Flap / down elevator mix

[TH]

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 H]

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 Lee]

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 H]

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 Bowlan]

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 - Moderator]

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 Lee]

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 Chaddock]

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 8]

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 H]

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 Bowlan]

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 Bowlan]

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!

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

[Russ Barnes]

I'm late to the party sorry but just a friendly answer to your question. I've been an A-10 pilot for over 20 years in the USAF retiring in the next couple years so full scale just like your Carbon Z or any RC plane use of flaps is essential. Yes it produces drag but it also produces lift specially if the nose is pointed into the wind. You are adding extra camber to the wing with flaps because you are changing the shape of the wing. Always use flaps on take off and once in the air level off gain airspeed then take out the flaps. Even if you want to descend use flaps as you can increase the angle to the ground without gaining air speed.

[Matt Carlton]

The use of flaps is not essential. I'm sorry, but that is just not correct, given that the majority of sport RC models do not have, nor need them.

Flaps are generally a means of reducing takeoff and landing distance, as well as increasing the margin between landing speed and stall speed.

We don't generally have those issues with most sport models (excluding fast/heavy/scale etc) so they don't need flaps. They are usually very much more overpowered compared to full size, plus contain reserves of strength that allow us to be a bit more rough about things.

If I drop a 1m span model weighing 2lb from 2m up, it'll probably be ok. Scale that up to a 10m span light plane weighing 2000lbs from 20m and it's a different story.

[Jon H]

I have to agree with Matt for the most part. Model aircraft are so lightly loaded and have substantially better power/weight than full size but drag is a much bigger factor as the light weight gives reduced momentum. As a result many full size practices are not directly applicable to model operation.

I mostly fly models of WWII fighters and would never use flaps on takeoff. I use them on landing for sure, but not for takeoff.

The only model i have that i use flaps on takeoff is a model of a piper tomahawk. I use the flaps as the high mounted tail is very ineffective at low speed and the model can leap into the air in an uncontrolled manor. With half flap used on takeoff the model is more inclined to rise from the ground without as much elevator yanking and this makes eveything a little safer.

[Martin Harris - Moderator]

Flaps may not be absolutely necessary but a scale warbird or relatively heavily wing loaded model such as a Chipmunk benefits greatly on the approach, both in scale appearance and handling quality. The difference in stability when flown at an appropriate speed is very marked.

My own radio allows me to assign a slider or rotary knob to a mixing function and especially when used in conjunction with flight modes, makes trimming pitch changes during the initial test flight a doddle. If I'm doing a maiden flight for someone else, I make a mental note of the stick deflection required with flap and then approximate it during setting up an elevator/flap mix. Surprisingly, n most cases, this results in successful mixes for the next flight.

My 1/6 scale Chipmunk, for example, has scale deflections of 15 and (a modest) 30 degrees. The effectiveness of the take off position is such that I habitually use half throttle for the flapped take off run in order to prevent lift off in a very non-scale couple of fuselage lengths - and I'm only using a 52 4 stroke when many people go for a 70 in similar models!

I'm also of the belief that using flap over the centre section (AOA is increased by the change of chord line) gives a small reduction in the likelihood of tipstalling by effectively introducing some washout.

Edited By Martin Harris on 21/12/2020 11:46:14