flaps + elevator
 

this is a question i never understood.

Why does DOWN elevator brings the plane down. but DOWN flaps makes the plane go UP (at a certain setting)

please help

RE: flaps + elevator
 

Both of these deflections do the same thing, increase the lift on the "flying surface". The difference is the location of these two flying surfaces in relation to the center of gravity of the airplane. The wing for example is very close to the CG so an increase in lift makes the plane rise without changing the pitch attitude very much at all (nose up, nose down movement is pitch). However, the tail is very far behind the center of gravity so an increase in lift there will make the tail of the airplane rise up, which in turn makes the nose of the airplane point down... this also tilts the wings down which lowers lift and the airplane goes down.

To add to the confusion airplanes usually pitch up when the flaps are lowered... this is not due to an increase in lift, but rather an interaction between the wing and the tail known as downwash. When the wing flaps go down they deflect air downward behind the wing... this downward deflected air strikes the horizontal tail and pushes it downward as well. The result is that the tail goes down, and the nose goes up.... which tilts the wings up and creates more lift.... it is exactly the same reason the airplane pitches nose up when you pull back on the stick, it's just caused by the wing flaps instead. This is why many high lift airplanes with large flaps have T-tails like the C-17... otherwise the airplane would pitch up dramatically with those big blown flaps.

Ty

RE: flaps + elevator
 

Both actions change the effective angle between the wing and stabilizer.

You need to understand that the effective angle of attack for a wing or stabilizer is the line from the leading edge to the trailing edge. So when you deflect the control surface that centerline also changes it's angle. In effect the movement of the surface adds camber to the airfoil section AND changes it's angle to the air all at the same time.

We get used to measuring the wing and stabilizer angles with reference to a center line. For example we set the wing for +2 and the stabilizer at -1 or somesuch. In truth the air does not see any stinkin' centerline. The air ONLY cares about the angle between the wing and the stabilizer. So the AIR sees a 3 degree difference at the stabilizer. The true angles of the airflow at the wing and tail will vary widely depending on the flying speed but whenever the controls are at neutral the air will see a 3 degree difference between the surfaces and the model will fly based on that. This difference angle is known as decalage. And yes it is French in origin.

But when you use the controls to change the effective angle of either surface you change the decalage angle. And as you well know if you shim the wing or tail to change the angle the model flies differently. Adding down elevator is like shimming the stab to a negative angle and this makes the decalage angle go more negative and the model responds by nosing down. Adding down flap is the same as shimming the wing to a more positive angle and the decalage goes more positive so the model noses up.

RE: flaps + elevator
 

I think it's because down flaps essentialy change the incidence of the wing to a more positive angle, thus creating more lift, which lifts the nose, which needs to be counteracted by down elevator. Put an incidence meter on a plane with flaps fully extended, and compare the incidence with the flaps retracted- big increase in incidence with flaps down. Either that, or it's something else.

RE: flaps + elevator
 

Umm..... That was DOWN elevator there Ty.... :D

RE: flaps + elevator
 

If you deflect the elevator downward... you increase the lift of the tail.. pulling the back of the airplane up... which points the nose... down.

RE: flaps + elevator
 

However, flaps down doesn't always cause a pitch up.

My SE will pitch down when I lower the flapperons. In this case, I guess the increased pitch moment from the wings is greater than the moment due to the tail incidence relative to the wing incidence.

RE: flaps + elevator
 

When flaps are deployed Down, there is a transient effect that causes the airplane to pitch Up momentarily; but once the steady state condition is attained, the airplane may require Up or Down trim depending on the downwash from the wing and the airplane tail configuration; ie, low horizontal, mid horizontal, or T-tail.
A good example is flying the Cessna 150: at 10deg flaps (first detent), the airplane requires Fwd (Down) elevator trim to maintain 75mph; but at 30deg and 40deg(full flaps) the airplane requires almost full Up elevator trim to maintain 75mph.
In most cases, most airplanes require a considerable amount of Up trim on approach and landing.
How do I know? I've spent 25 years flying full size airplanes.
feihu

RE: flaps + elevator
 

[link=http://adg.stanford.edu/aa241/highlift/highliftintro.html]Because....[/link]

Elevators work by deflecting the tail up or down so the nose will go down or up. What works to bring the nose down, when applied to the wing, also makes the wing lift more.

