Flaps

[Keith Evans 3]

I have a number of models which have flaps .they are all I.C. powered around 46 to 60 size and I would like to know the best procedure for carrying out a steep landing .I currently use them just for a slow approach to the field for a conventional landing . I would like them to be more like what a STOL plane can do . The planes I have with flaps are ---Beech Bonanza .---Bird Dog ---Sky 40 ---Snowy Owl . Any help would be received with thanks

[Former Member]

[This posting has been removed]

[Biggles' Elder Brother - Moderator]

Flaps can have two different functions Keith depending on how you deploy them.

At relatively small angles, say 20 degrees and under, they increase the wing camber and give you more lift. This is good for take offs. And if you can take a longish flat approach can be used to help you land slower.

At larger angles, 40 degrees plus, they don't provide lift they just make drag - lots of lovely drag! So, you want to approach steep? Well your main problem is going to be that you will pick up a lot of extra speed, but that is where your draggy flaps come to the rescue. The idea is they act like air brakes, stopping the aircraft from over speeding on a steep approach.

I say "the idea is" because it doesn't always work as well as you might wish! For a whole host of reasons some aeroplanes react better to flaps than others.

Some tips for trying this:

1. Try it first on whichever of your models has the lowest wing loading. That will be more forgiving.

2. If you can slow the flap deployment - try not to bring them in "bang" all or nothing. Some Tx's can do this or you can buy servo slowers quite cheap.

3. Establish what's going to happen when you dial in 40 degrees plus of flap at least 3 mistakes high - don't try it for the first time 30 foot off the ground on finals!!

4. You need to slow down to deploy the flaps - don't just put them down with the model doing full cruise speed. But - very important this - at the same time you must keep the power on at all times, right down to the ground. Slow down yes, but not glide.This is most definitely not "cut and glide" territory!!!

5. To pull this off you need to have mastered landing under power, controlling your rate of decent with the throttle and your speed with the elevator.

6. Don't "milk" the approach. The stall behaviour with the flaps may well be much less predicatable than for that model normally

7. Practice your timing!! You are going to have to level then flair at just the right moment - you don't get two goes at this. Also, be prepared for the fact that if you level/flair too quickly all sorts of things might happen! You might induce a zoom and head back up into the air! You might slow very suddenly - almost like you have hit a wall! Be vigilant in the early days to establish each model's reactions.

Don't be put off by what might look a daunting list - its lots of fun when you get it right and it isn't that hard as long as you understand the basic principles and take a cautious initial approach to it.

Have fun!

BEB

[Frank Skilbeck]

I had a Glenns Maule and now have a Seagull MaxiLift both with flaps, for steep approaches I drop the flaps to 80 degrees and can then come in on a very steep approach, both models need down elevator with full flap and I have this mixed in. This works up to around 1/3rd throttle after which it zooms up again.

As BEB says it's all about timing, but it is surprising how steep the Maxilift will come in and flare out very easily, the first couple of times I was having to add power to prevent an undershoot.

[Ron Gray]

My best ‘plane for flapped landing has to be the HK Grand Tundra. Approach landing zone at about 100’, deploy full flaps, cut throttle then vertical dive pulling out at last moment. Timed correctly it will grease in but does take a fair bit of courage and faith to pull out at ast moment. Even more impressive when 2 of you do it together. Have also tried it leaving full flaps until halfway down , really impressive air braking and scared me the first time!

Totally irresponsible but such good fun!

For full size STOL look up Draco on YouTube

Edited By Biggles' Elder Brother - Moderator on 15/08/2018 11:55:36

[Martin Harris - Moderator]

In terms of handling with flaps deployed, although there may be some negative effects from disturbed airflow, as the effective angle of attack is higher at the flapped part of the wing I have always felt this gives less tendency to tip stall as the inner part of the wing stalls a long way before the outer parts - i.e. using flaps provides wash out...

[Piers Bowlan]

Posted by Biggles' Elder Brother - Moderator on 15/08/2018 10:44:03:

At larger angles, 40 degrees plus, they don't provide lift they just make drag - lots of lovely drag!

BEB

Not wishing to be picky BEB (!) but at larger angles, at 40 degrees plus for example, flaps still provide the same lift they do at 20 degrees deflection, it is just that they produce a lot more drag as well (not instead of). Perhaps that is what you meant? In other words the stalling speed will be the same at 40 degrees as it will at 20. This assumes that all other variables like wing loading and air density remain the same. Aircraft with efficient double slotted fowler flaps for example, will also experience a small further reduction of stalling speed with greater flap deflection but still the principal effect will be an increase in drag with large flap deflections.

[The Wright Stuff]

I think the reference is to the marginal effect of adding more flap angle. At a small angle to the wing, an extra degree down produces more lift than drag, but if the flap is already at 40 degrees, that extra degree produces only extra drag, to all intents and purposes.

I asked a question last week about cutting and gliding, and since then I've been surrounded by envelopes with force diagrams sketched on the back. I've concluded it's too complicated for a general 'how to do it' guide, flaps or no flaps.

I note Martin's point about less tendency to tip stall, and I agree from a theoretical point of view, but it's not at all my experience in practice.

The bottom line is to remember that flaps are there to make landings slower. They do not often make it easier.

[Former Member]

[This posting has been removed]

[Nigel R]

oops!

