Tightest Radius Turn?

[Sunfish]

I'm getting myself somewhat confused about exactly how to make the tightest possible 180 degree turn (in the horizontal plane) in a non aerobatic aircraft. While I do not ever intend to do any scud running (who would?), the question of reversing course in a valley exercises my imagination.

Now the texts will tell you that as your velocity increases, then at constant G the radius must increase as the square of the speed. So in theory, if I pull 2G in a level 60 degree banked turn, I need to be doing 1.414 (ie square root two) times Vs to generate the required lift. In say an Arrow, Vs = 60 knots indicated, so I should be doing a little above 85 knots to do the minimum radius turn possible at 2G. Anything over 85 knots is going to widen the turn more than necessary. When you read the Piper POH, well bugger me, the stall speed at 60 degrees bank is....85 knots.

Then you can do the exercise with flaps down, Vs = 55 and you get 77 knots...although the book says 75 is the stall speed.

So in both cases we are flying just below the critical angle.

However Mr. Robson's excellent book on basic beginners aerobatics has managed to both educate and perhaps slightly confuse me at the same time, and I wonder here if anyone can elucidate some of the mysteries.

1. He explains that at Vs in level flight, no turns are possible because there is no excess lift - which is so blindingly obvious, I wonder why it never entered my head before, and it explains why approach speeds are what they are.

2. He also explains that if I wish to pull 3.8G, the maximum allowable on my theoretical Arrow, then I have to be doing very close to twice Vs for obvious reasons. I can't use flaps as the limit is 103 knots and I assume that they are rated for 2G max extended anyway.

So my question is, which of these approaches, 3.8G at 120 KIAS, 2G at 85 KIAS or 2G at 77KIAS flaps extended to 40 deg. (plus a gust margin and a little bit extra) is going to produce the tightest turning radius? :hmm:

[Brian Abraham]

Here you go Sunny. For homework I'll let you do the maths.
http://en.wikipedia.org/wiki/ROT

[ForkTailedDrKiller]

Why do you want to know?

If its just for the mental gymnastics - then skip this post. Its too long since I did all that stuff to be bothered with it now, but ........

..... if you genuinely want to know how to do a 180 to get out of IMC, then I would suggest that you focus on holding a Rate 1 turn on the T&B, but ......

...... if you want to know how to do a 180 in the least amount of sky, this is how I would do it -

roll into the turn and pull on 1st stage of flap as you speed washes off, and continue to increase bank and increase back pressure until you feel the start of buffet or until the stall warning starts - then adjust your angle of bank to stay just above the stall until you are ready to roll out.

Something you probably should practice!

Does a Max Rate turn = Min Radius turn?

Beggered if I know - but probably near enough.

I have done it only once in anger when I flew up a blind valley in NZ in a Traumahawk and realised the cloud covered mountains either side were closing in on me. Needed a change of undies afterwards but lived to fly another day. Others have been less fortunate.

Dr :8

[paul makin]

Sunfish

The only figure I have is radius of turn = V squared over 11.26 x TAN bank angle.

Mathmatically wouldn't have a clue how the various configs change that, but they probably do.

But at the end of the day. Is it really relevant cos if you have put yourself in that much sh1t, you probably won't have time to change your config any way.
PM

[tinpis]

I've turned so sharply once backing out of a New Guinea valley,that I nearly collided with myself coming the other way :hmm:

[RandyBandit]

Why use 60deg AOB? why fly it fast? You dont want to get out quick, just in minumum radius. Stick to a comfortable, safe Angle of Bank (even rate 1) and fly as slow as the a/c config will let you. The slow speed is what dramatically reduces the radius of the turn.

RB

[Sunfish]

Randy:

Going from 60 degrees bank to 30 degrees bank and dropping speed from 77 knots to 70 knots INCREASES radius from 303 feet to 751 feet! - Wrong way!

[Capnmakka]

Sunfish:

Maybe he's just trying to get you whacked.

[ForkTailedDrKiller]

if I wish to pull 3.8G, the maximum allowable on my theoretical Arrow

Which, of course, is why you should be flying a Bonza - where the limit is 4.4G.

Just rack it over and fly it on the point of the stall by feel!

Oh yes - and you can leave your feet flat on the floor too!

Dr :8

[Dixons Cider]

Too many numbers and too much to think about if you're in the shiite and about to pancake onto a valley wall - do as FTDK says...turn hard, keep enough speed/use enough flap/power, to keep you above the stall.

Definitely something you should practice. If you need it one day, guarantee you'll not be focusing on formula's or flight manual details.

Not being a smart arse here, kudos to you for digging into the theory, however reality is sometimes a tad different. Just go and try it.

