The dreaded down-wind turn

[Tim Mackey]

A couple of you have started discussing this fact / fiction over in another thread, so I have moved the posts over here to this new thread -

-  I look forward to the polite arguments

[Tim Mackey]

Simon Chaddock said....

This may cause a bit of a stir but:

To say a model may fall out of the sky when turning down wind is just not correct. The model has no idea which way the wind is blowing so it makes no difference to it. The problem is the pilot on the ground. When you turn down wind, particularly in a slower flying plane, it will appear to speed up dramatically so the pilot tends to slow it down (particularly when turning onto the downwind leg) and that's when the trouble starts.

So yes flying a bit faster with more throttle will increase the safety margin above the stall speed but its really for the pilots benefit, not because the plane needs it.

The engines in most full size WW1 types were very  heavy (when compared to the airframe) hence their short noses, so you really need to put in an engine of scale weight rather than carry nose ballast - of which you may need quite a lot - simply because of that short nose. So your smaller lighter engine could actually end up having to haul the same total airframe weight around as a bigger heavier engine.

[Tim Mackey]

Phil_G said ....

Simon, I'm sorry, I'm well aware of this view, it has been said many times,  but I just cant agree. Its nothing at all to do with aerodynamics, its pure Newtonian physics.

If the model had no inertia at all, then yes. But it has and, it cannot immediately change its speed to maintain flying speed above wind speed, it takes more time to change speed than air (wind) does.  The model is far more dense than air and so cannot make changes of speed as fast as air can. Do you think that an open cockpit pilot doesnt feel gusts?

"The model has no idea which way the wind is blowing so it makes no difference to it."

Its nothing to do with wind direction, its all about the difference in speed between an aircraft and the air through which its travelling, air which can and does rapidly change speed and direction. In terms of the  actual vector in space, not airspeed, the plane flies upwind at one speed, downwind at a higher speed. Air can change speed immediately, a heavy plane cannot.

A plane flying at 40mph will stall if hit by a 40mph gust from the rear, inertia prevents its vector from instantly increasing from 40 to 80 mph. What happens during that period of zero airspeed is down to inertia, gravity and the duration of the gust.

Phil

[Pete Rieden]

Timbo,

To understand this you need to dig a little deeper and look at the mechanisms of the turn. When the model turns id does so by grabbing the air it's flying through. The acceleration you're looking for occurs as an inherent result of using the lift vector to do work against the air and change the flightpath of the aeroplane. If you sum the forces acting throughout the course of the turn you'll see where it comes from.

Someone once tried to explain to me that there must be a huge energy loss through a turn by saying that an aeroplane travelling due north at 100mph has an energy of 1/2*mass*(100mph^2), but when it has turned south its energy would be 1/2*mass*(-100mph^2) and that the difference between the two was the work done in turning that appeared as a drag increase.

This is demonstrably untrue (run the numbers for a few examples and you'll soon see how it fails to match observation), and it is based on a falacy that kinetic energy is a vector quantity when it's actually a SCALAR quantity. The work done to turn an aeroplane is the centripetal acceleration (mv^2/r), which acts at right angles to the velocity vector and therefore has NO effect on it. The drag increase through the turn is simply the work done in centripetal acceleration - the kinetic energy remains unchanged.

You can verify this in many ways, but some simple thought experiments can help.

Consider a ball flying through the air. As it passes an infinitely stiff post it reaches out and grabs it (it's a clever ball), swinging around through 180 degrees before letting go. If there's no friction in the hand, what's the energy change in the ball through the turn? (the answer is "nothing" BTW, because the centripetal acceleration is provided by an inelastic arm so whilst there is a force thewre is no radial movement and so no energy required).

Consider swinging a ball with zero drag coefficient (another clever ball!) around your head on a string - how much energy does it lose whilst running on a circular path? (none).

Newtonian physics can only support the counter argument if you assume there is an absolute universal frame of reference - newton never suggested such a thing, and indeed the bedrock of newtonian kinematics is that there isn't one. In all the years I've been modelling aero performance in the aircraft industry I've never even seen a model that includes wind vectors. There isn't even a variable for it in the dataset!

 PDR

[Pete Rieden]

Timbo - forum moderator wrote (see)

A plane flying at 40mph will stall if hit by a 40mph gust from the rear, inertia prevents its vector from instantly increasing from 40 to 80 mph. What happens during that period of zero airspeed is down to inertia, gravity and the duration of the gust.

