Pitch hesitation on rotation

[flyboy44]

Just wondering why there is a hesitation at about 10' of pitch on rotation? any ideas??

[Fatter Bastard]

Tailplane flies thru the wing wash, stop me if i'm getting too technical... :O

[Notso Fantastic]

Mention of a type might be useful, but I will assume you mean Boeing types. Have you thought what the tailplane is doing when you rotate? There are compressibility effects as you sweep the tail near to the ground, and also you are blanking the tailplane behind the flapped wing. An extra pull is needed. Not something I would think DC9/BAC1-11/Tridents suffered from. Mentioning that amazing CAT3b aeroplane, why doesn't someone tell us how marvellous it was?

[flyboy44]

RRR yes Boeing! thanks, have heard about ground effect? what about shifting from the main gear to the centre of pressure? any thoughts??

[Notso Fantastic]

This picture shows how close you sweep the tailplane during rotation:
http://www.airliners.net/open.file/1176974/M/

Centre of pressure theory is interesting. You may be right. DP Davies in 'Handling the Big Jets' doesn't appear to mention it.

[forget]

:confused: :confused: Is it an illusion, or is that rotation just a little over-cooked?

[Notso Fantastic]

Not overcooked at all. People don't realise it, but it's not unusual for the aft fuselage of a 747 to get to about 18" from the runway surface on a heavyweight take-off. That's why to date the 747 has never been 'stetched'- it would take a complete new wing. Being taken towards end on with telephoto, there is a lot of foreshortening evident in the photograph that gives the impression of excessive steepness.

[ZeBedie]

Same hesitation happens at 10deg with the minibus as well. I always thought it was to do with the main wheels leaving the ground and rotation ceasing to be around the wheel axles and becoming around CofG instead.

Anyone able to offer proof, either way?

[BizJetJock]

Weelll,
Unless you're flying a taildragger jet, then the change as the pivot moves from main gear to CofG at liftoff actually tends to cause the pitch rate to increase! This is due to the tail arm increasing. This (increased pitch rate) effect is quite noticeable on light straight wing aircraft, so I suspect that the reduced rate is a ground effect result. Since it also noticeable on the T-tail types I fly, it may be due to the sweep causing the wingtips to get closer to the ground during rotation rather than just plain speed.
Interesting though:hmm:

[Kit d'Rection KG]

Ground effect, wing wash, down wash, relative development of CofG and CofL as the wing begins to lift, and (most critically) jet efflux.

:ok:

[FlapsOne]

Fatter B

You lost me after tailplane!

[XPMorten]

This is probably not a result Ground Effect.
Chord dominated GE starts at about 1 chord altitude, but
will not be noticable until far less. On most aircraft, the
horizontal stabilizer will never get into GE.
My theory is downwash and wake from the wing and fuselage.
Also, wouldn't the rotation in free air be around the center of lift,
not only CG?
If tailmounted engines on rotate, it could be the blast hits
the ground since the engine cant also is 2-3 deg negative and
give a slight reaction force.

Cheers,

M

[misd-agin]

Very noticeable on 727's.

[gimmesumvalium]

Not sure of the aerodynamics.

But, why don't we train to rotate (if the ops manual specifies it) smoothly thru the hesitation to the initial pitch attitude?

[error_401]

Same for the streched ATR72.

Rotate to about 8deg wait for unloading the main gear then follow FD bars to higher pitches.

If rotating through 10 deg a little quick - tailstrike.

[A Comfy Chair]

Gimmesumvalium,

I know at least one operator does!

[PantLoad]

With regard to the 727...and, please forgive me, since it's been maybe 23 years or so ago since my last flight on the 'three-holer'...but, as memory serves me, the 727-200 dragged the skid at about 10.5 degrees (struts compressed), and the 727-100 did so at about 11.25 degrees.

So, the two techniques used by many were either:
1. Rotate slowly so as to time the strut extension and subsequent lift off when going through about 10 degrees in the rotation, or
2. Rotate at the normal rate (i.e. about 3 degrees per second) to about nine or so degrees of pitch, hold it there, until the plane was ready to fly itself off the ground, then continue rotation.

It normally wasn't a problem, but on takeoffs where we did a Flaps 5 takeoff, you had to be careful. (e.g. Denver on a hot July day...heavy weight...long runway available) With the Flaps 5 takeoff, you definitely had to rotate to about nine or so degrees and wait until she was ready to fly off the runway.

I know this response doesn't directly address the question in the initial thread. Just wanted to add my opinion...for what it's worth.


PantLoad

[ZeBedie]

So change from rotating around the axles to rotating around the CP/CofG is obviously ruled out. Jet blast impinging on the R/W? Well if that was a cause, wouldn't it be much more noticeable on a/c with tail mounted engines? Tailplane flying into ground effect: Sounds good to me. But some people have said types with T-tails also have this effect (727). Tailplane getting into the wing-wash? Same as above, what about T-tail types? Maybe we should ask the test pilots forum?