Torque reaction

[Martin Harris - Moderator]

3 hours ago, Doug Moss said:

Rich asked which is the safest direction to turn, suggesting an emergency. If you're marginal on power or airspeed, then convert your momentum into airspeed by turning gently into wind.

Unfortunately, this is an old chestnut that continues to be propagated. The only advantage would be a better angle of climb over an obstruction - but I can’t visualise a flying site which would make this a consideration. 

[Peter Jenkins]

5 hours ago, Doug Moss said:

Rich asked which is the safest direction to turn, suggesting an emergency. If you're marginal on power or airspeed, then convert your momentum into airspeed by turning gently into wind.

Sorry Doug, turning into wind does not convert momentum into airspeed.  All you do is reduce your ground speed so that the length of landing run is effectively increased.  if you turn into wind and push the nose down then you are converting potential energy into kinetic energy.  Momentum is mass x velocity.  Unless you are flying in gusty conditions and low turning into wind or downwind does not affect your airspeed but ground speed can alter significantly but that's relative to your stationery position in the pilots' box.  As has been pointed out above, in an emergency turn away from people and no fly zones regardless of wind direction.

[Trevor Crook]

In my experience with electric warbirds, a major factor affecting torque reaction is prop size. I had a Durafly Me109 which had a scale 3-blade prop, and the safest way to take off was to apply power gradually, and no more than half throttle. My initial full throttle take-offs resulted in a very quick unstick, followed by a vicious bank to the left. 

I applied this experience to my Avios Spitfire, also with a large prop and loads of power, and fuss-free lift-offs resulted. Even my more modestly powered FMS 109 takes off nicely on about 60% power.

As someone said earlier, best to climb out straight and let the airspeed build before turning, then it won't matter which way you turn.

I.C. models generally run smaller props at higher rpm, this seems to generate less torque reaction.

[Peter Jenkins]

It might be worth remembering that the WW2 fighters, such as the Spitfire, could only have half power used for take off as the torque reaction was so strong that the aircraft would hop sideways on the undercarriage.  In fact, reading a recent article on how to fly the Spitfire, that was the message the aircraft sent to the pilot to back off the power.  Once airborne the power could be fed in of course but smoothly.

 

I've also seen a horrifying set of photos showing the "death" of a Mustang following a long straight in approach.  The pilot had not seen that there was another Mustang line up for take off until it suddenly appeared to grow out of the nose!  The pilot was too enthusiastic in his application of power and the airborne Mustang rolled to starboard before crashing to the right of the Mustang on the runway.  In the sequence, you can see the airborne Mustang's prop chopping through the Mustang on the runway and only missing that pilot because the airborne Mustang was rolling and turning to the right.  A terrible way to learn about the dangers of straight in approaches with WW2 fighters with long noses and the sudden application of power from a low speed.

[Simon Chaddock]

"then convert your momentum into airspeed by turning gently into wind."

Is the momentum of the plane dependent on the direction of the wind?

Not according to the Newtonian Laws on the Conservation of Momentum.

 

 

[Zflyer]

Your right momentum isn't but lift certainly is.

[Don Fry]

I’m getting a bit uncomfortable here.
We are discussing collision avoidance between a toy airframe and a human being. And suggesting turns at low altitude and low speed, in an emergency.

 

As the old bit of wisdom goes, no hight, no speed, no ideas is a route to disaster. 
 

Low, slow, don’t mess about, hit the ground. If you then chose to pull the trespassers arm off, and beat him to death with the wet end, that is your choice, but not with an airframe.

[Peter Jenkins]

1 hour ago, Zflyer said:

Your right momentum isn't but lift certainly is.

When your aircraft has left the ground, it has no concept of wind.  It has airspeed while the block of air in which it is is moving relative to the ground - not the aircraft.  Turning into wind while maintaining height will not result in increasing airspeed.  If you let the nose drop during the turn the airspeed will increase.  Equally if the nose is pulled up by over appkication of the elevator the aircraft will climb but airspeed will fall.  If the aircraft was flying close to normal stalling speed at the time and corrective action is not taken then the aircraft could stall.  Equally, turning down wind doesn't cause airspeed to drop.  These are the rules of physics and not my theories.

[PatMc]

6 hours ago, Peter Jenkins said:

When your aircraft has left the ground, it has no concept of wind.  It has airspeed while the block of air in which it is is moving relative to the ground - not the aircraft.  Turning into wind while maintaining height will not result in increasing airspeed.  If you let the nose drop during the turn the airspeed will increase.  Equally if the nose is pulled up by over appkication of the elevator the aircraft will climb but airspeed will fall.  If the aircraft was flying close to normal stalling speed at the time and corrective action is not taken then the aircraft could stall.  Equally, turning down wind doesn't cause airspeed to drop.  These are the rules of physics and not my theories.

Nothing to do with the OP - but how do these rules of physics square with dynamic soaring also with the fact that without a change of elevator input c/l models will rise as they come into wind then descend 180 degrees later ?