Sorry - I was running out of message space so I split this one off seperately. The point here is that, whilst you're quite right, it's not really relevant. Transient gusts produce (and remove) forces as you'd expect, and that's why aircraft get bumpy in turbulent air. But all turbulence cancels out, and all these gusts are of very short duration. In the absence of both of these points there would no longer be an atomsophere around the planet! Net turbulence is always zero when averaged over time, and the time doesn't have to be very long (a few seconds) to observe this in practice.

 PDR

[Myron Beaumont]

Peter   May I question your statement regarding your "clever ball! going found a post ?  As it circumnavigates it is actually accelerating towards the pivotal point and so energy is required to make this happen !  There is an input then ?  If there wasn't ,then the velocity of the ball after its trip changing direction would be less .  If I'm talking rubbish don't be afraid to tell me   It is early in the morning for me so my brain cell is not up to its normall 5 % efficiency  . I'm not quite sure how this affects my model in its downwind turn  - All  i have ever done is make a sweeping /longer/exaggerated turn -from ground / pilots perspective that is in highish winds or turbulence just to make sure I take the same TIME to turn as on the upwind leg  In other words -pretend you're travelling in a car or something & controling through the roof opening whilst going with the wind & at its velocity .In other -other words Ignore the ground completely Just think of the wind blowing in your face in the cockpit   Slghtly off track but as I fly I sub conciously tell myself  left ,right up down throttle  etc but thats just me .Others I know look for dipping wings etc & react accordingly , I daren't even think about it 'cos I know I'd get it wrong

Difficult trying to explain how you do things by "feel" isn't as in so many other scenarios you encounter ? . Makes me think of Dads army for some answers-    _DON't PANIC  !!!

[Tim Mackey]

Small point of order...none of the comments made were mine - the post states clearly that Simon and Phil were having this discussion

Interesting info though - which is the point .

[Bert]

It certainly is interesting - these are the best sort of threads IMHO

I subscribe to the 'flying too slowly downwind and tip stalling on the turn'  theory

Bert

[Gemma Jane]

What was the question

[Simon Chaddock]

Thank you Timbo for putting this thread in the right place!

As a long time full size glider pilot (they have no engine so there is no change in the energy available - unless you dive) I can assure you that you do not have to speed up to turn down wind, the ASI and the aircraft attitude stays exactly the same but your ground speed changes.

Yes in windy conditions close to the ground (below 500'?) turbulence, caused by the wind, can create localised changes in both wind speed and direction and this can indeed cause a significant temporary change in the aircraft's airspeed so, I agree entirely, the stronger the wind the more you must raise your airspeed on the landing circuit to prevent turbulence (and wind shear) causing any trouble.

I subcribe to the "fly too slowly and the inside tip will stall in any turn" theory.

[Bert]

Simon Chaddock wrote (see)

I subcribe to the "fly too slowly and the inside tip will stall in any turn" theory.

I subscribe to that one as well

[Gemma Jane]

Oh I get the question now!

You will slow down a little in your turn Simon, maybe a few kts? I've not yet flown a glider but as you bank the lift vector will still act through the normal axis of the aircraft (i.e. it would appear inclined if the ground was the reference), it therefore now has a horizontal and vertical component, the vertical component being smaller than when in straight and level flight for a given airspeed. This must be so as it is the horizontal component of the lift vector that provides the force to turn the aircraft.

As the vertical component has reduced you must pitch up slightly and increase the angle of attack to maintain height (the vertical component of lift must equal weight if we are not to descend) and as the airspeed is proportional to the angle of attack, the aircraft must experience a small reduction in airspeed as the angle of attack increases.

In a PA-28 at 100kts I would see maybe a 5kt drop in airspeed for say a 30 degree banked turn which is acceptable and it isn't usually corrected for. The effects become more obvious in steeper, say 60 degree banks where it is usual to add thrust in a powered aircraft to counteract the loss in airspeed due to the further increased angle of attack if one wants to maintain height.

Your quite right though, the aircraft does not 'see' the air packet it is moving in, we have a little loss of airspeed due to the inclined lift vector in the banked turn, but it certainly isn't because the wind is now coming from behind in a downwind turn, the aircraft has been flying through an air packet and continues to do so regardless of the vector displayed by the air packet, to the aircraft it is identical to a flat calm, it is only as observers on the ground when flying models that we perceive a change in velocity.

I would say Myron has it, you can't fly anything if you cannot feel it and that is where pilotage is a different thing to theory, where one tends to run into problems is when trying to fight what is naturally happening, maybe literally a case of learning to 'go with the flow' (appologies for bad joke)  Which I'm very sure you understand Simon with your glider flying and hence why you can see perfectly well what is happening to the model in the downwind turn.

