Turnbacks
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As has been said already, go and practice at a safe ht. In most light a/c including the Pitts, from an engine failure in the after take off mode (again, at a safe ht) I can be pointing the other way with a loss of 200ft. Just stuff the nose down, roll on 60 or so bank, pull to the stall warner, roll out on heading. After a bit of practice you can refine it of course! It is a bit of fun just seeing what your a/c can do. Please no hands up in horror replies.
eeeerr... Croqueteer, You are an experienced (?) aerobatic pilot.The advice you are giving may not be suitable for low time/experience pilots. I think what most in this thread are thinking of is how best to approch turn back training and theory.
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After reading all he posts and adding my experience, i would sum up by saying that turnbacks are a possibility given a whole lot of considerations:
a) it should only be considered in aircraft which have good L/D ratios and don't lose speed quickly at higher angles of attack (like most non aerobatic biplanes). It also helps if the aircraft has a high power to weight ratio and therefore gains height quickly after takeoff.
b) Given the above, the exercise must be practised at a safe height first.
c) Before takeoff, the parameters off minimum and maximum heights and therefore distance from the airfield must be decided.
d) The direction of turn must be considered depending on the crosswind (personally I favour turning towards the wind) and the airfield configuration.
e) The absolute necessity of maintaining safe airspeed in relation to the dynamic stall speed in a steep gliding turn.
f) Lastly, the first part of the turn must be flown without reference to the visual effects of slip and apparent increase in groundspeed.
Given all the above, anyone presented with a donk failure after takeoff and hasn't made all the correct decisions beforehand is better off going straight ahead.
a) it should only be considered in aircraft which have good L/D ratios and don't lose speed quickly at higher angles of attack (like most non aerobatic biplanes). It also helps if the aircraft has a high power to weight ratio and therefore gains height quickly after takeoff.
b) Given the above, the exercise must be practised at a safe height first.
c) Before takeoff, the parameters off minimum and maximum heights and therefore distance from the airfield must be decided.
d) The direction of turn must be considered depending on the crosswind (personally I favour turning towards the wind) and the airfield configuration.
e) The absolute necessity of maintaining safe airspeed in relation to the dynamic stall speed in a steep gliding turn.
f) Lastly, the first part of the turn must be flown without reference to the visual effects of slip and apparent increase in groundspeed.
Given all the above, anyone presented with a donk failure after takeoff and hasn't made all the correct decisions beforehand is better off going straight ahead.
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I'm not sure how applicable this is to GA but low turns in gliders are discussed here..
http://www.abqsoaring.org/misc_files..._Accidents.pdf
Basically the message appears to be not to float around the turn slowly trying to preserve height but the get the turn done positively. A slow turn has a larger radius, longer distance to fly therefore potentially a greater loss of height. I don't believe this contradict the advice to fly the turn "just above the stall" because the stall speed will vary with rate of turn.
See page 15..
"A common mistake by many glider pilots in situations when height becomes critical is to instinctively keep the glider’s nose up while entering a dangerous low-banked turn with the intention of preserving height. Knowing that at the same rate of lift while circling in a thermal, a lower bank angle will increase the rate of climb the pilot assumes that a lower bank angle in a no lift situation will preserve height due to the lower sink rate. This can be a fatal mistake. As demonstrated above a low-banked turn takes much longer to complete and as a result significantly extends the glider’s exposure to its inherent sink rate. This leads to a higher total loss of height despite the lower inherent sink rate as compared to a high-banked turn with its relatively higher inherent sink rate."
continues..
http://www.abqsoaring.org/misc_files..._Accidents.pdf
Basically the message appears to be not to float around the turn slowly trying to preserve height but the get the turn done positively. A slow turn has a larger radius, longer distance to fly therefore potentially a greater loss of height. I don't believe this contradict the advice to fly the turn "just above the stall" because the stall speed will vary with rate of turn.
See page 15..
"A common mistake by many glider pilots in situations when height becomes critical is to instinctively keep the glider’s nose up while entering a dangerous low-banked turn with the intention of preserving height. Knowing that at the same rate of lift while circling in a thermal, a lower bank angle will increase the rate of climb the pilot assumes that a lower bank angle in a no lift situation will preserve height due to the lower sink rate. This can be a fatal mistake. As demonstrated above a low-banked turn takes much longer to complete and as a result significantly extends the glider’s exposure to its inherent sink rate. This leads to a higher total loss of height despite the lower inherent sink rate as compared to a high-banked turn with its relatively higher inherent sink rate."
continues..
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The perspective, visual horizon, and ground rush are enormously significant.
