ahhhh-
OK lets look at one more "defination"
- If---- flaps add more lift -- why not leave them deployed for maximum cruise efficiency?
The obvious reason is that they are ONLY better at lower speeds.
And each increased angle is better ONLY at a lower speed
Now there is one interesting plain flap for fully symmetrical foils
That is a flat plate flap added to trailing edge the curved shape - such that there is a reflex top and bottom at the juncture of the flap and the wing
when the flat plate is lowered (or raised)
one surface becoes a smooth path from leading edge to end of flap the other side becomes an undercambered shape.

You are correct. It is amazing to see the conclusions people come to. Flaps change the shape (and sometimes enlarge the area) of the wing. The new shape generates more lift at a given speed; the speeds associated with T/O and landing. So why not fly with flaps extended all the time? Because their lift/drag ratio is worse than a clean wing. Once speed is sufficient, the lower lifting ability of a clean wing is able to do the job because it is moving faster, and of course is also more efficient.
I flew B737's out of short slippery runways. That meant that one wanted to get off at the lowest possible speed, in case one needed to abort T/O. Yet one couldn't just select the largest flap setting, because drag would become a factor. You have to be able to maintain a minimum climb gradient in case you lose one engine right at V1 decision point. We would get into the books to select the highest flap setting that would allow a one engine climb out.
Conclusion: Flaps are used to make up for the inadequate lift supplied by a clean wing at landing speeds. The clean wing is suitable for cruise, not landings.

It all comes down to how you miscomprehend a Cl-Alpha plot..
If your Alpha is at A, your Cl at B, due to your wing area, your weight, and airspeed, and you pop 60 degrees of flap,
Your Cl moves to C. Which is 4 degrees -past- your stall alpha.... well past it. Lots of lift... except the plane has departed long ago..
Since you can't change your wing area, or your weight, you have to change your alpha.. to the -same- Cl you had... after all, the weight, area and airspeed haven't changed... to D.. which corresponds to the alpha at E.. which happens to be 12 degrees -less- than that at A.
Unless you have a pivoting wing like a Crusader, you have to nose down... just to keep the -same- lift.
Your drag goes way up as a consequence of the flap deflection, and your Cm takes a walk towards instability.
But if you're one of those old timey round-motored planes, you come downhill nicely after adjusting alpha, without speeding up... even some kerosene stoves approach properly.. not wallowing around with their skinny noses in the air...

When you say flaps add lift do you mean you will gain altitude at the same power setting? Am I confusing lift with altitude gain? I thought flaps were used to steepen the glide path while keeping the plane at a proper speed and attitude. They are also used to steepen and shorten the takeoff run, but I would think this requires more power because of the drag.

If -nothing- else is changed when putting flaps down, yes, they "add" lift. The airplane will change its attitude due to the increase in lift... and the increase in drag... and the change in the pitching moment.
All of these sum together.
For a takeoff, the "extra lift" is removed after getting airborne.
Consider a plane of X pounds, dumping flaps that give .25X more "lift". The weight of the plane is still X. -Something- has to change in response to the flap change which would, uncorrected, do something (probably drastic) to the plane.
The controllable variables are: power, and pitch angle.
The drag increase will require power to maintain level flight.
Or, the plane's nose can be lowered to the desired descent rate/path, and power reduced.

to ozspit - lowering the flaps does not decrease the AOA, it increases it. Hence the increased lift. Since the AOA is now higher than the wingtips, the root gets to stalling AOA before the tips.

