileron authority, but a tip stall caused by the additional camber at the tip. Nothing should go amiss travelling in a straight line if you keep the flap deflections low, but as you turn and put in some elevator you're asking a lot of that inner wingtip as it slows down (see my post on page 1). I know you want to slow your landing speed, but remember that maintaining control authority is always more important.

Edited By MattyB on 08/10/2020 09:25:48

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David

I can only endorse the point made by MattyB. A high aspect ratio wing has its own issues when in slow flight with a tendency to initiate a spin in a turn where trying to raise the low wing may just make the situation worse worse.

Altering the tip wing camber tends to aggravate this effect hence the use of differential aileron action, more up than down or in extreme case no down and only up, bearing in mind this beneficial effect really only applies to surfaces that are not otherwise being used to reduce flying speed.

The advice is to be cautious and progressively explore the flaperon effects and benefits in safe situations (height) remembering that slow speed and big control deflections are the most likely combination when on a landing approach with any sort of turbulence. wink 2

Do let us know how you get on.

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Thank you Steve, MattyB, and Simon.

Following earlier comments I did stop the flaperons 4cm short of the tips and build in some tip washout on that outer 4cm of trailing edge so I'm hoping that that will have reduced the probability of tip stalling. As previously mentioned, I have designed in some dihedral and a big rudder so I'm hoping that may be sufficient to initiate some banking without resorting to ailerons for wide turns.

Given the dangers of full length flap use on high a/r wings, and given my complete lack of experience with flaps, I think the best thing I can do is forget it has flaps for the time being. Get used to flying it just using just ailerons, elevator and rudder and learn all it's characteristics before gradually trying the flap function.

Another challenge is that I'm using mechanical function mixing rather than Tx programming. **LINK**

No matter what happens, I have loved the design and build process. I find these conversations fascinating, and the prospects of flying it quite exciting (and a bit scary!). If I have to do major repairs that's fine. If I have to re-design and build a new pair of wings then that's fine too. To me, these are all part of the journey and with all your help (ALL of you) I'm learning a lot along the way.

Many thanks,

David

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Posted by David Ramsden on 08/10/2020 12:15:01:

Thank you Steve, MattyB, and Simon.

Following earlier comments I did stop the flaperons 4cm short of the tips and build in some tip washout on that outer 4cm of trailing edge so I'm hoping that that will have reduced the probability of tip stalling. As previously mentioned, I have designed in some dihedral and a big rudder so I'm hoping that may be sufficient to initiate some banking without resorting to ailerons for wide turns.

Given the dangers of full length flap use on high a/r wings, and given my complete lack of experience with flaps, I think the best thing I can do is forget it has flaps for the time being. Get used to flying it just using just ailerons, elevator and rudder and learn all it's characteristics before gradually trying the flap function.

I think that is very wise course - just make sure you keep the speed up at all times in the early flights, and check the stall out 2-3 mistakes high before you slow it down near the ground.

Posted by David Ramsden on 08/10/2020 12:15:01:

...Another challenge is that I'm using mechanical function mixing rather than Tx programming. **LINK**

Holy cow, I've not seen one of those since about 1992! Had something similar in my fist SAS Thing slope wing (a white foam precursor to the Wildthing) - on warm days it worked nicely, but when it got cold differential expansion took hold and the tray would stick in the runners! Not much fun. Whilst I admire you for trying this method I would heartily recommend you invest in a new TX that is capable of some glider mixing - if you do go 4 servo wing it will make it hugely easier to setup and fly. They are not expensive either now, with an OpenTX radio you could sepnd ~£100 to get something ith mixing capabilities that were formerly the reserve of £1k+ sets.

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Funny I just found an old RM Zippy flying wing in the garage and flew it yesterday, first thing I did was take out the mechanical mixer, a pivoting servo system, and use the Tx mixing.

Of course with the mechanical mixer David is using then he can't change the aileron differential as flapperons/spoilerons are deployed.

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Maiden day

dr420 black hill 081020.jpg

My DR 420’s ‘maiden' consisted of two flat field test glides and two 40 min test fights in a gusty 6-12 mph on Black Hill, Dartmoor. She flies(! ) and no damage (yet).

