A question on flaps ?

[Piers Bowlan]

Lightly loaded aeroplanes just fly better, whether that is a model or full sized B747. They will fly on less power, land and take off at lower speeds. Needlessly adding more materials to make them ever stronger is counter productive because of the weight gain.

One complication I am keen on however are flaps, not just for low speed arrivals but to stop the long float in ground-effect before touchdown. Some years  ago I remember watching a friend’s Mossie come creaming in over the threshold at low level before floating the length of the strip. Finally he landed in the ‘rough’ and ignominiously ending up inverted. That is when you do need a tough model! Of course he was coming in like a train because of the fear of the dreaded ‘tip-stall’. (I would have done the same)
 

Flaps? A  good servo is all you need in terms of added weight, a good trade off I feel.

[Maurice Dyer]

Question for the experts. Got a Chipmunk 68 inch span that tip stalls at the merest suggestion of landing. Would flaps help ??

Maurice

[Paul Johnson 4]

Could try wing cuffs on outer 70% of leading edge.

[Paul Johnson 4]

Link

[Maurice Dyer]

1 hour ago, Piers Bowlan said:

I don’t pretend to be an expert but it depends….

 

Is the model over weight, a high wing loading? Does it always drop one wing? Were the wings built with any washout? Are the ailerons differential in operation (more up than down)? More questions than answers here unfortunately.

 

Small deflections of flap can lower the stalling speed effectively to some degree. How much (%) depends on the type and area of flap. Further  deflection of flaps will principally increase drag with little further reduction of stalling speed. 

At the point of stall if an aircraft is being flown out of balance (yawing) this will often precipitate a wing drop. Adverse yaw from ailerons can cause yawing at slow speeds. This is reduced by differential aileron deflection which is why most full sized aircraft has this.

 
To answer your question Maurice (!) flaps can actually exacerbate a wing drop at the point of stall!

 

Not too sure what this has to do with ‘The Big Question’ though 🤔.

I read Piers talking about flaps. Differntial aileron: no washout, ARTF: 68 inch chipmonk, weighs 8lbs.

[RICHARD WILLS]

It is a bit of a digression to our thread Maurice , but perhaps we can clear it up then get back to our flapless aeroplanes 😁

Two comments above are significant . Piers asked , does the tip stall happen both sides or just one ? I would qualify that with low power setting to take torque out of the picture . So just go to height and stooge into wind . 

If C of G is  well forward, I would suspect a problem in the wing shape or angle . Paul made the next significant comment . Try cuffs . 

Lets guess that the left wing drops . It has lost lift and the suction on the upper skin lets go . 

To get the wing to hang in a bit longer , we need to make the airflow go over the top longer , rather than taking the quick route underneath . 

Quick bodge is to glue some balsa under the leading edge , in order to lower the most forward point of the wing section where the airflow decides to separate and take two different paths . This will move the "decisive separation point " lower making more air flow up and over and creating more suction . Moving A to B . 

The modification only needs to be on the outer half of the wing . The root is rarely a problem . Ive tamed many a beast with this method . 

If its stalling both ways then add to both sides . 

If you need more info , PM me . 

Regards

Richard 

 

 

[Jon H]

Without knowing the exact model being discussed i cant say if the following will apply directly. For very light foamy models will will not be applicable, but for everything else it should apply. 

 

Flaps tend to be a little misunderstood in modelling circles with a great focus on the increase in lift they provide. This is true of course, but the big thing they add is drag. 

 

This is important as it completely changes the way you fly the approach. 

 

Many modellers fly long an fairly flat approaches with the nose high in an effort to bleed speed off. This is not at all uncommon and i do it myself with some of my more slippery sport models. 

 

However, this is not ideal from a tip stalling point of view as you already have the wing near its critical angle of attack, and as we know stalling is all about angle of attack and not airspeed. 

 

If you add flaps to a model, and give them a fairly healthy deflection the added drag means you can fly a steeper nose down approach with power off and not accelerate to warp speed in the process. The added lift will also mean a lower angle of attack is needed on the wing as a whole, further improving tip stall margins. As your efforts in this configuration are mostly aimed at shoving the nose down, rather than holding it up, you cannot stall the model.

 

21 hours ago, Maurice Dyer said:

Question for the experts. Got a Chipmunk 68 inch span that tip stalls at the merest suggestion of landing. Would flaps help ??

Maurice

 Yes

 

As i have said many times before, one great help is an elevator flap mix. I use these all the time on my warbirds to re trim the model for landing as lowering the gear and flaps along with reduced airspeed usually causes a significant nose down trim change. Set up correctly this will make your model 'stall proof' if the correct technique is used. It should also help in your case with the chipmunk. 

