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Old 11-05-2013, 05:29 PM   #1
CNY_Dave
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Default Can 'excessive' flaps cause pitch sensitivity?

I have a kyosho 50 ep spitfire, very mellow for a warbird.

I like to use the split-flaps it has on landing as it lands a good bit slower and the rollout is less. I have noticed it seems to be a good bit more pitch sensitive with full flaps, was curious if:

- Flaps in general cause pitch sensitivity (larger reaction to el inputs)
- Overextended flaps can cause pitch sensitivity
- It's just me

If flaps cause the center of lift to move backwards (which I may have wrong/backwards), then that's akin to moving the CG forwards relative to the center of lift, which I would figure would lead to less pitchiness.

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Old 11-05-2013, 05:55 PM   #2
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Default Flap pitch change

So, what flaps do is change the camber of the airfoil (creating more lift and drag by percentages based off of amount of flaps and airspeed) and in "most" cases it will case a nose pitch down attitude once airspeed has stabilized. The leading edge of the wing to the trailing edge of the wing has a line known as the chord line and that will change the pitch attitude of the lateral axis of the aircraft if either edge move up or down. The best way to counteract that sudden pitch is to mix in down elevator with flap application. it will take a trial period to get it where you want it but remember that typically flaps are applied at slower airspeed and in real aircraft there are speed restrictions as to when they can be applied without ripping off. Hope this helped.
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Old 11-05-2013, 06:06 PM   #3
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I do need down el mixed in to maintain a constant attitude with full flaps, just a little.

I find I still need to hold some down elevator as I slow down over the runway.

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Old 11-05-2013, 06:06 PM   #4
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Also if you want to get technical, your center of lift will change because of something known as induced drag but at this scale I would just keep it as simple as possible. Keep in mind also that typically past 15-20 degrees of flaps the percentage of lift to drag become more drag than lift and more of an air braking effect happens hence a steeper nose pitch down is required to keep from inducing a stall.
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Old 11-05-2013, 06:10 PM   #5
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yes the down elevator in the round out phase most likely has to do with flap configuration and that is where you will have to mess with how much down elevator mixing will be needed with the application of flaps. its kind of trial and error. Makes for some good "wheel landing" experience!
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Old 11-05-2013, 06:15 PM   #6
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if you want to get crazy you could try slotting the leading edge… could be interesting!
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Old 11-05-2013, 06:41 PM   #7
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I just want to improve my landing consistency!

I seem to get a few good ones in, then I am all 'thumbs' at it for a few.

I noticed I was pitching it up and down a bit when I started getting close to the ground, and was wondering if the plane was more sensitive to this with the flaps down.

I am also much better at coming in steep with no power for some reason.

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Old 11-05-2013, 06:42 PM   #8
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yes, flaps can cause pitch sensitivity. this can be bacause when flaps are extended beyond maybe 20 degrees they cause flow separation which causes a very turbulent wake downstream of the wing. If the horizontal stab is submerged in this turbulent wake then it's efficiency is much reduced. If the tail pops in and out of the wake you will get some erratic pitch behaviour.


PS.. in response to previous comment.. 'centre of lift' is not a recognised aerodynamic term. Possibly the poster meant 'centre of pressure' or 'aerodynamic centre'? Anyway, induced drag doesn't not cause either one to move. Centre of pressure does indeed move when flaps are applied but it's caused by the change in pressure distribution due to the flaps, not induced drag.
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Old 11-05-2013, 06:54 PM   #9
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Yeah Dave, as long as she's till "flying" the flaps will cause more pitch sensitivity but slower airspeed obviously makes all other control surfaces less effective. The key to RC and real aircraft landings initially is having a "stabilized approach"(basically meaning consistent pitch attitude airspeed and power setting, trying not to oscillate) whether it is for short field, soft field or normal landings. One thing I used to tell students was "pitch for airspeed and power for altitude" if ya smell what I'm stepping in.
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Old 11-05-2013, 07:03 PM   #10
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I started the whole 'center of lift' thing, knew it was close but not quite the right term.

