Couple of unusual visitors to Duxford today

[Paul K]

Polish Army ( or possibly Airforce ). No idea what they are doing here, and nobody nearby to ask. Interesting to see them, all the same.







I also saw a Eurofighter Typhoon for the first time 'in the flesh'. Odd that it should happen in a museum. Very nice, I thought...then quickly headed for the Lancaster, Sunderland, York etc.

[DaveB]

COOL That's one of the nice things about Duxford.. you're never quite sure what you're going to see. Anyone know what type of helo's they are?? I notice the one on skids doesn't have a rear rotor

ATB

DaveB

[forthbridge]

Gimme a few pints at the bar Dave, and i'll try and spelll/pronounce the actual names, but in the meantime:

http://en.wikipedia.org/wiki/PZL_SW-4_Puszczyk

http://en.wikipedia.org/wiki/PZL_W-3_Sok%C3%B3%C5%82

I think the tailrotor's just hiding behind the tail

[DaveB]

Hi Jim..

Yes.. you're right. I looked hard and couldn't see it and couldn't imagine a blade small enough to hide but having looked again.. you can just see the lower tip. Well done that man

The other one (Sokol??) has been around for years in one shape or another but I'd not seen the top one before

ATB

DaveB

[forthbridge]

Yo Dave....

This leads onto a question..... why the tailrotor os on the opposing side to say Lynx, Seaking, Bell etc......... anyone know?

[SkippyBing]

This leads onto a question..... why the tailrotor os on the opposing side to say Lynx, Seaking, Bell etc......... anyone know?

I think it's because the main rotor rotates in the opposite direction to most Western types (apart from France because they're difficult), you have to use the opposite pedal when you apply power in say a Squirrel rather than a Lynx.
I think the idea is to have the tail rotor in clear airflow, rather than being in the turbulent flow behind the tail fin, remember the down draught from the main rotor isn't coming straight down. I'm not sure how much of a difference it makes in the grand scheme of things but having been in a situation where I couldn't turn back to the right after a lookout turn due to a lack of tail rotor authority every little helps.
Of course getting the tail rotor to spin in the right direction in the first place helps, yes Westlands I'm looking at you.

[DaveB]

Of course getting the tail rotor to spin in the right direction in the first place helps, yes Westlands I'm looking at you.

Funny ya know.. I've spent an innordinate amount of time in a certain Alouette model of late and knowing little about rotary winged aircraft, I was still confused to see the tail rotor spinning away from the aircraft though I'm not really sure why Is that good or bad Skip??

ATB

DaveB

[SkippyBing]

Dave,

The tail rotor will be more effective if the forward half is sweeping up towards the main rotor, this will accelerate it into the downwash from the main rotor. With the lift/force from an aerofoil proportional to its velocity squared using the downwash is an easy way to get extra authority from the tail rotor.
Of course the flip side to that is you're putting extra stress on the tailboom which is why I think the Lynx originally had the tail rotor go the wrong way (and still does in the Mk3). With the extra weight of the Mk8 it was obvious they needed to get more control from the tail rotor but if you ever compare one to a Mk3 you'll see there's a lot of extra plating on the transport joint of the 8 to cope with the bending moment.
Ultimately there's no right way if you're getting enough control from the tail rotor, and I think Westlands may have also been trying to reduce the noise signature with the original Lynx, however you'd need a good reason not to use the 'free' energy in the main rotor downwash.

[forthbridge]

Fascinating stuff...... so in general, it's a little bit like LHD/RHD cars (except if you compensate for power with the wrong pedal you'll be inventing a new display).... is there actually any benefit to the engines, flying the machine etc depending on what way the rotors go?

And second..... on tailrotors, the Gazelle's is an 'internal' tailrotor - does this help in any way? Does the fact it's 'housed' have any tangible control benefit?

[SkippyBing]

As far as I'm aware there's no advantage having the rotors turn one way or the other. However once you've started building helicopters with the rotors going one way you'll tend to stick with what you know as everything will be geared to that, both in terms of design skills and engineering production.

The internal tail rotor or fenestron (from the French for window) has the advantage that it's much harder to damage in a confined area landing or with someone's head. However early fenestron equipped helicopters had some control problems where a small pedal input could lead to a large rate of yaw in certain conditions due to interaction with the main rotor tip vortices, these have I believe now been overcome, do a google for Fenestron Stall. Having never flown one I'm not sure if there are any other issues although I believe the shape of the duct aids the performance of the fan.
Another limitation is that there's less flexibility over where you can place the tail rotor with a fenestron, if you look at most military helicopters the tail rotor is aligned with the main rotor disc (e.g. Lynx, Sea King, Blackhawk etc.) whereas for most civilian helicopters it isn't. This is done by the military to allow for a more level attitude in the hover as they're willing to use your taxes to cover the extra expense and they spend more time in the hover, civilian helicopters (of which the Gazelle sort of is one) don't need the extra expense as they'll spend more time in transit flight, and really only hover during take off and landing. It's down to the couple between the centre of the two rotor discs and the direction of total rotor thrust from each.

Before anyone asks, the NOTAR helicopters bleed air from a slot on the side of the tailboom which due to the Coanda (mad Italian) effect and the increased speed of the flow on one side of the boom turn it into a wing which counters the main rotor torque. There's then I believe a set of vanes or flaps in the box on the end of the tail which release the rest of the air in the tailboom one way or the other to provide yaw control.