Scott Gardner opined:
>Well, teetering rotors are good for half a loop at least. The second half
>would be kinda ugly. You would have a good view of the rotor departing
>though.
Whyzzat? As long as you kept it "positive" all the way through it should be no
problem. The great Maury Paquette, who flew for Seagram's for years and years
and had so many engine failures in early 206's that he kept a paddle strapped
to the bulkhead behind his seat for his NEXT water landing, told me a story.
He said that in the late '60s when the 206A first came out everyone was
impressed with its speed and maneuverability compared to the 47J-models they
had been flying. One day, he went by somebody's house (I forget who) to "say
hello." During the ensuing buzz job, he initiated aft cyclic for the
pull-up...too much! The ship went so vertical so fast that he felt the only
thing to do was to keep it coming around. Which he did, doing an unintentional
loop. The rotor did not come off, he did not die. But he said it scared the
crap out of him and gave him a healthy respect for this new helicopter.
I have heard other anecdotal stories from people (my father, for instance)
who've seen other two-bladed helicopters do unintentional loops or otherwise
get upside down. And one day, I watched in horror as a fellow PHI pilot (a
friend and former Cobra driver) who was passing underneath me in cruise decided
to do a Split-S for effect. And I have to say, this guy flipped it as upside
down as it gets. All I saw was belly, landing gear and a float bottle. But it
was from such a low altitude that I thought I was watching a crash-in-progress
and was getting ready to pull the trigger and report it. Somehow, he managed
to pull it off without pulling the tranny out, although I'm sure his heart was
pounding as hard as mine was (not that he ever admitted it). Dumb.
Ray Prouty tells us that all helicopters have more than enough cyclic power to
do a loop, probably even from cruise airspeed. Unlike airplanes where the
pitch-authority is limited by diminishing airflow over the horizontal
stabilizer/elevator as the maneuver progresses (especially nearing the top), a
helicopter retains its control power all the way 'round. Rotor blade/tailboom
contact would be just one of your worries.
The question would be: If you had an engine failure at the top of a loop in a
206, would you put the pitch down? (My guess is that it pretty much wouldn't
matter what you did with the controls at that point.)
Bob -hoping never to find out- Barbanes
Petroleum Helicopters
"The dignity of the craft is that it creates a fellowship."
"Bob Barbanes" wrote ...
(snip)
> Ray Prouty tells us that all helicopters have more than enough cyclic
power to
> do a loop, probably even from cruise airspeed. Unlike airplanes where the
> pitch-authority is limited by diminishing airflow over the horizontal
> stabilizer/elevator as the maneuver progresses (especially nearing the
top), a
> helicopter retains its control power all the way 'round. Rotor
blade/tailboom
> contact would be just one of your worries.
You need more than pitch authority to complete a loop, you also need forward
airspeed, and to maintain airspeed, you need thrust. The more you pitch a
helicopter nose up, the less forward thrust you're generating. My first
concern would be when you're just past the vertical, at a low airspeed and
you need a lot of aft cyclic to maintain a pitch rate, so you have little if
any forward thrust, so you can't accelerate until your nose is below the
horizon. Scary.
My second concern would be if you carried too much airspeed over the top and
you exceed Vne on the pull out of the loop. You'll be excessively nose
down, vibration increasing from advancing blade tip compressibility, and
then retreating blade stall requiring more and more right cyclic (ccw rotor)
until you run out. Very scary.
If I were to throw myself into helicopter aerobatics, I'd start with barrel
rolls. Smaller pitch changes, easier to work into gradually, and easier to
bail the manoeuvre if you are outside parameters you've established.
To "wade" into looping you could try the slant loop where you start by
rolling about 30-45 degrees and then fly your loop along the z-plane of the
helicopter. Advantages to this are the same as the barrel roll, smaller
pitch changes and easier to bail.
I talked with a Westland Test Pilot about the British Army Eagles Lynx that
does the 'backflip' from the hover. Essentially the pilot (I think his name
is Mike McKeown) learned he could do it gradually by rolling and pulling aft
from a high hover and following that through to an inverted but banked
position, then pulling it out. Gradually he lessened the amount of roll
input until he had the Lynx doing a backflip, essentially a loop starting
from a hover.
> The question would be: If you had an engine failure at the top of a loop
in a
> 206, would you put the pitch down? (My guess is that it pretty much
wouldn't
> matter what you did with the controls at that point.)
Same with all engine failures you would want to decrease the blade's angle
of attack. If you allow the situation to progress to the point that air is
flowing down through the rotors because the helicopter is falling inverted,
then it's best to let go of the collective, shove your head between your
legs and pucker....
Remember that in a positive g loop, the rotors are always pushing air
towards the skids. When the engine fails, lowering collective will preserve
Nr but you need the lift to pull yourself out of the loop, so only lower
collective to the point that you balance available lift with maintaing
Nr....about the same for a min rate of descent auto. You then want to get
upright ASAP so start barrel rolling to the nearest horizon. Once you have
your nose down enough I don't think it will be difficult to maintain Nr with
aft cyclic, so increase your collective to provide more lift to get the
aircraft level.
This is all theoretical, of course. You won't catch me practicing this in a
fully articulated tandem rotor helicopter.
Matthew.