Apache gurney flap
 

Does anyone know why they added a gurney flap to the Apache? It has two on the stabilator, left side down, right side up. Happy how a gurney flap works (black magic clearly), but what did it cure on the Apache and why?

My guess is that it cures tail rotor roll in forward flight as the TR sits higher and higher (in relation to the rotor disc) even with a moving stabilator. Other options might be uneven EFABS, dutch roll. Anyone know any fabled design stories about the Apache?

I have looked at the old closed threads on gurney flaps, and they don't really answer this question.

Thanks in advance

Jeep

Various sources online suggest it improves performance in high powered climbs, but without much more detail.

When I was a mechanic on them I think I had heard that it was something to do with how the rotor wash flowed over the stabilator at a hover, making it more stable as a firing platform.

But, that could have been just someone’s opinion on it or a complete fabrication, I haven’t touched an AH-64 in about 16 years thankfully so I’m not in the know on Apache anymore.

FltMech

I have a feeling Prouty talks about that exact topic in one of his articles. I will browse later

You have to come up with a solution for the hover, high power climb, low power descent and the cruise, each of which will have differing airflows over the tail and tail boom so it's probably a bit of a fudge.

To quote Ray 'What you gain in the hover you lose in forward flight and vice versa' with regard to tweaking designs.

I am hoping that John Dixson can share a few thoughts on that from the Sikorsky perspective. Some years ago, there were some Blackhawk mods being sold to various places like Thailand that had various arms stations on the stub wings that the US Army usually uses to hang fuel tanks on.

So Prouty does talk about the flow around the tail of the AH64 in several chapters of his Helicopter Aerodynamics book. He states "...the much stronger downwash behind the advancing side than behind the reverse flow region...". This would lead to a significant roll attitude in low speed forward flight which would muck up a rocket attack. The gurney flap may also be to remove instability by reducing the chaos and tripping the airflow to separate in a more predictable manner.

But the AH64 tail went through many many variations as they tried to iron out snags (Low stabiliser, T-tail, vibration dampers, slots etc). The flaps are definitely a bodge!

“But the AH64 tail went through many many variations as they tried to iron out snags (Low stabiliser, T-tail, vibration dampers, slots etc). The flaps are definitely a bodge![/QUOTE]

I didn’t realize they had so much trouble on the AH-64. I’ve seen the vibration dampers, there are elastomeric mounts where the tail pylon attaches to the tail boom and the stabilator is solid mounted at its hinge fittings.

Which is opposite of UH-60, it has elastomeric mounts for the stabilator hinge fittings, the pylon is solidly mounted to the tail boom.

I wonder why the manufacturers chose their respective vibration absorber locations? It seems that they were both aiming to mitigate vibration induced by the rotor wash impacting the stabilator perhaps?

The stab mount bearings cop a flogging on both aircraft, the UH-60M folding stabilator center box bearings get worn quite frequently, to the point that the allowable vertical deflection of the stab wings at the tips is 7/16” vs 5/16” for the one piece stab on the A/L models.

FltMech

Just FYI.... Gurney Flap Could be the Key for Better Helicopter Rotors | Dan Gurney's All American Racers

Sorry, LW: other than the stub wings that were initially designed for the aux tanks to enable a flight from Newfoundland to the Azores to deploy UH-60 aircraft to Europe, I didn’t get involved in any other mods like that. However, going back to the picture of the UH-60 involved in the Bin Laden situation, its pretty clear there has been some “unofficial” mod activity.

Forgot: we did fool around with a Gurney flap on the S-76 horizontal tail surface-don’t recall keeping it. Nick Lappos will remember-his recall is awesome.

​​​​​​​Like yours isn't!!

Another Gurney Flap Youtube….a bit of a sleep aid.

There are some interesting videos on race car applications.

Good to hear from you John Dixson hope all is well. Cheers.

I think the first Gurney Flap I saw was on the right side of the lower vertical stab on an AS350D Astar.

Hey Jeep. Is that you, Ginge?

Still alive and kicking :)