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Strong x-wind and prop aircraft...

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Strong x-wind and prop aircraft...

Old 28th Oct 2014, 15:49
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Strong x-wind and prop aircraft...

Dear All,

I was told about a type of effect that I couldn't imagine having such a big impact on a single prop aircraft, in particular if taildragger, when taking off under strong x-wind.

The said effect is described as resulting from the deflected propwash, as the pilot releases the brakes and starts his takeoff run.

Imagine a powerful P51, taking off, with the first detent of flaps, with a 15-20 knot direct x-wind component... As full power is applied, the propwash will be deflected downwind, and "wash" the right wing, and probably also the right horizontal stab. It can reach as much as 30m/s while the aircraft doesn't get to takeoff speed.

The result, given the additional lift generated on the right lift generation surfaces and the associated drag, can make the aircraft want to turn downwind, opposite of what I always thought was the only possible outcome, weathervaning... So, this will require from the pilot, the use of left rudder ( upwind rudder ??? ) and even a bit of right aileron ( downwind aileron ??? ) in order to counter the tendency to lift the downwind wing...

Now, being only a glider pilot, I usually use this forums to seek for opinions from you "lucky" guys with a prop ahead or by your sides, and in this particular case, I am really interested in hearing from taildragger pilots "mad" enough to takeoff under extreme x-wind situations :-)

What do you think? I am really confused about this one, and swear I thought that by far the wind played a much bigger role on every x-wind takeoff :-/

EDIT: On "TAILWHEEL THOUGHTS" I found described, under "CROSSWIND TECHNIQUE", an effect that can cause a slight moment opposing weathervane, but it's a completely different effect. Yet, it's caused by drag from the deflected aileron on the right wing, when the pilot applies "into the wind" aileron, so, it reinforces my beliefs in the effect I mention above as it was described to me, because it involves not only the drag from the aileron, but also from the "washed" portion of the right wing, considerably covered by the deflected propwash crossing it at a top 30m/s at the beginning of the takeoff roll...

Last edited by jcomm; 28th Oct 2014 at 19:23.
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Old 28th Oct 2014, 17:12
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Well there are a whole lot of factors that play a role in high-power single-engine taildragger takeoffs.

Not just wind (weathervaning) itself, and perhaps this effect (sounds plausible - sort of like running into your own drifting wake turbulence vortex ). But also engine torque, gyroscopic precession (when raising the tail), p-factor, the regular prop vortex effects.

In the wind-tunnel, one can probably filter for just one of the variables and study it.

But in the real world, for a pilot, all that stuff is happening all at once - some effects reinforcing other effects, or neutralizing other effects. It is a constant juggling act with rudder and stick, not a "put the stick here and the rudder there, and hold them" situation.

In the really powerful ships like the P-51 and the Corsair, the practice was to NOT use full power, until one had built up enough speed for rudder effectiveness. Partial power, pick up speed, raise the tail (into the propstream) - and then bring the power (with all its side-effects) up to maximum.
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Old 28th Oct 2014, 23:14
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In an American-powered single (or any with RH prop rotation) of say 450 hp or more, a left crosswind can be quite a challenge. The combination of "torque" (actually the propwash vortex pressing the LH side of the fin) plus the x-wind (more of the same) can send you off the left side of the r/w into the weeds - or the ditch.

Awareness is the preventive action; be prepared to use more right rudder than usual, and open the throttle slower, so the powerplant doesn't get ahead of the rudder control authority.

And it would take a really really strong wind to create the situation jcomm describes.
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Old 29th Oct 2014, 08:54
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Ok,

thank you for your excellent comments, which pretty much concur to accept that the common technique should be, as usual, "aileron into the wind" and "rudder away from the wind", exception being made to situations under which probably one shouldn't really even consider to takeoff.

Under such extreme situations, use of progressive application of throttle, waiting for tail effectiveness, followed by the necessary rudder and aileron inputs, without trying to follow a predefined, "linear" sequence, will make the difference :-)
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Old 29th Oct 2014, 16:44
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barit1 ....The combination of "torque" (actually the propwash vortex pressing the LH side of the fin)....
Not really - they are two different (and real) effects.

The force (torque) the engine applies to the crankshaft and prop to spin them clockwise also applies a counterclockwise rolling force to the engine and what it is attached to (the rest of the aircraft). Basic Newtonian "equal and opposite" reaction.

Easy to see this in a helo that loses its tail rotor. Free of ground friction and a countervailing yaw force, the engine torque spins the airframe as well as the main rotor.

