Of Props, Rudders and
Thrust and How it Can Help or Hinder
Article by Ted Collins who is currently flying
a C441 out of Abuja, Nigeria.
It is my
humble opinion that not enough attention is given to asymmetric thrust and
I will tell you about how three airplanes got bent as a result. We covered
it in ground school and we know that right rudder input is needed when
taking off and when raising the tail due to gyroscopic precession and that
corkscrewing slipstream, but there is more to it. Oh yes, much more.
I am not too concerned with gyroscopic
precession and slipstream effect although they do contribute to asymmetric
thrust. I’m talking blade angle relative to airflow when taking off and
landing and especially when combined with a cross wind on single engine
airplanes. For simplicity I’ll just talk about clockwise rotating
engines although the opposite will be true with those CCW designs.
First a quiz to see if you understand the
basics; the wind is from the North. Which runway, 09 or 27 will give you
the shortest takeoff run? If you said 27 you are right. The right
crosswind will counteract the three left turning prop forces lessening the
required right rudder input. Less rudder deflection equals less drag and a
shorter run. Simple, eh? Also you would be able to handle more crosswinds
on takeoff and landing because when the rudder is not enough to counteract
a weathercocking tendency, you can add power as required to keep straight.
Asymmetric thrust is your ally when taxi
turning a single engine floatplane downwind. By increasing the power to
where the floats are plowing and the nose is up, the right and down going
blade has a higher angle of attack than the descending left blade
therefore helping you turn left. (You don’t want me to talk about
lateral movement of the thrust vector do you?) Unfortunately some water
thrown up by the floats and driven by the wind will likely get at your
prop tips but that’s a different matter. At least you got to taxi that
mile downwind when sailing would take too long in a stiff or freshening
In a similar vein, when you want to takeoff
in a small rounded or dog-legged lake with a gross load of successful
moose hunters, which way you gonna turn as you come up on the step? You
got it, left. If you back off the power and prolong the plow before
getting on the step you can modulate the amount of left turn you can pull
off. Allowing the plow to continue as you gain the step will add to the
left turn. Once you are ready to commit to obtaining the step the downward
pitch of the nose and the added speed increases the radius of the turn
drastically as you turn toward the open water.1 With the level
attitude the asymmetric thrust won’t be able to help you anymore.2
Also, on a left turn, you can see your left flank better if you are a left
Now the sinister side of asymmetric
thrust… and to the accidents. In northern Ontario I had occasion to take
a buck eighty-five on skis to a lake that was mostly void of snow. I could
have elected to land with the skis up (wheels down) but there were some
remnants of drifts I was afraid the wheels would dig into. So I says to
myself, ‘Self, land skis down, into wind as per normal.’ All was good
and fine, until I lowered the tail and Mr. Aysmmetric came into play and
Sir Rudder became less effective as we slowed. We (the plane and I) became
like a curling stone describing a straight line of travel but with a slow
uncontrollable left turn at idle RPM and with full right rudder.
Moving like a tugboat traversing a low ocean
swell, we slid sideways over a hard packed snowdrift without digging
in…luckily. As we slowed the C of G wanted to go first and the rate of
turn was somewhat, shall I say, accelerated…as in a ground loop. We came
to a stop facing the direction we just came from and the passenger let out
a “Yee Ha.”
A week later and a little wiser I was called
to collect a competitor who did the same thing in a 180 but his ski dug
into the drifts and the lateral load on the gear was too much and it
collapsed. Wanting to keep my longitudinal axis corresponding more or less
to the direction of travel this time, I amended my procedure and planned
to land with a right crosswind. Since there was no cabin smoke or drifting
snow to indicate the wind I had to do a MacGyver and improvise.
I took the Northern Store plastic bag my
lunch was in and dropped it on the ice tied by the bag handle to a sneaker
to make a windsock on the frozen lake. Clever, eh? (Probably violated some
Air Reg in the dropping, but not twisting a gear leg off and resultant
prop strike seemed somehow more important.)