Lowering flaps will increase lift, as I said to a point. The flaps change the camber of the airfoil, and increase the AOA, and sometimes increase the wing area (fowler type). If all things remain equal, then yes, the plane will continue to climb if the AIRSPEED (not the power) remains the same. Well, since we all know that flap deployment, in addition to those other benefits, also increases drag, then it will require an increase in power to maintain the same airspeed. If you want to keep the climb with flaps, you have to increase the power to overcome the drag. So, why does the aircraft balloon when the flaps are lowered? The aircraft is trimmed for level flight at a certain stable airspeed. Change one parameter - the flaps. Now with this huge increase in lift, the aircraft thinks it is flying much faster. With the plane trimmed as when it was configured clean, it begins to climb and tries to return to balanced, trimmed flight. It slows down when the nose balloons, until the airspeed again stabilzes to the trimmed airspeed - nose lowers and eventually begins a stable descent at the trimmed airspeed. Why? Because now at that same airspeed, there is increased drag - so to maintain that airspeed (remember, we have not changed anything other than lowering the flaps), the nose has to come down, hence the plane descends, but at the original airspeed. So we can descend with flaps at the same airspeed we were maintaining level flight with a clean wing. Which is why they are beneficial at slow speeds on approach. You can maintain a safer margin above the stall by carrying more power (to maintain your airspeed) but also be descending. Balanced flight has four EQUAL parts - lift, weight (gravity), thrust and drag. Change one, and it will cause the plane to try to reach that equilibrium again all by itself.

Try it. Take your plane to a safe altitude at say 1/3 throttle. Trim for level flight hands off. Lower the flaps to say 1/2. Do not change the power setting. Watch what happens. The plane will balloon, then slow, then the nose will fall through. It will stabilize in a descent at the original airspeed you deployed the flaps.

That is what I thought (I think). In other words you can't get something for nothing???

Depends on your definition of "nothing"...
With my Senior Kadets, full flap and full power results in a uncontrollable pitch up. I plain run out of down elevator to push the nose down. The -only- way to get out of the situation is to raise the flaps.
OTOH, putting the flaps down at moderate to idle power, and it needs just a tad of elevator to keep the nose where it should be, and the plane will fly at about 12 mph, without any excessive pitch.

Navy flyer,
I can't let that `The airplane thinks it's flying much faster' go. I would have thought a better explaination would have been that as the camber increases with flap application, lift and drag generated increases until equilibrium is again reached (ie the model will `balloon' and bleed off airspeed, inertia effect) at a lower airspeed than before, where L & D again = T & W. The airspeed will change, effectively the cl of the wing has changed, so the lift generated at any airspeed will change but the weight will not. For there to be equilibrium then L still equals W, so the IAS must reduce in this case if the craft is not to descend or climb. To descend then, L must be less than W, and pilots do this with both trim and power changes, but then you knew that.

Hmmm....dunno about an airplane keeping the same airspeed with flap deployment, unless it's related to the amount of flaps deployed.

Real world experience: Used to fly a Cessna 150. Would set up for hands-off level flight speed of 80 MPH on downwind leg. When abeam of the landing point, it would be carb heat on, throttle to idle, and then hit the flap switch to lower flaps until they were fully-deployed. You used the controls to basically hold the nose level as the airplane slowed as the flaps came down. As soon as the flaps were fully-deployed, you turned base leg. The elevator was not re-trimmed, and the airplane wound up being trimmed for a hands-off 65 MPH. Stay power off until landing, with only a few blips of throttle to 'clear' the engine. I believe the pattern altitude was 800 feet AGL.

If power was added, the airplane would tend to hold the 65 MPH. The elevator had to be re-trimmed for the airplane to want to hold a different speed. This showed that while the trim setting did not actually change, the relationship of the elevator trim setting and the flap deployment made for an effective change in the actual trim of the aircraft. It would not hold the original trim airspeed when the flaps were deployed.

Same thing for flap retraction. You could be in a climb after takeoff, trimmed for constant airspeed. Slowly retract the flaps, and the airspeed would increase as they came in. Didn't change the trim, but the speed would increase...again, it's because of the complex relationship between the effective angle of attack change, the effective decalage between the wing and the tail, and the change in drag caused by the flaps.

Make no mistake, they all change when the flaps change, and it's not a simple relationship...and models will basically behave no differently than full-size aircraft.