Corrections for gusts in the PA-28 out of interest would be to bung about 5kts on the approach speed whilst flying directly into 25-30kt gusts, clearly aircraft do not stall 'when hit from behind by a 40mph gust' (fortunately).

In all I would say there is a case for increasing the throttle a lilttle in the turn when flying a model simply because one doesn't have an ASI to check, it is more than just for the sake of the model pilot, the model is reducing in airspeed to maintain height in a turn yet the ground based pilot is perceiving an increase in velocity when turning downwind, so it certainly won't do it any harm to increase the throttle a little and keep the bank angle reasonable.

[flytilbroke]

I wish at least one "qualified" model flying instructor could understand what Simon and Gemma are so clearly saying. He instructs novices to reduce engine speed when completing a downwind turn, he might have the skill to hold a decent position, the novice is now so often in trouble with the models loss of airspeed while trying to maintain height..

[Guest]

I always thought that it was more likely the whole inboard wing stalled due to reduction in airspeed from

 1 )  change of true airspeed from "ground speed + wind speed " to "ground speed - wind speed"

 2) the air speed of the inboard wing being slower than the outboard wing particularly if the turn is tight.

 If the change in true airspeed is sufficient to bring the wing down to stalling speed, then the inboard wing will stall earlier -  between that and the turn, the efect appears to be a tip stall but as I indicated above, it is possibly the whole inboard wing which is stalling.

 moral - near the ground , keep speed up and turns wide.

[Gemma Jane]

Wings do not stall at a given speed John, they stall at a given angle of attack  Your formulae are I'm afraid quite incorrect see above where it has been made clear that the ground speed is entirely irrelevant to the aircraft, only the airspeed through the air packet has any relevance. Your model is not somehow flying outside of the wind and back into it, it is flying within it so the wind has no bearing on the airspeed.

True airspeed is something completely different btw, a correction applied to full size aircraft due to density changes at altitude.

One of the problems here is that any tendency for an aircraft to drop a wing is often referred to as a 'tip stall'. Very often it isn't a tip stall at all. One of the major reasons (despite the often quoted reason of faster moving outer wing) that the inboard wing goes first is the outboard wing tends to have a down going aileron on it, which makes a very effective flap and therefore increases the angle of attack required to reach the stall, the outer wing can therefore experience a higher angle of attack than the inner without stalling, so the inner is the one that will stall first.

[Former Member]

[This posting has been removed]

[Gemma Jane]

Well excuse me then Tom I best bow out.

I was not actually arguing with John I was attempting to explain what was actually happening. If he is also an aerodynamicist and aeronautical engineer  I would understand your comment.

I shall go re-write the text books with the formulae that the change of airspeed is from ground speed + wind speed to ground speed - wind speed which is simply nonsense.

And also that an aircraft does not lose speed in a turn because of the Newtonian solution. (oops we will have to totally explain away lift induced drag, oh well it will give me something to do)

Bye.

[Guest]

Tom,

how did you arrive at me being a "god" ?????

 Gemma,

I am no aerodynamics expert but why -in full size aircraft - is the stalling speed always quoted re safe flying  and not angle of attack ?? 

, Without getting into aerodynamics of which I know very little, i believe speed is quoted because in the final analysis that is the parameter which defines for the given aircaft ,including its wing, when all the bad things happen re lift.

 Re "ground speed" - perhaps the more correct term should be "observed speed" when viewed in the context of we, the pilots on the ground" flying the aircraft at a speed consistant with keeping it in the air.  Pilots of fullsize have the benefit of true airspeed by virtue of the pitot tube - something we dont

Of course the model is flying in the air , but we have to be aware of both that and the wind when we elect to turn down wind and be prepared for any adverse reaction of the model should we be flying at low speed as in turnig in on finals.   .

John

[Pete Rieden]

Timbo - forum moderator wrote (see)

Small point of order...none of the comments made were mine - the post states clearly that Simon and Phil were having this discussion

Interesting info though - which is the point .

A thousand apologies - I was confused by the transcribing from elswhere.

 PDR

[Pete Rieden]

Myron Beaumont wrote (see)

Peter   May I question your statement regarding your "clever ball! going found a post ?  As it circumnavigates it is actually accelerating towards the pivotal point and so energy is required to make this happen ! 

OK, stop there and think.

From the definition:

Energy = force times distance moved

Except that there's an extra bit people seem prone to forgetting. the ACTUAL definition is:

Energy = force times distance moved in the direction of the force.