 

First, make sure your c/g and elevator rates are half decent. IF the model has massive elevator rate and/or is out of balance your are on a looser from the get go.

 

With that covered, set your flaps to the desired deflection. Dont be shy, get them down a good 40 degrees or more (89 degrees for a spitfire!) and enable the elevator flap mix on your tx. It should be set up so that as the flaps are lowered elevator position changes to deal with the trim change. The flaps are the master channel, moving the elevator should not move the flaps. Generally, models will need up trim with flaps deployed if they have retracts as well. The added drag of the gear below the model causes a nose down pitch, so i always start with about 5% up on the mix just to make sure its working as intended. 

 

With that done leave the mix menu open on the radio and go flying. Get the model into the landing configuration and observe the response. Adjust the mix while in the air until you get to a point where the flap mix is set to hold the model in level flight at just over idle power without stalling. The way i set this is to increase the up elevator on the mix until the model stalls, note the value, and then knock a bit off. Sometimes i will get to a comfortable feeling setting before a stall, and in those cases i give the elevator a few tugs to see how close i am to provoking a stall, then make adjustments as needed. If you are not confident beeping your tx buttons without taking your eye off the model employ an assistant to beep for you. if it all goes wrong be on a hair trigger to get the flaps up so the mix is disabled. If you have to look at your radio to find your switches, spend a little time before flying finding them on the radio by feel until you can do it with your eyes shut. Having a standard switch layout for all models is also very handy. I cant believe the number of people i have flown with who have a different layout of switches for every model. Its like they loaded a shot gun with switch assignments and just fired it at the radio each time. 

 

With your mix set up nicely its time to land and you need to fly a more formal landing circuit than most are used to and at a higher altitude. If you whizz downwind and just dump full flap the model will damn near do a backflip as the model will be way too fast for your flap mix trimmed speed. A 3 position switch is needed as a minimum, a slider or servo slow on the radio is ideal. Either way, the general idea is to slowly add flaps as you slow down on your downwind leg so that when you turn base they are full down and you are holding a small amount of forward elevator pressure on the stick. You can then bin your forward elevator pressure as you roll into your base turn using the slight up trim condition to float you round. Once lined up leave the elevator alone. If the model is falling too fast add power, if its going long pull some power off. Use the throttle to control the rate of descent and dont touch the elevator. We know the model is trimmed for a just above idle level flight airspeed/aoa which is below the stall so it will naturally fly to that trimmed speed and so cannot stall. The energy to maintain that speed can come from the engine, or from gravity. More more engine you use the less gravity, so a lower rate of descent. The less engine and more gravity...you get the idea. The point is if you just leave it alone it cannot be stalled like this as its just parachuting down with those big draggy flaps keeping it all under control. All you have to do is decide when you level off, and use just a tiny squeeze of elevator with a little added power to achieve this. Ideally you are now about an inch off the floor in level flight. Chop the power and slowly tug on the elevator until you either stall or mush onto the runway. 

 

Taxi back looking smug as your club mates look on after your perfect landing. 

 

This is a very different landing technique than most modellers are used to and it demands accurate use of throttle as well as resisting the urge to yank on the elevator. If you can do that, your scale models should be a great deal easier to get down. 

 

 

 

 

[Martin Harris - Moderator]

Great description - done right, there’s an almost surreal connection between the throttle and its effect on the model’s position on the glide path. Difficult to visualise in words but get a steady approach set up - particularly with draggy flaps - with power balancing drag and the connection is unmistakable. 

[Jon H]

One thing i forgot. 

 

Aborted landing/go-around procedure. 

 

With your elevator flap mix enabled if you need to go around do not under any circumstances firewall the throttle. If you do, especially on a 50cc class model, you will have a massive shot of torque rolling you left and because of your low speed your ailerons will not cope and may in fact induce a stall. The rise in speed will also cause an almost uncontrollable pitch up if you just boot it. 

 

If you need to go around rapidly advance the throttle to about half, hold the model level (forward elevator pressure), flaps up to half, gear up, increase to 75% throttle, flaps up, climb away. 

 

I use a variation of this technique and just go to flaps half and cruise round the circuit at half throttle holding the model down, but i am very used to this type of flying so for the newcomer to this style its best to just tidy the model up and go again. 

[J D 8]

       In the early days of RC with just single channel or just rudder elevator we would tend to have a high approach to ensure arrival at the strip on the glide engine having stopped when it ran out of juice.

            Thing is you often see type of approach used even today with throttled reliable engines.

