Crazy question

[baobab72]

Hi guys
Another one of those questions which i am having a hard time to visualize:
WHY WHEN YOU LOWER THE COLLECTIVE YOU HAVE TO APPLY AFT CYCLIC IN ORDER TO MAINTAIN THE ATTITUDE SPEED? AND viceversa?

Thanks

Baobab72

[KG86]

It depends upon the individual helicopter but, in general, this is caused by the horizontal stabiliser at the tail.

As you lower the collective, a rate of descent builds up. This rate of descent flow impinges on the underside of the stabiliser, pushing the tail up and the nose down. Hence aft cyclic is required to stay pitch level.

Simples!

[baobab72]

Thanks very much for your explanation.
So if i get it right the nose down tendency is the result of the relative flow induced by the rate of descent and acting upon the tail boom and the horizontal stabilizer, pushing the tail up and thus causing the heli to pivot about its center of gravity, assuming a nose down attitude.
Am i correct? Cos i don t recall any significant horizontal stabilizer in the r22.

Thanks

Baobab

[FullTravelFree]

Has to do with flapback of the MR, explained here:

http://www.pprune.org/rotorheads/224249-why-does-nose-pitch-forward-when-you-lower-collective.html

[Thomas coupling]

OK: here goes:

Lower the collective and the amount of pitch deselected in the rotor head causes a reduction in lift due to that reduction in AoA.
The lift reduction is dissimilar (unequal) around the rotor plane due to the required AoA's on the retreating and advancing side.
More lift is lost on one side compared to the other side due to this reduction in AoA - as a proportion of lift generated in those areas.
Due to phase lag the net result in dissimilar lift in the rotor head rotates "x" degrees and causes the helo to pitch up or down dependent on the direction of rotation of the rotor head.
This phenomena is not divergent and the overall lift almost instantly equals out around the disc and it returns to a steady state.

The mention of stabilisers is a secondary effect on those helos that have them fitted.

See posts #9 and #24 in the previous posters link.

[Dennis Kenyon]

TC ... couldn't agree more ... but ... I know you expected a BUT ... It doesn't explain why - when conducting a constant attitude autorotation to the ground, AFT cyclic is required as the collective is raised to maintain a level attitude and cushion the landing. Dennis K.

[Thomas coupling]

Hi DK: hope this finds you well.

As I said in the previous explan. It depends on the a/c type as to whether the helo pitches up or down when lowering the lever. Thus the opposite happens when the lever is raised.
In your case, at the bottom of the auto.

Observation:
Do you mean a constant attitude auto with fwd speed, or a zero spd auto.
Similarly, do you mean auto to the hover/ground or EOL to the ground? :uhoh:

[Cheeto9]

What does the acronym AFT mean?

[AnFI]

causes the helo to pitch up or down dependent on the direction of rotation of the rotor head.
Really? Why's that then?

[tottigol]

AFT stands for backwards, in helicopterese.

The opposite direction is FWD, as in forward for short.

HTH.

[Thomas coupling]

ANFI: Quite right. Apologies - it matters not what direction the rotor turns in. What was I thinking of?? It will always pitch fwd.:ugh:

[Dennis Kenyon]

Hi Thomas C ... thanks for the good wishes and yes, DK is still well and breathing! And for some obscure reason I keep passing the CAA's 'stress ECG' thrown at me annually. Last time the LIQ did get me into a sweat at the 15 degree slope stage. Being a cheeking S O D I put my hand on her shoulder and said something like ... "its been a long time since a pretty lady like you made me sweat that much!" I thought the Doc was going to chuck me out!

Back to helicopter attitude related to collective. On the right occasions I demonstrate a 'constant attitude' ... 'zero speed' autorotation to the ground, usually starting directly above the target depending on W/V. Donkeys ago, the wonderful Ken Reid of the CAA, showed me a good timing technique. I've never forgotten his advice regarding the use of collective in the last twenty feet or so. "NOT now ... he used to say ... NOW" a technique that worked well. Another BUT, but as collective is raised, a trifle aft cyclic is required to prevent the airframe moving forward a few feet past the target. (all with due regard to the T/R strike possibilities!

Thanks for the input. Dennis K.

[Ascend Charlie]

Dennis, we reckoned the Broomstick Technique would work best for zero-speed autos.

Cut a hole through the floor, under the collective, insert a broomstick pointing down. When the aircraft gets so close to the ground that it hits the broomstick, it automatically pushes the collective up and makes a smooth touchdown. :eek:

[Thomas coupling]

Dennis, I have a theory - you only fly so you can get a full blown medical twice a year. You're a hypochondriac really :uhoh:
Surely the day must dawn when you'll think - perhaps I should stay firmly planted on terra firma today - no? God bless you. You are the patron saint of helicopter pilots.
I used to adore constant attitude EOL's...especially at night.
As AC mentioned, that feeling of floating down vertically with limited external references, digging around for the right visual acuity with your landing light and playing chicken with the student with the hoiking in of collective seconds before oblivion and then a gentle tilt of fwd cyclc and a smooth low speed (5kt) running landing to polish the act off as the Nr virtually sagged around you. :ok::ok:

Ooooh brings back spine tingling memories.

Have you still got the E Type?

[AnFI]

cheers TC.

[Dennis Kenyon]

Ah .... for Charlie and Thomas ... so good to read your sweet words. Perhaps the reason I keep flying. And I suppose the answer is 'yes' to the hypo accusation! But now in my 62nd year of aviation, I thought I'd heard most of the stories, then you come along with your 'broomstick' technique ... how I can I beat that?

Anyway, lads, I'm sure a wee bit of happy banter will help the day along.

Take care to all out there. Dennis K.

[Dennis Kenyon]

Sorry to say TC ... I turned the lovely Jag into a bit of my country retirement home on the Welsh Wales border. I think she went to a good home after 45 years with me.

Dennis K.

[heli1]

No E Type? What next! Have you stopped display flying too?