...and THAT's the point. In centripetal acceleration the force is at right-angles to the direction of motion and so it CANNOT extract kinetic energy from the ball. The radius of turn is constant, so there is no movement in the direction of the force or force in the direction of the movement. Thus the turn is a zero-energy manoeuver (QED). That's why satalites in orbit will stay there forever without energy input unless there is some external factor that takes energy away (atmospheric drag, usually). It's also why a simple piece of string (or the arm of our clever ball) can charge the direction of a moving object without providing (or possesing) any energy of its own.

PDR

[Gemma Jane]

Hi John,

Firstly apologies for throwing my teddies earlier but I find it can be very annoying on forums when one has taken a great deal of time to explain something only to be told off like a naughty schoolgirl for debating an issue, particularly when the same poster does not offer any alternative argument.

Firstly the reason we have stalling speeds quoted in a Pilots Operating Handbook (POH) is because, well put simply, nobody every came up with a useful angle of attack indicator and so it is what pilots are use to!

Pilots think in terms of airspeed, aerodynamicists think in terms of angle of attack. Both are linked by the lift equation. What is actually quoted in a POH is a demonstrated stalling speed for a given configuration taking into account weight and balance etc etc. It is a fixed speed at which the aircraft stalled under test conditions. As the test conditions rarely exist in reality it therefore becomes a guide to at what speed the stall will occur. Many things can upset this such as G loading, increased AUW etc. So full size do not actually stall at a given airspeed, they stall at a given angle of attack just as any wing does, but this information is not easily presented to the pilot whilst IAS is. There is also a hint in the name 'indicated' it is just that the airspeed indicated on the dial not the actual exact airspeed due to many other factors and errors involved.

Your second point is a case of terminology nothing more however I think what is actually being experienced is a decrease in airspeed due to the bank, the aircraft loses height and the instinct is to pull back on the stick. This is exactly what full size pilots are taught not to do because the result is inevitable that the angle of attack is increasing, the airspeed is decreasing due to lift induced drag and the wing is beginning to reach the stall. I have seen this at model shows with experienced model flyers a number of times and it was blamed on 'tip' stall.

I would maintain that on the turn downwind the effect of the wind has no bearing on the model, only on the ground observer, one way to imagine this is to visualise the aircraft manoeuvring through a fixed block of air as it turns in that block there is no force from the wind that was not already there, the block is the wind moving along the ground, the model appears to speed up to the ground observer, but is actually flying exactly the same airspeed through the block as it was before the turn all other things being equal and accepting as discussed above that the model lost a little airspeed whilst banked in the turn due to the effect of lift induced drag from the banked wing. Loss of control is more likely to occur due I believe to the ground observer perceiving an increase in airspeed (which is actually an increase in ground speed) and therefore reducing the throttle, pulling the nose up to maintain height and again reaching the scenario where the critical angle of attack is exceeded.

At the end of the day your moral is entirely correct, keep the banks shallow near the ground and don't let the model get slow...... shame really I never learnt this with the PZ Mustang, crunch... crunch... crunch again (but I did spend some time trying to figure it all out )

[Pete Rieden]

Gemma Fairchild wrote (see)

One of the problems here is that any tendency for an aircraft to drop a wing is often referred to as a 'tip stall'. Very often it isn't a tip stall at all. One of the major reasons (despite the often quoted reason of faster moving outer wing) that the inboard wing goes first is the outboard wing tends to have a down going aileron on it, which makes a very effective flap and therefore increases the angle of attack required to reach the stall, the outer wing can therefore experience a higher angle of attack than the inner without stalling, so the inner is the one that will stall first.

I agree with most of what you say Gemma, but a few pedantic bits about the above section. The downgoing aileron is certainly a flap, but just what it does isn't really that simple, and it most certainly doesn't simply "increase the angle of attack" (although we often reference it in that way in analyses in the interests of a common datum). The trouble is that as soon as the aileron is deflected we get a different wing section altogether, and it has different characteristics (there are a few example polars in Abbot & Von Deonhoff showing this).

Now you can look at this in two ways - you can relate the angle of attack to a line drawn through the centre of the LE radius and the point of the TE. In this frame of reference the wing's stalling angle will increase dramatically with flap [aileron] deflection, so the whole issue becomes moot. My preference is to look at it in the other way - to leave the Angle of Attack datum where it was (a line drawn through the  LE centre and the zero-flap-deflection position of the TE). When looked at in this way the changes in stalling angle are dependant on the particular wing section and flap design - A&vD shows some polars where the angle is increased with flap and others where it's reduced.

IMHO the reason for wing-droping in turns is simply that no aeroplane is perfectly symetrical and so it's going to break to one side or the other, and the steep turns are usually acompanied by a side-slip which biases the spanwise flow and creates the tendency to stall towards the inboard wing. When I flew chipmunks I could demonstrate this by stalling in a 40 degree bank with small degrees of sidelip - the aeroplane would invariably drop a wing away from the slip ball!