  

Edited February 10, 2022 by PatMc

[Martin Harris - Moderator]

Dynamic soaring uses sharp edged transitions at the point where two different air masses coincide. 
 

Control liners aren’t flying free - they are held at a fixed point on the ground and the additional energy is applied through the pilot’s arm. 
 

I spent many hours circling in gliders and never had to make any corrections for wind induced speed variations. At low level in a meaningful wind, where relative ground movement became apparent, it was important to ignore your eyes’ impression of airspeed and continue to fly by attitude and airspeed - neither of which varied in a constant circle.  
 

Getting back towards the  topic on torque reaction, isn’t a lot of the thinking behind the possible B test amendment learning to react safely to a situation such as that faced by the Mustang pilot mentioned above?

[PatMc]

Martin, I've never taken much interest in DS so won't pursue that line. 

However I don't see how the C/L pilot's arm provides extra energy. Also if & when the lines go slack control liners are flying free but they still climb into wind at that point.

BTW I've no axe to grind on this subject, I've spent a lot of time circling model gliders without finding it necessary to make corrections. OTOH I've usually been several hundred feet vertically & horizontally distant from the model so not in the best position to judge any small rise or fall in altitude. Also the circles have been greater radius than the average C/L model's lines so any variation is likely to be less pronounced. Against that I would often be flying thermal gliders when the (near to ground) windspeeds would put most people off flying the average power models (RC or C/L) so any induced rise or fall, if it happened, would likely be enhanced. 

I'm not totally convinced one way or t'other.         

[Martin Harris - Moderator]

The reaction comes from the interaction between the wind and the ground and shows up as a varying force through the pilot’s arm (I didn’t mean to imply that it was supplied by the pilot). Think of it in terms of a kite - which will rise if downwind of the flyer but would fall if the flyer ran upwind. 

[Robin Colbourne]

1 minute ago, PatMc said:

Martin, I've never taken much interest in DS so won't pursue that line. 

However I don't see how the C/L pilot's arm provides extra energy. Also if & when the lines go slack control liners are flying free but they still climb into wind at that point.

BTW I've no axe to grind on this subject, I've spent a lot of time circling model gliders without finding it necessary to make corrections. OTOH I've usually been several hundred feet vertically & horizontally distant from the model so not in the best position to judge any small rise or fall in altitude. Also the circles have been greater radius than the average C/L model's lines so any variation is likely to be less pronounced. Against that I would often be flying thermal gliders when the (near to ground) windspeeds would put most people off flying the average power models (RC or C/L) so any induced rise or fall, if it happened, would likely be enhanced. 

I'm not totally convinced one way or t'other.         

Martin, a free flight or radio controlled model allowed to circle will fly a steady state turn whilst moving downwind with the body of air in which it is flying.  The circle will perfect relative to the body of air, e.g .when viewed from a balloon also moving iwht that body of air, but from the ground the model will appear to slow down when heading upwind (ground speed = airspeed - windspeed) and faster downwind (groundspeed = airspeed + wind speed) .  A control line model, on the other hand, is circling around a fixed point on the ground, whilst the body of air in which it is flying moves downwind.

The suggestion to gently turn into wind in the event of a problem is incorrect.  Get the nose down and turn at a 45° bank.  The shallower the bank the more height is lost, as more time is spent in the turn.  One must also remember wind gradient.  When landing into wind, the air's friction with the ground results in the wind speed reducing closer to the ground so the model needs to accelerate to maintain airspeed.  The way to do this is to get the nose down.  A downwind landing is less likely to result in stalling on approach as the model will gain airspeed as it descends into relatively faster moving air.  The landing will also be faster relative to the ground and will result in more damage if it goes wrong.

[Martin Harris - Moderator]

I think on the first part of your reply Robin, you’re preaching to the converted - these were exactly the points I made. 
 

I’m not so sure about the second half though. At low level during the final approach, you should be flying at an appropriate speed, well above stall speed with allowance for expected turbulence and wind gradient until the commencement of the flare. To abort and avoid a hypothetical obstruction (which we have to assume is far enough away to allow an avoidance turn rather than sacrificing the model) means applying power appropriately and making a safe climbing turn. Getting the nose down from the approach attitude may not be appropriate in a typical low level abort - as would a dramatic nose up without appropriate airspeed and power. The bank angle will need to take into account the urgency of the turn and the energy available - remember that with increasing bank comes increased G loading and higher stalling speed. 
 

These requirements make learning to execute the proposed manoeuvre safely and efficiently a valuable exercise in my opinion. Although I’ve yet to see a demonstration of the proposed manoeuvre, I’m pretty certain it shouldn’t be seen as requiring a pylon turn on the edge of flicking out…

[Peter Jenkins]

Hi PatMc

 

When you are flying in a thermal the air can be very rough caused by the fact that a thermal is triggered by slightly warmer air breaking away from the ground in the form of a ring doughnut.  Think of a smoke ring.  This provided the varying lift you expect to find in a thermal.  I have many hours spent thermaling in full size gliders and in very strong thermals I would see air speed vary greatly as I flew round in a circle.  In fact, the rising air in the centre of the thermal was so strong on one occasion that even with full aileron in the direction of turn my glider was rolled out of the turn and out of the lift.  I had to increase speed significantly to get the required control authority before attempting to centre in this particular thermal.  As you say, it's difficult to see small changes in attitude when model gliders are at height.

