Turnbacks

[212man]

Shytorque, without trying to make excuses (apart from the memory loss!) I thought Vy was 80 kts. Now I'm curious: what was/is it?

[actus reus]

Loss of airspeed turning out of a headwind

I think there is a great deal of discussion that could be had re a loss of airspeed turning out of a headwind into a tailwind. Apart from the obvious discussion of CAS versus EAS and the ramifications of very slow speed aircraft such as a First World War type, there is the issue of slip and skid, whether the aircraft is in or out of balance, whether the aerodynamic forces involved are in equilibrium and, not a insignificant factor, the rate of turn used. Instantaneous effects, regardless of the overriding criteria of 'airmass movement' versus 'ground observation' will show that the airspeed will change during such a turn. If you take the manoeuvre to criticality, i.e approaching the 1G stall speed, in a twin engined aircraft with the critical engine (the engine inop that requires the highest speed for controllablility) shut down; I think you will very well find a decrease in airspeed turning out of a headwind. All thoughts welcomed.

[Flash2001]

Neville Shute Norway in his autobiography gives an interesting account of F/L George Stainforth, who had been hired to test fly the first Airspeed aircraft, sitting for hours in the cockpit prior to the first flight asking the same questions over and over again and running his hands over the controls repeatedly. The Airspeed group actually became rather uneasy in the situation and thought perhaps they had selected an unsuitable test pilot. On the first flight the engine stopped at less than 500ft after takeoff, F/L Stainforth did a 180 and returned to the field for a perfect downwind landing. Then they figured it out.
After an excellent landing you can use the airplane again!
(and Airspeed did)

[Megaton]

In a previous life I used to occaisonally back-seat while the pilots practised turn-backs. I seem to recall that certain combinations of height/speed/wind in the Hawk would provide insufficient energy for a safe return to the field. It was always interesting and sometimes exciting to watch :-)

[tonker]

The RAAF did indeed look into this matter and there is a document on the net somewhere detailing the results of their experiment.

In short 3 of the 5 test pilots on the programme died during the trials.

[H Peacock]

Turnbacks were (are) routinely practiced by QFI/QPNIs at Linton in the Tucano. You needed to have achieved a speed/height combination (130kts/500ft from memory). Without this you simply did not have the energy to complete the manouvre and still have the energy to safely eject if needed!

Options available included a 'dumb-bell' back on to the reciprocal, or a less agressive turn onto finals for another runway. The former involved a hard turn (45aob), invariably away from any crosswind (to help blow you off the centre-line) prior to a swift reversal onto the the reciprocal. (A bit like a procedural turn!). In the Tucano a top-tip was to take mid-flap ASAP to arm the stick shaker and give a slight increase in performance. Needed plenty of awareness re: speed, height , bank, ROD, seat parameters etc. The aim was simply to see if a safe landing could be made rather than jettisoning the aircraft! Landing ahead in a field was not recommended.

Another option was to simply fly a 180 as discussed by others, but this was rarley practiced due to relative position of the ac v runway. It was only an option when the ac was well upwind or had more energy when you could glide ahead for a few seconds prior to initiating the turn.

Take-off brief included details about the turnback to be flown if required. You needed to think about the planned departure; you were often planning to turn at 500ft anyway, so this needed to be included in the turnback plan. Have heard some people call it a turnback only if you go for the reciprocal rather that an alternate runway.

Finally...

There's no loss of airspeed turning out of a headwind to a tailwind. If there were, you couldn't fly a 360 at height without constantly changing attitude to maintain a constant speed, 'cause there's head and tail wind there as well.

However, if you turn back in these circumstances your groundspeed increases markedly (by 2 x windspeed once you've completed 180 degrees). As all this is happening close to the ground, and is thus very noticeable, the natural tendency is to pull off the apparently excessive speed. Result, tears all round.

I'm with JF - I don't agree! Wish I could prove it with the numbers, but I can personally testify that, in a light/slow aircraft, a slick turn onto a downwind trk will temporarily reduce your IAS and not instantly increase your gnd speed.

