Flap increment questions

[aeromed202]

I may be over thinking this or just haven't a clue but here's the question(s). In general, do aircraft designers design flaps to deploy in increments that would graph as an increasing parabolic curve, straight line or decreasing parabolic curve? Does it vary widely? I ask because for fun I often graph the flap increment values in the CFG and get widely differing, sometimes bizarre curves. I would think that in general, flaps of several increments would deploy from small changes to large changes in accordance with decreasing air speeds so what I then do is graph the values in Xcel then adjust them to create a smooth increasing parabolic curve. This sometimes messes with the animations but I wondered if the simulated lift effect would be truer to life that way.

[Milton Shupe]

Interesting question.

First, what the visual model does or looks like has no bearing on the flight model performance. You could have the flaps animate in circles without impact.

Second, I usually model flap animation to coincide with the increments documented for the aircraft so that visually, the flaps appear to be at 15, 25, or 45 degrees as an example. Even with this, some flaps operate as start/stop by implementation so the increments are simply "waypoints" for reference.

And, I would ensure that the animation agreed with the aircraft.cfg.

Third, when creating the flight model, the characteristics of lift, drag, and pitch with flap movement is dependant on the kind of flaps installed in the aircraft. Simple, slotted, hinged, fowler, etc. types all have different effects on reaction moments. Throw in CoG differences, root cord size, CoL changes with flap deployment, and it gets complicated.
And simple flaps at 15 degrees would effect different moments than a fowler flap at 15.

When I do Flight Models, I use Jerry Beckwith's FDWB to get me to a starting point, then tweak from there. It's an excellent tool once you learn to "manage" it.

Since the sim must "fly" a whole world of various aircraft sizes and weights and configuration, there are many things that can be used to tweak results.

So, to answer your question, I try to get the aircraft to perform based on the data that we have for the aircraft at various flap settings. Tom Fliger who does most of our FM's now is very good at doing that very thing. (Thank you Tom :-)

[pfflyers]

I'm not an aircraft designer - real or virtual. However I did spend some time as an airframe mechanic at a shop that specialized in replacing GA aircrafts' plain and split flaps with Fowler type flaps. In the case of those flaps there was definitely a non-linear increase in flap angle, the first couple steps were pretty small becoming increasingly bigger step increases as the angles got bigger.

[Sunny9850]

The design criteria as outlined by Milton of course also depends of course on the aircrafts intended role. Flaps can perform different tasks at different settings.
The early stages may be designed to mainly increase lift by changing the profile of the wing without a large increase in drag.
The later stages might be designed to mainly increase the drag component to allow a steeper approach profile without the aircraft picking up speed in the process.

Stefan

[aeromed202]

That's what I meant Milton, that changing flap values might well cause a disconnect with the animation ("And, I would ensure that the animation agreed with the aircraft.cfg"). Like if a visual deployment to position 1 is only a horizontal extension, then subsequent extensions increase the downward angle and can be disagreeably changed by changing the original values. I certainly appreciate it when the time is taken to have things look and work like the real thing. Makes all this just coffee table discussion. And Stephan, I think we're in agreement, the old increased lift=increased drag, and nobody wants to shove lots square footage into too fast of an airflow. So I kind of think the parabolic increase is probably what designers shoot for.

So here is the next question. Having learned of other factors involved with modelling flap affect and other concerns, does changing the CFG flap values alone really give the expected effect? It sounds like it may not because of all the other considerations. I can only say that it "appears" to have effect that I expect, that is I get a visual and performance response that seems to match what I changed. Again I have little flight time but lots of narrated vids which led me to some of these conclusions. I defer to those with deeper knowledge knowing I am just scratching the surface here.

[Milton Shupe]

Just to embellish my previous response, there are also 3 ways to employ animation for the modeler.

One is standard simple flaps like on a Cessna 172. Here we name the flaps appropriately and the sim does the animation. Degrees of movement is dictated by the flaps section(s) of the aircraft.cfg.

Second, the key frame animation is based on the modeler's animations done by keyframes. The aircraft.cfg flaps section(s) looks the same but the degrees of deployment (of the visual model) is based on the modeler's animations. (In other words, the cfg may say 15 degrees but the modeler may have used 30 for the first stop and 30 is what the visual model would show.)

Third, the percent key frame animation allows the modeler to animate the range and the sim will deploy a percent of the range based on the aircraft.cfg flaps section positions. Here you might have 10 flaps positions 10-100%. The C-130 comes to mind. If I recall correctly, the C-130 has up 50 degree flaps. So, in this case each 10% would give you 5 degrees of animation assuming the modeler animated 50 degrees.

So, with the first and third techniques of animation, you as the user can change and dictate degrees of deployment by changing the flaps section of the cfg.

