Tail Boom Strake on the Sea King/Merlin

[Hueymeister]

What exactly does it do? Does it turn the boom into a lift producing aerofoil, thus taking the load off the tail rotor ? Trying to explain it to a colleague.

Mauch appreciated.

HM

[SiClick]

As I understand it it brakes the aerofoil effect of the downwash on the strake's side of the tail boom, thus ensuring that any remaining aerodynamic effects can only assist the tail rotor, not work against it
but aerodynamics always was about telling a plausible story, not about understanding what actually happens!

[NickLappos]

Fundamentally, the downwash on the tailcone behaves like NOTAR, but unfortunately in the oposite direction, so without the strake, some helos spend pedal margin fighting the tail cone side force. (More detail: picture the tailcone as a wing flying in the downwash of the main rotor. Picture the "lift" that wing makes as trying to rotate the aircraft, but in a pro-torque direction. This means the anti-torque must be greater).

The spoiler trips the smooth flow over the tail cone and thus spoils the lift.

Not all helos see the benefit (unless you read the web sites of the guys who try to sell the spoiler to you!) For helos with good, powerful tail rotors, you often can't see any benefit from the spoiler. For helos with marginal designs (older criteria called for much less powerful tail rotors) the spoiler can make the difference between adequate control and the dreaded "LTE"

[Graviman]

"picture the tailcone as a wing flying in the downwash of the main rotor."

Do you know of any attempts (other than NOTAR) to physically put an aerofoil on the tail boom to counteract rotor torque? Tail rotor loss would be reduced, although design attention would be required for no adverse autorotation effects. It is something i have often wondered...

Mart

[John Eacott]

My understanding links the strakes to Coanda Effect, whereby a symmetrical tail boom would have equal airflow either side of the boom.

The strake breaks up the airflow (downwash from the main rotor) on one side, creating a low pressure airflow past the strake. The 'normal' (higher pressure) airflow on the unobstructed side of the tailboom thus turns the tailboom into a wing, and creates lift toward the strake side of the boom. This offloads the anti torque demand on the tail rotor, giving greater tail rotor authority in the hover.

The 109E Power has a strake as a standard fit these days, and there are kits available for quite a number of helicopters. Apparently the B205 unit makes a large difference, according to some.

[wishtobflying]

I wonder if they used strakes on the helicopter used for the Everest landing? Would strakes change it enough to make it technically NOT a stock issue helicopter?

[HeliMark]

WTB, the B2 has the strake, not the B3.

[spinwing]

Gosh ... I wish the B412s I'm flying at the moment were fitted with "strakes" .... it would make putting up with pitching and yawing on trubulent decks much easier to live with !!!!!

;) ;) ;)

[Heli-Ice]

Strake or no strake...

Why does it have to be this complicated to fly a helicopetr...or was it a hecilopter ? or a helicopter?

How much does a blond, blue and clear eyed helicopeter aviator have to know much tech stuff like this to be able to... do the simplest jobs?

Anyway.. which one of you helicopter pilots hasn't done an impressive pinnacle landing?

[S92mech]

Spinwing, strake kits are made for the 205/212/412, we have a kit installed on one of our 412s. I'll try to find a link to the maker of the kit and post it for you.

[212man]

Thes guys perhaps:

http://www.blrvgs.com/helicopter/aircraftpages/bellaircraftpage.htm

[spinwing]

S92 mech ....

Thanks ... I know about the kits from "Boundary Layer Research" ...

What would be really useful would be some kinda device (preferably non-explosive) which would convince my employers that the kit would be a great mod to increase the productivity/longevity of their aircraft and that they should retrofit same to their 12 BH412s ?????????



:rolleyes: :rolleyes: :rolleyes:

[Wunper]

Graviman

Raoul Hafner did that years ago!! the yaw pedal control the pitch on these tail feathers

http://www.helicoptermuseum.co.uk/aircraft/r2.jpg


W

[NickLappos]

Go back to day 1 in the practical helicopter world, here is the VS-300 in the months before first flight, note the airfoil tail cone:

http://www.qsl.net/w1ors/images/0vs300.jpg

[Dave Anderson]

Our Tail Rotor Enhancement Kit for the UH-1H series uses a single BLR strake on the tail boom. There is definitely an increase in authority over not having one, but the main difference is a reduction in pilot work load in crosswind hover. By spoiling the airflow over the left side of the tail boom the pedal excursions are reduced. In a cross wind from the left front quarter there is usually a pedal reversal required to keep the aircraft straight at about 10 knots of wind speed. The strake reduces that effect. I have also tested strakes on Bell 206B and L and found the effects to be pretty small, likely because the cross section shape of the tail boom is small and circular in shape.
The EH101 has a big single strake. NASA did a strake test on a Bell 204B and I think the test report is available, if your interested.

[Graviman]

"...VS-300 in the months before first flight, note the airfoil tail cone"

Yes, i thought i remembered the aerofoil on this. The final single rotor design that Les Morris flew so convincingly (after the lead-lag dampers) appeared to lose this feature. I did wonder if the "Vought-Sikorsky" bill-board was angled to make up. Any thoughts as to why a true aerofoil was dropped, in favour (later) of less efficient strakes?

Mart

[NickLappos]

Graviman,
There is no "free Lunch" and I would guess the airfoil never earned its keep. This is speculation (informed, perhaps) that the airfoil actually cost more vertical drag then it gained in anti-torque.

