Gust Loading

Geico266

Geico266

Touchdown! Greaser!
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Geico
Hearing about the in flight break up of the Angel Flight got me thinking about discussions on POA about gust loading.

There hasn't been much. :no:

When you are flying along and turbulence hits, slow the plane down out of the yellow and into the green. :D. I'll let you guys discuss "V" alphabet soup nonsense. :rolleyes:

Gust loads can occur in a split second and exceed the Max "G" loading of an airplane. We all know what "G" forces are when we push the stick around ( yokes for you other guys, ;)). Gust loads occur as the plane is being buffeted around. Up one second and down the next, that is gust loading.

Serious gust loading can cause catastrophic failure of the airframe if it exceeds the "G" forces your plane can take. Any plane is susceptible to these gust forces. So what to do? :dunno:

Slow down, change altitude, divert and land if conditions don't improve.

Okay, that gets the conversation started. :D

Can someone smarter than me explain gust loading?
 
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I have always considered "Gust Loading" to be a factor while the plane was tied down.... In the air, while flying, the plane should deflect and absorb some, if not most of the loads put on it by a gust..... Also, in my opinion , gusts hitting the plane while airborne is windshear, not gust loading. IMHO...:dunno: :confused:
 
Time is an important factor as well, you can throw eleventy billion Gs at my plane and if the load doesn't last long enough to move the structure beyond it's plastic deformation limits no damage (short of cold working for us metal types) will occur.
 
Negative Big Ben.

Gust loading is an in flight phenomenon. A sudden gust (turbulence) can change the angle of attack of the wing to inflict serious G forces as the plane corrects itself. This is how planes break up in flight.

Maybe this is a good safety topic to bring up. :yes:

Gust loading could also be defined as Geico talking about RVs. :rofl::rofl::rofl: :nono:
 
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Geico266 said:
Negative Big Ben.

Gust loading is an in flight phenomenon. A sudden gust (turbulence) can change the angle of attack of the wing to inflict serious G forces as the plane corrects itself. This is how planes break up in flight.

Maybe this is a good safety topic to bring up. :yes:
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Yup!:yes:
 
Geico266 said:
Negative Big Ben.

Gust loading is an in flight phenomenon. A sudden gust (turbulence) can change the angle of attack of the wing to inflict serious G forces as the plane corrects itself. This is how planes break up in flight.

Maybe this is a good safety topic to bring up. :yes:

Gust loading could also be defined as Geico talking about RVs. :rofl::rofl::rofl: :nono:
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Key word there is " phenomenon ".......

Does it exist or not ?????:dunno::confused:
 
N801BH said:
Key word there is " phenomenon ".......

Does it exist or not ?????:dunno::confused:
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Imagine your airplane as a weight suspended from the middle of a pole. Now suspend the pole at the tips with your fingers. Bounce it up and down. Now harder. Now hard enough to break the pole. There is gust loading simplified. It is real and it can ruin your day. The only recognized remedy is to slow down (Va in most cases),and get out of the activity if possible.
 
Threefingeredjack said:
Imagine your airplane as a weight suspended from the middle of a pole. Now suspend the pole at the tips with your fingers. Bounce it up and down. Now harder. Now hard enough to break the pole. There is gust loading simplified. It is real and it can ruin your day. The only recognized remedy is to slow down (Va in most cases),and get out of the activity if possible.
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I agree on exceeding VA will damage the airframe when hitting turbulance or an abrupt manauver.... But.. I want to go back to Scott's in flight break up...... Was he at or below VA ???
 
As Threefinger said, stay below Va. Anytime I'm penetrating turbulence I immediately slow to Va (145 kt ish). Too many people get over confident in their 40 yr old airframes and go bouncing around the country well above Va thinking as long as they're below Vne they're good. Not true.

Crossfield. I'd say he was above Va since he broke apart in flight. Either it was from wind gusts or his over stress due to loss of control. His accident site is actually only about 10 miles from where I'm sitting right now.
 