Only turn back if you have planned to do so on the grounds that you knew it was safe. This requires a highish performance aircraft (either in terms of climb angle, glide performance, or both) and a skilled and determined pilot.
Only turn back if you have planned to do so on the grounds that you knew it was safe. This requires a highish performance aircraft (either in terms of climb angle, glide performance, or both) and a skilled and determined pilot.
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cwatters wrote:
The BGA training (backed up with the maths and empirical investigation, see e.g. Longland, Gliding (A&C Black 2002) pp 105-9, though this discusses turning generally and not this specific case) is that the minimum height loss in a gliding turn is at 45 degrees of bank. This is the best compromise between time in the turn and sink rate, for gliders at least, and produces the minimum loss of height in the turn.
So far as speed is concerned, such a turn would normally be flown at at least approach speed (1.3 vs), some draggy types maybe faster. This would allow a (for gliders) comfortable margin above the turning stall speed of around 5-7 kt, so it might be "close to the stall" as discussed in this thread.
I only fly gliders, so can't say how far this translates to a powered a/c with engine failure. My sink rate in such a turn is less than 3 ft per second, so at 50kt I'd expect to lose no more than 50ft if I fly it properly. My guess is that the 45 degree turn is optimal for all a/c forced to become gliders, because the maths is the same, but this is only a guess.
Glider turnbacks would be from a failed aerotow, which would likely be caused by engine failure of the tug aircraft, and this is something which we consider and in some countries train for (though not the UK). The UK briefing is to set a decision height below which you will not turn back, even if you might theoretically make it. For experienced glider pilots that height will be between 200 and 300 ft - this gives a margin of, say 100ft for a hurried turn and 100 to 200 ft to make it back to the airfield. As the climb rate on an aerotow ought to be at least twice the sink rate of a glider, this leaves an adequate margin.
I'd say do the calculations on the ground - a failed engine a/c which has a sink rate higher than its climb rate with the engine working would never be in a position to turn back, even if it could make a no loss turn!
Basically the message appears to be not to float around the turn slowly trying to preserve height but the get the turn done positively. A slow turn has a larger radius, longer distance to fly therefore potentially a greater loss of height. I don't believe this contradict the advice to fly the turn "just above the stall" because the stall speed will vary with rate of turn.
So far as speed is concerned, such a turn would normally be flown at at least approach speed (1.3 vs), some draggy types maybe faster. This would allow a (for gliders) comfortable margin above the turning stall speed of around 5-7 kt, so it might be "close to the stall" as discussed in this thread.
I only fly gliders, so can't say how far this translates to a powered a/c with engine failure. My sink rate in such a turn is less than 3 ft per second, so at 50kt I'd expect to lose no more than 50ft if I fly it properly. My guess is that the 45 degree turn is optimal for all a/c forced to become gliders, because the maths is the same, but this is only a guess.
Glider turnbacks would be from a failed aerotow, which would likely be caused by engine failure of the tug aircraft, and this is something which we consider and in some countries train for (though not the UK). The UK briefing is to set a decision height below which you will not turn back, even if you might theoretically make it. For experienced glider pilots that height will be between 200 and 300 ft - this gives a margin of, say 100ft for a hurried turn and 100 to 200 ft to make it back to the airfield. As the climb rate on an aerotow ought to be at least twice the sink rate of a glider, this leaves an adequate margin.
I'd say do the calculations on the ground - a failed engine a/c which has a sink rate higher than its climb rate with the engine working would never be in a position to turn back, even if it could make a no loss turn!
Last edited by ProfChrisReed; 19th Nov 2007 at 21:45. Reason: To clarify stall speed in turn
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The Professor wrote:
...and he clearly hasn't considered runway length, wind speed and direction, or the possible desireability of getting back to the airfield over, say a landing on water. It is climb angle, descent angle (both taking into account wind speed and direction and pilot actions), and airfield environment, that count. 
a failed engine a/c which has a sink rate higher than its climb rate with the engine working would never be in a position to turn back
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Sorry Chris, but I think you're wrong. I rarely like to deal in such black and white terms, but while the idea of the aircraft moving relative to a package of air is a fine way to think about navigation and the like, you really can't forget that an aircraft's inertia comed from its groundspeed (well, technically we need to get even more complicated than that, but bear with me!). An aircraft's airspeed will determine the aerodynamic forces available for it to turn, while its groundspeed will determine its momentum, and thus rate of turn, turn radius, etc.
