A Question on Flaps

[Levanter]

Latest project is to complete all the old "latest projects" and Oodalally is back on track. For a bit of fun and an experiment I decide to fit flaps and at the moment these are simple flat plates.

I have two options:

One is to have a simple split flap with the plywood plate flap hinging down under the fixed trailing edge.

The other would be to have a loose section of the trailing edge glued to the top of the flap so that the camber of the wing would be increased, albeit having sharp exposed edges on the top surface of the wing with the flaps down.

It would be easy to build either way but I wonder which option might work the best?

Seasonal Greetings to all.

Levanter

[Frank Skilbeck]

A full flap will give more lift at small deflections for a small increase in drag, so useful to assist in shortening a take off and provide more drag at larger deflections allowing a steeper approach on landing. Split flaps won't provide as much additional lift at small deflections but provide the drag on landing.

It all depends on what you want the flaps to do.

If it's just to experiment then on the Oodallay I'd be tempted to make it a full section and then if your radio allows it you could mix the flaps with ailerons to retain full span ailerons and also maybe try some snap flap where the flaps deflect in the opposite direction to the elevator for tighter loops etc, this is a normal set up on many gliders that have flaps.

[Piers Bowlan]

I concur with Frank. Just fit regular plain flaps and bottom hinge them. I wouldn’t worry about ‘sharp exposed edges with the flaps down’, half the function of flaps is to generate drag to steepen the approach. As Frank says, small deflections increase the camber of the wing and consequently lift. Actually, the same lift at a lower speed and hence lower stalling speed.

Full span ailerons are good as are snap flaps but avoid full span flaperons if you want to avoid tip stalls and unintentional spins.

 

Edited By Piers Bowlan on 23/12/2020 09:21:41

[Simon Chaddock]

Levanter

As others have said is does rather depend on what you are trying to do.

if the airframe is 'slippery' then drag to steepen the approach angle is the priority. If it has a high wing loading then creating lift to safely reduce the landing speed.is more important.

Note the early full size Spitfire split flaps were either up or down to 90 degrees. In that case the requirement was for drag rather than significantly reducing the stall speed.

[Levanter]

Thanks for the advice everyone.

Being a Peter Miller design I think he uses a fairly low wing loading so lift should not be too much of a problem as he says he designs to "fly on the wing" I suppose instead of brute power.

I think I am heading towards a full flap for its merits on lift. The power comes from a Saito 40a swinging a decent sized prop that should give some braking along with the fixed undercarriage.

In a way I also think it will look better in use.

Levanter

[Tim Ballinger]

Levanter,

Oodalally is certainly a slippery beast and I have considered retro fitting flaps but never got round to it. One thing for sure sacrificing some inboard aileron for flaps will not compromise the roll rate ( it is really fast) and I can guarantee you would not need to go the flapperon route.

Tim

[Chris Walby]

Only my opinion but worth asking what you want to do with them?

Additional lift with minimal drag so deployment up to about 45 degrees, beyond this there is little more lift, however the drag increases significantly.

Do you just want to slow up a bit and float in then a few degrees of flap or dive at the ground with maximum drag with flaps deployed 90 degrees roll out under power and nail it to the floor?

The tech guys will put me right however I find the addition of flaps produces a more abrupt stall all be it at a slower air speed, ok if you are on the ball with the thought that your once slippery model now has the characteristics of an angry manhole cover.

My twin ic powered Dual Aces does not have flaps and comes in like non stopping train, but has a very benign stall, it will sit with a high AOA and just bleed the speed off, does not tip stall and just descends on to the runway. Just saying that the designer has very carefully selected many parameters to produce a very good flight envelope and its not a problem changing things, but don't be surprised if it spoils the overall performance.

[Levanter]

I will confess to everyone that I have not yet learned how to fly so to answer Chris's point truthfully, I have no idea what I want to do with it. Someone else hopefully will maiden it and if they produce nasty characteristics they won't get used.