All items on a flying machine are resolved into a universal coinage that allows the item to be comparitavely valued. That coinage is usually weight.

The airfoil itself is probably extremely light, but it creates additional download with its drag (the sideward lift it creates has a drag at 90 degrees to the lift - thus downward). Most lifting bodies have a lift-to-drag ratio of between 4 to 1 to 12 to 1. Since that airfoil is low aspect and probably operated at a high angle of attack to squeeze as much anti-torque as possible, let's guess it is in the middle somewhere, 6 to 1. If the surface has appreciable anti-torque it probably reduces payload quite a bit.

Some examples: if the helo was 10,000 lbs and uses 1000 HP to hover, its tail rotor must produce about 550 lbs. If we use the airfoil to produce 30% of that, the airfoil must produce about 200 lbs of thrust (it is inside the tail rotor, so the thrust it creates acts on a shorter lever arm, and must be proportionately larger). If the airfoil had a L/D of 6 it will reduce payload by 33 lbs, at a disadvantageous aft CG.

BTW, Sikorsky never used the strake in any model, those fits were done aftermarket by others. The flight testing we did showed no significant gain, IIRC.

[Graviman]

Nick,

Interesting analysis - thanks.

Would it be fair for me to comment that a more holistic design would be required with this approach? With your well informed example, the tail rotor power goes down from 55HP (5.5% of 1000HP) to 38.5HP. This increases main rotor available power from 945HP to 961.5HP, allowing a total lift of 10,175 lbs. Subtract the 33 lbs, you neatly worked out, still gives a net gain of 142 lbs.

I realise that exact figures require several iterations, since increased lift means more torque. The tail aerofoil is also operating in the whirl component of downwash, so that the drag vector will be rotated to reduce download further. The practical upshot is that either helo weight can go up or power required goes down. I suspect that the rearward shifted downforce, along with the adverse torque reaction in autorotation, was the real reason Igor Sikorsky dropped it (bearing in mind the control problems overcome).

Could a conclusion be that Sikorsky should reconsider the merit of tail boom aerofoil torque reaction? Clearly the camber (or AOA depending on design) would need to be linked in with the collective.

Mart

[Edit: 'cos hot choc and typing don't mix...]

[NickLappos]

Mart,
Your analysis is correct, and is the bread and butter of NOTAR, but (you knew there is a but there!):

The airfoil only works when the downwash is on the tail cone. In as little as 6 knots of wind/aircraft speed, the wash moves considerably, and the airfoil on the cone is does not produce the anti-torque needed.

The first NOTAR actually lost yaw control in 6 knots rearward flight, so they invented the tip nozzle to fill in.

In the example we have worked, you must leave the tail rotor power where it is for some conditions that are in the hover. That is why NOTAR lifts about 2 passengers less than its tail rotor equipped clones.

[Graviman]

Nick,

"The airfoil only works when the downwash is on the tail cone."

Good point, well made. So the design needs to include the vertical tail surface to work well, which basically leads back to angled vertical surfaces and tail boom strakes for hover - if they work...

I can see myself slipping over to the "dark side" of symmetrical helicopters.... :D

Mart

[NickLappos]

graviman,

No, the point is that you need to account for the full power in the tail rotor, because if you count on the airfoil, you will lose control or descend and crash.

[Graviman]

Nick,

"No, the point is that you need to account for the full power in the tail rotor, because if you count on the airfoil, you will lose control or descend and crash."

Yes, i understood what you were pointing out in your post. What i am curious about is whether there is any design of tail boom aerofoil that would make the most of the available downwash. I accept that you still need a tail rotor, but just want to reduce it's power requirement.

One suggestion would be an aerofoil which was a blended boom and vertical stabiliser, going above and behind the rotor. Forward velocity would always result in downwash going over some portion of the aerofoil. The front portion would only really be there to stop end leakage, since at high speed the downwash from the front of the rotor would impinge on this.

Rearward flight, autorotation, and any heli aerobatics might well present difficulties. Aerodynamic studies may have already rejected this approach, but it just seems logical to me to reduce power for most flight conditions.

Mart

[NickLappos]

The most developed system to try to harness the downwash is the NOTAR, Mart.

There is no free lunch, the downwash is too flakey to depend on, unless you want to lose your helicopter every now and then so you can "gain" a little efficiency.
The papers written by the NOTAR folks show all the tribulations they went tjrough to make it work, and then they ended up with a full variable pitch fan in the cone, a full tip thruster to fill in when the downwash went missing, and the fins and rudders for forward flight. Sort of reaching a bit for that downwash "efficiency".

[Graviman]

"There is no free lunch, the downwash is too flakey to depend on, unless you want to lose your helicopter every now and then so you can "gain" a little efficiency."

Point well noted. This probably serves to justify the electric control systems you support so strongly. An augmented tail rotor would respond far more rapidly to a complete loss of anti-torque than any pilot, especially if flight regime caused downwash to just miss the tail boom (but floating).

I have been trying to think of a gyro sytem which could be easilly fitted to a small (non-active augmentation) helicopter. I won't present the headaches here, but suffice to say that electric servo rudder trim looks preferable (like some of the e-steering trim systems on the auto market now).

Mart

[NickLappos]

If your point is to cure failing tail rotor systems, perhaps you should not try to invent a new control mythology, depending on downwash.

Its good that you've found your goal, 23 posts into the thread.