gurney flap

[oldbeefer]

OK, so I'm turning into an anorak but, why do our AS350BBs have a gurney flap on one side only of the horizontal stabiliser. I'm happy about the workings of the flap itself (thanx to Prouty), but what's it for and why only one?

['India-Mike]

Could it be because of lateral asymmetry in the wake, resulting in one side of the tailplane flying at a different angle of attack from the other?

I think that the A109 has the starboard and port horizontal tailplanes mounted at different incidence angles on the fuselage for this reason as well.

I stand to be corrected or better-informed....

[Nick Lappos]

A gurney flap is a simple tab applied to the trailing edge that stands 90 degrees to the airfoil, usually only a short height above the trailing edge, perhaps 1 to 3 inches (perhaps 5% of the airfoil chord). It was developed by the race car driver. It adds drag, but effectively modifies the air flow to behave quite differently. A simple gurney is easy to install, but can add 5% to the drag of the aircraft!

If a full gurney is installed (basically a T across the trailing edge) the air flow is disturbed so that the airfoil "looks" longer to the airstream, so the total area of the airfoil is greater. Any force generated by the airfoil becomes greater for the same angle of attack.

If the gurney is on only one side, then the flow behaves as though a flap or aileron was deployed along that side. This changes the zero lift angle, so it basically retrims the airfoil without changing the angle of the whole device. In this case of the horizontal tail, it is used to fix an incorrect angle with a quick and dirty device. THis could retrim the nose slightly down in cruise flight (if the flap is on the bottom).

[BlenderPilot]

The Bell 230 has a pretty large gurney flap from top to bottom of the vertical fin, my manual says that its for stability in the yaw axis, it must help since supposedly the 230 is the most stable helicopter so far, it was this that allowed it to be the first Single Pilot IFR Machine (I read this somewhere).

[Flight Safety]

The basic idea behind a gurney flap is to increase the lift of an airfoil at higher angles of attack. When the flap is on one side only (most applications) it is located on the high pressure side of the airfoil. As Nick said, it's positioned at the trailing edge, and stands up 90 degrees to the airfoil surface.

The flap creates a small vacuum behind it, and since the flap in on the high pressure side of the wing, it forms a dam to the airflow on that side, so that the only way to fill the vacuum is from the low pressure side of the wing. The benefit to high angles of attack, is that the vacuum pulls the airflow over the low pressure side of the wing back down unto the wing surface at higher angles of attack. This helps to control the boundary layer separation that occurres on the low pressure side at high angles of attack, and the loss of lift associated with it. The price for this however is more drag.

Up to a point, taller gurney flaps further enhance the high angle of attack performance of a wing. but always at increasing drag penalties. The size of the flap is usually referred to as a percentage of wing cord, the same way that Nick referred it.

[CTD]

Flight Safety, the gurney flap may also be used to reduce the airfoil efficiency alone.

On the 407, for instance, the flaps on the horizontal stab reduce the lift (downward, because the airfoil is 'upside down') above 130 kts, and allow the tail to migrate up, reducing mast bending.

The vertical fin also has one, which reduces the kick in the event of an engine failure above 130 kts, permitting a smoother entry into autorotation.

Why does EC uses them on the 350? If it's only on one side, it's likely to control a rolling moment at high speed. On your a/c, which side is it on (port or starboard), and is it on the top or bottom?

[oldbeefer]

CTD, the strip is on the left-hand side at the top which, it seems to me, will generate a rolling moment to the left as speed increases. The cambered fin (above tail boom) generates thrust to the left as speed increases (to offload the tail rotor), so this also generates a rolling moment to the left! Could there be something in the rigging of the main rotor that causes a roll to the right which these other moments offset?

[CTD]

Yeah, makes sense, but that's a lot of roll. There must be an EC guy on here somewhere?

[Flight Safety]

Here are links to 2 interesting articles on the gurney flap and its use on helos. The articles are part 1 and part 2, written by R.W. Prouty.

Part 1 (http://www.avtoday.com/reports/rotorwing/previous/0200/02rwaero.htm)

Part 2 (http://www.avtoday.com/reports/rotorwing/previous/0300/03rwaero.htm)

(edited for a typo)

[Arkroyal]

You're all wrong. Gurney Flaps were invented by racing car mechanics to stop their spanners, (wrenches) coffee etc. falling off the convenient table/wing during maintenance.:cool:

[Holly_Copter]

I'd check on the other side to see if there's any sign the other one fell off!
:)

Holly_Copter

[Shawn Coyle]

Ray Prouty says it best:
"No one really understands what happens aft of the rotor mast"
The number of different aerodynamic fixes that have been tried in helicopters to sort out the airflow is amazing, and the AS350/355 family is a case in point.
If you look at the vertical stabilizers on different models of these machines you will see every variation of gurney flaps possible. One side on the top fin, both sides of the top and bottom fin and so on.
Other machines are not much different - I was amazed at the size of the Gurney flap on the port side of the AH-1W fin when I looked closely.
In the same vein, the Gazelle has a simple cambered fin, and it is quite happy to stay at a sideslip angle anywhere plus or minus 5 degrees of zero sideslip in cruise - makes life difficult when you want to maintain a heading. I wanted to put a gurney flap on one to see what it would do, but didn't ever have the opportunity.

