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From: (Nick Lappos)
Subject: Re: Fantail TR
Date: 04 Jan 2000
Newsgroups: rec.aviation.rotorcraft

In article <>, wrote:
>What are the advantages/disadvantages of a "fantail/fan-in-fin"
>shrouded tail rotor (Dauphin, Comanche) vs a traditional tail rotor?
>I suspect a shrouded TR would be better protected if it hit something
>and would form less drag when cruising, but are there any negatives?
>What about noise, efficiency, reliability, maintenance, etc.?

Dan Hollenbaugh's separate post is right on the mark, but let me add these

When well designed, the shroud of the fan controls the fan plume very
efficiently, kind of like a rocket nozzle, so the power efficiency of the
small fan can be equal to a much larger tail rotor.  On the Fantail
demonstrator, we could overlay the power requirements of the fan right on top
of the original (and quite efficient) S-76 tail rotor.  By comparison, the
Notar is much less efficient, mostly because the tip nozzle is so inefficient.

The weight of the fan is a bit higher, as Dan has said, since the structure
must be fairly stiff, even though the fan has one less gearbox than a typical
helicopter tail rotor (with intermediate and tail boxes).

The fan shroud must be somewhat deep to control the plume, so the drag of a
fantail is higher than a skinny tailed helicopter with a tail rotor.

The main advantages of the fan are quite compelling, especially in a scout
helicopter role - protection and maneuverability.

The shroud protects the fan quite well, so it is very much less vulnerable to
tree strikes.  Not as well protected as the Notar, but still a leap better
than a tail rotor.

Maneuverability is a whole other animal.  The fan doesn't see the relative
wind that a tail rotor sees, since the duct sorts out the inflow.  This means
that the fan can be whipped around at speeds where a tail rotor would be badly
stressed (this is caused by the strong flapping forces the tail blades see as
the relative wind, at higher speed, is pushed over the disk).  The snap turn
is mostly due to the fact that the fantail can be angled at will to the
airspeed, with almost no regard to the structural penalties.  This allows the
Fantail to snap turn at speeds up to about 120knots, a real treat when it
comes time to point and shoot.

Nick Lappos

Newsgroups: rec.aviation.rotorcraft
Subject: Re: Fenestron/ Advantages/Disadvantages
Date: Sun, 21 Jan 2001 21:49:06 GMT

In article <>, (ASW24) wrote:
>Can anyone with knowledge on the subject (ie no Ronpilot) enlighten me as to
>the avantages and disavantages of a Fenestron.  I have heard that the fenestron
>on the AS365 can take up to 40% of available power.  Why not just keep the tail

As with all of the configuration choices, there are strengths and weaknesses
-Notar, Fenestron, tail rotor, co-axial all have advantages and disadvantages.

Fenestrons advantages are:

Protection - hard to stick it into trees, ground etc.  About half way between
tail rotors (no protection) and Notars (really hard to screw up)

Impervious to flapping/sideslip structural problems.  The fan is centered in a
duct that basically presents the air to the rotor in an axial column, so no
skewed inflow forces are presented to the blades or hub.

Disadvantages are:
Heavier - lots of weight in the ring and support structure, so the designer
wants to make the rotor a small as possible.

Tail contact angle - the fan almost has to be on the tail cone centerline, or
an intermediate gearbox is needed, and the weight really blows up.  This means
that the fan hangs pretty low at the tail, and the bottom of its ring has to
be beefy enough to take a whack.

small fan - weight issues make the fan want to be small, but small fans take a
lot of power, so fans often have compromised sideward flght capability.  This
is offset by the efficiency that the duct adds to the whole system - a good
duct can double the thrust and almost offset the power issue.  The S-76
Fantail used exactly te same power as the original tail rotor (in a steady
hover) so the duct really earned its pay.

The Eurocopter folks found out when they built a fenestron Puma that the fan
doesn't scale up very well, (and bigger helos don't skunk around the trees as
much) so they abandoned it.

It is true that a fan can consume lots of power (the 40% estimate is probably
true) but this is only while really yawing the aircraft in a hot maneuver.
The average power needs while hovering is probably very cvlose to a normal
tail rotor.

I was part of the team that selected the fan for Comanche (we had an internal
Notar team working to sell us that configuration!)  The Comanche fan makes the
Snapturn and the 80 knots sideward flight possible, as well as the protection
from trees and such.  We slowed the fan way down so the typical fenestron
siren is gone.  Throughout the shakedown,  we never found a reason to doubt
our choice.

PS for all the Notar folks, please remember that there is a full variable
pitch tail rotor inside the tail cone, with all the parts and failure modes of
a fenestron.  There is also a rotating can and its mechanism, and in most
models a sas required, as well.  Everything comes at a price, and nothing is

Nick Lappos

From: David Lednicer <>
Newsgroups: rec.aviation.rotorcraft
Subject: Fenestron/ Advantages/Disadvantages
Date: Mon, 22 Jan 2001 23:10:20 GMT

I've got to disagree with you a little Nick.  The RAH-66's fan tip speed
is not the complete reason that the shriek is gone.  Here is a summary
of the data I have:

Helicopter         Fan RPM         Fan Vtip (ft/sec)
RAH-66             2865            675
H-76               2935            605
S-67               3000            732
XOH-1              3504            662
EC135              3545            607
EC120              4567            588
SA366              3740            705
SA365N             4693            725
SA342              5774            695
SA341              5774            695
Cabri G2           5734            531

As you can see, the tip speed of the Comanche is low, but really on par
with a number of other ships, some that shriek (SA341/342) and some that
don't.  A good part of the quietness is the treatment of the hub and hub
supports.  The shriek on the S-67 was partly tip speed, but mostly the
hub supports.  What Aerospatiale has found to really eliminate the
shriek is to use non-uniform blade spacing.  This way, the blade passage
frequency is non-periodic and the noise is distributed across the
frequencies (at lower frequencies and reduced amplitude).  The Comanche
has uniform blade spacing..