 

Around 1:20 for a pretty good high power flaps on steep approach short roll!

Edited By Nigel R on 15/08/2018 13:22:37

Edited By Nigel R on 15/08/2018 13:26:36

[The Wright Stuff]

Is that 'milking' the approach?

[Nigel R]

WS, I posted the wrong video first, check the approach at 1:20 in the second one...

[Ron Gray]

https://youtu.be/G46w3ZkLwxA

YouTube vid showing the Grand Tundra’s vertical landing

[Piers Bowlan]

Posted by Martin Harris on 15/08/2018 11:43:52:
In terms of handling with flaps deployed, although there may be some negative effects from disturbed airflow, as the effective angle of attack is higher at the flapped part of the wing I have always felt this gives less tendency to tip stall as the inner part of the wing stalls a long way before the outer parts - i.e. using flaps provides wash out...

Selecting flap effectively increases the camber for that section of the aerofoil. The coefficient of lift increases as does drag for that part of the wing. The more cambered section (inboard) will stall at a lower airspeed than the tips so the tips of the wing will tend to stall first as they have a less cambered aerofoil. That is my understanding and is what I experienced when I instructed on C152s many year ago.

I would demonstrate a clean stall first followed by a power-off stall with flap and finally a power-on stall with flap. The clean stall was usually pretty benign in a C152. The stall with flap at idle power might provoke a bit of a wing drop but the stall with power and full flap would often provoke a rapid wing drop and autorotation into an incipient spin if you didn't correct in good time with opposite rudder. In truth the C152 didn't spin that well. The important thing was to keep the slip ball in the middle at the point of stall, particularly with flap out. With flap and power it was critical. My conclusion was that flap increased the likelihood of a wing drop (tip stall).

Edited By Piers Bowlan on 15/08/2018 15:14:58

[The Wright Stuff]

Posted by Piers Bowlan on 15/08/2018 15:04:21:

The more cambered section (inboard) will stall at a lower airspeed than the tips so the tips of the wing will tend to stall first as they have a less cambered aerofoil. That is my understanding and is what I experienced when I instructed on C152s many year ago.

I agree that we often see that effect in practice, but it still isn't a completely satisfactory explanation. Another way of explaining it, I guess, is that when you lower the flaps, the additional overall lift provided can be partly offset by reducing the angle of attack, whilst still maintaining a net increase in lift coefficient (and hence slower stall speed*). In turn, this reduces the AoA of the tips, making them less likely to tip-stall than without flaps.

In that way, the tip-stalling behaviour is attributed not directly to the position of the flaps, but the resulting AoA that is presented by the flap-elevator trim.

*yes, yes BEB, I know...

[Nigel R]

"power-on stall with flap"

Interesting point Piers.

Just a thought, and it may not be correct.

If you have power on, there is a big dose of prop blast over the inner section of the wing.

Obviously different aircraft are all different, but I can see a situation where that blast keeps the centre of the wing flying, at a speed where the tips want to fall out of the sky. Clearly this wouldn't happen on a jet or pusher.

Summed up above - don't milk the approach!

[Biggles' Elder Brother - Moderator]

TWS has it right on both counts!

1. yes that is what I meant. All lift produces drag, there is no such thing as a free lunch! But at smaller flap deployment angles the balance between lift and drag is very much in favour of lift. Whilst at the larger angles the balance is in favour of drag. So we can talk, lossely admttedly, about small angles "producing lift" whist large angles produce drag"

2. Yes I am gritting my teeth at all this talk of "stall speeds", yulk! But I shall hold my peace, as many already know my views on that topic!

BEB

[Piers Bowlan]

Yes Nigel, 'if have power on you have a big dose of prop blast over the inner section of the wing' not only that but the prop slipstream will be rotating causing a different angle of attack between the two wings and therefore asymmetric lift and drag. Probably more importantly, the increase in lift produced by the flapped inboard section of the wings will cause the centre of lift to move inboard, reducing lateral stability, which at the stall will exacerbate any tendency to drop a wing.

Yes, I thought talking about 'stalling speed' will cause apoplexy in some people. Apologies all!

[andyh]

my absolute favourite illustration of how flaps can change the effective angle of incidence of a wing:

Edited By andyh on 15/08/2018 18:13:12

[Piers Bowlan]

I think that video should come with a health warning:- Don't try this at home!

[The Wright Stuff]

Absolutely try that at home!

[with a suitable model, of course]...

[Keith Evans 3]

Hi all . Thanks again for all the valuable info .The next time I fly , weather not too great at the moment ,I'll try out the suggestions at a good hight in my SKY 40 .

[David Ovenden]

Flaps make most models more versatile and fun. My favourite is my "old" Great Planes 82" span PT-19. It didn't come with flaps but I added some inset flaps to the wing underside. Scale size, shape and position. They really transformed a nice model into a great one. A favourite occupation with the PT is take off and fly at full throttle and max climb until the end of the downwind leg. (Idea is to get as high as possible) Then cut the engine to tickover, apply full flap and adjust the descent with elevator so as to arrive at the runway ready for a touch and go then go round again! It's amazing how steep a descent and how much height one can lose with flaps!

Worth noting the model has lots of wing area and is lightly loaded. Maybe not something to attempt with a highly loaded brick!