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shumway76
13th Feb 2015, 23:30
I understand that different aircraft behave differently when flaps are extended / retracted. Some with flaps extended tend to pitch up, some pitch down. Some pitch up initally slightly, then pitches down more etc etc.
I believe this depends on aircraft design.

I have 2 questions:

1. Can someone explain the reason why some aircraft pitch up while some pitch down? I think it's related to C of P position on the airfoil when flaps are operated in relation of C of G of aricraft, and possibly to some extent the direction of the airflow from the wing to the tailplane due to flaps position.

2. Specifically to Zenair CH701 which has one 1 flap setting (used for landing), just to confirm with others who flew this aircraft - with aircraft in trim for straight & level:
- when flaps is extended aircraft noses down
- when flaps retracted aircraft noses up.
Anyone can confirm this?

Cheers!

Helicopterdriverguy
14th Feb 2015, 00:01
An explanation by Oktas8 in an older thread:

http://www.pprune.org/flying-instructors-examiners/222519-best-approach-land-pa28-180-cherokee.html#post2536580

Just a thought for the pitch up & down people:

(1) In a low wing aircraft, application of flap will tend to cause a pitch down.
(2) In a high wing aircraft, application of flap will tend to cause a pitch up.
This is because the increase of drag at a low point (high point) on the airframe will give a pitch down (pitch up) tendency.

(3) In an aircraft where the tailplane is in the downwash of the wing, application of flap will tend to cause pitch up. This is because downwash increases with flap application.

(For those thinking "why?" consider this: Relative airflow from the wing will be approaching the tailplane from ahead & above it. The tailplane will be operating with negative angle of incidence, and generate a downwards lift force which will increase with increased downwash.)

- T-tail aircraft such as the Arrow and Tomahawk have tailplanes well clear of downwash, so might be expected to pitch down with flap application.
- High-wing Cessnas and similar aircraft, where the tailplane is squarely in the wing's downwash, experience (3) quite strongly, so might be expected to pitch up with flap application.
- Warriors and similar have middling effects, because the stabilator is affected by the wing's downwash but is not in the strongest part of it. You might have to go and fly them to figure out whether point (1) or point (3) wins out when the pilot extends flap!

- Designers use lots of subtle tricks to minimise trim changes with flap, so all of the above is only a crude explanation... :ok:

skyhighfallguy
14th Feb 2015, 00:51
I followed the link to the discussion of landing a cherokee 180

Since it was closed, I have to point out here that the whol darn series is called a CHEROKEE with an add on for each version.

Cherokee Cruiser
Cherokee Warrior
Cherokee Archer
the 235 was called a Cherokee Pathfinder Or Cherokee Dakota

etc even cherokee arrow

and the Cherokee Six.


Call up atc and say: Piper 12345 six miles south landing with lima and they will ask you what kind of piper you are...cub, tripacer, navaho etc

or if you call up and say CHEROKEE 12345 six miles south landing with lima and they will know what to look for.

Difference in wings and horsepower are relatively minor things.

garrya100
14th Feb 2015, 00:57
Just to stretch the brain cells while you think about this one..With my Arrow, extending one stage of flap produces a nose down change as expected. With no flap, extending the gear also produces a nose down change. With one stage of flap extended and the gear out there is no pitch change.....

bookworm
14th Feb 2015, 08:27
I understand that different aircraft behave differently when flaps are extended / retracted. Some with flaps extended tend to pitch up, some pitch down. Some pitch up initally slightly, then pitches down more etc etc.

It can be helpful to distinguish between the short term transient effect and the longer term effect on the steady state.

Flaps (unless they are very strange ones!) always increase the lift coefficient for a given angle of attack (measured relative to the original chord line of the aerofoil). Thus if you want to maintain the same speed in level flight, it will be at a lower nose attitude if the flaps are extended. If the flaps increase drag-to-lift, then the aircraft will slow down, and the AoA may need to increase again, and it will also enter a descent, and I think it's rare that the pitch attitude would exceed the original pitch attitude before flap application.

Transient effects depend on many other factors.

rapidshot
15th Feb 2015, 16:40
it is the effect of change of downwash on the horizontal tail due to flaps

9 lives
16th Feb 2015, 02:14
Can someone explain the reason why some aircraft pitch up while some pitch down?

Per aircraft type, perhaps. Generically, no.

There are many configurations, and therefore differences in effect and behavior of aircraft resulting from flap application. Type of flaps, tail configuration are main factors. The double slotted flaps of a Twin Otter will allow so much pitch change that it is possible to touch the nosewheel down before the mains while landing - I've seen it done, it's not pretty. It usually results in what's called (Station 60 damage). The Cessna 310 or DC-3 on the other hand have split flaps, which do lots for drag, but little for lift, and therefore have a much lesser effect on the pitch of the aircraft.

Anticipating the effect of flap application while flying is excellent, but it will not carry over from some types to others, so just learn the nuances of each type.

I have not flown the Z 701 since 1984, and that was serial number 1. It had an unsatisfactory flaperon arrangement, which I believe was changed for the subsequent models, but I never flew one.

Johnm
16th Feb 2015, 07:08
The Socata TB 20 balloons when landing flap extended on final and requires a substantial push to maintain the glideslope

englishal
16th Feb 2015, 07:28
Yes, in fact pretty much every aeroplane I have flown balloons, and requires a push when flaps are extended.

mary meagher
16th Feb 2015, 07:57
Stepturn, touching the nosewheel down before the main wheels? over here I think we call it performing a wheelbarrow maneuver....

