Autorotation ? R22 for the Experts

Mike...

Would you consider autorotation a LOW LIFT condition??? I kinda thought you
would...so please refer to the following and read about the CONING HINGE. I
and I'm sure the group will await your response.

http://www.copters.com/mech/mr_semi.html

No. A steady autorotation is not a "low lift condition"; it is a low shaft
power condition. In flight the lift is equal to the aircraft's weight times
it's load factor, g's. Low lift means on the ground, or in pushovers, or at
unusually low weights. Autorotation gives zero or negative steady blade
lag, but the coning does not decrease. In fact, if you measure it closely
enough you will see that the coning actually increases slightly in
autorotations because the blade lift distribution shifts outboard during
autos.





"Bob" wrote in message
...
Mike...

Would you consider autorotation a LOW LIFT condition??? I kinda thought
you
would...so please refer to the following and read about the CONING HINGE.
I
and I'm sure the group will await your response.

http://www.copters.com/mech/mr_semi.html

(Eric D) wrote in
om:

Here is a good question for the more experienced helicopter pilot or
mechanic. A friend of mine that flies a late model Robison R22 Beta
II asked this question of a number of individuals and no one seem to
have an explanation. This included people at the manufacturer. I
believe I know the answer, however, I would like to put the question
out and see what others think before tainting their thoughts with my
ideas.

So here is the question: With one person flying the R22 (170lb),
simulating an autorotation by rolling off throttle, the collective
goes full down and pretty-much stays there to maintain proper rotor
rpm. With two people, same maneuver, some collective has to be pulled
to keep the main rotor from over-speeding. The amount of collective
will obviously vary dependent on a number of factors, but for this
example let's say about 1 inch off the stop. One more time with the
same maneuver, with two on board, except let the governor control
engine speed, and not rolling off throttle. In order to maintain
rotor speed the collective has to be on the stop. The question is
why? Please post your thoughts.

Thanks,

Eric D

This gets a little complicated, but I'll give it a try. You'll probably
need to refer to some pictures, so try page 3-10 of the FAA's Rotorcraft
Flying Handbook, or pages 139 and 140 of Principles of Helicopter Flight,
by W.J. Wagtendonk.

First off, one of the previous replies mentioned that the ship is set up to
autorotate correctly by mechanics. In the R22, this is done by entering
autorotation at 50 kts and known ship weight with the collective full down,
and then adjusting the pitch links to set the full down collective blade
angle to obtain the correct rotor RPM from a chart in the maintenance
manual. For example, at max gross weight at sea level density altitude, the
RPM should stabilize at about 108%. At 2000 feet density altitude, it
should be about 110%. As you observed, at a lighter weight, the rotor RPM
will be less. This is the reason for the minimum pilot weight limitation in
the R22.

During autorotation, there are three regions of the rotor disk. In the
first, close to the hub, the amount of air flowing up through the disk
(remember, during autorotation, air flows up, and in powered flight, it
flows down) is large compared to the rotational velocity of the blade. The
result is a high angle of attack, and a stalled airfoil. As far as
generating lift and govering rotor RPM, not much is going on here.

As you move out towards the tip, the rotational veloctity increases and
thus the angle of attack goes down and the airfoil is now flying. The
upward airflow has the effect of changing the angle of the relative wind
across the blade, tipping the vector forward (here's where the pictures
help). Since lift is produced perpindicular to relative wind, this tips
the lift vector forward. Drag is relatively low, as rotational velocity in
the center of the blade is not too big, so the total resultant force vector
is still tipped forward. This forward vector is what drives the rotor
system during autorotation. That's why this part of the blade is called the
driving region.

As you continue to move outward along the blade, the rotational velocity
increases. This has two effects. First, since upflow is constant, and the
relative wind is the vector sum of the upflow and the airflow due to blade
rotation, the relative wind is more flat to the blade (aligned with the
chord). Lift is thus tipped forward less. Second, drag increases. Both of
these effects tip the total force vector aft (towards the trailing edge of
the blade). Once you've moved out far enough, the force will be pointed
backwards, and you will have entered the driven region. The total force
generated in this region tends to slow down the RPM.

Now, the size of each region is governed by several factors. Higher blade
RPM means more rotational velocity and thus a larger driven region. That's
why the rotor RPM tops out in the case of the 170# solo pilot. A higher
collective setting increases drag across the blade, also shifting the
driving and driven regions, and resulting in a lower RPM. That's why you
are able to keep the rotor RPM in the green at a higher weight.

Now for the tricky question. Why is rotor RPM higher for a given
collective setting with more weight on board. More weight means you come
down faster, and thus you have more upflow through the disc. Remember, the
upflow causes the relative wind to be tipped forward, which tips the lift
vector forward all across the blade. This has the effect of increasing the
size of the driving region, which causes the rotor RPM to increase. Drag
will then increase until a new steady RPM is reached.

By the way, when you enter a turn during autorotation, you effectively
increase your weight by pulling more than one "G". As you know, the RPM
tends to increase here also.

As for your last question, if you let the governer control the RPM, you are
probably not autorotating. Ask your friend if the needles are split. The
governer WILL maintain engine RPM at 104% (top of the engine tach green),
so in order to be autorotating, the rotor RPM would have to be higher than
104%. Since you said that the collective had to be full down to maintain
RPM, I'm guessing that it was not. I'll bet the descent rate was less than
in an autorotation, indicating that the engine was helping out.

Hope some of this makes sense. If there are any errors, I'm sure someone
will point them out.

Jerry