[rotorfossil]

Having practised turnbacks as a routine exercise as a QFI in the RAF, I have always had the thought as a possibility in later life, but practices (at a safe height) in draggy light aeroplanes convinced me that it wasn't a goer. However, for the sake of interest, I investigated the technique again as a part owner of a Europa. I found that in this slippery aeroplane, it was perfectly viable given certain criteria as mentioned by previous guys above.
You must pre brief before takeoff as to the lower and upper height limits and the direction of turn. These vary considerably depending on the wind but roughly in the region of 600 - 1200 ft for this type.
Looking out in the initial stages of the turn is not good news due to the apparent slip and acceleration over the ground. Once round the problem is nearly always a tendency to overshoot rather than drop short.

[ShyTorque]

Shytorque, without trying to make excuses (apart from the memory loss!) I thought Vy was 80 kts. Now I'm curious: what was/is it?

Do you know- I can't remember it and can't find that figure in my notes - which are now some 18 years old! Having said that, turning at 45 degree AOB increases the stalling speed by a factor of approximately 1.2 hence the need for an increase over normal glidespeed.

I don't think "Vy" is the correct term to use for a glide because it refers to best climb speed.

[Flying Binghi]

This may be simular to an EFATO turn back and what may be encountered -

When I do fixed wing or Heli cattle mustering, I encounter what I think of as a rolling turbulence (Roller) that travels with the prevailing wind.
In the Super cub, when I do a come back turn to keep the pressure on an animal,[ once I have the aircraft pionted down hill ] I sometimes get the down side of this roller and see the effect of a rapid increase in ground speed and desent, yet a fixed airspeed (Due to the position of the main herd and other facters, a musterer can not always operate into wind)
What I do in windy conditions to compensate for rollers and wind shear, is to manuver at a faster airspeed, then if all is right when pointed down hill, I do a full slip to slow down if needed.

[172_driver]

I used to believe myself turning from headwind to tailwind would result in a marked decrease in IAS, but this is a common mistake. The only loss in IAS results from increased induced drag during the turn.

No matter how rapid your heading change is when turning head- to tailwind there will be no decrease in IAS. Your airspeed doesn't change in relation to the cube of air you are in, only to the ground beneath you.

In order to understand this fact (and I still doubt myself sometimes) I imagine the cube of air being still and the ground map moving like on a rollerfloor below me. No matter what my heading is the air around me doesn't move and IAS remains the same. On the other hand, my speed in relation to the ground does vary with my heading.

[John Farley]

I72 driver

Sorry – not very fair of me to set you up like that. Well not you particularly but just anybody who came out with that sort of standard comment.

I said it depended on how quickly you turned and I stick by that. If your mount is a 172 you are quite correct. However some other aircraft have very different capabilities which can change the situation.

Take a Harrier on a heading of north doing 60 kt IAS on a flat calm day. Now change you heading to south in three to four seconds (easily done with full rudder I can assure you). I think you will probably be able to see for yourself that you will end up not with less than about 45 kt of backwards velocity (a bit more than a ‘reduction’ in IAS) because the drag that acts for that brief period of sideways flight is fighting the momentum of eight tons of aircraft doing 60 kt and does not represent much of a 'brake'.

Do this same downwind turn manoeuvre into an original headwind of say 30 kt and you might even die should you let the nose drop a tad so that the backwards IAS can get under your tailplane and blow it over your head.

BOAC knows ‘cos he was a Harrier pilot too.

Regards

JF

[Islander2]

Rogers' Paper

We are dissuaded from turning back following EFATO because experience has shown this all too often ends with stall/spin fatalities.

Am I alone in thinking that, in advising this maneuver be flown using a 45deg banked turn at 5% above the stalling speed, Rogers must therefore be completely barking?

Whilst some pilots may remain calm and collected following an actual EFATO, thereby fitting the profile of the simulator test group that supported the feasibility of his recommendation, I suspect the vast majority would exhibit varying levels of panic and be unlikely to fly with a very high degree of precision. Is it a terribly good idea, then, for pilots to be taught to turn back flying a profile where, using numbers appropriate to Rogers' Bonanza example, a stall would follow from them getting the speed low by 4 kts or from overbanking by 5deg?