RE: flaps + elevator
 

thanks for the help guys

RE: flaps + elevator
 

feihu-RCU

this may sound not so good but...(im not a full scale pilot, but it intregue`s me):

you are saying flying (a cessna 150) with 10º flaps it takes fwd trim....to maintain 75mph, but.. when adding 30º or 40º of flaps...it takes back(up trim ) to maintain the same 75mph.... why would you want to maintain the same speed>?(with 10º or 40º of flaps) i thought the idea of flaps was so you could approach at a slower speed>?..and land. does it become more stable with the flaps at max deflection>?

you may of just used this as an example for arguments sake....

so ..when coming in for a landing you start with 10º of flaps ...it pitch`s up slightly .you compensate by pushing fwd.....then once it begins to "settle in".. and you add more flaps.(and i would guess throttle)then you begin to pull back. to maintain same speed,, during aproach>?... then it sinks in at what ever speed the aircraft was designed to land at>?

RE: flaps + elevator
 

Well... you have a good point. I think that 75 mph is pushing VNE (if not exceeeding it) for 40 deg flaps on the 150. (can we rip a flap off the plane please?)

Yes the whole point of flaps on a full scale aircraft is to be able to make a slower landing approach and still maintain control. (or at first detent.. aid in preventing tip-stall on take-off)

The normal way they deal with flaps on full scale airplanes is... add more as speed decreases... not put them down and try to hold speed.:eek:

Another use of flaps is to increase descent rate without allowing airspeed to build up. You can point the nose upward, slow down, add flaps and power to descend almost like on an elevator. The flaps add a tremendous amount of drag. The center section of the wing would be stalled and the tips would have full aileron authority.

Full flaps and some power added on my Kadet Sr... You'd think you had a Fokker Tripe's glide slope (45 deg nose-down or stall if dead stick on the Tripe) instead of the glide slope of a Kadet Sr's that wasn't using flaps (float forever). Except the Kadet with flaps has a 10 ft roll-out... the Dr1 rolls 100 ft minimum.

RE: flaps + elevator
 

thanks for the info...
looking forward to learning to fly(full size) in the future..(before i get to old) :)
i have rode around in the 150`s.. were not exactly doing anything impressive ... something a little more aerobatic may be in order.

RE: flaps + elevator
 

Chris,

No time like the present, go to Chandler Air Service www.aerobatics.com get an intro flight in a piper cub or a great lakes open cockpit biplane... they are the best school in Arizona, and they are close by.

Ty

RE: flaps + elevator
 

thanks acropilot_ty ,

I was thinking of Glendale airport... and there school , since it is very close to my home..(less then 5 minutes).. I am not sure if school is as good..or if instructors are as knowledgeable... but ..for the basic course may be the best thing.

http://www.glendaleaviation.com/

I have a few questions... I need to run past them....
one is: how much money you need to come up with initially . to get started.... and how all that works.( I have an idea on basic costs. on basic license, then IFR training.

and ...what physical aspects to they require(health issue’s). is there a physical you need to complete>?

(I am healthy, but have broken quite a few things.<have loads of hardware. :) > )

thanks again for the input.

RE: flaps + elevator
 

Geh...

So confusing. My name is also Chris. I also live in AZ. So many Chris'...

RE: flaps + elevator
 

heh how ironic.. figures huh>?
i can be called elmerfud.... no biggie

RE: flaps + elevator
 

Always glad to see someone interested in flying!
You can get a head start with the Microsoft Flight Simulator - it is in no way a substitute for flying the real thing - but it will give you an insight to cockpit familiarization and flight procedures. And you should have a good flight stick that has a throttle and trim controls.
(But you probably have these things already).
Good luck with your flying.
feihu

RE: flaps + elevator
 

thanks again... i spoke to a flight instructor today..(on the phone)... im going to go meet him on saturday(at the school) and get the perverbial ball rolling...
its looking like i can swing this..i had allot of questions answered today...

