Aircraft that pitch down with flap extension

[russfarris]

This came up on another board - every airplane I've ever flown had a pitch up or nearly zero trim change with initial flap extension (The Piper Apache/Aztec and DC-8 had a VERY pronounced pitch up!) Since there is a wide range of experience on the forum, can anyone come up with an aircraft type that pitches DOWN with initial flaps? Russ Farris

[GAHorn]

Beech Starship. (I've never flown one but that's what was reported to me.)
I believe it has to do with the fact that most conventional aircraft experience an increased AOA on the tail as the result of flap down-wash, coupled with the aft-shifting C/L as flaps extend. It's a major contributor to large lawn-darts due to tail-plane stalling in icing conditions when the pilot extends flaps on short final.

[cessna170bdriver]

Is it a high-wing/low-wing thing? I seem to remember Cherokees pitching down with flaps, but it's been many moons since I flew one, so I could have it wrong... My hypothesis is that drag above the vertical CG would cause pitch up, drag below the vertical CG would cause pitch down? (I hope to learn something here. )

Miles

[SteveF]

Miles,
That is the way I remember Cherokees also. It has been a long time but it seems to me that low end cessna's pitched up and low end low wing Pipers pitched down. I don't have a clue as to why.

SteveF

[4stripes]

The 170 will pitch down with full flaps in a sideslip...
I remember the 172 would as well.
Cheers Eric

[PilotMikeTX]

Change in pitching moment is due to the the increase of lift and center of center of pressure moving aft as the camber and wing area are changed. It's more pronounced with fowler flaps, obviously. A high wing airplane would pitch down, (because the arm between the center of lift of the wing and the horizontal stab is shortened), except that the increased airflow over the tail has a much greater effect. I would wager that a high wing airplane like the Dash-8 which also has a T-tail would probably pitch down with flaps extension. As flap position is increased, the flaps start creating more drag than lift and the and the plane will tend to start pitching down again at some point. Of course, you're usually slowing and increasing up elevator and retrimming so it's hard to notice.

I'm not sure what happens with the PA-23 and DC-8, I'll have to think about it before I start making WAGs. I have about 200 hours in Apaches and Aztecs, but I really don't remember. Never flown a DC-8. The jets that I fly incorporate a system that automatically adjusts the stab trim with flap application. I guess it's a fringe benefit of having a mach trim system that the DC-8 may not have had.

[russfarris]

Good responses, guys!

It's funny, but with all the different types I've flown over the last 36 years, I have trouble remembering exactly which way a particular airplane pitches with flap extension. I would say most do pitch up, some more than others. The PA-23s and DC-8 stand out, because those had a dramatic nose-up - the DC-8 technique was to start running the stab trim forward just before calling for flaps 10 or 12, depending on the series. It had Mach trim, but that didn't come into effect until around .65 or so. The Airbus I fly today trims itself in-flight, so I can't really tell you about that one! I do remember the Boeing 727, my personal favorite of all time had virtually no trim changes with flaps, speed brakes or gear extension - the last of the airliner "pilot's airplane", in my opinion.

I'm even current in a Comanche 180, and I can't remember what it does! This discussion came up on an RC model forum - some guy building a Douglas A3D Skywarrior was advised by another guy who is an MD-80 F/O to mix up elevator in his transmitter with flap extension because all airplanes pitch down with flaps!
I questioned that, but the plane truth is as pilots we just fly the airplane without giving things like pitch behavoir much thought, unless it was memorable, which the PA-23 and DC-8 are to me. Since this is a friendly discussion, I don't think high or low wing makes any difference. Now I have to go fly the Comanche
and see what it does...Russ Farris

[oz502man]

Russ,

I fly the Air Tractor 502 Ag Plane. It has 30' flaps and as well the ailerons droop in conjuntion with the flaps, although not to the same extent - maybe 15' . When the flaps (and flaperons) are extended the nose pitches down considerably. To counteract this, a cable runs from the flap torque tube to the elevator bellcrank through some pulleys and a long spring. The result is that when the flaps are extended the cable pulls spring pressure on the elevator, effectively retrimming the elevator to conteract the nose down pitch.

At the end of each spray run, you pull up turn around dive back in on the next run and the 502 needs flaps pretty much every time you turn and as you can imagine there can be a lot of turns flown in a day. So you can spray at 120kts, pull up into a steep turn slowing down to about 80kts(depending on the load) at the top of the turn, rolling out some flap as the speed decays, then dive back into the next run while retracting flap as speed increases and not have to touch the trim lever once throughout the turn. All in all a very effective system.

Where it can get a bit funny is when you are light and pull up into a wide lazy turn, if you roll out to much flap the system can over compensate causing the nose to pitch up, requiring forward pressure on the stick and a quick shuffle to retrim or retract the flaps a bit.

Just another perspective.

Here is a link to video made by company about 100 miles north west from where I work, similar country, similar work...

http://www.youtube.com/watch?v=65KLN5gDHzI

Cheers,

Noel

[russfarris]

Very cool video, Noel - it's a very demanding job. Doesn't seem like ag flying is what it once was in the U.S. Used to be a regular sight when I was a kid, I haven't seen a crop duster in years. I spent the summer of 1977 spraying mosquitoes from C-47/DC-3s in the Florida Keys. Now THAT was fun!

I spoke with my buddy with the Comanche 180 - he says it pitches down with the first notch of flaps. Russ Farris

[Jr.CubBuilder]

The 74 172 I spent a few years flying before I got the 170 pitched down when you put the flaps on with the power set for approach. Add some flaps, set the trim, ad flaps, dial in some more trim.

One side effect of this is that if you powered up for a go around with full flaps you needed to be ready for a lot of back pressure on the yoke, so much that you almost need both arms on it to keep the nose down and not stall.

[170C]

Neat video on the Air Tractor spraying in the Land Down Under! Must be a real rush to fly those spray planes. I have always wanted an opportunity to fly one just for fun, but that opportunity isn't likely. I'll leave the spraying to the professionals. As a kid, growing up in west TX, I watched a lot of Stearman rigs and others too. Figured spraying mesquite must have been fun. A cousin did so for a while and said occasionally he found thorns in his tires. That's a bit close for me. I guess flyin Air Tractors is much like a lot of other jobs as it gets to be old hat after a while, but one sure has to keep on one's toes to do the job and stay alive to tell about it. What a contrast between those Stearman's, Cubs, Pawnee's and flying an airconditioned Air Tractor. I have been through the factory a couple of times and crawled over a couple and wished I could get in and go flying. Its fun to fly down low (not as low as you spray pilots) over a nice grain field, put in 10-20 degrees of flaps and make like you are in an AT, but I have never had the guts, or maybe I had better sense than to fly under the power wires.