Well, me too - USN flying for 22 years around big grey hulls (CVN's) so I know just a little something about AOA flying. About 3000 (give or take) hours. Now fly for fun the C-150/172/177RG/T-34B - and of course my small RC fleet. We are saying the same thing - albeit a bit differently. You mentioned the C150. Ok, let's use that to illustrate what really happens with flap deploment. When you deploy flaps (full scale) in the pattern, you must maintain the pattern altitude until abeam the landing position - so you pull the throttle back to 1600 rpm (not idle) entering the downwind, carb heat on, slow to within the white arc (flap operating range). At mid-field on the downwind leg you drop the first 10 degrees of flaps. Plane wants to pitch up, so you hold it down with the elevator to stay on altitude. Target airspeed is 'best glide', in this a/c that's 70 mph. Best glide gets you best L/D in case you lose the engine, and best chance to put the mains on the asphalt if in the pattern when it happens. As she slows, you drop another 10 degrees when abeam your touch down point (should be on the rwy numbers). When you turn to Base, wings level, you drop another 10 (30 total now), maintaining best glide (70 mph), and still at 1600 rpm. To maintain 70 mph, you have to ... yes, you can say it.... change the trim! Or, fight the yoke and get really tired. You were trimmed to 80 entering the downwind, now you are trying to maintain 70, so you have to trim nose up, as the nose wants to drop to pick up airspeed to the trimmed speed of 80mph. So, 30 degrees of flaps, now trimmed for 70 mph, power at 1600 rpm. The drag induced by that extra 10 degrees to 30 increases your descent rate. The VSI goes from about 200' per minute to about 300-350 fpm. Turn final - use rudder to keep the ball centered- or risk an approach turn stall caused by cross controlling (opposite (right) aileron to keep from over rolling (left) while turning left). Decrease throttle in small increments to 1200-1300rpm. When you have the field made, put all of the remaining flaps in (40). The nose wants to pitch down (lots of drag) to maintain the trimmed airspeed of 70. But you don't let the nose drop - you keep the rwy numbers in your windscreen, holding the nose attitude steady, and the result is that airspeed begins bleeding off, you're looking for the over the fence speed of Vso plus 10 (60 mph). Vso is stall speed in the landing configuration. A bit more back on the power, holding that nose attitude steady. Over the numbers, start the round out (flare) and power all the way back to idle. Ease the yoke all the way back, in your gut, using rudder only to maintain centerline. Squeak squeak. Flaps up, carb heat in (off). Get off the active.

Hopefully this illustration points out the aerodynamic forces at work with flaps. If not - go do it your way - have fun anyway.

Cor, that brings back memories! Used to fly with Grumman AA1's, trim for airspeed, control height with throttle. Models do exactly the same, only not being in the cockpit with the numbers right there in front you need visual clues, so what looks like is happening often isn't, hence all the usual misconceptions about what things do. Trying to illustrate things with words usually only makes things foggier, but that last post, I could see it all again, line it up, check the drift, watch the hens teeth and real gentle with the throttle. Thanks.
Evan.

Ok. Lots more experience flying the real thing out there than me with 8 lessons! It's been a while too so my memory of what actually happens is a bit vague! [&:]
So.... when flaps are deployed and the plane balloons it pitches up, not just gains altitude at the same pitch. I believe you guys with heaps of flying but why? If the CG is forward of the CL, an increase in lift, given the moment arm should pitch the nose down while gaining altitude. Thus the nose down attitude with flaps extended so I thought. Is there something else happening or is my thinking just plain wrong?

Some pitch up, some pitch down, depends on the flap type, wing position, airspeed, aircraft design.
Evan.

ozspit,
What pimminz just said and..........

the science of aerodynamics offers some explanation with something they call pitching moment. They offer measurements of it in plots and formulas to predict what'll happen because of it. And in effect, what happens because of it depends on lots of things, most just mentioned.

The wings configuration at any moment gives you an airfoil that's a resolution of the position of all it's parts. If the flaps are up, the spoilers down, no inspection covers flapping, etc you've got an airfoil that probably is producing a pitching moment, and it's probably more or less stabilized by the entire airframe and specifically by the horizontal tail. When some of those movable parts are moved all that changes. And one of the forces in play is a new pitching moment. And if you can find a plot that matches the new configuration, you just might find a pitching moment quantified. With pencil and paper you can figure it all out. And get a better insight into the whys and whats. And you're going to have to take more than just the pitching moment into consideration.

Of if you know a pilot, and especially one with hours in the particular subject airplane, you can just ask what happens. You'll certainly find out the "what".

And a number of people have mentioned that different things happen with different airplanes. And different things happen with the same airplane when different amounts of different things like settings or speeds are part of the picture. There isn't going to be one simple answer to cover even one simple configuration of flaps. or spoilers. or airspeeds. etc

I DID?????

