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Turbulence and feet per second

Old 22nd Apr 2012, 20:20
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Turbulence and feet per second

The area that I have been flying is bumpier than what I'm used to, and I want to calculate or find a cruise speed that will not 'break' the plane if turbulence is encountered, but also maintain a reasonable airspeed.

I've been doing lots of research, and it seems like people are pointing to using Maneuvering speed. The problem is that Maneuvering speed in my plane is rather slow....about 110-120 kias. The aircraft that I'm using at the moment is the Cessna 210. For the most part, I find that people cruise around 145 kias, but can go up to 155 or even Vno. They tend to maintain that speed through light-moderate turbulence and not bother adjusting the power. My concern is the potential of increasing the load factor on the wings and thus over stressing the plane.

This has lead me to become curious lately about V speeds, specifically Va (maneuvering speed), Vb/Vra (rough air penetration speed), and Vno (maximum structural cruising speed). I have been looking, with no joy, for a formula to calculate Vra.

From what I've been reading, in GA airplanes, Vno or the top of the green arc is a speed at which the aircraft is able to withstand up or downdrafts of up to 30 feet / second without over loading the plane. The question then arrises, What category of turbulence is 30 FPS? Light? Moderate? In any case, if severe turbulence, or even sustained moderate turbulence is encountered, slowing to or below Va is probably a good idea.

To sum it all up, what I'm looking for is a way to find or calculate Vra, and understanding turbulence in terms of feet per second.

Thanks
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Old 22nd Apr 2012, 21:08
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Admittedly, it's been quite a few years since I flew lighties, & furthermore I never had the pleasure of flying the C210. That said, most aircraft flight manuals publish a turbulence penetration speed, & failing that: a manoeuvring speed.

My point being: If the manufacturer publishes a recommended speed, then that's the speed they recommend, so that's the speed you fly. Simple. And with that being said, why bother trying to reinvent the wheel if the manufacturer has already done the math for you? Of course, it will mean having to slow down - which can be frustrating, I know - but them's the breaks. And 'break' you just may if continually ignored, especially with the age & fatigue issues facing so much of the GA fleet these days.
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Old 22nd Apr 2012, 21:19
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From what I have found.

Light - 5 Feet Per Second - 20fps
Mod - 20fps - 35fps
Severe - 35fps - 50fps
Extreem- >50fps

Maneuvering speed Va is the speed at which the wing will stall at the limit load of 3.8g
( remembering 1.0g of that is in level flight)

The wing will be over stressed at 3.8g above Va and potentially fail at 5.7g ( 150% above the limit load )

That's positive load factors, I think most aircraft are stressed for less in negative g ? At least the Vn diagram I was looking at has that.
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Old 22nd Apr 2012, 22:20
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Sagan is right.

From the Manual of Aviation Meterology, chapter on Turbulence

Light = 300-1199 fpm
Moderate = 1200-2099 fpm
Severe = 2100-2999 fpm
Extreme = >3000fpm

If you divide the above numbers by 60 you will get the numbers quoted by Sagan. Just highlighting where the numbers come from.

My rule of thumb when flying survey operations. Avoid Extreme turbulence at all costs. Avoid severe turbulence where possible and, if encountered or flying in forcast area, fly at Va.

Fly at Va if your in moderate turbulence.

Proceed with caution in light turbulence.

Its also worth noting that Va can change with aircraft weight. For the aircrat I flew Va was painfully low when you where light. The heavier the aircraft the better. We used to calculate it for take off and landing weights for each flight. This info should be in the AFM

Alpha
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Old 22nd Apr 2012, 23:09
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Yes Imagined I too have asked the same question and never got an exact answer.

On descent in the 210 you easily approach the top of the green arc and must be vigilant to keep from exceeding that.

My Manouvre speed is 125 knots which is a hell of an excercise to reduce to on descent if things get rough.

I will watch this thread with interest.
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Old 22nd Apr 2012, 23:29
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The C210 is deliberately limited by Cessna to avoid overstress in the un strutted wing/fuzelage attach area.

This has been the subject of debate for years but the end conclusion is that the way the wing attaches does warrant care in turbulent conditions.
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Old 22nd Apr 2012, 23:35
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Good question, and one I've wondered about too. I have no written references, so what I say is open to correction. However...

My understanding is that Va is a little too slow for best performance in turbulence. Yes, it protects the aircraft from structural damage due to excessive g load. But you could inadvertently stall in severe turbulence, which is just as bad.

Rough air penetration speed, or turbulence penetration speed, is around 5% to 10% faster than Va to reduce the risk of stalling. I don't know the precise calculation. The manufacturer expects the pilot to maintain an attitude in turbulence, specifically not to try to maintain an altitude, and this will reduce the chance of structural overload.

