Today's ridiculous question regarding the Lynx.
 

The downward cant of the tailboom: what's that all about? I've searched around on and off for years now and have yet to find an explanation for the design.

Supposition? Forward flight attitude places the stab in a more neutral position - but it seems as though a fixed AOA for the stab itself would take care of that.

Someone clue me in, please.

Mmmmm ...

SIMPLE ......

So that the tail boom does NOT get the chop (so to speak) during extreme aircraft attitude changes ......

i.e Main Rotor clearance



:}

That was another theory I'd tossed around. Got a reference?

Mmmmm ....

..... Got a reference? ....

Nah .... but if you wrote to or contacted the ETPS I'm sure they would be able to give you chapter and verse ....

Cheers ...

Damn. I'm a design geek, so such things interest me.

Maybe fuselage drag (as a whole) has something to do with it as lynx has very little forward tilt of the main rotor, so to prevent the tail sticking up in the air - a downward cant.

I was told...years ago..
 

..by an old Middle Wallop QFI that it kept airflow from the TR away (as far as possible) from MR downwash... also that it eased TR inspection/maintenance..

certainly credible but just his opinion. Cheers bm.

As mentioned earlier it is probably to make sure the blades don't strike the tail. The Lynx has a semi-rigid rotor so the ammount of blade flapping is limited but the aircraft also has a very compact main gearbox, in terms of height, and a short rotor mast.

Mmmmm ...

Like the Bolkow 105 ... the Lynx has a 'Rigid' rotor ... and as said above a short mast ...

In fwd flight the T/R is clear of the rotor airflow by being above the downwash.

And apart from inflight Aero's etc .....


Rotor clearance problems can arise with touchdown autorotations (like with the blo cow) it is possible to chop the tail because of blade flex at low Nr especially if it is kicked up by an over 'positive' touchdown.


:ok:

Lynx tailboom
 

Main rotor blade clearance is the answer. The Lynx collective range goes into negative pitch, primarily to 'push' the aircraft down onto the deck during shipboard operations in rough seas. It stops it being thrown off before the groundcrew have secured it.

I flew Army mk 7s and 9s and after an engine or governer change, torque matching was done on the ground in negative pitch. The RRPM was plotted against torque for each engine to obtain the best FTG matching. Pushing 100% torque with the lever down reverses the coning angle somewhat. The introduction of the BERP blades with their paddles and greater washout even more so.

Also, the Lynx and the Bo105 both have semi-rigid heads. They both have rotation around the pitch change bearings and as such have tie-bars fitted. the only 'rigid' head I am aware of is the EC135. It has the blades bolted directly onto the rotor mast. No hinges and no bearings! The best design yet.:D

Mmmmmm ..

....Also, the Lynx and the Bo105 both have semi-rigid heads. ...


Blind Bob ... could be semantics but I disagree and so do others ...



"Rigid Rotor ... This system allows the blade freedom to feather only .... etc"

(Principles of Helicopter Flight 2nd Ed. W.J Wagtendonk Chap.6 P.45)

And the Bolkow105 and Bk117 certainly have Rigid Heads with only feathering axis freedom .... NO Lead or Lag hinging.


;)

The 135 is not rigid, it is just (nearly) bearingless.

Why is 135 head not rigid? It is bearingless yes, but rigid, no?

Aircraft in Detail - Helicopter Rotorhead Image Gallery Index

The best site I have found for rotorhead pictures.
EC135 is described as hingless bearingless , did not know that Comanche had Bo108 rotor head .

Semi-rigid heads
 

Further to my last........................

Agusta Westland Lynx manual states it has a semi-rigid main rotor system.

Eurocopter Bo105 and Bk117 training manuals both state semi-rigid rotor system with rigid hub.

The EC135 doesn't have a rotor head as such, it has a rotor hub shaft, to which the blades are attached. All flapping, dragging and pitch change movement is done in the blade spar. It is described as hingeless and bearingless. The only bearings on the mast assembly are on the rotating pitch control rods to the blade cuffs and in the swashplate.

NB. The EC135 was a Bo108 until the French got involved and replaced the conventional 2 blade tail rotor with their 'Fenestron'.

This goes back much further than the Lynx. Look at pictures of the original Sikorsky S55, and you will see it has a straight tail boom. Now look at the Westland built version, the Whirlwind, and you will see it has a bent down tail boom. We were told it was for main rotor blade clearance.

DL are you referring to LH blade on # 14 ? , where those last 3 at full RPM ( not sure if any one in that pic is still reading PPrune )

Semantics...
 

I think the original definition of "rigidity" referred to the presence or absence of hinges.

Fully articulated = flap and lead/lag hinges
Semi-rigid (semi-articluated) = flap hinge(s) only (UH-1, R-22)
Rigid = no hinges

The use of these terms became somewhat ambiguous with the advent of flexible blade elements and/or elastomeric bearings.

A rigid rotor may be hingeless, but still have bearings, e.g. Bo105 and Lynx for blade feathering.
A hingeless and bearingless rotor, e.g. EC135, may not be "rigid" at all, as the blade root is is highly flexible in bending (for flapping) and torsion (for feathering)
To add even more confusion, there are quite a lot of fully articluated rotors that have no hinges. Here, the function of hinges (elimination bending moments between the blade and the rotor head) is taken over by elastomeric bearings.

BTW, manufacturer's manuals are not fully reliable, neither is my website :)

Burkhard,
www.b-domke.de/AviationImages/Rotorhead.html