I didn’t touch the flap function for the first 20 mins just used aileron and elevator and struggled to get above 30-40 feet in less than ideal conditions. Applied about 30% down flap and she was 80 feet up quite quickly.

Stalling straight showed no tendency to drop a wing and recoveries were easy and with little loss of height. She did sometimes drop a wing in turns but usually not. I couldn’t establish exactly why but suspect that variations in wind speed caused the inconsistency. Mostly she flew pretty well for a first attempt. I found that:

  1. Flaps make more difference to airspeed than altitude and no constant elevator input is needed. With full up flap she does come down gradually but gains considerable energy. At full down flap she flies close to stall speed but with careful handling is still well behaved. 25-35% down flap was best for climbing.
  2. The aileron response is disappointingly slow even at high airspeed. Increasing aileron throw a bit made little difference. I couldn’t roll her past about 80 degrees no matter what I did and even that took ages and height was lost. I expected to be able to do quick 90 degree banks and turns using up elevator but she was having none of it. Most turns were fairly close to flat.
  3. When I flew her rudder/elevator the flight characteristics were almost identical (even with very little dihedral).
  4. Remarkably, flap position made no difference to aileron authority and no difference to the likelihood of dropping a wing. That’s good.
  5. She looked majestic in flight and sounded great in passing.

I'm going to try increasing aileron throw some more before the next test flights. All good fun!

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Sounds like a successful maiden! Comments below against your numbered ones above...

1) How much reflex have ou got set for spoilerons? I would say the minimum amount to get an actual braking effct is 30 degress, 45 would be better. If you've only got a mm or two all it will do is speed up the model and increase the stall speed.

2) I suspect your disappointing aileron response is due to those long ailerons bending as they are driven from the root. Try upping the movement if you can (how much is it at present?) but I probably wouldn't expect it to be that effective.

4) That's good news - you should be able to try a bit more flap then next time.

Edited By MattyB on 09/10/2020 10:33:05

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Hi MattyB. I haven't actually measured the flap or aileron movements in degrees but the up flap function produced nothing like 30 degrees reflex and I programmed the aileron servo to only 50% of it's movement. I've now increased that to 80%. I could increase the flap movement too. I intentionally limited all flaperon movements following the warnings I received earlier in this thread. I guess increasing them is dangerous but I'm going to try. After that the next move will probably be working out how to increase the differential.

I did test the flaperon twist last night and from the root to the tip they are losing about 8 degrees of deflection. The film hinges are full length. I could try cutting through the film hinge say 3cm out of every 5cm. Not sure about that.

Thanks for all your help. Hugely appreciated.

David

Edited By David Ramsden on 09/10/2020 12:33:40

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Test flights 3 & 4

Beforehand - I increased and aileron throw AND the up-flap throw but decreased the down flap. Conditions were far better, steeper slope with a much stronger wind than yesterday gusting 7-17mph at eye level .

In these conditions she flew like a different bird possibly because of the overall higher airspeed but the increased aileron throw was obviously making a huge difference. Mostly I just tested her responses and watched for tip stalls:

1) Aileron response was good at a wide variety of airspeeds and was not affected by up or down flap input. She still won't do a complete roll but did pretty fast 80 degree banked turns (both into wind and downwind) and half rolls from inverted were no problem. She even did a half-decent Cuban 8.

2) Up flap (now about 30 degrees) response was much stronger and looked identical to down elevator input (it had zero braking effect - quite the opposite!). I guess the next thing to try is up flap and up elevator at the same time(???).

3) No sign of tip-stalling in any sort of turn or at any groundspeed (but probably the airspeed was constantly higher than yesterday).

4) Rudder response is great. She can be flown rudder/elevator and does great stall turns.

She really was a great pleasure to fly in these conditions. To anyone reading this who has not had a go at 'designing your own' I would say have a go.

With traditional spruce spars, her thin wings are pretty flexible and at speed she literally 'wriggled' her way though strong gusts - like a falcon rousing her feathers in flight - quite something to watch. In recoveries from failed manoeuvres I pulled far more G than intended which greatly increased by faith in the light balsa wing structure.

