Moe142

My Feedback: (7)

Ok. So I get why flaps are used. But.... why on some jets does it push the jet down and on other one's it pushes the jet upwards? When the flap switch is actvated, flaps go down, making the plane baloon upwards. Why would it push the plane up? When down elevator is used it pushes the plane down. As true when I put in up elevator the plane goes up... Wish I understood air flow or aero dynamics more.. Thanks guys, Mario (Moe)

Terry Holston

My Feedback: (1)

Depends on the flap location and whether the wing is swept or not. When flaps are applied, most of the time, they will add lift to the wing. The added lift causes the aircraft to rise unless down elevator is added at the same time. If the wing has a sweep back to it, then flaps may cause the nose to drop as they may be behind the aerodyamatic center enough to act like the elevator, then you will need to use up elevator to counter act the flaps........................Clear as mud????



edited for typos

Harley Condra

My Feedback: (4)

Don't forget....the by product of lift is drag.........

Here's a link to a simple explanation of aero theory:
www.travel/howstuffworks/airplane.13/htm

If you can't find it, just Google "How things work" and you will find it right away.

Harley Condra
BVM REP
JetCat REP

squirt!

I think you are right to a point...different degrees of flaps create different amounts of lift AND drag. For example, the first 20 degrees of flaps (depending on the airframe) may create more lift than drag and cause a rise. However the last 20 degrees create more drag than lift causing the nose to lower.

GrayUK

This is an interesting one that has been the point of discussion many time on my field.[sm=confused.gif]
On straight wing the simple theory should apply the flap is always behind the MAC and the CG (through which all forces operate) therefore when it is lowered, it acts like an elevator, so down elevator puts the nose down.

The increased lift will make the plane balloon (first 20 deg.) but the lift should not really affect the pitch. The plane should just 'float' more.

On the L39 Which fits all of the 'standard' criteria, when i drop take off flaps the plane becomes more 'floaty' as predicted but the nose goes UP!
If I drop full flap, which not only should produce more nose down but also applies significant drag, (more nose down in theory) the nose comes up even more and I have to push down to stay level.

Over the years I have flown many planes (prop and Jet) high and low wing, with flaps, some follow the rules other work backwards (as the L39).

I to would be interested in a definitive answer on this.
Could it be a down draft or turbulent affect on the stabs?

Paul

ravill

My Feedback: (11)

This very question is always on my mind on maidens. I go up high, slow down, cross my fingers, and deploy flaps. I really enjoy the airplanes that baloon when I deploy the flaps versus the airplanes that drop the nose with deployed flaps. I don't like it when airplanes point to the ground. []

Raf

Moe142

My Feedback: (7)

Thanks guys! I have to agree with ravill. I like when I deploy the flaps the plane points up and NOT down..

DCM

My Feedback: (15)

Then,this comes along........................[:@]


http://www.youtube.com/watch?v=Yux0FfdhI9g


David Hudson

www.dcmodelshop.com

HarryC

My Feedback: (1)

The effect of flap on the wing's various forces is always to make the wing pitch nose down, taking the rest of the aircraft with it. Any wing with camber has a nose down pitching force related to the amount of camber, lowering flap increases the camber and therefore increases the nose down force.

Lowering flap will increase lift in the early stages of flap, in its extreme down stages it barely changes the lift force. The increase of lift will cause the plane to rise, which will cause the tail to pitch the plane nose up, providing a pitching force opposing the effect of camber. This force is transient and is gone when straight flight is resumed.

The tailplane often lies in the flow affected by the downwash of the wing, and the affected airflow can extend to quite some height above the wing. As the flaps are lowered the angle of the downwash increases, so if the tailplane is affected by the downwash its angle of attack is made more negative and it pitches the plane upwards.

So you have a nose down effect from increasing the camber, and perhaps a nose up effect if the tail is in the downwash. Which of these is the more powerful determines whether the plane has a net nose up or nose down effect. High wing low tails such as a Cessna 150 have a pronounced nose up as the tail is firmly in the downwash. Low wing high tails tend to have a nose down effect as the tail has little or no flow in the downwash.

Most model fliers tend to lower the flap at much too high a speed, resulting in a ballooning climb due to the huge increase in lift, often exacerbated by lowering flaps instantly at full servo speed. Once the excess speed has bled off the true character such as a nose down pitch becomes evident, leading to the model porpoising as it balloons upwards then pitches nose down and dives, requiring up trim to stop it. If this happens, learn to slow down before lowering the flap, and use servo slowing to soften the effect as well.