Take for example an aeroplane flying at 60 knots IAS with a 60 knot tailwind. Fairly obviously, it will have a groundspeed of 120 knots, and so 120 knots worth of momentum. If that same aeroplane turns into wind, with sufficient added power to compensate for the extra induced drag, then, assuming a relatively instantaneous turn, it would suddenly find itself with 120 knots groundspeed into a 60 knot headwind, resulting in 180 knots airspeed. The large resultant increase in drag would rapidly reduce the airspeed to 60 knots, resulting in zero groundspeed. Of course, this drag doesn't wait until the aircraft is pointing directly into wind to take effect; it will begin to bite the moment the aircraft turns slightly from the wind, and is therefore easily disguised by induced drag effects in the turn. It will be more noticeable in heavier aircraft, as JF's Harrier example shows.
Chris - your statement about sink/climb rates is also suspect - you've neglected to consider the effect of runway length. With a sufficiently long runway a turnback would still be viable even if your sink rate was higher than your climb rate, as you won't be aiming to arrive back at the same point you got airborne from. Feel a bit like I'm having a personal dig at you, promise I'm not, sorry!
All of which is fairly well off topic, apologies!
Turnbacks are still a currency item for RAF single engine FW QFIs. Tucano turnbacks have already been thoroughly described here. Hawk turnbacks are only routinely practiced to runways other than the reciprocal of the departure runway, with an additional consideration being that the approach end barrier of the runway in use (ie the one likely to be needed to stop a heavyweight aeroplane landing downwind at high speed on the reciprocal of the departure runway) is generally electrically isolated, and would therefore likely be unavailable. Tutor turnbacks are very similar to the Bulldog variant described - 80 knots and an immediate 45 deg AoB turn into any crosswind being the key parameters.
For my money I'd always get the 180 done first, rather than worry about jinking away from the crosswind for separation from the centreline. I'd rather sort out the line up after at least pointing myself in the general direction of the airfield crash services. That is, howver, just my thinking, and jinking first may well be a better plan in aeroplanes with steep climb angles, to leave more runway available for the downwind landing by tracking away from the runway initially. So many variables!
Take for example an aeroplane flying at 60 knots IAS with a 60 knot tailwind. Fairly obviously, it will have a groundspeed of 120 knots, and so 120 knots worth of momentum. If that same aeroplane turns into wind, with sufficient added power to compensate for the extra induced drag, then, assuming a relatively instantaneous turn, it would suddenly find itself with 120 knots groundspeed into a 60 knot headwind, resulting in 180 knots airspeed. The large resultant increase in drag would rapidly reduce the airspeed to 60 knots, resulting in zero groundspeed. Of course, this drag doesn't wait until the aircraft is pointing directly into wind to take effect; it will begin to bite the moment the aircraft turns slightly from the wind, and is therefore easily disguised by induced drag effects in the turn. It will be more noticeable in heavier aircraft, as JF's Harrier example shows.
Chris - your statement about sink/climb rates is also suspect - you've neglected to consider the effect of runway length. With a sufficiently long runway a turnback would still be viable even if your sink rate was higher than your climb rate, as you won't be aiming to arrive back at the same point you got airborne from. Feel a bit like I'm having a personal dig at you, promise I'm not, sorry!
All of which is fairly well off topic, apologies!
Turnbacks are still a currency item for RAF single engine FW QFIs. Tucano turnbacks have already been thoroughly described here. Hawk turnbacks are only routinely practiced to runways other than the reciprocal of the departure runway, with an additional consideration being that the approach end barrier of the runway in use (ie the one likely to be needed to stop a heavyweight aeroplane landing downwind at high speed on the reciprocal of the departure runway) is generally electrically isolated, and would therefore likely be unavailable. Tutor turnbacks are very similar to the Bulldog variant described - 80 knots and an immediate 45 deg AoB turn into any crosswind being the key parameters.
For my money I'd always get the 180 done first, rather than worry about jinking away from the crosswind for separation from the centreline. I'd rather sort out the line up after at least pointing myself in the general direction of the airfield crash services. That is, howver, just my thinking, and jinking first may well be a better plan in aeroplanes with steep climb angles, to leave more runway available for the downwind landing by tracking away from the runway initially. So many variables!
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Wind effects on airspeed in a turn
Most definitely a potential 'killer', one of the major problems being the loss of airspeed going from a headwind to a tail and the tendency to stall in the turn therefore.
I've not got any instrument rating so haven't been able to experiment... does this loss-of-airspeed effect manifest itself above cloud and out of sight of the surface? I look forward to reading responses from people who've tried it.