Coming back to the theme of this thread 'Flaperon Differential', I still think I need to increase it (reduce the amount of down aileron) and hope to further improve the aileron response. Mind you, if it doesn't work I'm still a happy chappie. If I'd wanted an out-and-out aerobatic glider I would have designed her differently.

I guess that'll be my next project.

Edited By David Ramsden on 09/10/2020 22:25:09

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Posted by David Ramsden on 09/10/2020 22:22:31:

2) Up flap (now about 30 degrees) response was much stronger and looked identical to down elevator input (it had zero braking effect - quite the opposite!). I guess the next thing to try is up flap and up elevator at the same time(???).

In effect, you are rotating the wing downwards and dramatically reducing the angle of attack - the chord line of what is now an odd shaped wing section is measured from the rear edge of the control surface so you're quite right in that you need to increase the angle of attack with up elevator to control the airspeed which will add drag resulting in what should be a rapid sink rate.

The first time I tried this system - on an old F5b hotliner - I didn't find it very effective but controlling the airspeed with the elevator made all the difference and I've used it on several gliders since, including a cheap and cheerful 4m Discus with no provision for air brakes. Trying to get that down in our relatively tight field without effective approach control would have been "interesting" to say the least!

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David

Don't expect 'up' flap at small angles (up to 10 degrees) to do much apart from making the plane fly faster Good for penetrating into wind.

Similarly down (up to 10 degrees) significantly increase the lift with little increase in drag so it will the plane can flly slower at the same sink rate thus making better use of any the lift.

You have to go to 20 degrees (up or down) before the drag starts to increase significantly allowing you to put the nose down without increasing speed. At 45 degrees up or down the drag rises rapidly allowing a steep dive but as flaperons your aileron roll control would be seriously impaired.

Gliders such as your are aerodynamically efficient so your flaperons will be more 'speed adjusters' rather than providing any serious braking effect.

Full size soaring gliders, even with flaps, also have airbrakes otherwise landing at a given point would be virtually impossible.wink 2.

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Thanks Simon for your comment. So basically, don't expect much braking effect from up flaps unless they are well over 30 degrees.

Nevertheless, I still want to see what happens to the dive when I add up elevator to my 30 degrees of up flap. Martins comment makes me think it will have at least some braking effect. (thanks Martin). If aileron authority is reduced perhaps she will still respond to rudder? I've been very surprised how much directional control the rudder has in spite of her having very little dihedral.

I've never had a model with flaps before let alone full length flaperons so I'm on a steep learning curve.

All comments welcome!
David

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David good stuff, it's always interesting to experiment. One reason why you might have problems rolling is that when you get inverted you've now got "reversed" differential laugh.

Your right spoilerons don't really provide any "braking" but they do reduce the lift so you can control the landing approach more, putting the nose up to compensate for the reduced lift will provide some braking. But it's not as effective as crow braking.

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Posted by Frank Skilbeck on 10/10/2020 08:13:29:

David good stuff, it's always interesting to experiment. One reason why you might have problems rolling is that when you get inverted you've now got "reversed" differential laugh.

Have you? Isn't the rolling action still being applied in the same direction whichever way up the model is? I believe that the downgoing wing is transformed into the upgoing wing during the roll but other than gravity being applied differently the aerodynamic effects haven't changed...

Edited By Martin Harris on 10/10/2020 11:59:31

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Posted by Martin Harris on 10/10/2020 11:56:20:
Posted by Frank Skilbeck on 10/10/2020 08:13:29:

David good stuff, it's always interesting to experiment. One reason why you might have problems rolling is that when you get inverted you've now got "reversed" differential laugh.

Have you? Isn't the rolling action still being applied in the same direction whichever way up the model is? I believe that the downgoing wing is transformed into the upgoing wing during the roll but other than gravity being applied differently the aerodynamic effects haven't changed...