My flying opportunities are limited (I live in Mallorca) but I love building and try to do something different on each build. So far I have not built anything with retracts, ducted fan or sea-plane and all are on my list.

A good point though Chris and again thanks to all.

Levanter

[Piers Bowlan]

I agree Chris flaps can and will make a stall more abrupt if large deflections are used. Just because the spitfire had two flap settings, namely up and 90 degrees, is not a reason for doing the same on your model aeroplane! If your normal approach involves 'cut and glide' the use of a large amount of flap will probably ruin your day. The idea is that the drag from the flaps is balanced against thrust of the engine on a powered approach. The rate of decent can then be controlled accurately and once in the flair and the throttle closed, the aircraft will quickly settle to decelerate once on the ground. The full sized Boeing 747 has very effective double slotted fowler flaps but the maximum setting is flap 30 and if the runway is not limiting, 25 is normally used. Many light aircraft will have a maximum flap deflection of 40 degrees but in some instances it may be easier to land if only 30 flap is selected. The point I am making is do not use excessive amounts of flap if you want an easy life and don't want to re-kit the model.

The Seagull Duel Aces wing appears to have a fairly blunt leading edge and thickish wing section, both conduce to a forgiving and benign stall.

 

Edited By Piers Bowlan on 23/12/2020 13:34:26

[Chris Walby]

Thanks for a really good summary Piers and I don't think its too far off topic to mention the great Capt Eric "winkle" Brown landing 7,000 Kg of Mossie on a carrier. I think they extended the flap range to produce more drag just for this task as well as 4 bladed paddle props.

Amazing skill to hold a Mossie on the point of stall while landing on a carrier and well worth a watch

Capt. Eric 'Winkle' Brown: the first Mosquito carrier deck landing - YouTube

Mossie Carrier landing

[Martin Harris - Moderator]

Absolutely fascinating link - I've not seen that interview before. Well worth 15 minutes of anyone's time!

The description of high power/minimum touchdown speed was brilliant - I've flirted with the technique of getting on the back of the drag curve and it can make an astounding difference to the landing run...if you get it right!

P.S. After a little pondering, perhaps I should have said that it still reduces the landing run if you get it wrong too - but may require retrieving in a plastic sack!

Edited By Martin Harris on 23/12/2020 14:32:34

[Nigel R]

"Note the early full size Spitfire split flaps were either up or down to 90 degrees."

I seem to remember reading that they were sometimes (often?) set to 15 deg for a dollop of extra lift, by closing them up against wooden blocks.

The blocks fell away after takeoff (presumably by a quick toggle of flap down / flap up).

[Martin Harris - Moderator]

It was certainly done in order to fly them off a carrier at maximum possible range/fuel load when they sent a number to defend Malta as urgently as possible. I'm not aware of this being necessary otherwise.

[Frank Skilbeck]

Posted by Nigel R on 23/12/2020 22:12:38:

"Note the early full size Spitfire split flaps were either up or down to 90 degrees."

I seem to remember reading that they were sometimes (often?) set to 15 deg for a dollop of extra lift, by closing them up against wooden blocks.

The blocks fell away after takeoff (presumably by a quick toggle of flap down / flap up).

This was done on the Spitfires sent by carrier to Malta.

[Piers Bowlan]

Posted by Chris Walby on 23/12/2020 13:22:32:

Amazing skill to hold a Mossie on the point of stall while landing on a carrier and well worth a watch

Capt. Eric 'Winkle' Brown: the first Mosquito carrier deck landing - YouTube

Mossie Carrier landing

I expect everybody on the forum has read his extraordinary story and autobiography 'Wings on my sleeve' but if not, you should. Unbelievably brave and so modest too. They don't make em like that anymore.

Edited By Piers Bowlan on 24/12/2020 05:40:17

[J D 8]

Plus 1 for Wings on my sleeve.

His book " Wings of the Luftwaffe " which covers his test flying of many of the 55 German types he flew. Just amazing stuff.