The second and third will use the animation flow of the flaps as dictated by the model, but you can choose to alter the degree or position settings as well as the amount of lift/drag/pitch applied.

Another technique to tweak the lift/drag/pitch curves is to have multiple flaps sections (3 is good), with each one addressing one of the lift/drag/pitch characteristics. This also allows you to cheat a bit and show for example positions 15, 25, 45 for one set, 0, 15, 15, for another, and 0, 0, 45 for another. This gives you a lot of control of the effects of lift/drag/pitch.

I learned this from Jerry Beckwith's FDWB techniques. You can see this in action on several of my aircraft.

Example using two flaps sections:
[flaps.0]
type = 2 // 1 - trail, 2 - lead
span-outboard = 0.8
extending-time = 22
flaps-position.0 = 0 //normal flaps positions stated
flaps-position.1 = 15
flaps-position.2 = 25
flaps-position.3 = 45
damaging-speed = 170
blowout-speed = 200
lift_scalar = .7 // reduce overall lift characteristics
drag_scalar = 1.0 // full drag effects
pitch_scalar= 1.0 // full pitch effects
system_type = 1
　
[flaps.1]
type = 1
span-outboard = 0.8
extending-time = 11
flaps-position.0 = 0
flaps-position.1 = 0
flaps-position.2 = 0
flaps-position.3 = 45 // Only 45 degree flaps use these effects below
damaging-speed = 155
blowout-speed = 170
lift_scalar = 0.3 // Very little increase in lift
drag_scalar = 1.0 // I want full impact on drag
pitch_scalar= 2 // I want extra pitch moment applied at 45 degrees
system_type = 1

[Tom Clayton]

If you want to experiment, try adding more increments to the default C172. It's been a while since I did it, but it seems like I remember getting the visual model down to 60°, but if the flight model went any further, the visual just stayed put. I thought I might be able to "break" the animation, but no such luck. I never did try to fly with the increased flap angle.

[Dev One]

Certainly an interesting discussion!
From my real world design experience on flaps & spoilers (the C5A) the fowler type flaps extend aft about 75% with very little angle change about 5° initially - this gives increased wing area with a bit more camber effect, increasing lift with little drag. The next extension is probably out to 95% & at 30°, with the last probably about 45° & 100% extension. The critical distance is that between the underside of the spoilers (or trailing edge of the wing) & the leading edge of the flap to energise the airflow & is usually defined by the aerodynamicists. (BTW don't quote me on the above figures - its over 40 years ago!).
As for FS9/FSX - I have only modelled simple hinged flaps - split type & plain, but what I did find in those is that in Gmax I animate the flap to 90° even though it will never go there! The aircraft.cfg then uses the angular values to deflect the flaps correctly. (If you try my Proctor you will see that the flaps do not visually deflect the 42° called for in the .cfg - thats because I modelled the 42° in Gmax & the .cfg uses the 42° as a percentage of the animation. For my Provost & Satellite I modelled 90° & then the .cfg deflects it correctly.
Confused????
Thanks Milton for the info on having multiple Flap sections - I hadn't realised that!
Good luck
Keith

[Motormouse]

I was going to recommend you get a copy of Air Commodore Kermode's book,
'Flight without formulae'; but a quick google and wikipedia has it covered here ---> http://en.wikipedia.org/wiki/Flap_(aircraft)

It all depends on what the particular designer is trying to achieve, certainly on some of the rw types I've test flown in you get a more marked change as some of the 'middle' increments take effect, not so much from middle to full, as you don't want a significant flight upset at lower speeds.

As Keith (DEV One) says, its all worked out by the aerodynamics boffins.

ttfn

Pete

[Dev One]

Crumbs - I'm very surprised that you quote Kermode - I still have my fathers 1930's 'Mechanics of Flight' publication!!!
It does explain things very well though. No jets or supersonic stuff, just plain slow aerodynamics.
The Wiki version though on Flaps is very good.
Keith

[Motormouse]

Crumbs - I'm very surprised that you quote Kermode - I still have my fathers 1930's 'Mechanics of Flight' publication!!!
It does explain things very well though. No jets or supersonic stuff, just plain slow aerodynamics.
The Wiki version though on Flaps is very good.
Keith

he-he, Kermode was required reading when I did my apprenticeship back in 1979, likewise Neville Duke's book,
'Through the sound Barrier' for all the jets and supersonic stuff.

ttfn

Pete

[Sunny9850]

One of the beauties of the flaps on most older Piper singles including my Saratoga is that the flap extension speed is pilot controlled by way of the manually operated Johnson Bar between the seats.
I can easily tell a comfortable with his airplane Piper flyer and someone who does things by the numbers but in a mechanically disconnected fashion.

