Puma with a fenestron

[NorthSeaTiger]

Does anybody have a picture of a prototype puma with a fenestron, it was the sa330z reg f-zwrr ?

[Hedski]

There was an article in French in the last Helico Revue (No.64) on French test developments and there was defenitely a photo of a test Puma with one, along with the AS351 and others. Maybe they can help you.
It did look strange, the thing was huge, CofG must have changed drastically.:8

[NickLappos]

when we brought the Fantail aircraft to Paris in 1991, I spoke to the head of EC test, a real professional named Bernard Fouques, about fans. He told me that they had built and flown an experimantal Puma a few years earlier that had a fenestron, and were not particularly happy with the results. he indicated that the slight penalty of a fan on smaller helos became a much greater liability as it scaled up toward the Puma sized aircreaft, so it was dropped.

Perhaps that test aircraft is the one in the sighting?

[TheFlyingSquirrel]

What are the penalties for Fenestrons on larger aircraft Nick?

[212man]

"he indicated that the slight penalty of a fan on smaller helos became a much greater liability as it scaled up toward the Puma sized aircreaft, so it was dropped."

Er, weight?

It's a pity as they are a delight to use compared to a conventional TR (well, the modern ones are anyway!)

[Ian Corrigible]

Voila...

http://helimat4.free.fr/2002/SA33X/m135.jpg

I/C

[TheFlyingSquirrel]

Rather than weight, I was led to believe the penalty was in the complexity of construction thus increasing airframe cost. Nick, is this correct? Thanks.

[NickLappos]

I wrote a bunch a while back on this, having helped on the Fantail conversion for the S-76 when we were demonstrating in the LHX competition.

The fan/fenestron is never as efficient as a tail rotor (nor is a notar, BTW), it only barely breaks even power-wise. Remember every wasted HP is about 5 to 7 pounds of payload you left behind. Also, the weight and drag of the fan is always worse than a tail rotor. The shroud alone is heavy, it must fit perfectly, and must not warp when you whip the aircraft around, because tip clearances beyond a fraction of an inch waste lots of power, but shroud contact is a real no-no. The drag of the fan (the shroud has to be deep enough to guide the air, so it is boxy) is about 15% of the total drag of the S-76 it was on!

Also, the fan does not want to have two gearboxes, so it must be on the shaft centerline. this means that the fan and shroud are real ground strike hazards, and therefore need even more beefing up to keep the shroud from deforming when the ground is struck, leading to a major blade strike problem if you whack the tail bumper.

Why did Comanche have a fan? Because the Army said, he who has an unprotected tail rotor automatically loses. We could read their lips.

Like NOTAR, Fenestrons are really design power and weight wasters, but they have a passionate following, and they sell a little, so what the heck!

BTW, the fan does not handle well at all, 212man, but the big endplates that fan equipped helos have do handle well. A naked fan is a pain in the neck in cruise flight, because the fan does not have any real yaw stability of its own (unlike a tail rotor, which behaves like a big, helpful fin).

[212man]

Nick, I agree with what you about the weight. power etc but when I was referring to it being a delight to use (you use the term handling) I meant at low speed. I doubt that there are many conventional TR aircraft that can be 'booted' around with such control and abandon, regardless of wind strength and direction, as a good, modern fenestron. Well not a civil aircraft anyway. Having seen the video of Nick flicking his Commanche from side to side at 80 kts I'm sure he'd concede that point!

Nick mentions the fan being on the driveshaft centreline and this has the effect of pronounced 'tail rotor' roll, compared to a conventional TR with an intermediate gearbox and vertical fin. Up to 4-5 degrees in the hover. Slightly uncomfortable but helps visual reference for vertical work (from the RHS, hinders for the LHS).

If you are going to have a driveshaft failure, have it in a fenestron aircraft; as long as there's no secondary damage and you have a runway to go to, it should work out quite well. On the other hand, if you have no runway and opt for an autorotation, you get some unwanted input from the vertical fin as you increase the speed.

[NickLappos]

212man,
You are right there, but be careful, because the Fantail was considerably more robust than a fenestron on a Gazelle! I would bet the EC folks might really complain if people kicked around a fenestron helo as if it were Comanche!

The high speed handling of the Gazelle I once compared to a good airplane, it was so nice. Those endplates make it really uncoupled and docile. You are aslo quite right about the effects of an anti-torque system failure. Because virtually all antitorque at high speed in a fenestron comes from the fin and endplates, loss of the fan at cruise is a much more managable affair.

[212man]

The fan on the 155 (and 365 N3) is as pretty beefy affair compared to the Gazelle. Assymetric spacing, so quiet too. the limiting factor for sideways speed becomes pedal travel at about 40 kts.

[steve_oc]

I remember seeing the picture when I was at EPNER umpteen years ago and one of the issues said to have curtailed the programme was that the aircraft ran out of pedal in the OGE hover as the weight/altitude increased.

[NickLappos]

212man,
Most of the beef must be in the gears that carry the power, which is hard to tell by inspection. Considering the relatively low max allowable power in EC gearboxes and the low max transients, I would bet non-essential fan maneuvering would raise the hair on the neck of the EC designers.

Where is helicomparitor when we need him?

[212man]

He's a conventional TR man, I believe.

[Thomas coupling]

Shame really, I was hoping the EC145 would eventually go the fenestron route. It's apparent now why it won't.

[NickLappos]

212man,

The truth be known, I'm a sucker for a pair of pretty eyes!

One of the fun things at pprune is that we can debate as though the words count for something, and we tend to think it is opinion that makes one configuration better. As Lenin once observed, "Facts are stubborn things!" The observations I offer above are based on facts and data.

[212man]

Nick,
out of interest, did you (Sikorsky) consider assymetric blade spacing for the S-92 to reduce the noise signature? It's oviously common place on military a/c now.