The other major use of the Gurney flap is to give a repeatable separation point for airflow. Blunt trailing edges do the same thing (look at the Dauphin or Chinook vertical stab to see this).

[Nick Lappos]

The rule of thumb for gurney flaps is simple:

They are fast expedients, dirty fixes, that help solve minor problems the occur in flight test. They have high drag, but get the job done. They often survive on the design for decades because they are dumb, but they work, so they are not dumb!

[md 600 driver]

the gurney flaps on the md600 are taken off when the ysas s fitted
last week they took all of mine when the ysas was fitted

what you want to make out that is up to you

[flyer43]

BlenderPilot

Not sure when the Bell 230 was cleared for single pilot Ops, but myself and 5 colleagues were the first cleared in the UK for single pilot IFR in the Bell 212 in Jan 1980. Can't say whether the 230 is more stable than the 212 as I've never experienced the 230

For those interested in more reading on Gurney Flaps. The following URL's to articles in Rotor and Wing might be enlightening.

http://www.aviationtoday.com/reports/rotorwing/previous/0200/02rwaero.htm

http://www.aviationtoday.com/reports/rotorwing/previous/0300/03rwaero.htm

Apologies here to FlightSafety as I noted later that he has already included these links, slightly in disguise......

[Letsby Avenue]

Hey Oldbeefer.. I can't believe this argument is still raging? Surely the school of excellence has come up with a satisfactory answer after all this time?

PS: Which model helicopter are you up to now?:cool:

[GLSNightPilot]

The rule of thumb for gurney flaps is simple:

They are fast expedients, dirty fixes, that help solve minor problems the occur in flight test.


So Nick, how come the S76C+, which one would think is a mature design, with a smaller vertical fin & a smaller horizontal stab, has a full gurney on the horizontal stab? One who knows as little as I do would think the same stab as the A would provide enough force, plus commonality of parts, thus cheaper to build & operate. Something must have happened somewhere along the line.

[Nick Lappos]

GLSNightPilot:

The S-76 horizontal tail was sized for the gross weight and CG of the A, with a maximum flight load (as dictated by its total area) just right for balancing the longitudinal axis during high G pullups. As you do a pullup and build G, the main rotor tries to take you deeper into the maneuver, and flap even more than the cyclic pitch asked for. The horizontal tail's main purpose is to develop an upload during this pullup. As the nose rotates upward, the tail is swept downward. This builds up an upload that helps cancel the main rotor's increase in nose up moment.

As the gross weight was grown from the A (10,700#) to the B (11,700#) we found that the aft CG case was only marginal in pullup stability.
I personally flew the development flights (in an A at high gross weight) that showed that the double gurney flap "tricked"the air into thinking the horizontal tail had much more chord. This extra area increased the total lift from the tail, and restored good stability.

There is somewhat more drag using the gurney flap, but it was easy, and did not require a redesign of the horizontal tail tooling (the B tail was an A tail with body filler to make it symmetrical!)

Much later, we went to a light weight new horizontal tail that was designed from scratch to be the same as the older, jury rigged one. Rather than redo all the handling qualities tests, we just made it aerodynamically the same as the original B tail.

[CTD]

...proving yet again, that when Sikorsky takes an old design, and massages it into working on a newer one, it's pure briiliance in its simplistic efficiency.

However, when anyone else does it, it's junk. Hmmmm :) :rolleyes: :p

[Nick Lappos]

CTD,

You sound somewhat sarcastic. Got up on the wrong side of bed?

Our choice was to re-tool the horizontal tail for some millions of dollars, and raise the price of the aircraft, or do a reasonable job for less. Our customers actually helped make the decision.

Brilliance had little to do with it, of course. Practicality did.

Nick

[GLSNightPilot]

I'll take your word for the history, but I'm still a little confused. The horizontal stab on the C+'s I've seen aren't at all the same as an A tail with a gurney, nor a B tail. They're much smaller, as is the vertical tail.

I didn't really mean to start a long thread, I was just curious about the full gurney along with the smaller size. What I don't know about aerodynamics would fill a very large book. :)

[Nick Lappos]

GLSNightPilot,

The C+ tail is identical to the B tail in every way except angle of incidence of the horizontal tail, which is adjusted about 2 degrees to keep the same nose attitude for both models.

[GLSNightPilot]

OK, it changed with the B model. I've never seen one of those in real life.

[oldpinger]

O'Beefer (not one of my old beefers I hope...)

As far as I know it's to help out with tail rotor roll in forward flight.(amongst other things)

AS350 tail rotor roll gives right wing low, so as the horizontal stab gives a nose up pitch via lift acting 'downwards' in flight, then if the port side of horizontal stab gives more lift 'downwards' therefore opposite rolling moment to tail rotor roll!

Alternatively it could be there to stop tools rolling off!:D