Va is not necessarily sufficient to prevent overloading the aircraft in extreme turbulence.

First, Va decreases with weight, at approximately 5% for every 10% reduction in gross weight.

Second, Va is based upon the power-off stall speed at the limit load factor. Since stall speed is lower with power, it can be possible to exceed the limit load factor despite being below Va whenever power is on.

Third, gusts can suddenly increase airspeed to above the target maneuvering speed and leave you vulnerable to excess load factor.

A speed approximately halfway between stall speed at your actual weight and the Va adjusted for actual weight provides an extra margin of safety when operating in severe conditions. Of course, all the usual guidelines still apply--get a block altitude and maintain pitch/bank attitude with smooth gradual control application and limit turns to shallow bank angles.
 
dmspilot said:
Va is not necessarily sufficient to prevent overloading the aircraft in extreme turbulence.

First, Va decreases with weight, at approximately 5% for every 10% reduction in gross weight.

Second, Va is based upon the power-off stall speed at the limit load factor. Since stall speed is lower with power, it can be possible to exceed the limit load factor despite being below Va whenever power is on.

Third, gusts can suddenly increase airspeed to above the target maneuvering speed and leave you vulnerable to excess load factor.

A speed approximately halfway between stall speed at your actual weight and the Va adjusted for actual weight provides an extra margin of safety when operating in severe conditions. Of course, all the usual guidelines still apply--get a block altitude and maintain pitch/bank attitude with smooth gradual control application and limit turns to shallow bank angles.
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We always limited penetration speed to Va +-5% for normal ops, (around 92k lbs.) Which gave us about 178-185kts. Va is a good rule of thumb for the average GA guy.

I've never experienced a significant increase in airspeed in turbulence.

Can you expand on your statement that "stall speed is lower with power"?
 
dmspilot said:
Va is not necessarily sufficient to prevent overloading the aircraft in extreme turbulence.

First, Va decreases with weight, at approximately 5% for every 10% reduction in gross weight.

Second, Va is based upon the power-off stall speed at the limit load factor. Since stall speed is lower with power, it can be possible to exceed the limit load factor despite being below Va whenever power is on.

Third, gusts can suddenly increase airspeed to above the target maneuvering speed and leave you vulnerable to excess load factor.

A speed approximately halfway between stall speed at your actual weight and the Va adjusted for actual weight provides an extra margin of safety when operating in severe conditions. Of course, all the usual guidelines still apply--get a block altitude and maintain pitch/bank attitude with smooth gradual control application and limit turns to shallow bank angles.
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Agreed with everything you said...:yes:..

I am still waiting for a link to an "official" report that says Scott Crossfields plane failed because of "Gust Loading"
 
N801BH said:
Agreed with everything you said...:yes:..

I am still waiting for a link to an "official" report that says Scott Crossfields plane failed because of "Gust Loading"
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I doubt they will crawl that far out on a limb. "Inflight breakup due to excessive aerodynamic forces" will be about as far as they will go.
 
N801BH said:
Agreed with everything you said...:yes:..

I am still waiting for a link to an "official" report that says Scott Crossfields plane failed because of "Gust Loading"
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Can you please hold your breath? :rofl:


Kidding.

Gust loading (as far as I know) is never an "official" cause of break up according to what I have read. It is always blamed on "convective sigments" or "sever turbulence". Gust loading is more of an engineering term, that if not respected will tear your wings off. ;)

Ben, you can exhale. :lol:
 
Velocity173 said:
As Threefinger said, stay below Va. Anytime I'm penetrating turbulence I immediately slow to Va (145 kt ish). Too many people get over confident in their 40 yr old airframes and go bouncing around the country well above Va thinking as long as they're below Vne they're good. Not true.
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I think you nailed it.

