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Flap / down elevator mix

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TH02/04/2020 12:04:27
18 forum posts

Hi All,

Trying to avoid going stir crazy, I'm thinking of setting a flap / down elevator mix on my, as yet unflown, Carbon Z Cessna. Has anyone any advice on this, is it necessary, if so what % to set and how will affect performance on take off. Thanks in advance,

Regards,

TH

Edited By TH on 02/04/2020 12:04:50

Jon - Laser Engines02/04/2020 12:16:12
5422 forum posts
263 photos

dont use flaps for takeoff as they add drag.

Beyond that i would set the radio to the elevator flap mix menu and then fly the model. Once its up put the flaps down and dial in the offset you need while up in the air. If you need it, get someone else to beep the buttons while you fly.

Also be aware that all of the models i fly with elevator flap mix need up trim as the drag of the flaps pulls the nose down. If you are ballooning up with flaps down you are likely to be either too fast, or the model is so light you dont need them really anyway.

Clearly there are always exceptions, but generally thats how it all works out.

John Lee02/04/2020 13:19:20
720 forum posts
62 photos

I have a Carbon-Z C150 & have -10% (ie down elevator) at half flap and -22% at full flap with an offset of 30%.

The C150, both model & full size (I have a share & about 500hours in the full size) both have a strong pitch up tendency when you lower the flap.

And I disagree with Jon, half flap helps shorten the take off run considerably, the extra lift more than compensates for the extra drag.

It's a good fun model, you will enjoy it. If you haven't already done so check out Duncan McClure flying his demonstrating the A Test on the BMFA Achievement scheme video.

Jon - Laser Engines02/04/2020 14:05:35
5422 forum posts
263 photos

As i said before, it depends on the model. In my case i am flying big/heavy WWII fighters vs your example of a light weight foamy.

In real terms the foamy dosent need flaps which is what i was on about before with the pitch up. Again on takeoff the power/weight/drag ratio of a foamy is not the same as 24lbs of warbird with flaps like a barn door!

Piers Bowlan02/04/2020 14:47:06
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2120 forum posts
53 photos

In general terms, the increased camber with small flap deflections (5-10degrees) will increase lift whilst producing a small amount of drag. Large deflections (30-40 degrees) will only add a small additional amount of lift but a very substantial amount of drag. As Jon says, for a model with a high power/weight-ratio and low wing loading like a foamy, little is to be gained by using flap for take off. Most aircraft, models and full sized alike, experience some pitch up as flaps are lowered. How much will very much depend on the design of the aircraft. Also, small flap deflections may not cause a significant pitch change, however large deflections usually will. In other words the pitch change with flap deflection is not a linear relationship.

Edited By Piers Bowlan on 02/04/2020 14:49:54

Steve Hargreaves - Moderator02/04/2020 15:18:00
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6756 forum posts
197 photos

Surely the pitch up/pitch down with flaps is dependant on the wing position...?

With a high wing the flaps create extra drag above the CoG thus pitching the nose up.....

With a low wing the flaps create drag below the CoG thus pitching the nose down....

John Lee02/04/2020 15:25:36
720 forum posts
62 photos
Posted by Jon - Laser Engines on 02/04/2020 14:05:35:

As i said before, it depends on the model. In my case i am flying big/heavy WWII fighters vs your example of a light weight foamy.

The 'light weight foamy' in this case (Carbon-Z Cessna 150) is 2.1 meter in span & weighs 10lbs! Not as big as your warbirds maybe but I think still reasonably substantial.

Simon Chaddock02/04/2020 15:32:19
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5692 forum posts
3026 photos

The drag of a flap is only one of the forces involved. The change to the wing camber as the flap deploys also alters the wings centre of pressure which can result in a significant pitch change force.

J D 802/04/2020 16:04:39
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1447 forum posts
84 photos

The Seafire had multi position flaps, unlike the earlier versions of Spitfire on which only up or full down.

When taking off a carrier a Seafire pilot could select some 15 degrees flap.

When launching Spitfires from a carrier as in the resupply of Malta Spitfire pilots would select down flap and then deck crew would hold in position wooden wedges [ fingers ] and signal the pilot to select up flap. The flaps would then be held in take off position. Once up a safe altitude the flaps were cycled for the wedges to fall away.

Jon - Laser Engines02/04/2020 16:52:21
5422 forum posts
263 photos
Posted by John Lee on 02/04/2020 15:25:36:
Posted by Jon - Laser Engines on 02/04/2020 14:05:35:

As i said before, it depends on the model. In my case i am flying big/heavy WWII fighters vs your example of a light weight foamy.

The 'light weight foamy' in this case (Carbon-Z Cessna 150) is 2.1 meter in span & weighs 10lbs! Not as big as your warbirds maybe but I think still reasonably substantial.

10lbs is really light for a model with that sort of wing area. My Seafury is 14lbs heavier with nearly 3 inches less span! Light weight it is not and with flaps and gear down i need nearly half throttle just to keep it moving.

Simon is also quite right about the c/p moving about.

Piers Bowlan02/04/2020 22:17:26
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2120 forum posts
53 photos
Posted by Simon Chaddock on 02/04/2020 15:32:19:

The drag of a flap is only one of the forces involved. The change to the wing camber as the flap deploys also alters the wings centre of pressure which can result in a significant pitch change force.

Simon, you are right, there are several forces involved. The point I was trying to get across is that small amounts of flap can produce more lift (or the same lift at a lower airspeed) for a very small penalty in terms of drag. Conversely for landing, large deflections of flap will additionally produce lots of drag - very helpful for terrain clearance.

John, it is the low wing loading that is important not the weight.

 

Edited By Piers Bowlan on 02/04/2020 22:45:50

Piers Bowlan03/04/2020 07:07:29
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2120 forum posts
53 photos
Posted by Steve Hargreaves - Moderator on 02/04/2020 15:18:00:

Surely the pitch up/pitch down with flaps is dependant on the wing position...?

With a high wing the flaps create extra drag above the CoG thus pitching the nose up.....

With a low wing the flaps create drag below the CoG thus pitching the nose down....

Steve, this is true in general terms if it wasn't for the fact that drag isn't the only force at play as flaps are lowered. Also significant is the movement of the centre of pressure as the lift distribution across the chord of the wing changes (it generally moves back). Flap design plays it part too. Fowler flaps for instance increase wing area rearwards improving lift, split flaps produce lots of drag (to generalise).

Then there is the increased downwash produced by the flap which can effect the airflow over the tailplane and consequently trim of the aircraft. This will in turn be affected by the aircraft design, high wing/low wing and tailplane position; T-tail, cruciform tail or conventional. As already mentioned, large flap deflections will cause drag to increase with the consequential loss of airspeed and/or an increased rate of decent. Whilst both may be desirable (or intended), at some point more power will usually be required to check the rate of decent - or the continued loss of airspeed. Therefore, on a classic three degree glide path, as full flap is selected power will need to be applied smoothly commensurate with flap movement. This in turn can produce a pitch change dependent the aircraft's pitch/power couple. Although not an aerodynamic effect of the flaps themselves, power changes with flap can produce pitch changes. If you are flying a bush plane a very steep approach may be the desired effect but it is a brave man that uses full flap with no power! smiley

The last aircraft I flew pitched up as flaps were initially lowered but pitched down as full flap was selected (it was low wing). They are all different.

 

Edited By Piers Bowlan on 03/04/2020 07:27:14

Edited By Piers Bowlan on 03/04/2020 07:37:20

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