I do believe that a turn back plan is good to have in mind ... but one where the numbers are based on Eckalbar's 45deg bank at 1.3x Vs45deg (which Rogers rubbishes as "from the popular aviation press" and "demonstrably incorrect") strikes me as infinitely preferably to the truly dangerous approach proposed by Rogers!

[Islander2]

JF wrote:

Take a Harrier on a heading of north doing 60 kt IAS on a flat calm day. Now change you heading to south in three to four seconds (easily done with full rudder I can assure you). I think you will probably be able to see for yourself that you will end up not with less than about 45 kt of backwards velocity (a bit more than a ‘reduction’ in IAS) because the drag that acts for that brief period of sideways flight is fighting the momentum of eight tons of aircraft doing 60 kt and does not represent much of a 'brake'.

BOAC knows ‘cos he was a Harrier pilot too.

Not that I was Harrier pilot (although I was a design engineer at Hawker's during your era, JF!), but do you not think your momentum example applies equally in turning upwind? Just posing the question!

Regards
Islander2

[172_driver]

JF, now I see what you mean but then we aren't talking about the wind anymore, are we?

I'll try to turn around quickly next time I ride my Cessna... maybe if I pull really hard?

[H Peacock]

So guys (specifically the increase groundspeed - don't lose airspeed - when you turn downwind brigade). Tell me why you think you feel any turbulence in a gusty wind? If your little box of air keeps moving about then surely your aeroplane inside does???


Turning a slow Harrier, or helicopter downwind is still flying. The 'problem' is that they can both achieve high rates of turn and fly at a low IAS at the same time, so the problems of losing IAS when turning downwind are far more pronounced.

[212man]

invariably away from any crosswind (to help blow you off the centre-line)

Surely if you are trying to return to the runway, you want to minimise the turn radius and therefore the number degrees of turn required to return. In a utopic world you would turn about the centre axis and point back at the reciprocal heading.

[Islander2]

H Peacock wrote:

so the problems of losing IAS when turning downwind are far more pronounced.

You were doing fine, until you got to that bit. Maybe the head-banging, though, suggests you won't want to re-consider!

There is no difference between turning down upwind and turning downwind at a given IAS. In both cases, a high rate of turn will result in loss of airspeed due to inertia and/or increased induced drag (the amounts of each depending on how the turn is achieved), and the loss will have exactly the same magnitude in both cases.

For conventional fixed wing aircraft flown according to IAS, there is no difference between upwind and downwind turns in steady wind conditions.

Harriers and helicopters are different! A hover stationary with respect to the ground is the extreme example. Turning infinitely rapidly from a 30 kts headwind to a 30 kts tailwind produces a 60 kts loss of airspeed. Vice versa produces a 60 kts increase in airspeed. This is the effect JF got very accustomed to and very much needed to concern himself about with respect to the wind getting under the tail. BUT, the IAS is completely different in each case. Turning downwind, you're starting with an IAS of +30kts. Turning upwind, you're starting with an IAS of -30kts. Try this now without keeping the Harrier (or helicopter) stationary above a point on the ground, and arrange for it to have a +30 kts IAS in both cases, and the loss of airspeed will now be 60 kts whether turning downwind or upwind.

[Genghis the Engineer]

Originally Posted by Islander2

Am I alone in thinking that, in advising this maneuver be flown using a 45deg banked turn at 5% above the stalling speed, Rogers must therefore be completely barking?

I was reading through it yesterday, and it did strike me as a nice bit of theory mercifully untainted by too much contact with the real world.
Whilst without doubt, it's a useful start to any analysis of the problem, and that you'd not really want to go below 1.05Vs - one's ability to fly this would depend a lot upon the aeroplane. I dont know the Bonanza personally, but if taking (say) a PA28 or Hawk you'd probably get away with it - the combination of high stick forces, clear audible stall warning at about that margin, and benign stalling characteristics should allow you to nibble the stall effectively. On the other hand, there are aeroplanes with poor stall warning, low pitch forces less than glaring stall warning, and less than benign stalling characteristics that I suspect would offer much too high a probability of crossing over into the stall - most likely fatally.