Here's what the good people at FAA.gov require (gusts are assumed to be 50fps according to rule 23.333, and the speed Vc is equal to 33 times the MTOW (lbs) divided by wing area (sq feet) according to rule 23.335.)
(d) Design speed for maximum gust intensity, VB. For VB, the following apply:
(1) VB may not be less than the speed determined by the intersection of the line representing the maximum positive lift, CNMAX, and the line representing the rough air gust velocity on the gust V-n diagram, or VS1√ ng, whichever is less, where:
(i) ng is the positive airplane gust load factor due to gust, at speed VC, and at the particular weight under consideration; and
(ii) VS1 is the stalling speed with the flaps retracted at the particular weight under consideration.


Couple of extra things, from others' posts above:

- Va protects against a single full and abrupt control input. It does not protect against multiple controls used simultaneously, or against one control input made multiple times.
- If using aileron and elevator together, the rule of thumb is that the actual 'g' limit is only two thirds of the basic limit. This is called the rolling g limit. Note also that for aerobatic aircraft, the entry speed for a flick roll is very much lower than Va.
- Va does not protect against multiple sequential inputs of the same control surface. If for example, at Va, you apply full left rudder then immediately full right rudder, you can expect to break something.
- Va does not necessarily protect against full forward stick. If you want to mess about with high negative 'g', Va is not the number to use.
- Va is not necessarily about the wing. The wing spar is usually over-engineered. Va might be limited by the tailplane or fin strength, or even by the strength of the aft fuselage (which takes a huge bending load at high g.) It is possible that the first item to fail might even be the engine mounts, or other high-load points inside the fuselage.

(Edit - I don't know much about the C210. My comments in the last sentence are for light aircraft generally, not the 210.)

Cheers,
O8

Last edited by Oktas8; 22nd Apr 2012 at 23:58.
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Old 22nd Apr 2012, 23:56
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Smile

IBG I've just retired after completing 18 years and 4000+ hours flogging a C210 around the northern parts of Australia. I too used to worry about what was acceptable turbulence until I attended a Systems and Procedures course for C210's put on by the Cessna Pilots Association. The instructors were very knowledgeable and one had worked for Cessna as a test pilot during the testing of some of these aircraft. Someone asked him when was turbulence bad enough to require cruise speed reduction. He said when you're hitting your head on the roof every 30 seconds or so. He said they are very strong and we shouldn't worry about moderate turbulence.

I tend to adjust my speed to the level of comfort I need. I'm too old to be bounced around in a hot uncomfortable cockpit in order to save 5 minutes. If it's hot and you just know it's going to be rough below 8000 stay up in the cool smooth air until the time to destination in minutes equals the number of thousand feet you have to lose. Then drop the U/C. A C210 will come down nicely @ 1000 fpm and you will see a stable 140-145 K indicated without any of those scarey 160K excursions sometimes seen descending in turbulence with U/C up.

We were told that wing failure in strutless 210's invariably occurred beyond VNE. The wing fails at the point where the flaps end and the ailerons start. The outboard section bends and breaks downwards because at very high speeds this outboard section is at a negative angle of attack due to the large washout in C210 wings. All it takes is a big aileron input (such as our hero pilot would do as he came out the bottom of a cloud and found himself looking at the ground sideways) to provide enough additional downward force to fail it.

One other worry I used to have. In dry season if you take your headset off in flight you often hear dreadful noises which sound like the carry through spar is on it's last legs. It is actually the windshield which is fitted loose so it doesn't crack. When it moves about only a minute amount it makes these noises that you would never want to hear from your spar.

So to sum up: to fly smooth, safe and worry free fly high, descend slow, don't exceed VNE and don't take your head sets off.

Cheers, RA
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Old 23rd Apr 2012, 00:19
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I think I'd rather inadvertently stall in turbulence than have something break.

That said, whenever I've flown in fairly bad turbulence, on the "airspeed dropping rapidly" side of the 'bump' there seems to be a corresponding large decrease in loading. Down to zero G, maybe a bit negative. Stall speed at zero g is zero. (control will be a tad difficult at this speed, though.)

I think it would be difficult to stall in turbulence, and the only severe encounters I've had, I've reduced to below Va, because the airspeed excursions easily shot the airspeed to well above Va, albeit briefly.At the other end of the green range, in the same turbulence, the stall warning operated, once again, briefly, and this also corresponded with much reduced loading.
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Old 23rd Apr 2012, 16:17
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Light;

Occupants may feel a slight strain against seat belts or shoulder straps. Unsecured objects may be displaced slightly. Food service may
be conducted and little or no difficulty is encountered in walking.

Occasional Less than 1/3 of the time.

Intermittent 1/3 to 2/3.

Continuous More than 2/3.


Moderate;

Turbulence that is similar to Light Turbulence but of greater intensity. Changes in altitude and/or attitude occur but the aircraft remains in positive control at all times. It usually causes variations in indicated airspeed.
or
Turbulence that is similar to Light Chop but of greater intensity. It causes rapid bumps or jolts without appreciable changes in aircraft altitude or attitude.

Occupants feel definite strains against seat belts or shoulder straps. Unsecured objects are dislodged. Food service and walking are difficult.


