How Far do Flaps Need to go Down?

[EvilC57]

I assembled and setup an ARTF EDF jet recently, and noticed that the while the instructions gave suggested movements for ailerons, elevators and rudder, they said nothing about the flaps (which the model also has). So as I’ve done before, I just set them for full up, a mid half extended position, and what seemed like a reasonable fully down position. And the model has flown and landed OK since.

 

I was just watching full size jets landing on Big Jet TV during an idle 10 minutes while having a cup of tea, and it occurred to me that rather than setting flaps to an arbitrary down position, presumably aircraft designers extend the flaps to the ‘right’ amount. So how? Is there a formula or rule of thumb? I can’t believe they do it empirically.

[Paul De Tourtoulon]

So different with each type of flap, plane etc, My first plane was a Robbe Charter, I installed small flaps square around 5cm long and 3cm deep and probably went down 70°, whereas my 1.8m warbirds which are totally different maybe as little as 30°, suck it and see.

Edited September 6 by Paul De Tourtoulon

[Jon H]

I tend to operate a 'barn door' policy with flaps on my 30-50cc class warbirds as they behave much as the full size do and they need both the lift and the drag. Spitfires for example have 89 degrees on their flaps i believe, so generally i tend to eyeball it vs full size and then make a guess. If i have a radio with a slider on it i can feed in the flap slowly and get a feel, on radios with a 3 position switch i set a half flap setting as well in case full flap causes some drama. 

 

Be aware that you may need to set up an elevator flap mix to retrim the model. In the case of my WWII models i need masses of up elevator trim with flaps and gear down to prevent the thing wanting to dive into the earth. Lighter models may not need this. 

[Peter Jenkins]

Rule of thumb.  The first 30 degrees of flap generate mostly lift with a little drag.  After that, the lift increase is relatively small but the drag increases all the time as the flaps are lowered.  The Spitfire had around 80 deg of flap with nothing between fully up and fully down.  Apart from the occasion they had to take off from aircraft carriers when on the mission to relieve Malta.  Then, the flaps were lowered, some brave/stupid person held some wooden spaceers and the flaps were realised till they met the spacers!  After take off helped by the bit of flap, the flaps were cycled so that the wooden spacers dropped out.

 

On some of today's airliners, the flaps are generally split into 2 or more slotted flaps and they can be be lowered as much as 110 degs for the last flap section.  Needless to say, there is a huge amount of lift and drag generated when the flaps are fully lowered.

[Piers Bowlan]

Peter is spot on with his ‘rule of thumb’. As for some airliners having as much as 110 degrees flap deflection, this seems unlikely although I am always happy to be proved wrong. 😑 

The B744 had flap 25 or 30 landing flap positions on the flap quadrant selector although  the third segment of it’s triple slotted TE flaps appeared to have a far greater deflection when deployed. All I can say is they worked very well! 
 

How much landing flap an aircraft has available depends on what the designers are trying to achieve. Large deflections facilitate a steeper approach and therefore terrain clearance. Also they will usually improve  the pilot’s view over the nose of the aircraft (think Spitfire) or if trying to land a jet on a carrier deck (😬). Shorter, slower landings on a limited or contaminated runway are other benefits but not really relevant to our models. However large flap deflections introduce complicated structures with a weight penalty. They also reduce lateral stability and while enabling slower flight, increase the tendency to ‘tip stall’ near the point of stall. Not good if near the ground 😱.
 

[Simon Chaddock]

I built this a few years back to specifically find  out what the effects of flaps might be.

Counter rotating props to eliminate the effects of motor torque and two section flaps that go from 0 to 90 degrees fully variable controlled by a transmitter knob.

The flaps do have to to be applied progressively to allow the pitch trim to be adjusted. 

At 90 degree flap the drag is such that almost full power is required to maintain level flight.

The flight speed between 0 to 90 flap is remarkable.

https://www.youtube.com/watch?v=6EyiWQ7YnlA&t=26s

 

It made for a good learning experience.

 

 

Edited September 7 by Simon Chaddock

[GrumpyGnome]

Similar experience with my Valiant - flaps come down to 90 degrees so certainly more air brakes!

 

Thanks to modern radio, flicking a switch will deploy/retract them slowly. and gradually feed compensating elevator with it's own curve 🙂  

[Mark Hewett]

On 06/09/2024 at 17:40, EvilC57 said:

I assembled and setup an ARTF EDF jet recently, and noticed that the while the instructions gave suggested movements for ailerons, elevators and rudder, they said nothing about the flaps (which the model also has). So as I’ve done before, I just set them for full up, a mid half extended position, and what seemed like a reasonable fully down position. And the model has flown and landed OK since.

 

I was just watching full size jets landing on Big Jet TV during an idle 10 minutes while having a cup of tea, and it occurred to me that rather than setting flaps to an arbitrary down position, presumably aircraft designers extend the flaps to the ‘right’ amount. So how? Is there a formula or rule of thumb? I can’t believe they do it empirically.