When it comes to pulling up a glider with a Supercub, in most cases the performance is much better when NO flaps are used. But if you are towing an antique and require a very slow airspeed, why then flaps are used on takeoff.

As for approach and landing in a cross wind, intending to touch down at a higher speed with more control over travel when rolling out after landing, half flaps is recommended. But if landing into wind, the slower the better, so full flaps, please. I did land the cub in a wind so strong that on touchdown, we enjoyed a ZERO groundspeed ! And so waited for a few heavy chaps to walk us backwards to the hanger. Enough is enough on a day like that.

Shaggy Sheep Driver
16th Feb 2015, 10:01
It is indeed possible for a ham-fisted 'pilot' to land the likes of the benign PA28 nosewheel first. Usually it just overloads the nose gear strut and if that's done enough some innocent future pilot gets a nose wheel collapse on landing.

But if it's done with significant yaw on, it can result in the mother and father of ground loops, wiping off the main gear.

9 lives
16th Feb 2015, 11:21
An aircraft may balloon with the application of flaps, as it may settle with the retraction of flaps - if the pilot is inattentive to flying the plane, and fails to manage speed and pitch. Otherwise, things should be fine. The first extension of flap will nearly always result in an increase in lift - different amounts for different aircraft type and flap configurations. So, yes, if you add lift, without compensating in another way, the plane is going to go up! Similarly retracting flaps when the aircraft configuration is not optimum - it's going to go down.

As Englishal has said, most every plane I have flown will require a nose down control input to maintain path, with flap extension. I seem to recall the C 310 and DC-3 less so, as they have split flaps, which contribute more to drag than lift.

So, as the pilot new to the type, you can allow yourself one excursion from your intended path while you understand how much nose down control should be anticipated with the application of flaps in that type - after than, you're responsible for applying your skill to anticipating the affect, and applying control to result in a smooth flight.

Piper Cherokees have delightful manual flaps, which are moderately effective through their range. But, you can apply them all at once, if you choose to. Most Cessnas have really effective flaps which are electric, so it's really the speed of the flap motor which governs the pitch change associated with flap application. However, much older Cessnas and all 180/185s have manual selection of those same flaps, so you can get all of it at once - you've got to be ready for that!

Contrary to the practices of others, I do not reduce away from use of full flaps for GA aircraft I fly)* exception once). If a Flight Manual presented a limitation to this affect, I would follow it, but I've never seen one. The crosswind capability of the aircraft was demonstrated with full flaps. With full flaps, you're going more slowly. Were something bad to happen on the surface, going more slowly is better. Slowing the aircraft on the surface with greater aerodynamic effect saves tires and brakes.

*The exception was when I was flight testing the first Z701 with a test pilot friend. It had an abomination of flaperon, the result of which was that as flap was applied, aileron travel was used up, so at a slower speed, you had less roll control available. That is the reverse of what was needed. I did have to use less than full flap for some crosswind landings in that. I commented this back to Chris Heinz. I'm confident that was resolved somehow, though I have not flown a 701 since, so I do not know.

The wheelbarrowed landings I have witnessed on Twin Otters were unintentional and bad! We don't do that! When I was trained in Twin Otters, I was limited to 20 flap, until I demonstrated that I had the skill and feel for the plane to maintain good pitch control with full flaps.

Shaggy Sheep Driver
16th Feb 2015, 12:11
GA in the Chipmunk I would not initially reduce flap until the aeroplane is climbing and airspeed is good. Touch & go however, I'd reduce to 'flaps 1' from 'flaps 2' on the roll, tail up, (having put full power on first) which involves an interesting but by no means difficult change of hands on the stick so the right hand can operate the flap lever.

The only thing I'm not happy about with this is that the there is no free hand to keep on the throttle for the second or so it takes to do this. Would add that I generally operated out of quite short grass fields.

ChickenHouse
16th Feb 2015, 12:22
As flaps are meant to reduce stall speed and allow slower approach velocity one can expect additional lift from the extension of flaps. If this lift produces nose-up or nose-down attitude depends on the relation between the lever of the additional lift to CoG. It may appear wise to construct nose-up behavior by means of support for landing, but the balance has been defined by the manufacturer and we simply have to know.

BTW: there is a reason why flaps are nowadays limited to a certain amount and that is - capability of pilots. The more pilots you need, the lower the requirements for them have to be, because not all pilots are brilliant aces. Not every pilot is able to safely go-around with a C172 on 40 degree flap configuration in final ...

9 lives
16th Feb 2015, 12:59
BTW: there is a reason why flaps are nowadays limited to a certain amount and that is - capability of pilots.

A sad truth! This is coupled by the fact that the added capability provided by lower stall speeds, generally benefiting STOL operations, is being dumbed out of training and operations. Instead of interesting turf runways really close to where you need the people and cargo to be, we are trending toward building expensive mile long paved runways a car ride away. The will to precisely take an aircraft in and out of a smaller runway has yielded to flying behind glass, just like the airline pilots do - mile long runway to mile long runway.

ChickenHouse
16th Feb 2015, 14:13
The will to precisely take an aircraft in and out of a smaller runway has yielded to flying behind glass, just like the airline pilots do - mile long runway to mile long runway.

Sad, sad, indeed and with Part.OPS this is forced further towards mile longs. It is amazing to see what one can do with a proper flown plane, but this needs also very frequent flying to short runway locations.