The first 10° of flaps in the Saratoga can come down at 110 KTS. But since they mainly increase lift by changing the wing camber but do not add a lot of drag the airplane pitches nose down because the mean aerodynamic chord shifts. Because one hand is on the bar and the other holding on to the yoke the aircraft tends to wallow up and down a bit....which is very uncomfortable for passengers in the rear seats. That type of pilot usually tries to remedy that problem by yanking the bar up even more rapidly to get both hands back to the yoke quicker.....in the end making things worse.

Let the aircraft slow another 10-15 KTS and then smoothly pull in that first notch just as the nose starts to get a bit heavier and there is virtually no pitch change at all.
The next 10° come with about equal lift increase and drag penalty....they work best during the descending turns onto base and onto final.
If I have a long runway and no early turn off I leave the draggy 40° stage and she lands with a nice chirp every time in exchange for a 100-150ft of real estate.
The 40° setting with power at idle requires a very deft touch to get that same chirp because deceleration and loss of lift then come very rapidly and if you are just a few inches too high the landing score goes from hero to zero quickly

However if there is that proverbial 50ft FAA tree at the end of the runway and that runway is short the 40° setting allows a much steeper final without picking up speed we then need to scrub off with expensive Bendix brake pads only seconds later.

The flap design is of course very important in a couple of ways. Fowler style flaps are of course very nice to have but generally speaking are more complex and therefor weigh more.
The Saratoga flaps are in some way ingenious because the manage to be a simple version of fowler flaps using the hardware of plain flaps.
Their main drawback comes from their location on a low wing which limits the chord size that could be used.

That is one reason why the same degree of flaps does "more" on a C-182

Stefan

[Sundog]

Certainly an interesting discussion!
From my real world design experience on flaps & spoilers (the C5A) the fowler type flaps extend aft about 75% with very little angle change about 5° initially - this gives increased wing area with a bit more camber effect, increasing lift with little drag. The next extension is probably out to 95% & at 30°, with the last probably about 45° & 100% extension. The critical distance is that between the underside of the spoilers (or trailing edge of the wing) & the leading edge of the flap to energise the airflow & is usually defined by the aerodynamicists. (BTW don't quote me on the above figures - its over 40 years ago!).
As for FS9/FSX - I have only modelled simple hinged flaps - split type & plain, but what I did find in those is that in Gmax I animate the flap to 90° even though it will never go there! The aircraft.cfg then uses the angular values to deflect the flaps correctly. (If you try my Proctor you will see that the flaps do not visually deflect the 42° called for in the .cfg - thats because I modelled the 42° in Gmax & the .cfg uses the 42° as a percentage of the animation. For my Provost & Satellite I modelled 90° & then the .cfg deflects it correctly.
Confused????
Thanks Milton for the info on having multiple Flap sections - I hadn't realised that!
Good luck
Keith

Just an FYI, part of the reason you see the flaps deploy the way they do is for take off you want Maximum L/D. This gives you maximum acceleration (low drag) for minimum ground run (high lift), assuming they remain static through the take-off roll. For landing you want maximum lift, for slow speed flight while landing, and a lot of drag to keep from accelerating while going "downhill." It also relates, the drag, to what is good to have the engine set at for responsiveness while landing. But that is the simplistic description of how flaps are designed for planes like the C-5.

[PRB]

The A2A P-47 has a unique flap system in that the pilot apparently can set any desired degree of flap, just by holding the flap handle down for longer or shorter times. So there are no “notches” at all. You just feed in the amount you think you need. In the sim, if you just hit the down flap switch, they’ll come all the way down. You have to hit the “down flap” key, wait for the desired flap extension amount, the hit the “flap up” key, which simulates returning the flap handle to the center (neutral) position, which stops movement. To bring them back up you do the reverse ("up" to start retraction, "down" to stop movement.) Is this flap operating mechanism possible in FS9 models, or something unique to the FSX modeling environment?

[Milton Shupe]

The A2A P-47 has a unique flap system in that the pilot apparently can set any desired degree of flap, just by holding the flap handle down for longer or shorter times. So there are no “notches” at all. You just feed in the amount you think you need. In the sim, if you just hit the down flap switch, they’ll come all the way down. You have to hit the “down flap” key, wait for the desired flap extension amount, the hit the “flap up” key, which simulates returning the flap handle to the center (neutral) position, which stops movement. To bring them back up you do the reverse ("up" to start retraction, "down" to stop movement.) Is this flap operating mechanism possible in FS9 models, or something unique to the FSX modeling environment?

That would be unique if featured in FSX however it is certainly doable in FS9 using an XML based animation. Next best or closest animation technique would be to use the percentage based animation with a lot of stops.