The safe plane is the one that pitches slightly nose down when flap is lowered as this helps to maintain speed against the increase in drag. A plane that pitches up when flap is applied is unsafe as it faces two forms of loss of speed - height gain and more drag.

Harry

HarryC

My Feedback: (1)

Paul, the ballooning should cause the plane to pitch up. The purpose of stability in a plane is to keep realigning the plane to point into the headwind. If the plane balloons upwards due to an increase in lift from flap, the tailplane sees a more negative angle of attack and pushes the tail down more so that it points the plane up into the new headwind. This will be opposed by the nose down pitch of the flaps but the net effect will be whichever force is stronger - usually the tail due to its greater lever arm.
The goal should be not to balloon at all, by getting the airspeed down, lowering flap slowly, and reacting with pushing or pulling the stick at a rate to counter any tendency to change flightpath.

Harry

BeanerECMO

My Feedback: (1)

If you've ever seen the F-8 off the cat (or even at the field), you will notice a huge settling when the cantilevered wing is lowered AND max burner is still required until ~500'AGL. It is built for speed, and dislikes slow speed and balloons when the cantilevered wing is raised, but still has a high approach speed.

Rbeav533

The primary reason there is a nose down pitching force is due to the net lift vector of the wing being behind the CG. This results in the nose down rotation. The horizontal stab's lift vector is a downward force and negates this rotation during straight and level flight. When flaps are deployed there will usually (depends on flap type) be an increase in lift due to the camber change, however the lift increase doesn't always result in a more pronounced nose down rotation because of how the tailplane interacts with the downwash from the wing. As you've mentioned, the wing downwash can be substantially higher than the nose down rotation caused by the increase in the wings lift. The net result is the nose up pitch as you've mentioned in your C-150 example.

This correlates to the concept of how the wing's lift vector causes a nose down rotation. This results in the need to increase the downforce the tail provides, or nose up trim.


Rich

HarryC

My Feedback: (1)

The raising of the wing has zero effect on the approach or stall speed which is why it still has a high approach speed. The wing produces the same lift at the same AoA, and has the same stall angle regardless of which way the fuselage is pointing. The purpose of raising the wing is not to increase its lift any more than it could already achieve, because it won't, but simply to allow it to be taken to high AoA without scraping the back of the fuz on the ground. The wing raising is there for the sake of the fuz, because they didn't design in enough ground clearance. Much longer undercarriage legs would have solved the ground clearance problem but the designers must have decided that wing raising was the preferable option to much longer legs.
H.

BeanerECMO

My Feedback: (1)

I guess they wasted all that money & time to have the cantilevered wing since it has no effect. As a matter of fact, the stall speed does increase significantly and the approach speed does increase 50 -75 knots even if the leading edge slats (flaps) and trailing edge flaps can be deployed, which is why, in such a configuration, the barrier is erected on board the carrier to assist in bringing the Crusader to a halt besides the normal arresting gear (which, if set for that speed of the F-8, would pull the hook out of the plane) as was done on the Coral Sea in '73 with Mark Weatherup at the controls (I was flying Whales during that time. Whale = EKA-3B.

HarryC

My Feedback: (1)

That is not what I said, go back and read it properly. Simply raising the wing can't magically increase its stalling angle or the lift coefficient at any particular AoA. It can not generate one ounce more of lift at any AoA just because you alter its rigging angle compared to the fuz. If that wing stalls in level flight at say 150kts and say 15 degrees AoA, then it will stall at 150kts and 15degrees AoA whether it is raised or not. The only effect it has is to alter the angle of the fuz relative to the ground at any given AoA and the reason it was done is because the undercarriage does not allow the whole plane to be taken to a high enough AoA when its wheels are on the deck. The back of the fuz scrapes the ground. Much longer legs would have solved the problem but every alteration creates its own problems and the designers clearly felt that the problems of raising the wing were less than the problems of longer legs. If the wing raising was not fitted, or failed in flight, the approach speed would be much higher simply because there is a limit to how much the pilot can raise the nose due to scraping the aft fuz and thus would be flying the wing at a relatively low AoA instead of a high AoA.

H

DCM

My Feedback: (15)

The wing was raised to keep the aoa of the fuse low so the pilot had better vision over the nose for carrier landings.
The fuse being at a lower aoa shorter gears could be used saving weight.
Harry I agree too much aoa on landing would get you a tail strike.


David Hudson

www.dcmodelshop.com