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Take for example an aeroplane flying at 60 knots IAS with a 60 knot tailwind. Fairly obviously, it will have a groundspeed of 120 knots, and so 120 knots worth of momentum. If that same aeroplane turns into wind, with sufficient added power to compensate for the extra induced drag, then, assuming a relatively instantaneous turn, it would suddenly find itself with 120 knots groundspeed into a 60 knot headwind, resulting in 180 knots airspeed.
In your example, for a hypothetical instantaneous turn the airplane would, as you say, initially continue to have 120 kts groundspeed. However, the point I think you may be missing is that it will initially maintain its same path across the ground, in other words it will also maintain its 60 kts with respect to the air mass, but it will now be pointed in the opposite direction, thus flying backwards and having a negative airspeed of 60 kts, not the positive airspeed of 180 kts you claim. Of course in the real world (everyone knows Harriers and helicopters are Alice in Wonderland!), we turn anything but instantaneously!
Last edited by Islander2; 21st Nov 2007 at 18:58.
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Knight Paladin
You are wrong wrong wrong and misleading many aviators.
Once you leave the earth the ONLY relativity your aircraft has with the earth is gravity. The aircraft performs relative to the air supporting it and to gravitation attraction. Groundspeed becomes entirely irrelevant to manoeuvering and the earth only becomes significant in the vertical plane to earth.
Your momentum becomes relative to the air molecules or to whatever you run into be it another aircraft or a rock filled cloud.
You are wrong wrong wrong and misleading many aviators.
Once you leave the earth the ONLY relativity your aircraft has with the earth is gravity. The aircraft performs relative to the air supporting it and to gravitation attraction. Groundspeed becomes entirely irrelevant to manoeuvering and the earth only becomes significant in the vertical plane to earth.
Your momentum becomes relative to the air molecules or to whatever you run into be it another aircraft or a rock filled cloud.
You are correct Milt.
What the Earth does add to the wind facter is Wind Shear and turbulance.
An EFATO turn back is a desending manuver with wind shear a facter in high winds.
What the Earth does add to the wind facter is Wind Shear and turbulance.
An EFATO turn back is a desending manuver with wind shear a facter in high winds.
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Milt - Sorry chum, A-level physics wins. Yes, the force produced by the air that acts on the aeroplane comes from the difference in momentum between the aeroplane and its surroundings, and yes, the only direct link between the Earth and the aeroplane is gravity, BUT, in just the same way that the Earth has it's own momentum travelling through space, so does the aeroplane. This momentum can most easily be referenced to that of the Earth itself, hence groundspeed is relevant. Of course, strictly speaking you need to consider the momentum of the aeroplane through 3-D space, probably referenced to orthoganal axes, but I fear that would be getting far too complicated.
At least, that's the way I've always understood and been taught it, the various poor buggers who've had to teach me could all have been clowns. Or... physics could work completely differently when you're on the wrong side of the world, surely you guys learn this from having to coordinate with opposite rudder?
Islander - true, instantaneous turns are indeed impossible, was just using a rather contrived example. As I said, the actual change is very progressive throughout the turn.
Edited to add lame Aussie banter
At least, that's the way I've always understood and been taught it, the various poor buggers who've had to teach me could all have been clowns. Or... physics could work completely differently when you're on the wrong side of the world, surely you guys learn this from having to coordinate with opposite rudder?
Islander - true, instantaneous turns are indeed impossible, was just using a rather contrived example. As I said, the actual change is very progressive throughout the turn.
Edited to add lame Aussie banter
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Knight Paladin said:
Oh well, there goes the last vestige of any notion that A-level is the gold standard!
And also:
That's as may be, but how do you account for the IAS you quoted for an instantaneous turn being 240kts in error (I assume by 'airspeed' you meant forward airspeed, i.e. IAS)?
Milt - Sorry chum, A-level physics wins. Yes, the force produced by the air that acts on the aeroplane comes from the difference in momentum between the aeroplane and its surroundings, and yes, the only direct link between the Earth and the aeroplane is gravity, BUT, in just the same way that the Earth has it's own momentum travelling through space, so does the aeroplane. This momentum can most easily be referenced to that of the Earth itself, hence groundspeed is relevant. Of course, strictly speaking you need to consider the momentum of the aeroplane through 3-D space, probably referenced to orthoganal axes, but I fear that would be getting far too complicated.
And also:
As I said, the actual change is very progressive throughout the turn.
Last edited by Islander2; 21st Nov 2007 at 20:48.
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Sadly my idea of "A-level" refers to the UK qualification, and is therefore, particularly in this day and age, far from being a gold standard.
