Of Props, Rudders and Wind

Asymmetric Thrust and How it Can Help or Hinder
Depending on You!

  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 wind.
     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 seat driver.
     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 save it.
     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 curled prop.
     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 you.”

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Last modified on March 05, 2006 .
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