I have tried to use the 'elevator controls speed, throttle controls altitude' method on this plane (which I have gotten pretty good at on other planes) and somehow when I get the 'right pitch' to do that at my brain screams 'it's gonna stall', heh.

I would like to hit the flaps and have just the right trim dialed in for the landing so it becomes pretty much a job for just the throttle.

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Old 11-05-2013, 07:26 PM   #11
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We could get into a pointless discussion of what induced drag is and how it happens but for this post Dave I would say that mixing the flaps with the elevator would be your best bet on finding the stabilized approach that will give you more consistent landings.
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Old 11-05-2013, 07:33 PM   #12
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Flaps can cause some strange stuff when you are in ground effect too and I think you are seeing that a bit as well. I have found it more with low wing planes too.

Along with the pitch sensitivity and the elevator compensation needed (can be either UP or DOWN by the way!) you need to manage power MUCH more carefully. We, as modelers, tend to get a bit sloppy with power when landing. Remember you should be using POWER not ELEVATOR on landing to extend or shorten the landing distance and approach.

I had a ultra cool and loved airplane - the wonderful Air Tractor 802. It was the only airplane I have ever flown that didn't require any elevator compensation during flap deployment. It was not sensitive to speed either (usually the faster you deploy, and the higher the airspeed the more correction needed). I talked to a full scale 802 pilot - he said exactly the same thing was true of the full scale. He said they deployed them at about any speed and they helped a great deal during spot dumps of fire retardant. Yes he flew the 802F used for firefighting.

This is why I usually don't recommend flaps to rookie pilots - you get to manage a great deal more during landing. It does make better pilot of us however.

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Old 11-05-2013, 07:41 PM   #13
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Given the field flaps are somewhat necessary to get the speed down, it's not very smooth.

I'm not sure I am having problems in ground effect, although sometimes I could be, where I can't seem to get it 'down' the last little bit until it plops down and digs in the gear and rips off the gear yet again. It;s hard to tell what height that happens at as no part of the field is really level, you are either landing uphill or downhill.

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Old 11-05-2013, 07:49 PM   #14
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More power - sounds like you are getting too slow. Try some nice wheel landings....
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Old 11-05-2013, 08:15 PM   #15
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I'm not trying to 3-pt it, although if I get just off the ground and have the rwy I will coast down into a really nice one.

But I also hate it when I touch down too fast and it noses over in a little divot or bulge in the runway. I think we need a roller.

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Old 11-05-2013, 08:39 PM   #16
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Originally Posted by CNY_Dave View Post
I'm not trying to 3-pt it, although if I get just off the ground and have the rwy I will coast down into a really nice one.

But I also hate it when I touch down too fast and it noses over in a little divot or bulge in the runway. I think we need a roller.
Yeah
I've got a couple of models with flaps.

What works well for me is using about 30 degrees or so of flaps during the landing approach. Then, at the instant the landing gear touches the ground, kill the flaps. Properly done, that plants the models wheels on the ground to stay. That really helps those bouncy landings when you get the timing right.

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Old 11-05-2013, 09:13 PM   #17
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Ha! I've thought of that but lack the coordination, by a wide margin.

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Old 11-06-2013, 01:05 AM   #18
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Originally Posted by CNY_Dave View Post
Ha! I've thought of that but lack the coordination, by a wide margin.

LOL
If you kill the flaps a second or two before touchdown, or even a second or two after touchdown, it will still work.

Just don't kill the flaps when your model is 5 feet from the ground. It might drop like a brick, depending on the model, and how close you are to the stalling speed.

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Old 11-06-2013, 02:07 AM   #19
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Sensitivity ? Well any model that is slowed down and especially more so with flaps will become less stable as it's so near to stall speed ....

The biggest problem is not sensitivity I find but the nose UP that occurs when deployed ... as usually they are deployed before the model is at ideal slower speed to match the new wing contour produced ... so you have to nudge in a touch of down till speed decays and she balances out again.