Severe;

Turbulence that causes large, abrupt changes in altitude and/or attitude. It usually causes large variations in indicated airspeed. Aircraft may be momentarily out of control.

Occupants are forced violently against seat belts or shoulder straps.
Unsecured objects are tossed about. Food Service and walking are impossible.


Extreme;

Turbulence in which the aircraft is violently tossed about and is practically impossible to control. It may cause structural damage.


The Vb speed (turb pen.) is not usually any different from the Va speed for light a/c and is defined at 66'/sec of vertical gust.

In general, any time you are in moderate turbulence you should be not above the Va speed adjusted for weight.

Note, even though it is expressed that you will not break the aircraft if you make full deflection at Va, DO NOT go from one full deflection (eg. Full left rudder) to the other extreme.

Last edited by MakeItHappenCaptain; 23rd Apr 2012 at 16:40.
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Old 23rd Apr 2012, 16:34
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There's some great information on this. Thanks everyone!
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Old 23rd Apr 2012, 20:45
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IBG
Wally is quite right-turbulence is a very subjective subject. Based on Makeithappencaptain's excellent definitions I slow down in moderate chop and the less moderate it is the more I slow. "Loose objects moving about " would be an easy way to judge that it's time to slow down. Perhaps you could place your mob phone on the glareshield and rename it your "Oh F*#k-o-meter" When the meter goes up you slow down.
Safe Flying RA
As the young pilots apple cheeked old grandmother used to say " I want you to be real safe so you fly real low and real slow"
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Old 24th Apr 2012, 05:52
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MIHC, Ive always found it difficult to walk around in a 210, regardless of how strong the turbulence is...
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Old 24th Apr 2012, 08:02
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Totally agree Lasiorhinus. LMAO

The above guide, whilst it is well written for an airliner, does it really apply to a lighter a/c ?

Ive been flying and encountered what I feel as top end of moderate turbulence in the circuit area in a C182. Later visually seen other light a/c getting bumped around in the circuit, but watching a 737 on downwind it appeared to be smooth as silk ?

Is it the same as comparing a 4.5m aluminium boat to an 80m floating gin palace ?
I can tell you the ride difference between a 4m tinny to an 8m fibreglass boat in 2 foot chop is spine breaking to champagne sipping.
Are these comparisons fair ? Both are basically vehicles travelling in a fluid environment although water is somewhat harder.
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Old 24th Apr 2012, 08:13
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There's the old coffee test for turbulence.

If the coffee is just slopping about within the cup its light.

If its slopping out of the cup its moderate.

If you cant find the cup its severe.
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Old 24th Apr 2012, 08:24
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Onya Mr RA for an excellent explanation.

I did not know about the windshield being fitted 'loose' in the 210.
However it does tell me why, in VH-KWW for example, the windscreen used to creak like crazy - especially on the descent, so I used to slow it down using the u/c method to keep it 'within my tolerances'.

But, alas, it was still full of 'internal' cracks which were a 'bugger' in the afternoon sun when landing to the west...

One 'famous' 210 driver in WA wrote this one up -
'Windscreen creaks badly on descent - frightens passengers - F#*in' TERRIFIES pilot!'

Cheers
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Old 24th Apr 2012, 09:00
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Las and Lanc,

Yeah, but these are the quoted "definitions" so I'm not complaining when the skimpy passes me a coffee from the middle row.

"Eh pahlut, you wanna drink? It's inna VB can but"

Actually, maybe this is a complaint in a way.......
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Old 25th Apr 2012, 19:13
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Now all of this food for thought has brought me to another question.....

How many G's can Light, Moderate, and Severe turbulence produce based upon their prescribed FPMs? I'll do some more digging and see if I can find a rate for 'Gravities'.
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Old 25th Apr 2012, 20:34
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Your question's answer is relative to the airspeed of the aircraft. The faster you travel for a given vertical gust, the smaller the resultant change in relative airflow.
WRT the lift formula, a 10% increase in speed will give an 11% increase in lift and (as LF=L/W) the resulting LF. Then you need to factor in the changing Cl as AoA varies, so you really need to look at it on a type by type basis if you want an accurate answer.

All you need to know is that going faster than Va in moderate or higher turbulence is bad.

(Another way to think of it is that it won't matter how deep it is, you'll be in the siht regardless.)
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Old 26th Apr 2012, 10:24
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If the coffee is just slopping about within the cup it's light. If it's slopping out of the cup it's moderate. If you cant find the cup it's severe.
That one goes on my "quotes to memorise" list. Thanks TB!

Just an extra thought. If you do go too fast in turbulence, don't expect anything to "break". What will happen is reduced airframe life and possibly some very slight bent parts, by no means necessarily the wing itself. Airframe abuse is often insidious and hard to detect.

But I continue to use turbulence penetration speed to penetrate moderate turbulence, assuming of course I haven't been able to avoid it. If it's not quoted, I guess Va is a close approximation, even if you're one of the nine out of ten pilots* who don't factor it for aircraft weight at the time.

* Personal experience from examining. Present company excluded, naturally!
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