I'm fairly sure that speed in the inertial plane is of (minor, relatively) importance in the general physics of flight, but we're deviating massively from the point of the thread, and I really don't think it's a critical issue when considering turnbacks. In addition, I'll be honest, I'm primarily an operator rather than a boffin, so we're getting well away from my Stick Monkey comfort zone.
Bit of a cop-out I know, but will gladly hear any more techniques/advice about the flying of turnbacks, have very much enjoyed the more practical discussion in this thread.
I'm fairly sure that speed in the inertial plane is of (minor, relatively) importance in the general physics of flight, but we're deviating massively from the point of the thread, and I really don't think it's a critical issue when considering turnbacks. In addition, I'll be honest, I'm primarily an operator rather than a boffin, so we're getting well away from my Stick Monkey comfort zone.
Bit of a cop-out I know, but will gladly hear any more techniques/advice about the flying of turnbacks, have very much enjoyed the more practical discussion in this thread.
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I'm fairly sure that speed in the inertial plane is of (minor, relatively) importance in the general physics of flight
I really don't think it's a critical issue when considering turnbacks.
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I can't believe downwind turns seem to have come up again...
I'm supprised that no one has mentioned what effect the length of runway has to do with whether or not a turnback can be done or not....If I take off from say 2000m of tarmac in my Super Cub and climb away....I'll be pretty high by the end of the runway and while I admit I've never tried it for fear of some instructor telling me off I'm pretty sure that I could land back again. There was an article in Flyer recently that covered them and actually it was quite positive. Just try finding an instructor who is prepared to teach them though.
I'm supprised that no one has mentioned what effect the length of runway has to do with whether or not a turnback can be done or not....If I take off from say 2000m of tarmac in my Super Cub and climb away....I'll be pretty high by the end of the runway and while I admit I've never tried it for fear of some instructor telling me off I'm pretty sure that I could land back again. There was an article in Flyer recently that covered them and actually it was quite positive. Just try finding an instructor who is prepared to teach them though
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I suspect the reality is somewhere between the two positions being taken on inertia.
Consider the example of wind shear. As a glider descends to land it experiences reduced headwinds due to wind shear. According to one camp a glider shouldn't notice this reduction in headwind - it's ground speed should "automatically" increase to maintain a constant airspeed (afterall it's lift drag and weight and trim hasn't changed so it's airspeed shouldn't change). In practice due to it's mass and inertia a glider cannot accelerate fast enough so it does experience a loss of air speed if it descends rapidly through wind shear. Glider pilots typically add half the wind speed to their landing speed to allow for this effect and for other reasons.
Consider the example of wind shear. As a glider descends to land it experiences reduced headwinds due to wind shear. According to one camp a glider shouldn't notice this reduction in headwind - it's ground speed should "automatically" increase to maintain a constant airspeed (afterall it's lift drag and weight and trim hasn't changed so it's airspeed shouldn't change). In practice due to it's mass and inertia a glider cannot accelerate fast enough so it does experience a loss of air speed if it descends rapidly through wind shear. Glider pilots typically add half the wind speed to their landing speed to allow for this effect and for other reasons.
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cwatters wrote:
I suspect this is right, too. However, I am sure from empirical means that the effects of intertia are very small. I have flown thousands of 360 degree circles at 45 degrees of bank and an IAS of 45kt, in airmasses which are moving at 30kt or more. It is possible to maintain an apparently steady airspeed doing this. If head/tailwind had any appreciable effect on airspeed I'd stall once on each orbit!
I can't believe that proximity to the ground makes any difference, and I have enough circles at 600 ft agl to know that, again, there are no detectable speed differences related to the wind direction and my heading.
My guess is that the force of the aircraft's intertia (is it a force? Why is there never a physicist on hand when you need one?) operates at a tangent to the circle you are flying. Thus as you turn through 180 degrees the inertial force is at all times nearly aligned with your heading, so that for practical purposes you can assume no headwind/tailwind difference.
Losing airspeed because of the higher induced drag in the turn is an entirely different matter - this would equally be a problem turning from downwind to headwind.
I suspect the reality is somewhere between the two positions being taken on inertia.
I can't believe that proximity to the ground makes any difference, and I have enough circles at 600 ft agl to know that, again, there are no detectable speed differences related to the wind direction and my heading.
My guess is that the force of the aircraft's intertia (is it a force? Why is there never a physicist on hand when you need one?) operates at a tangent to the circle you are flying. Thus as you turn through 180 degrees the inertial force is at all times nearly aligned with your heading, so that for practical purposes you can assume no headwind/tailwind difference.
Losing airspeed because of the higher induced drag in the turn is an entirely different matter - this would equally be a problem turning from downwind to headwind.