There is no, one, optimum flap extension angle to set, there are trade-off advantages and disadvantages between different settings and it will depend on the type of aeroplane and the size of the flaps and what the pilot is wanting to achieve.

 

 As has been said above, small amounts of flap, i.e. take-off flap settings, provide a useful increase in lift, with only a small increase in drag. A take-off flap setting of around 10 to 30 deg would likely be a benefit to your edf jet, especially if flying off grass. The increased lift will enable the model to fly at a lower airspeed, and so reach flying speed in a shorter distance. The increased angle of attack of the flapped part of the wing will increase lift before rotation has occurred, taking some weight off the wheels resulting in better acceleration.

 

As flap deployment increases beyond 20 or 30 deg, i.e landing flap settings, less and less extra lift is being produced with each increase in flap setting, hence smaller and smaller reduction in stall speed, but more and more extra drag. We already had most of the benefit of reduced approach speed when the flaps reached 30 deg, so why would we want to deploy flap beyond that, causing increasing amounts of drag, that require increased power settings to maintain approach speed? For our model planes the main benefit is increased speed stability on approach i.e. it makes it easier to 'lock in' and maintain a suitable approach speed. The large flap setting causes a rapid increase in drag if the speed tries to increase, and the higher thrust tends to prevent the speed decreasing. The extra drag also allows a steeper approach than the clean model can achieve, making it easier to judge the touchdown point, especially with an aerodynamically clean 'slippery' model like a typical edf jet. 

 

I'd probably set the flaps on a model like that to about 0 to 20 deg for take-off on tarmac, 20 deg for take-off on grass, 70 deg for landing on either. But the settings aren't critical, and there's no 'correct' or 'best' setting, just trade-offs for different circumstances or pilot preferences.

 

There is no one take-off or landing flap setting for a jet airliner. A 737-300 can take-off with a flap setting of between 1 (leading edge devices only) and 15 deg, usually flap 5.  The reduced drag of flap 1 might be useful if you had a long runway but hot and high conditions e.g. Madrid on a hot day. You can use the long runway to accelerate to the higher speeds required for flap 1, and the reduced drag and extra speed can enable you to meet your required engine out minimum climb gradient. Flap 15 might be an advantage when the most limiting factor is runway length available. For landing, settings between 15 and 40 deg are available with 30 most commonly used. 15 is used for an engine out landing, for low drag to enable the aeroplane to climb in the event of a missed approach. Flap 40 might be used for minimum landing distance on a short runway, or the extra drag might be needed to prevent the speed increasing on a steep approach.

[Nigel R]

Can't add much to the above excellent posts, low angles up to 30deg give lift for takeoff, over 60 deg is an airbrake. Both settings are useful in their own way.

[EvilC57]

Thanks for the interesting discussions after my OP chaps, most informative.

[Chris Walby]

Worth watching wrt flap setting on full size + after discussions with Richard Rawle I have found no take off flap for my (not so big 116 inch Tigercat, 21Kg) is easier to control the swing + it gets from take off to flying on one engine (minimum speed) quicker.

 

Richards vid is from last year, but he was flying the Mosquito at Much Marcle last weekend (vieo will come out at some point). 

 

 

PS My Seagull Mosquito does something very disturbing with UC down and flap at 1/2 where something blinds the ailerons, just what you don't need on the downwind turn. I leave flaps until I have completed the turn, but that increases the workload just when lining up with the runway is key!

[Colin Leighfield]

I recall that when flying Cessnas, mostly 152 or occasionally 172, 30/40 years ago, the flap settings were I think 10, 20 and 30 degrees. Generally used 10 for take-off, 30 for landing, don’t recall  using 20 much, although you would go back to that from 30 in a go-round. Bearing in mind that Spitfires use 86 for landings only, 30 doesn’t seem like much. However, I recall that when selecting full 30 degree flap on finals for landing, the effect on trim was remarkable. A very distinct nose-down, which you balanced with elevator trim, it needed considerable control back pressure otherwise. It certainly made you aware that flaps have a marked aerodynamic effect! 

Edited September 9 by Colin Leighfield

[David Holland 2]

If I remember correctly, Colin, and it’s many years since I last flew a 152, the flaps will go to 40 degrees but most flying schools blanked the 40 position as the rate of climb was almost non existent and the pitch change with power quite extreme and a bit tricky for a low hours student.

[Mark Hewett]

50 minutes ago, David Holland 2 said:

If I remember correctly, Colin, and it’s many years since I last flew a 152, the flaps will go to 40 degrees but most flying schools blanked the 40 position as the rate of climb was almost non existent and the pitch change with power quite extreme and a bit tricky for a low hours student.