believe it or not i do not have simulator stuff. i learned to fly(rc) choppers and planes.. the old fashion way i guess..
you would think i would though.. i do game some.. but never flying stuff..it seems.. heh just a kid at heart

ill keep you posted

RE: flaps + elevator
 

Hello, I am pretty new to flying R/C planes just coming up on one year. I need to ask for some help on this topic but i really need the answer in the simplest of terms. Normally I would just go to the feild and ask one of the regular old school guys but I live in N.E PA. and so far this winter I have been the only one going to the feild. (Best thing i ever did was put snow skis on my u can do ) and Practice Practice practice... ( I have seven different planes in my hanger at this time and three more to build (Ryan STA, Christen Eagle, & A Cap)
Anyway here is my Question : I just finished building an (world models super chipmunk 90 s) it is my first plane to have retracts and flaps, it has a magnum 61 2 stroke on it. i am itching to take it out and try it. But I need Help with how to set up the flaps.
I tried to use my Futaba 6 exs radios mixing function but I am not sure if I got it right. What I am hoping to accomplish is to be able to slow the plane down a little for landings. The way it is set up right now if i turn on the airbrake function the flaps lower about a quater to a half inch and the elevator goes down about a sixteenth of an inch. is this the right direction for both the flaps and the elevator to travel? and are the distances that they travel about the right amount???????????
Any help would be greatly appreciated.
Thanks alot
Bob

Need help with flaps + elevator
 

I have a full scale P 51 Mustang. I am having trouble setting my flps and elevator mixing so I can put in some elevator trim when my flaps are deployed. I am using a JR 662 radio. The only problem I do not like about this rabio is it ONLY has one setting for the flaps. I would I to set them for take off at 7 degs but I need more flaps for landing. Should I buy another JR radio taht has the landing mode and several flap setting or can some one give me some lite on what I am doing wrong.
I have landed the plane seval times with no flaps, but I am coming in pretty hot on a grass field and the grass does help to stop me. I have stoped flying the plane until I can get the elevator set with my flaps and also I need to settle on which flap setting I am going to stay with if I stay with this radio.

Pilot P51

RE: Need help with flaps + elevator
 

For a 2-position setup, you should use the case where you need the flaps the most, all the way down for landing, or retracted.
The takeoff condition doesn't normally need flaps.
I've tried a variable channel with all positions dialable via a knob, but that's much clumsier to use than switching.
A P-51 of course MUST have flaps down to land!
Anything else just doesn't look right! :)

RE: flaps + elevator
 

Don't preset any mixing -BEFORE- you fly the plane.
See how much elevator and direction you need to hold it in flight, then set up the mix.
And don't look to slow down.
Flaps are more useful for steepening/shortening the approach, without adding speed.
You have to maintain a bit of power, as slowing the plane with flaps can lead to stalls.
Relying on them without using power to keep the speed up won't work for what you want.

RE: flaps + elevator
 

Unfortunately... without having experience with the particular kit... we can't answer about exact control throw amounts and mix combinations such as this. I have never had any Chipmunk...

airbrake function is typically a very large deflection setup... adding massive amounts of drag (and hopefully not destabilizing the model...) The small deflections you note are nowhere near an airbrake function. (though they may be useful for getting a slower landig approach...)

Some models pitch up with flaps... some pitch down (more often down) Pre-mixing down with flaps may be quite a serious error.

Remove the elegvator mix from the flaps... slow the model down and lower the flaps. See which way your model pitches. Yuo will also see wghat happens with the airplanes stall speed and control responses. If the model is HIGH, you have TIME to pull the flaps uyp, add power and recver from whatever unexpected result the flaps may give.

Always change just one thing at a time... and test at considerable altitude. when experimenting with control function mixing. When experiment with a mix... I like to do it on one of the switchable program mixes.. so I can flip the switch and kill the mix if it does something I don't like and didn't expect.

RE: flaps + elevator
 

you have to understand that the effects of flap is not a linear think........

flaps from zero to lets say 10 degrees are making the wing produce more lift for only a small penalty in drag. if you extend the flaps more the increase in drag is much more important for only a very small gain in lift.

in a cessna the approach speed flaps 20 is aproximatly the same than the aproach speed flaps 40. But when aproaching flaps 40 you will need much more power on the glide.
the main reason to approach flaps 40 is that the plane will decelerate much more faster when you will cutof the power and make your landing short.
in a cessna 172 aproaching full charge flaps 40 the go around can be a problem because of the enormous drag generate by the flaps. So it's A GOP to select landing flaps when you are sure you are going to make the runway with that kind of airplane.

extreme positions like "flaps 50" on some airliners has a other main reason.
such a flaps seting will keep your engine runing at very hig RPM during aproach (to compensate the enormous drag). keeping the engine at hig rpm makes the spoolup time in case of go around much more short , that's great in case of go around at very low altitude (go around in 747 can be performed up to 20 Ft without touching the runway)