i thought that with flaps one has to mix a bit of UP elev input.

to my surprise when flaps are deployed (without any mixing) the plane starts climbing.

is that due to the added lift to the wing? i thought the flaps would "push" the leading edge downwards...

do the flaps have different effect at various speeds ? can anyone explain?

thanx

I fly sailplanes and use lots of flap to slow for landing. Of the 8 planes I have used flaps,on all required a good bit of DOWN elevator to keep the nose from pitching up when the flaps are dropped.
When you drop flaps the trailing edge of the wing drops. If you draw a line from the leading edge to trailing edge of the airfoil flaps up and flaps down you see the incidence angle of the wing increases when flaps drop. The incidence angle of the horizontal tailplane is not changed so decalage increases when the flaps come down, so the plane pitches up. It is similar to rotating the wing such that the trailing edge moves down. To restore decalage to the value for balanced flight you need to change incidence of the tailplane by dropping the trailing edge of the elevator.

Vasek,
I had the same experience that you with the use of flaps. The climb effect depends on two variables: air speed and incidence. At a given combination, you will not need up elevator, and eventually you will require down elevator to bring the plane to land (that also happened with my designed plane). Certainly all the above is related to the flaps area and drop angle. Another parameter is the relative position of the flaps center of force related to the CG. (momemtum cause by the drag created by the extended flaps). There are also planes that require up elevator.
Richard

What effect flap deployment has on pitch trim depends on several things. Deflecting flaps results in a nose down pitching moment on the wing, however at the same time the air leaving the wing is deflected more sharply downward. If the horizontal stabilizer is in this area of increased downwash, the increase in down force on the tail will tend to pitch the nose up. It will also be affected by the location of the cg. A more rearward cg will usually cause a nose up moment while one more forward will cause the nose to pitch down. The final net pitching moment is hard to predict and depends on which of the several effects wins the tug-of–war. It’s not so much a design thing as a try-it-out-and-see thing.

If you have a computer radio mix in 5-10% DOWN elevator with the equivalent flap deployment. This helps as it will increase down elevator with increasing down flaps. I found that 7% was about optimum with my model. There are so many variables to take into account (as mentioned) that you will need to experiment. Start with 5% and go from there.

thanks to all for the info, always good to learn something new! i will try 5% down and go from there

i was just wondering in real life scenario (lets say a Spitfire) deployed flaps, did the pilot trim the plane? or hold the stick? or was it linked so as when flaps were lowered the elevator ajusted simultaneously?

just curious

I don't know about the Spitfire, but in the many full size aircraft I've flown, You just maintain attitude with the stick then trim out any force you're holding. Some want to pitch up, some want to pitch down and some have little pitch change. I don't know of any that have a mechanical connection to change trim with flap deployment. The amount of pitch change also depends on speed and a constant correction would be too much at times and not enough at others. The way full size aircraft are flown is to fly the airplane with the stick then make trim changes as necessary to reduce stick force.

LouW , makes sence THANX

I haven't seen it mentioned yet, but tail moment has a lot to do with a planes tendancy to pitch with flaps or flaperons. Short coupled planes tend to pitch down, planes with longer tail moments tend to pitch up.

I'm not sure why this is, just that it's pretty consistantly true. As mentioned above, there are a ton of other factors as well.