   Flyer sets up in the circuit turns into finals takes throttle to tick over and that is where it stays. The elevator then induces a roller coaster approach where air speed can get low as it levels out to slow down, this high from the ground and here is where the tip stall can bite some types.

  Something left over from the days of no power approaches or poor low speed reliability with engines of old and was often taught even when full house radio became the norm about the time I started in RC.

  I still fly single and two channel types an if approach is to high [ better than to low ] a few gentle S turns looses hight and gets one on the strip.

[Piers Bowlan]

Whilst I have to agree with everything Jon says here (!) I think there is a bit more to it.

Certainly a stabilised approach;-a/c configured early on glide path in trim with approach power set, is the basis for a good approach and landing. To say the aircraft can’t stall is a bit of an exaggeration. Unlike a full size aircraft we have no idea how near the critical pitch attitude the aircraft is to the stall. There are these things called gusts, thermals and sink. Yes we can apply power to arrest the sink but can’t see the airspeed as the model approaches, unlike a full sized aircraft. It’s all down to guess work with a model - no stall warner. So when it’s gusty we fly a little faster, as you do with a full sized aircraft. We know when we have got it wrong however when it departs and ends up in bits 😭.

 

This doesn’t help Maurice with his Chipmunk ‘that stalls at the merest suggestion of landing’.

The question is why is his aircraft so unforgiving? As I said before (now deleted!) there are questions?

1. Is it overweight (high wing loading)

2) is there any inbuilt washout.

3) is it always one wing that drops (warp?)

4) differential ailerons would be helpful.

5) is the aerofoil ‘thin’ and unforgiving which won’t help matters.

6) C of G position?

As for flaps helping, most aircraft are actually more likely to drop a wing and depart into a spin (incipient spin) with full flaps selected if the speed is allowed to decay below a minimum safe approach speed (especially a high power settings).
 

Answers on a post card please! A little more information about Maurice’s Chipmunk might help possibly. 

 

 

[J D 8]

      Full size Chipmunk has a simple stall warning system on the wing at the wing root called stall strips, often called Toblerone after the triangular chocolate bar.

  They disturb the airflow over the center section causing the elevator to shake the stick. Also the disturbed flow triggers a stall to begin at the middle of the wing.

 At least that is how I understand it.

[Jon H]

17 minutes ago, Piers Bowlan said:

Unlike a full size aircraft we have no idea how near the critical pitch attitude the aircraft is to the stall

 

Very true, which is exactly why you trim it for a speed just shy of the stalling angle. The model will then naturally fly at this speed no matter what you do with it and you do not have to judge the speed as its locked in by the trim. 

 

Of course extreme gusts of wind etc can rock our applecart, but its by far the easiest and most unstallable a model is going to get. 

 

When i flew my sea fury the other weekend i had 3 great flights with easy landings, but the 3rd was a real challenge with 2 go arounds due to a shift in the wind causing turbulence over the patch. Could it all have gone wrong? yes, would have been a tip stall in the usual sense? no. The wind will always ruin our day by micro bursting us into the floor or similar, but most models are stall/spun on base entry or halfway down the approach and the method i describe will get most through that phase. 

 

19 minutes ago, Piers Bowlan said:

This doesn’t help Maurice with his Chipmunk ‘that stalls at the merest suggestion of landing’.

The question is why is his aircraft so unforgiving? As I said before (now deleted!) there are questions?

1. Is it overweight (high wing loading)

2) is there any inbuilt washout.

3) is it always one wing that drops (warp?)

4) differential ailerons would be helpful.

5) is the aerofoil ‘thin’ and unforgiving which won’t help matters.

6) C of G position?

 

Well it does help. While i agree in principal the issues you raise are important, and covered rates and cg in my post, he has the model he has at this point and there is likely not much that can be done about the washout or wing section. So its a matter of dealing with what is i front of him and flying approaches in the way i describe will help as it takes the airspeed judgement call out of the equation. 

 

All that said, more details on its bad habits would be helpful. 

 

 

[Martin Harris - Moderator]

If they’re scale style ailerons, setting some up on both ailerons can tame a nasty tipstaller by introducing some pseudo washout. 

[PatMc]

8 minutes ago, Martin Harris - Moderator said:

If they’re scale style ailerons, setting some up on both ailerons can tame a nasty tipstaller by introducing some pseudo washout. 

Really ? 

Surely the airflow over the top surface of the wing will have become detached & turbulent well before reaching the ailerons.

[Martin Harris - Moderator]

I’ve used this method to good effect on several occasions.  While laminar flow may be lost, the aerofoil is modified enough to reduce the effective angle of incidence. 

