The Strake on the Seaking/Merlin tail.....what exactly does it do?

[Hueymeister]

Just trying to explain to a colleague how it works, but not being an expert I thought I'd ask here. Does it off load the Tailrotor by turning the boom into an aerofoil, or........

Thanks

HM

[exleckie]

HM,

You may find this link useful. It has a little about strakes but I will keep on looking.




Click here (http://ettc.usc.edu/dryden/tos/helicopter.html)


exleckie

[jayteeto]

The opposite!! If it is the strake I think it is, then it disrupts airflow on that side while the other side is smooth. Every little bit of tail rotor support helps!!

[exleckie]

Inclined to agree, but I don't really know how strakes on tailbooms affect flight performance. I am looking into it right now.

The article I put up is food for thought though as far as tail rotor, main rotor and engine power are concerned. Maybe a combine of venting and straking could be a winner.

exleckie

[Cyclic Hotline]

BLR website (http://www.blrvgs.com/helicopter/tailboom_strakes.htm)

[Hueymeister]

TVM........very useful

HM

[goffered again]

When hovering, especially at high AUW the not very aerodynamic tail cone acts like an aerofoil under the main rotor downwash and lift is produced on the port side causing the aircraft to yaw without pedal input. The strake simply stalls the airflow and gives Johnnie Driver all of his boot back. Cheap and cheerful wins again!

[dangermouse]

The downwash over the tail cone produces low pressure areas on both sides of the fuselage, by using a strake to disrupt the flow on one side the remaining low pressure area acts like an aerofoil and produces lift in the direction you need. It should work on any fuselage with some dgree of depth to it, therefore the best benefit is from a slab sided fusealge such as S61 and Merlin

Nice to see all the US web sites ignore the fact that WHL first did this on the SK in 1982!!

DM

[Grey Area]

As described above it disrupts the airflow over the tail boom reducing the Coanda Effect, ie kills the unwanted "lift" (perpendicular to the airflow) generated by the rotor downwash. Drag is still there of course.

Of course on the NOTAR it is the Coanda Effect that is utilised along with the thruster to oppose torque and provide yaw control. Along with the fly-by-wire fins.

GA

[John Eacott]

Normal airflow over a symmetrical tail boom will generate (via the Coanda effect) an equal high pressure flow along both sides of the boom.

By installing a strake on one side of the tail boom, the airflow is disrupted, creating a low pressure airflow on that side of the tail boom. With the undisturbed (high pressure) flow on the other side, the boom then takes on the attributes of an aerofoil, creating lift toward the side with the strake, thus offloading the anti torque demand on the tail rotor, and effectively increasing tail rotor authority. Most efficient in the hover, with the full downwash of the main rotor flowing over the tail boom.

Various after market kits available, plus the A109E Power and the AS350B2 have strakes as standard.

[Paul McKeksdown]

I seem to remember that the 'strake' was fitted to helos when they started putting them to sea. As described above the effect is to disturb the airflow over one side of the tailboom to enable it to act as an aerofoil in the rotor downwash and off load the tail.

Back in the glory days when the blessed Sea Pig only had five tail blades and a fat all up weight the cone heads discovered that tail rotor authority was getting a bit tight over the deck with red and green relative winds and devised the idea of the strake.

Was explained to me by someone that it's similar to tampering with a cricket ball and rubbing yours balls raw down one side with sand! (ooooh er!) This causes the ball, when correctly bowled, to curve through the air. (Might be B******ks but seemed a fairly good analogy to me!).

Interestingly not seen on Lynx aircraft (last time I looked) which have a fairly good amount of tail rotor authority and a semi rigid head. Might be a good cause for discussion?