The flaps go to 40 on the Cessna 172, and 30 on the 152, and yer, you can get yourself into trouble with flap 40 on the 172 😐

[Mark Hewett]

I was climbing away in a 172 one windy bumpy day and at about 500ft lowered the nose and selected flap 0. Over the next 10 seconds or so I found I was having to push harder and harder to maintain a level attitude with the speed reducing at full power down to about 50 knots and the climb rate to zero. I didn't know what the hell was going on, if we'd still be alive in another 30 seconds and whether I'd have to try and put the thing on the ground somewhere. Then I realised it was no longer getting worse and we were still flying, I started looking around and, yep, found the flaps were at 40 deg! In the turbulence, as I had selected flap 0, my hand had knocked that horrid little lever down to flap 40 without me realising. I don't think the passengers even noticed. I wasn't the first to do that, and won't be the last.

[Colin Leighfield]

2 hours ago, David Holland 2 said:

If I remember correctly, Colin, and it’s many years since I last flew a 152, the flaps will go to 40 degrees but most flying schools blanked the 40 position as the rate of climb was almost non existent and the pitch change with power quite extreme and a bit tricky for a low hours student.

Thanks David, that might be right, although I was never aware of it. The rate of climb was non-existent with 30 degrees, in a go-round you immediately went to 20 and re-trimmed to climb out. I was alway surprised at how marked the effect was from just 30 degrees. I can imagine that 40 degrees would have been dramatic!

[Martin Harris - Moderator]

Probably what happened to an acquaintance many years ago after the instructor failed to raise the flaps after demonstrating an engine out forced landing. 
 

Both he and the instructor walked away from the crash - in the case of the instructor, never to be seen again at the flying school!

[David Holland 2]

Memory a bit foggy, Mark, confusing 152s and 172s. I did my ppl back in the days of the 40 hour course and I did the GFT after 38 hours. My instructor, lovely chap called Geoff Whitehead, checked me out on the 172 to make up the hours. I think, to demonstrate a point, he got me to do a full flap overshoot and it was all I could do to contain the pitch up while getting two stages of flap off. Lesson learned!!

 

[Mark Hewett]

21 minutes ago, David Holland 2 said:

Memory a bit foggy, Mark, confusing 152s and 172s. I did my ppl back in the days of the 40 hour course and I did the GFT after 38 hours. My instructor, lovely chap called Geoff Whitehead, checked me out on the 172 to make up the hours. I think, to demonstrate a point, he got me to do a full flap overshoot and it was all I could do to contain the pitch up while getting two stages of flap off. Lesson learned!!

 

You did well then, to complete the course in the minimum time - most students taking significantly longer. Yer, a flap 40 go-around is an essential item to practice for a pilot unfamiliar with the C172. Another worthwhile one is to set up a simulated flap 40 landing approach, but at 3000ft or so, commence a go-around, add full power and raise the nose to the normal take-off climb attitude, 'forgetting' to accelerate and raise flap in stages. The speed washes off rapidly with the drag of flap 40. Then 'accidentally' select flaps straight to up , the departure from controlled flight at full power can be spectacular, the aeroplane rolling smartly over on it's back if the vicious wing drop is not immediately corrected!

[Peter Jenkins]

If my memory serves me correctly, the C140 had 40 deg of flap available and could get into a field that was too short for take off!  Consequently, the C152 was restricted to 30 deg flap maximum.

 

I remember a check ride in the C172 where I was told to overshoot after an approach with full flap.  The nose up pitch on applying power required a pretty hefty shove with my left hand to prevent a disastrous nose up pitch.  He had not warned me of this before hand and it has remained a clear as bell in my memory ever since!  He told me that there had been some fatalities with light female pilots who were unable to push the control column forward sufficiently to avoid a low level stall and crash.  Sobering!

[John Lee]

Yes C150 had 40 degrees of flapped a flap lever that should have been spring loaded to neutral but most seemed to have a broken spring so that it was all too easy to end up with 40 degrees when you only meant to hold it for a few seconds to get 20. The flap indicator was on the left A pillar & outside your normal scan. It was considered normal when I learnt to fly in the 70's but caused plenty of accidents & the C152 limited the flaps to 30 degrees & has a pre select position lever.

 

I checked out a friend's wife on his Cessna 182 & she had a great deal of difficulty in holding a level attitude on a full flap go-around.

 

40 minutes ago, Peter Jenkins said:

If my memory serves me correctly, the C140 had 40 deg of flap available and could get into a field that was too short for take off!  Consequently, the C152 was restricted to 30 deg flap maximum.

 

Yep, my party piece when instructing on the C150 at Liverpool was to land & turn off at G, about 200 yards from the threshold and actually before the touchdown markers.

 

Edited September 10 by John Lee

[Martin Harris - Moderator]

I believe this extract was from the report from my acquaintance’s accident:

 

It certainly illustrates the difficulty climbing a fairly heavily loaded and low powered aircraft against the drag of fully deployed flaps - however, a typical model will be likely to have a much better power to weight ratio.