 

For control line flying, the model is restrained by the lines and this causes a force, centripetal, to accelerate the aircraft around the circle.  The model experiences a centrifugal force while it is flying attached to the lines.  It is important to remember that a C/L model is not in straight and level flight as it is under constant acceleration in order to maintain its position relative to the pilot.  That is why a C/L aircraft always climbs when flying into wind and descends when flying downwind.  As to why it climbs when the lines go slack, that could be due to the fact the aircraft is not trimmed to fly hands off as the airspeed varies all the way around the circle and as the pilot you would automatically compensate with input via the handle.  In that respect, a C/L aircraft is like a kite i.e. performing tethered flight.  So, the engine in C/L provides the motive power for the model while the pilot provides the acceleration force via the control lines to keep the aircraft accelerating around the circle.  If the lines break,  the aircraft will fly off in the direction in which it was pointing when the lines broke.  If the lines broke at the point they attach to the a/c the larger amount of lift from the inboard wing designed to support the line weight would roll the aircraft out of the circle aided by the off set of the rudder. 

 

Radio and FF aircraft are untethered and generally spend a good part of their time flying in free air where there is not much turbulence (apart from when landing and taking off) and hence will drift with the wind as do full size aircraft.  We correct for this drift by flying a suitable heading to give us the required track over the ground.  Unless you are flying directly into or down wind, heading and track will always be different - unless you are flying in a dead calm that is.

 

As soon as an aircraft flying S&L and in trim, is banked, the amount of lift holding it up reduces as the lift vector is no longer pointing directly up.  The part of the vector that is pointing towards the down going wing is responsible for causing the aircraft to turn.  Since the vector supporting the aircraft's weight is now less than the aircraft weight, the nose drops as the natural stability of the aircraft tries to regain the trimmed speed.  We counter this in model flying by easing back on the elevator to increase the lift force.  In full size flying, you would also advance the throttle to balance the additional drag induced by the increased lift we've just demanded.  As we are not sitting in our models, we can only approximate to what the attitude of the model should be and use the fact the aircraft might climb or dive as a clue to reduce or increase the amount of elevator we apply.  We don't bother with increasing engine power since we don't have an air speed indicator to tell us we're losing speed - unless you are close to landing and are also flying slowly.  Then adding power, not a wide open throttle, is a wise precaution to maintain flying speed.  

 

Unless you are aware of all the forces acting on your model as you fly it, you will believe the tales that the aircraft gains lift when you turn into wind and loses lift when you turn out of wind.  Unless you are flying in an air mass that is subject to considerable change in speed and direction e.g. wind shear when taking off and landing, the aircraft will not be aware of whether it is flying into wind or down wind or turning from one to another.  Something would have to change for the change of speed to occur which in C/L is the pilot holding onto the control lines via the handle.

 

I hope that all makes sense!

 

I won't go into Dynamic Soaring, as you said you were not interested in that.  Suffice to say that the Albatross uses dynamic soaring using the waves as hills in order to fly vast distances without needing to flap its wings.  It does, of coure, have recourse to instant power should it get the technique wrong! 

[Brian Cooper]

Meanwhile, back at the original question in the original post, yes, the torque reaction is opposite to the direction of rotation of the propeller. 

 

Assuming the propeller is turning in the traditional direction, the model will pull (swing) to the left. 

[Martin Harris - Moderator]

Agreed, but that’s really rather a simplification.

 

Torque reaction on the ground puts more load on one wheel causing a small swing. At the same time, helix effect applies propeller wash to the fin and rear fuselage in the same direction and is far more significant. On a taildragger, gyroscopic precession is involved as the tail is lifted which also adds a small component to the swing. 
 

Once in the air, torque effect primarily causes roll but as in all aerodynamics, secondary effects are involved as a consequence of bank or correction. 

[PatMc]

On 10/02/2022 at 22:54, Martin Harris - Moderator said:

The reaction comes from the interaction between the wind and the ground and shows up as a varying force through the pilot’s arm (I didn’t mean to imply that it was supplied by the pilot). Think of it in terms of a kite - which will rise if downwind of the flyer but would fall if the flyer ran upwind. 

Martin & Peter, the penny dropped as I was settling down to sleep on Thursday night. I could see exactly why the C/L pilot's fixed position caused the changes in airspeed according to the wind direction & part of the circle the model is entering. It seemed so blindingly obvious I don't know why I didn't grasp it sooner. As far as my point re the slack lines I think my memory was allowing me to remember it as I thought it must have been rather than the actuality, it is after all probably over 20 years since I last enjoyed a bash with a C/L model.

[Peter Jenkins]

PatMc

Glad to have been of assistance.

Peter