PDR

[Pete Rieden]

John Laird wrote (see)

I am no aerodynamics expert but why -in full size aircraft - is the stalling speed always quoted re safe flying  and not angle of attack ?? 

, Without getting into aerodynamics of which I know very little, i believe speed is quoted because in the final analysis that is the parameter which defines for the given aircaft ,including its wing, when all the bad things happen re lift.

John,

Airspeed is used because it's easy to measure on the aircraft (or was in the days before electronic instrumentation). Airfoils know very little about speed - you can't "stall" an airfoil in a wind tunnel simply by reducing the airspeed - you have to increase the angle of attack to see a stall. In an actual aeroplane the "stalling speed" is the speed at which the wing has to exceed its stalling ANGLE to keep lift equal to weight. this speed varies with all-up weight, bank angle and G-vector, but the stalling angle will (to be simplistic) be the same throughout. That's why a Stampe can be in unstalled flight at 15kts when going over the top of a loop, but will stall at (IIRC) 40kts when straight and level.

My aeroplane (ie as an engineer - I don't fly them) is the Harrier. You REALLY don't want to stall a Harrier - it has some very exciting departure characteristics and if mishandled can be stalled at airspeeds of over 300kts. But the aeroplane is routinely flown at airspeeds below 10kts, with the wing unstalled.

Final point - all fast jets and most modern airliners use airspeed indicators for simple things, but they also have an Alpha (angle of attack) indicator. When flying near the lower edge of the envelope the non-handling pilot calls the airspeeds, but the handling pilot monitors the alpha indicator. On the fast jets the alpha indicator is right next to the visual centre on the HUD because it's important. Airspeed indication is off to one side to be looked at occaisionally.

PDR

[Simon Chaddock]

Wow! It has certainly caused a bit of stir. I now see why Timbo put it in a new thread.

Gemma, you are of course quite correct that in a turn the wings are having to do a bit more work which means a bit more drag so in a glider you will either slow down a bit in the turn or you maintain the airspeed by decending just a bit quicker. In an efficient glider at normal angles of bank the effect is pretty small, probably below the ability of the pilot to notice it - unless of course you are already flying really close to the stall speed then he probably will!

[Gemma Jane]

Pete Rieden wrote (see)

Gemma Fairchild wrote (see)

One of the problems here is that any tendency for an aircraft to drop a wing is often referred to as a 'tip stall'. Very often it isn't a tip stall at all. One of the major reasons (despite the often quoted reason of faster moving outer wing) that the inboard wing goes first is the outboard wing tends to have a down going aileron on it, which makes a very effective flap and therefore increases the angle of attack required to reach the stall, the outer wing can therefore experience a higher angle of attack than the inner without stalling, so the inner is the one that will stall first.

I agree with most of what you say Gemma, but a few pedantic bits about the above section. The downgoing aileron is certainly a flap, but just what it does isn't really that simple, and it most certainly doesn't simply "increase the angle of attack" (although we often reference it in that way in analyses in the interests of a common datum). The trouble is that as soon as the aileron is deflected we get a different wing section altogether, and it has different characteristics (there are a few example polars in Abbot & Von Deonhoff showing this).

Now you can look at this in two ways - you can relate the angle of attack to a line drawn through the centre of the LE radius and the point of the TE. In this frame of reference the wing's stalling angle will increase dramatically with flap [aileron] deflection, so the whole issue becomes moot. My preference is to look at it in the other way - to leave the Angle of Attack datum where it was (a line drawn through the  LE centre and the zero-flap-deflection position of the TE). When looked at in this way the changes in stalling angle are dependant on the particular wing section and flap design - A&vD shows some polars where the angle is increased with flap and others where it's reduced.

IMHO the reason for wing-droping in turns is simply that no aeroplane is perfectly symetrical and so it's going to break to one side or the other, and the steep turns are usually acompanied by a side-slip which biases the spanwise flow and creates the tendency to stall towards the inboard wing. When I flew chipmunks I could demonstrate this by stalling in a 40 degree bank with small degrees of sidelip - the aeroplane would invariably drop a wing away from the slip ball!

PDR

I have no argument there Pete, indeed it is not quite as simple as I put it as we now have what is in effect a differently cambered aerofoil and I would totally agree sideslip is a major factor. Though I thought the simple explanation offered something in terms of visualisation. (Well I hoped it did)

You lucky thing flying Chipmunks, I hope to get some time on them this year, proper aeroplanes