I don't like to mix in elevator to flaps because of then needing to give UP elevator once slowed not only for landing flare but also to compensate for the mix input. I prefer to do it myself and ease of as speed reduces.

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Old 11-06-2013, 02:23 AM   #20
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I always come in under a glide and use up elevator to slow the model down.

if you want consistency, you need to just practice touch and goes. the lower the wing loading, generally, the more it wants to stay up as you get into the ground effect. I like to get it down to the point that.the plane starts to hit the bubble of air and slowly increase elevator until it starts to stall. should bring it down nice and level.

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Old 11-06-2013, 08:06 AM   #21
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The problem is that models can vary so much ... my big biplane - you have to fly that in and close throttle just as she touches ...

My EDF's can glide in dead-stick ...

My Cessna is brought in with throttle "pulsed" ...

The only similarity is really the ANGLE of glide slope to arrive at desired point ... and usually miss it !!

My ASK21 is the proverbial b******d to get on the spot ... you line her up nice ... she's sinking at a nice rate ... she arrives over landing zone and glides past into the rough !!

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Old 11-06-2013, 01:08 PM   #22
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Elevator to flair - but NOT to control landing sink rate guys - that is throttle!

While I agree some planes (my EDF's) do well under no power it actually helps to slow propeller models down at lower throttle settings as the prop disk slows the plane down.

Dead-stick approaches ESPECIALLY with flaps will bring you in way too slow and far to close to stall - this is why you get "plop" type landings. On my large Telemaster Plus with full flaps and NO power I have to use a roughly 40 degree approach to keep enough airpseed! It is a cool trick (coming down at that sharp angle) but when I flair I have to add power or it slows instantly to stall.

I realize our small light models do not need much power when landing - but as you advance to bigger heavier models like the OP is talking about dead-stick with flaps is the kiss of death. When you get to 30+ in/oz models doing that is almost a guaranteed landing stall, snap, spin and crash.

Think about it - on your last commercial flight - you see how much they play with throttle on landing? Every time they add more flaps - they add more power and there is a reason for that!

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Old 11-06-2013, 01:34 PM   #23
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I'm still pretty much the beginner but what I found helps with my planes that do have flaps is to deploy them early enough, ususally at the beginning of the downwind. Followed by gear down as you near the base leg. That way, you have everything stabilized well before turning base. No surprises with ballooning, etc. Once you turn base to final it's simply a matter of managing the sink rate and glide slope with the throttle. If it looks like I'm going long, I simply declare a go-around and try it again.

Most of my planes don't really need flaps. They're there for scale effect more than anything.

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Old 11-07-2013, 12:04 AM   #24
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Dead stick steep approach is what I'm good at- trouble started when I tried to do a more proper approach.

I don't mind having some pitch changes when i deploy the flaps at a higher speed, what will help me reduce the workload until I 'get it' will be to get the el trim with flaps (already in place, just needs adjustment) to hold the plane fairly level at a nice stable sink rate so I can focus on throttle.

I practiced this a *lot* on my trainer and got fairly good at multiple 'types' of landings (no flaps of course), but a bad power switch on my dx5e killed that plane.

I have an old (90s) LT40 kit I am thinking of getting put together and flying before getting the spit back out (now that it's cold and my fingers work even worse).

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Old 11-07-2013, 12:22 AM   #25
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Originally Posted by CNY_Dave View Post
Dead stick steep approach is what I'm good at- trouble started when I tried to do a more proper approach.
Yep but this dead stick is what causes your "plop" and gear rip out landings! You MUST land with proper power especially with flaps. If are dead stick you are just better off with no flaps.

I recommend just touch-and-go flights with a great model like the T-28 Trojan from Parkzone. Do 10 flights worth. Take off, then land, then do it all again. Once you have it down cold start really playing with throttle on the landing (just like you do ailerons, rudder and elevator). Just one more simple thing.

Then on to faster models - then flaps.

Then don't forget every model is just a bit different too! At any rate - it is all very manageable. Just takes practice. Then once you are good - practice right hand landing patterns. It is amazing how many RC pilots can't make right hand turns! Then you move on to something else.....

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