Little is said about what to do in turbulence. Turbulence is dangerous if not respected. Slow down to minimize the forces on the airframe. Change altitude, divert.
 
Geico266 said:
Can you please hold your breath? :rofl:


....
Ben, you can exhale. :lol:
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Too Funny..:D:D...

I flew this morning to check my irrigation ditches and got the crap beat out of me from the LLWS.... Thank god the 801 is a +6/-3 G rated plane........

Geico..... I will buy the first couple of rounds if and when we ever meet.:yes::yes:...

If I live that long.. :dunno::redface:
 
N801BH said:
Agreed with everything you said...:yes:..

I am still waiting for a link to an "official" report that says Scott Crossfields plane failed because of "Gust Loading"
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No way to tell AFaIK. But when the turbulence is really bad it's not easy to tell the difference between pilot induced loads and gust loads.

Turbulence generates a combination of AoA change and local airflow velocity and I don't think it's easy to distinguish between those two effects either. Chances are pretty good that any bump you experience involves both. If you're tooling along in smooth air and fly into rising air, the main issue is an AoA increase and since lift is fairly proportional to AoA at the same airspeed and it doesn't take much vertical air velocity to shift AoA up a degree or two from your typical +3° in cruise increasing lift by 33-66%. I think horizontal sheer is unlikely to be isolated from vertical movement and it seems unlikely that the gradient would be high enough for an airplane to experience a big change in lift. OTOH, lift is proportional to the square of airspeed at a constant AoA so the increase in lift would be approximately double the increase in speed.

Worst case would be crossing a shear line between air falling and moving laterally in the direction you're going and air that's rising and moving opposite your flight vector. That would simultaneously shift the AoA higher and increase the air velocity over the wing.

But to analyze the potential for damage would be very complicated because the elasticity of the structure would filter (delay) the rapid buildup of force while at the same time the aircraft would accelerate in the direction of the force. As the airplane accelerates both the AoA increase and the air velocity increase would be diminished, you might say the airplane was "protecting itself". I would expect wing loading (lbs of weight per sqft of wing area) to have a big effect on how well that works.
 
Standard design criteria for DOD aircraft is a "sharp edged gust" which simplified, means there's an instaneous increase in load factor on the aircraft. How much that is depends on the class of aircraft, transport vs fighter for example since a fighter can be pulling 9 g's while a transport is limited to a lower number by either the -1 or the FCS when it hits the gust.

It is a real phenomenon and one of the tests required for qualification is a demonstration either full scale or component.

Cheers
 
No need to be overly aggressive in fighting gusty altitude changes, go with the air flow a little.

Can't remember a primary flight student that didn't initially want to stiff-arm significant fight turbulence with both hands, even when at or below Va.
 
The best explanation of gust load, complete with the math, can be found in "Flying the Beech Bonanza" by John Eckalbar. I've never flown a Bonanza but this is still a great book. In it, he points out that certification standards are for a 30fps gust at Vno. At 3400 lbs, this creates a G load of 3.57 in a Bonanza at 165 knots. At 2400 lbs the same gust creates a load of 4.65 gs.

When I read the book and for about five minutes afterwards, I understood most of the math and it was eye opening how the forces experienced by a plane can change so dramatically with speed and weight. Now I just remember what he said at the end of the chapter: "When it's rough, slow down. When you are light slow down more"
 
Wingsofglass said:
"When it's rough, slow down. When you are light slow down more"
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This is the largest area of misunderstanding IMO. Everyone knows their aircrafts Va, but many do not know how much it changes according to weight.

The other issue is that Va was determined when the design was certified and the airframe was new. Take a 40 year old airframe, at light weight, and I wouldn't be surprised to see structural problems due to turbulence at Va.
 
Dave Krall CFII said:
No need to be overly aggressive in fighting gusty altitude changes, go with the air flow a little.