When you deploy flaps, you change the shape of the wing. One effect is more lift, which, at the current speed, translates into a balloon in altitude. The second effect, is a change in the centre of lift. This varies in different machines, which determines the pitching effect. If the CL moves back, the nose will pitch down for example. On the B737, as one took 40 flap, one had to push to stop the balloon, and start trimming back for the situation after the balloon settled down. Strange as it seems, you were pushing one way, and trimming another because you knew from experience what lay ahead required back trim.

Great feedback. Thanks. In short, there are no simple answers! Can't wait to get back home and try a few things on my models and get back in a Warrior with an instructor again to see what happens practicing circuits.

Dick, trust me, you can snap roll a control line model. And it's not purty when it happens. If it snaps to the outside it's often not a big deal. Just a nasty line snatch and hopefull a return to normal flying. But if it snaps to the inside it's baggie time.

I have had cl models come in at me - -been over 50 years tho- I never thot of it as a snap roll - just going too slow -stalled out etc..
actually out run some of these and saved em!
I flew Brave - Super Duper Zilch- Barnstormer and stuff like that -
then I finished up with 1/2A stunt stuf I flew on 50 ft of .008 braided wire - in 1969-
all that line drag assured no fast rolls just a gentle roll as the thing tried to get me --

Navy_Flyer,

I still don't know about the airplane keeping the same trimmed airspeed with flap deployment. My experience was that from a hands-off trim of 80 MPH on downwind to hands-off trim of 65 MPH with power off and full flaps deployed had zero change in elevator trim setting with the C-150's I flew. I'd cut the throttle, deploy the flaps, and turn towards the runway. It was a good exercise. I'd cut power and deploy full flaps when abeam the touchdown point and then turn as soon as the flaps hit the stop. I'd use pressure on the yoke to prevent the typical nose-up tendency of the airplane as the flaps came out, but did not alter the trim at all.

The airplane would change its trim airspeed with flap deployment if you did not re-trim the elevator...and that would be because flap deployment changed the complex relationship between all of the surfaces involved.

At any rate, it was a fun exercise to see if you could touch down just beyond the numbers from downwind without having to touch the throttle. My instructor used the full flaps example because he said I could lose all power, engine and electrical at any time, and should learn how to handle the airplane in the 'worst' configuration. Of course, you can't do this if it's bumpy or with much of a crosswind because those conditions mitigate against flap usage, per the Pilot's Operating Handbook.

I was flying a .35 powered Monoline plane which came in at me... that wire.. I believe it was .060", tried to wrap itself around me.. Would have been a serious situation had the thing not crashed about 1/2 way round.
With most 1/2A Monolines the line going slack was no big deal, you still had control.
A Ringmaster I was flying back in '56 just plain stopped flying at the top of a wingover.. dropped almost straight down.
Never could finger out the cause of that.
And at the '62 Nats in Chicago, at a pit stop with my FAI team racer, the lines snagged on a piece of grass about 20 feet away.. I fuelled the plane, started it, released it.. and it whipped around in a 360 around the grass, then pulled free as it came to the usual flight direction, and kept on going. I yelled to John Barr who was also flying then... "That's the way we do it on the East Coast!".. Came in second behind John.

hi guys
Flaps are high lift devices. There are many different types like: plain ones,split,slotted,Fowlers. All the flap types are used to increase the maximum lift coeficient for low speed flight. All flaps are applied to the trailing edge and are usually 15-25 percent of the chord. Now the deflection of a flap produces the effect of a large amount of camber added well aft on the chord. The effectiveness of the flaps on a wing configuration depend on many different factors. One of them is the amount of wing area affected by the flaps. If the wing has a low thickness any type of flap will be less effective than on a wing of greater thickness. Sweepback of the wing can cause an additional significant reduction of effectiveness.
1)Lowering the flaps requires retriming to ballance the nose down moment change.
2)The increase in drag requires a higher power setting to maintain airspeed and altitude.
3 The AOA required toproduce the same lift coeficient is less, e.g., flap extension tends to cause the airplane to "balloon".
These are the basics guys. If something is not clear I'll be happy to assist you to make it clearer.
Cheers
Johnnie