[E&H]

Thanks for that Tinpis....needed a good laugh

Probably not laughing at the time though:eek:

[Capt Wally]

Dr & 'dicko' is right, keep it simple & just remember that stall speed increases the more AofB you pull on, that's the basics of it & that's all you ought need to recall when the spam hits the fan!. Lower the nose slightly in the tight turn if alt permits also. And finally the BEST advice I can offer here as well as everyone else DON'T get yrself in a position where it requires very tight turns in the first place ! If you need to figure out from all the boffins in here as to how you can achive the tightest possible turn then yr going to be too damn close to the valley wall anyway.:bored:




CW

[elche]

Here's another way....

box canyon turn
http://www.pilotfriend.com/safe/safety/canyon_turn.htm

[bushy]

Figure out what the theory says is best for your aircraft, then go out and practise it at altitude.
Remember the Hamilton Island accident. Too many of todays pilots have not done spin training.
It's also smart to stay on the downwind side of the valley, so you will be turning into wind.
I remember once, a long time ago, flying through the Haast pass in NZ under the cloud. Fortunately we did not have to turn around, as we took all the right valleys.
NZ (south Island)and PNG pilots will have intimate knowledge of these techniques.
(and lots of ozzies who will not admit it)

[404 Titan]

bushy
NZ (south Island)and PNG pilots will have intimate knowledge of these techniques.
Correct. Smallest radious turn is achieved with full flap and as slow as you can go. Any technique that results in high g's being pulled will kill you in a tight enviroment.

[djpil]

aaaaaaaaaaaaaahhhhhhhhhhhhhhh Sunfish, this is one of my favourite discussion topics over a cup of coffee or glass of red wine.

So my question is, which of these approaches, 3.8G at 120 KIAS, 2G at 85 KIAS or 2G at 77KIAS flaps extended to 40 deg. (plus a gust margin and a little bit extra) is going to produce the tightest turning radius? The rest of the question is whether you can descend or not?

Refer figure 11 at http://www.avweb.com/news/airman/182421-1.html For the Cessna 172, the tightest turn radius is at 100 kts CAS pulling 3.8g but descending at over 4000 fpm - gives a radius of 350 ft. Unfortunately the graph is provided only at an altitude of 12,000 ft (I've been down a canyon at 12,000 ft in a 172).

If you want to maintain height then you've got to fly slower - that same chart suggests 58 kts CAS but again that's at 12,000 ft. At lower altitudes the performance is better so a higher airspeed.

[youngmic]

Yep dicko and FTDK are spot on.

Pretty much the same teqhnique as every Ag pilot around the world uses.

Cept they pull up a bit first.

[Captain Nomad]

From someone who has done them in anger, the practical tips above are worth noting. At least some flap (in the vast majority of acft) is very helpful - keeps the speed slower and usually gives you a much better margin over the stall. Secondly, telling you to suck eggs but you would be surprised how much speed can be washed off in these turns - especially in a loaded aircraft. Add power - probably more than what you would expect.

Know your aircraft and how it handles - especially at different weights. What's easy in a light aircraft can become very challenging in a loaded one.

When you are poking up a valley where you think you may need to turn around - ALWAYS fly on one side (downwind if possible).

As has been previously mentioned, if you can descend while turning - great - you don't have to load up as much - but watch the speed! If your speed races away you could find the radius increasing rapidly and you might be in trouble with your white arc also. If you are doing this in anger it could be harder than what you thought as you will have no proper horizon and your motor skills can become impaired with stress...

Wouldn't advise it but Smithy saved his hide in NZ doing a stall-turn to get out of a valley once...

[grusome]

Guys,
Take FTDK's advice and add full power (if you are saving your life, that is). Think about it - buffet threshhold, stall AoA, ability to gain a bit of height. You'll never find a knuck trying max rate/min radius without full chat+AB.
Gru

[scrambler]

Turn downwind initially until at about 30 deg to the side of the valley, when relatively close to the side turn into wind, slow lots of flap, at about 45 deg to the other side if you are certain you are going to make it around, rack it over (45 to 60 deg bank max) and continue the turn, lots of power, above the stall. Practice it first.

Turning into the wind for the reversal will make a huge difference, turning away (downwind) for the reversal kiss your @ss goodbye

[Mr. Hat]

I did some specific training on my first job for this.

From memory it was full flap, we were very very slow, rate one turns with a fair bit of power on adjusted to silence the horn.

It was a fairly uncomfortable feeling. I think we did it for over an hour, over and over.

If you got caught out you'd have to really be quite quick to react and slow down.

[ForkTailedDrKiller]

Take FTDK's advice and add full power (if you are saving your life, that is)

Oh yeah, forgot that bit!

If you are flying anything other than a Bonanza, ya might want to give it full power!

Dr :8

[ITCZ]

And what is the radius of turn, in nm?

Try this javascript Aircraft Turn Radius (http://www.csgnetwork.com/aircraftturninfocalc.html) calculator.

So my question is, which of these approaches, 3.8G at 120 KIAS, 2G at 85 KIAS or 2G at 77KIAS flaps extended to 40 deg. (plus a gust margin and a little bit extra) is going to produce the tightest turning radius?

Entry 120kt, AoB 74 degrees :eek:, 3.6g:
Turn diameter 736 feet, or 0.1nm. Time to turn 180 degrees = 5.7 seconds

Entry 85kt, AoB 60 degrees, 2.0g:
Turn diameter 743 feet, or 0.1nm. Time to turn 180 degrees = 8.1 seconds.

