From: nlapposNOSPAM@miami.gdi.net (Nick Lappos)
Subject: Re: Fantail TR
Date: 06 Jan 2000
Chris Johnson had some good comments and questions:
>>Any time Francois wants to match the tail rotor of a Bell 412 against the
>>Fantail of Comanche, let me know!
CJ : That don't seem like a fair, apples to apples comparison to me. :-)
NL: Yea, you're right, my comparison is kind of lame, two different aircraft,
different missions and different genrations of technology.
CJ: But seriously, while we're on the subject of tail rotor authority of
>fan-in-tail versus traditional rotor --
>There are some configurations and wind directions that result in the tip
>vortices of the main rotor being blown into the tail rotor. This produces a
>fairly, uh, challenging loss of tail rotor effectiveness.
>Nick, what's your take on how this situation affects a fantail ship?
NL: The real skinny on LTE is simple and quite easily stated:
All rotors lose thrust when operating in disturbed air, including tail rotors,
main rotors and fantails. LTE sets in when there is so little thrust margin
that the loss of the maybe 5 or 10% of the thrust is enough to kill you. With
a really marginal design, that slight loss of thrust unleashes the massive
main rotor torque, and the aircraft bites its pilot big time. I cut my teeth
on the old AH-1G snake, with its way too small tail rotor, and had at
least my share of 360 turns while trying to get over someplace to cut the
LTE only affects those helicopters that have too little tail rotor thrust
margin. Period. It is really not LTE it is "Not Enough Tail Rotor".
LTE happens when the tail thrust is consumed by several possible upsetting
factors, and when the remaining thrust, by marginal design, is simply
inadequate. Yes, inadequate. These possible tail thrust reducers are:
1) main wash into the tail rotor, as illustrated in the LTE handouts that
we've all seen.
2) using somewhat too much main rotor thrust (collective pitch) at the bottom
of an approach, especially in critical tail thrust conditions. I can touch
the left pedal stops on any helo by simply raising the collective pitch until
main torque washes out all tail margin. One inch more collective and WATCH
out! You get LTE, surprise, surprise.
3) terminating an approach with a critical wind condition, where the wind is a
few knots more than your helo can stand.
I did a study on "LTE" accidents to support a regulatory change meeting I was
attending, and , wow, it was some surprise to find that about 95% of all LTE
accidents were restricted to one brand name, and all LTE accidents were
experienced by helicopters with very little cross wind capability.
The cure for LTE is quite simple - get a helicopter with a bigger tail rotor,
as proven by the certified crosswind capability. Don't get in one that has an
LTE history, and don't buy the bull that LTE is a pilot error problem. Don't
buy into the new certification rules that allow you to operate with a helo
that has no crosswind capability. If you do, get a good helmet, and a good
BTW, the LTE study that I did was opposed by an engineering manager from that
particular manufacturer because "LTE is a pilot error problem, plain and
simple" I asked how so many bad pilots were flying his helos, and expressed
our luck that so few of these dumbos were flying all the other brands.
In the fantail, sometimes I can feel the main rotor wash flow into the tail,
the pedal moves a few percent the noise increases, and that is that.
CJ: I'm also curious -- does the typically larger vertical surface of the
>fan-in-tail result in a greater tendency to weather vane, and thus need the
>greater tail rotor authority otherwise made available by that configuration?
NL: The big tail surfaces on the fenestrons and fantails are because the fan
doesn't respond to small yaws, so the aircraft will snake a bit, unless the
vertical tail is big enouigh to keep the nose ahead of the tail. On regular
tail rotors, the tail rotor responds strongly to change thrust when some
sideslip develops, and the yaw stability to small disturbances is strong. In
fact the tail rotor area is as powerful as a vertical fin that is about 4 to 8
times bigger than the tail rotor.
This is one of the reasons why the Fantail is able to snap turn, because the
fan doesn't care where the wind is from, it keeps its thrust and bending
closer to normal. Under big sideslips, a tail rotor is positively screaming
from the big stresses it develops. With the same maneuvers, the Fantail is
calm as can be.