[Piers Bowlan]

Whilst it is true there is not much that can be done about the washout, wing section or a warp Jon, having a knowledge of why the model behaves this way might help us not to build, or buy one, like it again!

 

 If Maurice has not already done so he could try adjusting the C of G to see if there is any improvement? Alternatively he could try increasing the aileron differential incase adverse yaw is precipitating a wing drop or even adding a tad of coupled aileron and rudder ☠️.

 

I tend not to take the view, you are stuck with it, just trim for speed and all will be well. If the C of G is out then the model can be speed unstable. If a model is well designed, built and set up correctly, as I have no doubt yours are, the model will behave itself if trimmed for a stable approach.

[Piers Bowlan]

In cases like this more information can be helpful to get to the bottom of what is going on.

i remember reading a thread where someone was having problems with a Sopwith Camel (or similar) which had a strong tendency to ground loop. Many suggestions were forthcoming including gyros and altering the wheel toe in/ out etc. Eventually it transpired that a very large lump of led had been attached ABOVE the engine to compensate for a very heavy tail. The resulting high  c of g and narrow track u/c was the cause of the models errant behaviour.  A lighter tail would helped a lot!

Knowing about the lead earlier in the thread would have halved the number of posts. 🤣

[Jon H]

1 hour ago, PatMc said:

Really ? 

Surely the airflow over the top surface of the wing will have become detached & turbulent well before reaching the ailerons.

 

AS the ailerons work with whatever airflow they get as it is, setting them up a tiny bit will help as relative aoa will reduce. But its sub optimal. 

 

1 hour ago, Piers Bowlan said:

Whilst it is true there is not much that can be done about the washout, wing section or a warp Jon, having a knowledge of why the model behaves this way might help us not to build, or buy one, like it again!

 

 If Maurice has not already done so he could try adjusting the C of G to see if there is any improvement? Alternatively he could try increasing the aileron differential incase adverse yaw is precipitating a wing drop or even adding a tad of coupled aileron and rudder ☠️.

 

I tend not to take the view, you are stuck with it, just trim for speed and all will be well. If the C of G is out then the model can be speed unstable. If a model is well designed, built and set up correctly, as I have no doubt yours are, the model will behave itself if trimmed for a stable approach.

 

This isnt personal, but having left the modelling industry i find myself with a complete disinterest in explaining myself 500 times in order to get a point across. I have made my recommendation, and we are rapidly approaching take it or leave it territory as i no longer have  the desire to chew the fat ad infinitum. 

 

I am not saying your points are wrong, although aileron rudder mixing on the tx is the devils work (2 hands, 2 sticks. Do it yourself), but as some point you just have to fly what you have warts n all. Without input from Maurice on the specifics, we are just talking ourselves in circles and these days i just lack the enthusiasm for that sort of thing. 

 

All i can say is that using the technique i described i was able to take someone who's flying would not get them through an A cert, and within a day get them to a point where they could safely fly their 80 inch Spitfire. Arguments about the merits of that can wait for another day, but the point is someone with limited ability can fly a model like that purely by following procedures like the one i posted earlier. Its not ideal, but better than them having a crash. Even if our troublesome chipmunk can be tweaked, i would still land it with a flap mix if i felt it needed it as it takes the thinking out of itand allows brain power to to be allocated to other things. 

[Simon Chaddock]

I entirely agree that light wing loading planes have better "manners" on landing but I was interested in how flaps might perform on such a plane.

My most extreme experiment was this which I call the "Push Pull" with 2 section flaps. They are on a Tx knob so are variable from 0 to 90 degrees. 

 

A basically very stable design with inline counter rotating props to virtually eliminate motor torque.

The advantage of such a layout is that a big speed reduction is possible without requiring a nose high attitude so the wing tips do not stall although the ailerons are "weak" at slow speed and thus need big inputs.

This video shows how it flies with no flap and full flap. 

https://www.youtube.com/watch?v=2TNp_G2NQ00&t=1s 

   

[Piers Bowlan]

An interesting experiment and a nice video illustrating the effectiveness of flaps increasing a wing’s camber so enabling the ‘same lift’ but at a lower speed whilst increasing drag producing a steeper glide angle.

 

The ailerons or any control surface will be ‘weak’ at slow airspeeds, it is the first  feedback we model pilots have that we are flying slowly, maybe too slowly. What we observe is always ground speed but what we (and the model) are interested in is airspeed. The second indication we are flying too slowly is the incipient spin!

[RICHARD WILLS]

Maurice wishes he hadn't asked the question .

He's left the building 😕

I apologise in advance , I just can't help myself.