Entry 77kt, AoB 60 degrees, Flap, 2.0g:
Turn diameter 609 feet, or 0.1nm. Time to turn 180 degrees = 7.4 seconds.

You asked for the tightest turning radius, not whether it was a good idea to try it :=

Why use 60deg AOB? why fly it fast? You dont want to get out quick, just in minumum radius. Stick to a comfortable, safe Angle of Bank (even rate 1) and fly as slow as the a/c config will let you. The slow speed is what dramatically reduces the radius of the turn.

Entry 60kt, AoB 45 degrees, Flap, 1.4g:
Turn diameter 640 feet, or 0.1nm. Time to turn 180 degrees = 10 seconds.

Performance is secondary to controllability?

Remember Vmca demo during initial twin asymmetrics? "Five to the live" sacrifices a minute amount of vertical component of lift (performance) tilting it toward the live (for control). If you bungle the manouevre and cannot control the heading of the aircraft because you are below Vmca, reduce power to reduce the asymmetric yawing moment and restore control.

Sacrifice performance to maintain control.

[Sunfish]

Many thanks for all your advice. When I get a chance I'll try the tight turn at low speed......with a lot of altitude.

[reynoldsno1]

It's much easier in a helicopter....:}

[Brian Abraham]

Lifted from some notes.
Limit Turns and Manoeuvre Speed
The minimum radius or maximum rate of turn are limited by either the strength of the structure, or by the power available. At low speeds (where the angle of attack is already high), it is not possible to significantly increase the lift coefficient. Therefore the angle of bank is limited by aerodynamic considerations, and under these conditions, it will not be possible to produce the limiting structual load factor. The wing loading and maximum lift coefficient determine the load factor for any speed. See the “Aerodynamic Limit” curve in the graphic.
As the speed is increased from the straight and level stall speed, the radius of turn can be reduced by increasing the angle of bank. However, a radius and speed are reached where the load factor is at a maximum for the structure. Any further increase will overstress the structure. Thus the radius of turn will increase, as the speed is further increased at constant angle of bank. The speed corresponding to the minimum radius turn is called the manoeuvre speed.
At speeds below the manoeuvre speed, the maximum (or limiting) load factor cannot be produced aerodynamically i.e. the angle of bank is limited by the maximum lift coefficient. At speeds above the manoeuvre speed, an increase in the angle of attack to produce maximum lift coefficient, will produce a load factor higher than the limit load factor i.e. it will overstress the structure.
Except in aircraft with a very high thrust to weight ratio, the limit load factor cannot be obtained in a level turn as shown in the graphic. The available thrust (or power) sets a lower limit to the angle of bank. Thus the minimum radius of turn is larger and occurs at a lower speed. When height is increased, the limiting effect becomes more noticeable. Thus at low levels, aerodynamic and structual (if power is sufficient) limits will apply, while at high altitudes, aerodynamic and power limits will apply.

http://i101.photobucket.com/albums/m56/babraham227/t0010.jpg
Sunny, with your new found skills how about a stall turn, or perhaps even a Immelman. Zero radius, in the horizontal at least.

[Clarie]

Sunfish can I ask why the turn must be only in the horizontal plane? If you want to turn back with a tight radius you could use the vertical (as Brian said) and do a wingover, which wouldn't over stress a non-aerobatic aircraft.

[ITCZ]

Brian, that would be a graph from a Val Dyson-Holland ATPL text, no?

[Capt Wally]

Some very professional answers coming here but when you really think about it who has remembered in detail & has applied all we have learnt during our flying career studies the finer details of obtaining the min rad turn? Few if any I'd say! I for one admit that from all the ATPL subjects learnt years ago I probably couldn't pass any of the exams again if I sat them today without any study. We learn/pass them at the time & together with that knowledge we go out & really learn it, at the wheel. Tight turns are fraught with danger if you really need to do them then go out & practise at ALT a little at a time, that's where you really learn not in some book!



CW

[djpil]

turn back with a tight radius you could use the verticalI'm sure that others will chip in with additional reasons but one scenario is that you can't go higher due to cloud base otherwise an option might be to simply climb above the canyon ridge and turn around normally.
I often chose a canyon route as it gave me lots more options eg normally a road down below so one option was to land straight ahead and wait. Lots of issues with canyon flying, especially with winds and other weather considerations.

(My usual interest in turn radius has nought to do with canyon flying)

Sunfish's original questions were around statements in Dave's book - those statements made in the context of an intro to flight mechanics of a course in basic aerobatics. Sunfish, you might be interested in reading Rich Stowell's book "Emergency Maneuver Training" (http://www.richstowell.com/shop.htm) for more on the subject with different applications in mind - check it out at Skylines.

[Brian Abraham]

ITCZ - you may be right, have collected a library of odd bits of material from all over the place.
Wally - how right you are about stuff learnt to pass exams and promptly forget. As for passing exams doubt whether I could manage a PPL. Min radius turns are at the fore front of most military pilots skill set though, particularly fighter types.