Oh yes, and how accurately can somebody hold a precise bank angle? At low level, under stress, and whilst keeping looking out of the window most of the time.

I certainly hope that if I'd been asked to referee that paper (I wasn't), I'd have had the sense to ask for inclusion of a few test points in support of the theoretical analysis. Without that, the drawing of conclusions is arguably a bit over-confident.

G

[H Peacock]

Surely if you are trying to return to the runway, you want to minimise the turn radius and therefore the number degrees of turn required to return. In a utopic world you would turn about the centre axis and point back at the reciprocal heading.

212man, I take your point, but the aim here is to use the crosswind to help in a different way. A jink away from the crosswind will quickly displace the aircraft from the centreline such that a tight 180 (plus a bit) will line you up back on the reciprocal. If there is no crosswind and you fly a turn you will clearly be displaced by the diameter of the turn from the centreline. If you had a very strong crosswind you could dispense with the jink away and just turn towards it, this would again reduce most of this displacement. Wish I could draw it to make things clearer.


Islander2 wrote

For conventional fixed wing aircraft flown according to IAS, there is no difference between upwind and downwind turns in steady wind conditions.

Harriers and helicopters are different! A hover stationary with respect to the ground is the extreme example. Turning infinitely rapidly from a 30 kts headwind to a 30 kts tailwind produces a 60 kts loss of airspeed. Vice versa produces a 60 kts increase in airspeed. This is the effect JF got very accustomed to and very much needed to concern himself about with respect to the wind getting under the tail. BUT, the IAS is completely different in each case. Turning downwind, you're starting with an IAS of +30kts. Turning upwind, you're starting with an IAS of -30kts. Try this now without keeping the Harrier (or helicopter) stationary above a point on the ground, and arrange for it to have a +30 kts IAS in both cases, and the loss of airspeed will now be 60 kts whether turning downwind or upwind.

Think I follow that and agree; your IAS will change when turning upwind/downwind.

There is no difference between turning down upwind and turning downwind at a given IAS. In both cases, a high rate of turn will result in loss of airspeed due to inertia and/or increased induced drag (the amounts of each depending on how the turn is achieved), and the loss will have exactly the same magnitude in both cases.

Not true. A slow aeroplane/helo does not need lots of 'g' to achieve high turn-rates, and therefore doesn't suffer from a big increase in induced drag.
In the Puma I could fly downwind with a low (+ve) IAS, but when I flew a balanced turn in to a strong headwind the IAS would rise dramatically before steadily reducing back to the steady state condition. You gain IAS turning in to the wind, lose it turning downwind - is that what you're saying?

[Islander2]

H Peacock wrote:

Not true. A slow aeroplane/helo does not need lots of 'g' to achieve high turn-rates, and therefore doesn't suffer from a big increase in induced drag.

Have another look at my post: it doesn't say that, does it? What it says is: "a high rate of turn will result in loss of airspeed due to inertia and/or increased induced drag (the amounts of each depending on how the turn is achieved)"

Also:

You gain IAS turning in to the wind, lose it turning downwind - is that what you're saying?

Categorically not, 'cos it isn't necessarily true! What I am saying is: 1) that the impact on IAS is identical whether you're turning upwind or downwind for the same pre-turn IAS, and 2) that whether or not the turn results in an increase or decrease in IAS is dependant solely on whether the IAS is initially greater or less than zero, and is nothing to do with whether the turn is upwind or downwind. If the pre-turn IAS is positive ahead of a very rapid turn, you'll temporarily lose an equal amount of airspeed whether turning downwind or upwind; if it's negative (obviously not a usual fixed wing scenario, but certainly one for Harrier pilots), you'll temporarily gain an equal amount.

Edited to add an intepretation note: all the above assumes a steady wind.