The force is equal and opposite, but the acceleration (speed of rotation) is proportional to the mass of each object (and the prop/rotor masses a lot less than the airframe, so spins a lot faster). F=M*A > F/M = A

On the ground (takeoff roll), the counterclockwise rolling force is translated into downforce on the left main gear, trying to dig it in, and thus adding drag on the left side (left-turning tendency).
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Old 31st Oct 2014, 12:36
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Yes, the unbalanced torque created at the prop shaft, and reacted through the engine mounts, is real. And pilots have been taught this and P-factor for ages. There's also the explanation that the left wing has more "washout" to react against the roll.

But when the numbers are examined, and the engineering designs reviewed (wings are in fact symmetrical), then unbalanced torque is pretty minor compared to the propwash vortex striking the fin. This is further reinforced by experience in nonstandard aircraft designs with the fin (or most of it) below the thrust line; LEFT rudder is required at takeoff!

Check the fin of your plane; chances are the fin LE is slightly offset left to trim out the yaw resulting from the prop vortex.

40+ years ago there was an outstanding article in EAA's Sport Aviation, examining these and other aspects of what we've always called "torque".

Last edited by barit1; 31st Oct 2014 at 13:14.
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Old 31st Oct 2014, 13:37
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Jcomm, I had the good fortune to have trained in the RCAF at a time when the last of the big single pistons were still around, used only in second line units and non front- line duties mind you, they could be a handfull on T/O but nothing to compare with an 81 year old British single I now fly for fun. To begin with the fin/rudder combination is way too small, the aircraft does not have diferential brakes as most from this period have been rertofited by now, the tail wheel has no locking system {we intend to install one this Winter} and is free castoring, combine this with 80% of the mass being aft of the main gear and one has an aircraft which would like to "swap ends" every landing, on top of this being British, the prop, thus the T/O swing are backwards to us on this side of the pond! As with most aircraft from this period it was designed to land into wind on a grass field, however our present home airport has very limited grass area, so the briefing I recieved from a very experienced owner of one of this make was"dont even think about flying from blacktop if the cross wind is over ten knots", a point driven home last week when we stayed on the blacktop but it was a close thing indeed!We are building an RC model of this aircraft, would love to put it in a slow speed wind tunnel and see what is really going on, by the way, one cant use the "wing down" method in a cross wind, the bottom wing is too close to the ground, the mass balences on the ailerons show much evidence of past scrapes, all in all, the most humiliating aircraft to land and handle on the ground I have ever flown, but lots of fun, what is it? Its a DH 87B Hornet Moth, not a fire breathing Sea Fury as on might think from my previous ramblings!

Last edited by clunckdriver; 31st Oct 2014 at 16:05.
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Old 1st Nov 2014, 09:33
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@chunkdriver: Delightful reading!!!! Thank you so much for taking the time to write it here!!! I would really like to get additional feedback from any wind tunnel tests you can run in the future with the scale model of the Hornet Moth! I will try to PM- you ( don't know if possible here at the PPrune forums yet ) with my email!

@barit & pattern_is_full: Thank you for bringing the "torque" subject to this thread as well :-) I love that particular subject an the overall modeling of prop effects in flight simulation games ( used mostly all you can think of... ).

In a previous thread here at PPrune, I started a debate on that interesting matter :

http://www.pprune.org/tech-log/53516...cts-et-al.html
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Old 10th Nov 2014, 08:11
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I certainly concur with the comments on the Hornet Moth. In the air, it is a wonderful gentleman's conveyance and a delight to fly but in the landing phase it is certainly a handful. Add to all the previous comments a braking system that applies full brake differentially with the use of full rudder - even when selected fully off! - and you have a recipe for acute embarrassment.

The answer is to always 3-point her, use grass where possible, restrict the crosswind and do not try to land with more than 25 galls (UK!) of fuel onboard.
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Old 11th Nov 2014, 09:11
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"Torque" isn't a really big thing for normal sports planes.
Two things need to be made clear however: torque exerted during stabilised RPM, and torque created when the RPM is quickly increased, as in "gunning" the throttle.
The mass of the prop needs to be accelerated and that will try to "roll" the plane around its longitudinal axis. But only briefly. And it will produce NO movement around the top axis (yawing) directly, like the spiralling airflow coming from the prop does.
It will be concealed by the more pronounced effects that happen during Take Off. As long as You have no Griffon with 2420HP with a heavy (metal?) 4 blade in front of You, "torque" might be the least thing to worry about. Crosswind or not.

You can clearly see the phenomenon on a float plane, gunning the throttle dips the LH float momentarily (with clockwise turning engines), but comes back as RPM stabilises. Surely the float will be dipped somewhat lower in the end, but the resulting force counteracted by the huge wings it will not be noticeable, at least I have never really noticed it.

Come to speak of it, I wonder if torque also exists with counter-rotating props?
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Old 12th Nov 2014, 18:40
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Double Back,

thx for your observations regarding the significance of torque mostly when power varies, with increasing or decreasing throttle / thrust, with the effect dissipating as RPM stabilises.
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