The skis were alternately clacking on the
bare ice and silent on the swell like ridges of snow. As the rudder lost
it’s effect the right crosswind countered the nose high left turning
asymmetric thrust. I stopped pointed in the proper direction this time.
After throwing the engine cover on and collecting my shoe, I examined the
bird with the broken leg and bent wing time and curled prop and thought,
‘That could have been me.’
If I had misjudged and had too much right
cross wind I could always power out of it but not so with a left
crosswind. With no wind at all I wouldn’t recommend landing on glare ice
with patchy edge-catching snowdrifts.
My next asymmetric example relates the tale
of a single Otter that tried to takeoff in a sheltered channel, which ran
past a broad adjacent bay. As the Otter came on the step using the usual
right rudder, the broadside suddenly became exposed to a 90 degree left
crosswind from the bay on the left. There was not enough rudder to stay
straight and the centrifugal force of the veer plus a sudden gust of wind
lifted the left wing such that aileron deflection alone was too little too
late and the right tip dug in. The Otter nosed into the water and semi
sank on damaged floats and mangled float struts. I don’t know the
topography around the lake but I assume had he taken of in the other
direction the event could have been avoided with prop asymmetric thrust,
countering rudder, and a favoring wind in a controlled balance.
Finally, this story concerns all single
engine turbine floatplanes. A Caravan was landing into an ocean bay with a
moderate left crosswind to avoid heavy waves further out. In the bay, not
far from shore, there was a pick-up/drop-off dinghy moored to a buoy. The
pilot wanted to keep it in sight for reference so aimed to the right of it
(on his left side.) Well, due to a few gust-correcting stabs of power, the
landing got a little long but not to worry, reverse on touch down would
On touchdown the big water ahead was coming
up quick so he threw’er in hard reverse. Coming off the step with the
nose pitched up, the down-going blade is flat and the rising blade is at a
high angle of attack in reverse. Yet again the left turning moment veered
the nose left unchecked by the too little too late lost-my-effectiveness
rudder input. You guessed it…dinghy 12 o’clock…crunch, and a badly
Pilot anticipation of the oncoming yaw with
hard right rudder before the fact and leaving on just enough power to
clear the dinghy before committing to the beta (beta is much preferable to
reverse anyway) would have saved him face on this one. Even if he had
overshot the chosen landing area, big water is not so bad at the speed
that you fall off the step.
I’d hazard to say a greater appreciation
of the left prop yaw in nose high attitudes, the reduced effectiveness of
the rudder at slow speeds, and the effects of the wily wind, will prevent
accidents. Furthermore, appreciation will enable you to turn the three
yawing forces to your favor. In other words: “May the force(s) be with
The margin tile was made from a
image of a wooden prop on a SE-5A WW1 bi-plane. The same aircraft type
that Billy Bishop flew.
Note from the Editor.
Or at least: "may all the forces balance each other out." The
Caravan floatplane has an insidious left asymmetric yaw on landing in the
nose high, even slightly high, attitude. If you touch down on water in
this yawing condition your right float will dig as the deceleration from
the water friction veers you further left. One day I circled overhead to
watch a Caravan land, and witnessed a dig and spray that left the aircraft
90 degrees to the left of his landing direction…Luckily right side up!
If you want to visualize what happens if
you don’t stay blue sky up, see if Block Busters can get you Mckinna’s
Gold with Charlton Heston. That end over end was for real as the pilot
touched down on glassy water with an ever so slight left yaw.
Thus, anticipate the left yaw as you flare and
increase right rudder until the yaw is countered before you touch down.
Even lead in with a degree of right cock to anticipate the surface tension
grab, and you will land as sweet as wood smoke.
1As an opposite effect, if you are
near getting on the step the immediate effect of hauling back will
decrease the left turn.
2Torque and slipstream effect,
however, will help you to lift that right float going into the left turn.
The attitude indicator will guide you back to The
Bush Pilot Company.
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Last modified on
March 05, 2006 .
© Virtual Horizons, 1996.