CJ: As with all things helicopter, it seems this is yet another of those situations
>where there are so many different dynamic factors involved, it's a real
>challenge to figure them all out and measure them appropriately.
Thanks for the great insights,
PS A screen in front of the Fan or a tail rotor would cut the thrust by a
big amount, perhaps 5 or 10 % of the total thrust for a wide mesh fan. Kind
of like carrying around a permanent case of LTE, huh?
Subject: LTE does not affect all Helos!
Date: 11 May 2000
For Meshnet, and the others who express the universality of LTE, and who want
to blame pilots for airplane problems:
While your opinion is theoretically correct, the practical issue is that
several helos are woefully inadequate in basic tail rotor authority, and are
therefore more likely to experience LTE.
The term LTE is itself a misnomer, in that it infers that somewhere the
perfectly normal and acceptable tail rotor gets blanked, or stalls or somehow
becomes ineffective. This is simply not supported by the data, or by the
military tests that have been conducted.
LTE is found on helos that have poor tail rotors, where the yaw thrust margins
are low in normal maneuvers. With low margins, the type and severity of
maneuver that absorbs all the anti-torque is much more likely to be
encountered. On helos with powerful tail rotors, LTE is non-existant I defy
you to find legitimate LTE in a Black Hawk, or an Apache or an S-76.
LTE is cured in two ways:
1) In shitty helicopters, baby them as you maneuver at low speed, especially
with regard to collective pitch applications in descent with cross or down
2) get a helicopter with a better tail rotor, and you will never see LTE
unless you purposely induce it. The proof is the US Army yaw maneuver
criteria improvement over the last 30 years, which has eliminated LTE in the
new designs. In 1965 (H-1, H-58), all you needed was 17 knots of sideward
flight capability. In 1978 (Ah-64, UH-60) you had to show 45 knots at
altitude). In 2000 (Comanche), you have to show a snappy hover turn at 45
LTE is not a problem in any helicopter that meets the modern yaw maneuver
Why do I rag on this way? Because if we all don't help keep the issue
understood, we will always have to fly crappy helicopters where some well
meaning but misinformed guy can simply blame the pilot (Meshnet said "which I
prefer to call "LPE", for "Loss of Pilot Effectiveness").
Modern helos can eliminate the need to blame the pilot by eliminating the need
to baby the machine. The Army forced the redesign of the OH-58D to eliminate
LTE, which they found when doing simple bob-down maneuvers.
Why stop at LTE, Meshnet? let's blame all those pilots for the post crash
fires, and eliminate crashworthy fuel systems. Those stroking seats are for
wimps who can't really fly, lets just break the backs of the assholes, ("Loss
of Crappy Pilot") and eliminate the problem through Darwinism!
Seroiusly, I believe we must learn to criticize the job we are given, and the
machines we use, if we are to make progress in our industry. I joke that the
only difference between a test pilot and a regular pilot is that the test
pilot is allowed to blame the machine.
Learn to blame the machine, learn to fix the heliport, learn to say no to the
night semi-IFR mission, and maybe our loss rate will improve, our customers
will be safer, and our insurance rates will drop.
Every airliner lands on the same runway, same width, same markings. They make
their living changing the environment to fit their needs. They force standard
angles, clearances, dimensions and procedures, and so they get impeccable
safety - 10 times the safety we do.
We in Helo-land (45 clicks south east of Pepperland) are very willing to land
in an LZ with a telephone pole in the center, to fly VFR at night in 500 and
1, to lift the boss over a set of wires at his heliport without complaint.
And when we hit that pole, run into the terrain, or snag that wire, we blame
the pilot, a poor shmoe who only has 2500 hours and 7 tears experience.
This attitude has to stop - we have to stop trusting the "Right Stuff" to keep
us out of the woods. We have to start expecting to get aircraft that are
relatively easy to fly, systems that help us do the job and operating
environments that help us keep safe.
We don't have to look too far for examples. Lose 3 helicopters in ice fog,
anyone? Do you think the average person thinks helo pilots are fools? Do you
think we looked foolish? Do you think a guy in Alabama will think twice about
a charter because of that Alaska incident?
Wow! Did I get carried away, or what?