Edited By Martin Harris on 10/10/2020 11:59:31

This is an interesting question! Admittedly I'm a relative novice but I think Frank is right - differential is reversed in inverted flight - so does that mean that out-and-out aerobatic models are set up with zero differential? Maybe Martin is also right because a continuous roll is not the same as inverted flight (is it?).
David

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Posted by Martin Harris on 10/10/2020 11:56:20:
Posted by Frank Skilbeck on 10/10/2020 08:13:29:

David good stuff, it's always interesting to experiment. One reason why you might have problems rolling is that when you get inverted you've now got "reversed" differential laugh.

Have you? Isn't the rolling action still being applied in the same direction whichever way up the model is? I believe that the downgoing wing is transformed into the upgoing wing during the roll but other than gravity being applied differently the aerodynamic effects haven't changed...

Edited By Martin Harris on 10/10/2020 11:59:31

Martin said that the differential was reversed, not the rolling action.

OTOH the reversed differential will increase the roll power but also add yaw in the wrong direction.

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If you're happy to accept a barrel roll & are able to mix rudder with aileron input at the Tx, then around 30% - 50% mix will probably speed the roll rate. Or you could try the same manually, but release the rudder mix during inverted if you're fingers & brain is agile enough.
The 100" glider in the photo below will roll fine but a bit slowly using ailerons only but I often switch in 50% rudder just to speed it along.

PS don't raise the nose above horizontal during the inverted phase or the rudder will oppose the roll direction.

Diamond

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Posted by PatMc on 10/10/2020 12:20:52:
Posted by Martin Harris on 10/10/2020 11:56:20:
Posted by Frank Skilbeck on 10/10/2020 08:13:29:

David good stuff, it's always interesting to experiment. One reason why you might have problems rolling is that when you get inverted you've now got "reversed" differential laugh.

Have you? Isn't the rolling action still being applied in the same direction whichever way up the model is? I believe that the downgoing wing is transformed into the upgoing wing during the roll but other than gravity being applied differently the aerodynamic effects haven't changed...

Edited By Martin Harris on 10/10/2020 11:59:31

Martin said that the differential was reversed, not the rolling action.

OTOH the reversed differential will increase the roll power but also add yaw in the wrong direction.

I'm struggling to understand where the concept of reversed differential has come from.

The adverse yaw is produced by induced drag as a by-product of producing the extra lift from the (initially) upgoing wing being greater on the camber increasing wing. As the roll progresses this lift is still being produced but at different angles relative to the horizon. Although when inverted the lift from that wing is pushing it downwards, the aerodynamics haven't changed and looked at from the wing's point of view, it is still producing lift!

The principle of reducing movement relative to the upgoing aileron in order to attempt to balance the unequal drag holds fast whichever way up the wing is.  With a truly symmetrical wing section, there is little or no need for differential but glider and general sport or scale models don't normally have them.

Edited By Martin Harris on 10/10/2020 13:22:49

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Posted by Martin Harris on 10/10/2020 13:11:46:
Posted by PatMc on 10/10/2020 12:20:52:
Posted by Martin Harris on 10/10/2020 11:56:20:
Posted by Frank Skilbeck on 10/10/2020 08:13:29:

David good stuff, it's always interesting to experiment. One reason why you might have problems rolling is that when you get inverted you've now got "reversed" differential laugh.

Have you? Isn't the rolling action still being applied in the same direction whichever way up the model is? I believe that the downgoing wing is transformed into the upgoing wing during the roll but other than gravity being applied differently the aerodynamic effects haven't changed...

Edited By Martin Harris on 10/10/2020 11:59:31

Martin said that the differential was reversed, not the rolling action.

OTOH the reversed differential will increase the roll power but also add yaw in the wrong direction.

I'm struggling to understand where the concept of reversed differential has come from.

Although when inverted the lift from that wing is pushing it downwards,

 

Not if enough down elevator is being applied during the inverted phase.

Edited By PatMc on 10/10/2020 13:46:05

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I was referring to the force continuing, in concert with the other wing, to rotate the model around its longitudinal axis.

Consider a model with a non-symmetrical wing flying inverted. If you start a roll, the less cambered surface will produce more lift and therefore more drag than the (conventionally) upper wing surface - so the adverse yaw will still act in the same direction and any differential applied will still tend to reduce the effect.

I'm still trying to get my head round the idea that the initial roll rate might be faster starting from inverted - it does seem feasible.

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