Can't remember a primary flight student that didn't initially want to stiff-arm significant fight turbulence with both hands, even when at or below Va.
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Another good point. "Hold attitude, forget altitude." (unless of course you are crawling through rain 1500 ASL "sightseeing" )
 
WRT to gust loading, I am always amazed that gusts can significantly impact one wing, but not the other, creating roll rates that can exceed 90 degrees/second.

If you figure the distance between my wings is less than 15' (30', tip to tip), it's hard to explain such a huge disparity in "gust movement" between them -- but it happens.

Air is funny stuff.
 
Jay Honeck said:
WRT to gust loading, I am always amazed that gusts can significantly impact one wing, but not the other, creating roll rates that can exceed 90 degrees/second.

If you figure the distance between my wings is less than 15' (30', tip to tip), it's hard to explain such a huge disparity in "gust movement" between them -- but it happens.

Air is funny stuff.
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Air Pockets.
 
The USS Macon didn't even have wings or a Va speed and still had structural failure due to wind gusts.
 
There is a limit with this. I've seen way too many pilots slow to absolutely ridiculous speeds simply because they're encountering some turbulence. Just because you're encountering some turbulence does not mean you need to slow below Va. There are many aircraft to where doing so is going to slow you down to a rather unacceptable speed, especially if you're trying to get a job done.

The key is to understand. If we take a look at 14 CFR 23.333 you'll see that designing for gusts is part of certification.
(a) General. Compliance with the strength requirements of this subpart must be shown at any combination of airspeed and load factor on and within the boundaries of a flight envelope (similar to the one in paragraph (d) of this section) that represents the envelope of the flight loading conditions specified by the maneuvering and gust criteria of paragraphs (b) and (c) of this section respectively.

(b) Maneuvering envelope. Except where limited by maximum (static) lift coefficients, the airplane is assumed to be subjected to symmetrical maneuvers resulting in the following limit load factors:

(1) The positive maneuvering load factor specified in § 23.337 at speeds up to V D ;

(2) The negative maneuvering load factor specified in § 23.337 at V C ; and

(3) Factors varying linearly with speed from the specified value at V C to 0.0 at V D for the normal and commuter category, and −1.0 at V D for the acrobatic and utility categories.

(c) Gust envelope. (1) The airplane is assumed to be subjected to symmetrical vertical gusts in level flight. The resulting limit load factors must correspond to the conditions determined as follows:

(i) Positive (up) and negative (down) gusts of 50 f.p.s. at V C must be considered at altitudes between sea level and 20,000 feet. The gust velocity may be reduced linearly from 50 f.p.s. at 20,000 feet to 25 f.p.s. at 50,000 feet.

(ii) Positive and negative gusts of 25 f.p.s. at V D must be considered at altitudes between sea level and 20,000 feet. The gust velocity may be reduced linearly from 25 f.p.s. at 20,000 feet to 12.5 f.p.s. at 50,000 feet.

(iii) In addition, for commuter category airplanes, positive (up) and negative (down) rough air gusts of 66 f.p.s. at VΒ must be considered at altitudes between sea level and 20,000 feet. The gust velocity may be reduced linearly from 66 f.p.s. at 20,000 feet to 38 f.p.s. at 50,000 feet.
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What does the above mean?

It means the airplane has been certified to be able to take a 50 fps gust up or down at design cruise speed (Vc), which is often much greater than Va.

It also needs to be able to take a 25 fps gust at design diving speed (Vd). Design diving speed is a rather high speed.

50 fps is 3000 fpm. That's a rather impressive chunk of rising air you'd just have to suddenly run into. You're going to need rather special conditions to run into something like that. (Thunderstorms, maybe high winds in the mountains).

What I'm trying to say here is that there is no reason to be slowing yourself below Va simply because you are hitting some typical turbulence while flying along on a typical summer day. Stay out of thunderstorms. Use your brain.

If 3,000 fpm thermals were typical we'd just all fly gliders everywhere we needed to go.
 
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