One other problem with fan-in-fins that you didn't mention is
autorotation.  To get good autorotation characteristics, a ventral fin
is needed.  Coupled with the low mounted fan, there isn't much room to
do this.  Witness the ventral fin development on the EC135 and EC120.

I wrote the document outlining the design guidelines for the H-76 and
RAH-66 fan-in-fins back in 1985.  Since then, Aerospatiale has pioneered
the use of stators and non-uniform blade spacing.  With these features,
noise and power consumption have been reduced significantly.

As a side note, non-uniform fan blade spacing was actually pioneered by
the automotive cooling fan folks.  It was from some work I did with
Delco back in 1990 that I first found out about it, much too late for my
design guideline document.  Oh well...

                         -Dave Lednicer

Newsgroups: rec.aviation.rotorcraft
Subject: Re: Fenestron/ Advantages/Disadvantages
Date: Tue, 23 Jan 2001 04:13:00 GMT

In article <94iekk$flg$>, David Lednicer <> wrote:
>I've got to disagree with you a little Nick.  The RAH-66's fan tip speed
>is not the complete reason that the shriek is gone.  Here is a summary
>of the data I have:
>good stuff snipped
>                         -Dave Lednicer


Its always a pleasure to be corrected so accurately.  I do understand the fun
stuff about non-radial supports and the distance ratio of the supports.  The
tip speed of the Comanche tail fan is about 630fps at 100% Nr, but when we
boosted the rotor speed to 107% it drove the fan back up to "normal".  Good


Newsgroups: rec.aviation.rotorcraft
Subject: Re: Fenestron/ Advantages/Disadvantages
Date: Tue, 23 Jan 2001 04:20:22 GMT

In article <94iekk$flg$>, David Lednicer <> wrote:

>One other problem with fan-in-fins that you didn't mention is
>autorotation.  To get good autorotation characteristics, a ventral fin
>is needed.  Coupled with the low mounted fan, there isn't much room to
>do this.  Witness the ventral fin development on the EC135 and EC120.
stuff snipped
>                         -Dave Lednicer

Dave,   I kept the original post simple, and didn't mention the way most fans
need lots of tail fin area to make up for the fan's poor contribution to yaw
stability.  It isn't autorotation that is the problem for fans, it is normal
powered flight, where the fan really doesn't help in yaw stability. This is
caused by the fact that the fan doesn't see the sideslip that a yaw develops,
so it doesn't change its thrust very much to help restore the yaw trim.  A
tail rotor is quite effective this way, unlike a Notar or a fan.  That's why
Notars and fans have extra fin area and usually end plates.

  The new Comanche tail will be seen soon, and it has end plates to beef up
the yaw stability a bit, especially when the radar beanie is on and its wake
reduces the effectiveness of the fin.


From: David Lednicer <>
Newsgroups: rec.aviation.rotorcraft
Subject: Fenestron/ Advantages/Disadvantages
Date: Tue, 23 Jan 2001 18:19:13 GMT


Regarding fin sizing; I have found this subject to be very interesting.
When I wrote the guidelines back in 1985, the French advocated
simulating the fan in tunnel testing for directional stability by
blocking the duct off.  Greg Wright's great AHS paper on the H-76 showed
this to be not so.  When I designed a fan-in-fin for a certain European
manufacturer, we heeded Greg's advice and used a CFD code where we fully
simultated the fan.  This resulted in a larger fin than we would have
gotten using the French method.  Interestingly, both the EC120 and EC135
have gone through a vertical fin development program in flight test.
This makes me wonder if they have read Greg's paper.

Other fan-in-fin pro/cons that occured to me during a quiet moment at

1) Theory says that in hover, the duct should carry half the overall
thrust load of the fan-in-fin.  Hence, it is no surprise that you are
seeing this on the Comanche.  If you weren't, I would be worried...

2) Having the duct walls close to the tips of the fan blades reduces the
tip losses (induced drag) to almost zero.  This results in a power
saving compared to an unducted tail rotor.

3) Getting good, linear pedal response involves trade-offs.  The S-67
fan used high twist to get good disk loading and low power consumption.
This was found to produce a non-linear pedal response, due to the fan
going through a largish region of collective where the thrust was near
zero.  The answer that Aerospatiale found on the SA342, after having a
similar problem on the SA341, was to use low, non-optimal blade twist.
They bragged about this in an AHS paper and I included it in the design
guidelines.  This led to low twist being used on the H-76 and Comanche,
giving you the nice, linear pedal response you have.

4) Good, reverse thrust characteristics also involve trade-offs.  You
want the inlet (in hover) side of the duct to have a nice smooth
countour and the outlet side to be squared off, to get the flow to
separate.  In reverse thrust, the roles are reversed.  In worst case,
the exhaust doesn't separate cleanly from the rounded former inlet
contour.  Early in the Comanche design (1985), we made trips to a NY
Helicopters, were we examined a SA365 and a USCG station, where we
examined a SA366.  We found that Aerospatiale had tailored the shaping
of the duct and this too found it's way into the design guidelines.

There was a tiny picture in Av Week of the Comanche with the radar
beanie and additional finlets - I got a good chuckle out of it.

                            -Dave Lednicer

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