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Thread: "Maneuvering Speed", a discussion.......I hope

  1. #1
    Dave Calkins's Avatar
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    "Maneuvering Speed", a discussion.......I hope

    Maneuvering speed, is it a reality? I was trained to believe that an a/c flying in turbulence, at a legal weight, at "maneuvering speed" or lower, would not break with abrupt control input, but would stall instead of breaking apart. Anyone have a thought??

    Let's keep the answers short for now, just to get the discussion going and to keep it all "readable".

    Thanks to anyone interested in this topic. DAVE

  2. #2
    mvivion's Avatar
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    Dave,

    That was the classical understanding most pilots had for maneuvering speed.

    Now, review the arguments of the engineers who work for Airbus Industrie regarding the Airbus that lost it's vertical fin and crashed into Jamaica Bay, New York a couple years ago.

    Suddenly, that really isn't true of maneuvering speed any longer.

    The first officer deflected the rudder in both directions, and the vertical fin came off.

    MTV

  3. #3
    Grant's Avatar
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    Dave,

    I might restate the definition in finer terms to say that full nose up deflection at or below Va would not cause a structural failure. Mostly because the formula for Va is very similar to Vs. I would have to break out my aerodynamics book but thats what I remember.


    I would like to read the reports from the Airbus accident as I only vaguely remember the details. I would like to believe that our understanding of subsonic aerodynamics is solid and not prone to exceptions, but the airbus case could change my mind as mvivion suggests. Any web link to this would be appreciated.

    Good topic! Worth knowing if any SC have ever suffered any structural failures at Va

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    Believe this is the one mtv is refering too.

    http://www.ntsb.gov/ntsb/brief.asp?e...30X02321&key=1

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    Va is manuvering speed
    Some planes also list a "turbulent air penetration speed"

    Simply put Va is typically higher because the pilot can only exert so force much with the controls before the aircraft stalls. Full and Abrupt is defined somewhere in some manual.

    Turbulent air penetration speed is usually lower based on a given sustained gust load on the aircraft measured in time of distance of wind shear. i.e 30 ft per second vertical shear exerts Xg on the airplane.

    More simply put the airplane can withstand more of our monkeying about so we get to go faster, we have to slow down in rough air because God doesn't need us going fast to smack us apart.

    Mike

    I think

  6. #6
    StewartB
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    Re: "Maneuvering Speed", a discussion.......I hope

    Quote Originally Posted by Dave Calkins
    Maneuvering speed, is it a reality? I was trained to believe that an a/c flying in turbulence, at a legal weight, at "maneuvering speed" or lower, would not break with abrupt control input, but would stall instead of breaking apart. Anyone have a thought??

    Let's keep the answers short for now, just to get the discussion going and to keep it all "readable".

    Thanks to anyone interested in this topic. DAVE
    I was taught that at the prescribed weight and Va, that an airplane would stall before structural damage would occur. In my 180 my gross weight Va is 123 mph. I've been at that weight and speed in turbulence and had the stall horn blast intermittently in the hard bumps. While I wasn't intentionally changing the deflection of the control surfaces, the turbulence was dramatically changing my angle of attack by very quickly changing the relative wind direction. I didn't find it particularly useful to exercise full or radical control movements, but the net effect was the same. I did take some comfort in the knowledge that some engineer figured that my speed allowed safe, if uncomfortable, penetration of the turbulence. At the time it seemed more like survival in the turbulence.

    The practical definition is more important to me than the technical one.
    http://selair.selkirk.bc.ca/aerodyna...ft/Page12.html

    Stewart

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    One of the recommendations in the NTSB accident report for the above mentioned accident is: "Amend all relevant regulatory and advisory materials to clarify that operating at or below maneuvering speed does not provide structural protection against multiple full control inputs in one axis or full control inputs in more than one axis at the same time."

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    maneuving speed

    For most pilots it's inversly related to how close their wife is

  9. #9
    Dave Calkins's Avatar
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    Thanks guys for speaking up!

    What is the concensus of opinion on how higher or lower a/c weight affects 'maneuvering speed'? (higher weight= decreased speed.......or, lower weight= decreased speed)

    Thanks again. DAVE

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    clouds's Avatar
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    Maneuvering speed

    Regarding the Airbus accident, I attended the entire NTSB hearings. All agreed that at maneuvering speed one complete, full control deflection is within the design criteria for an airplane (not sure about our little guys) at maneuvering speed, which is roughly turbulence penetration speed. What is not acceptable is a full control reversal, or full displacement of multiple controls. In the case of a swept wing airplane and the rudder, reversal results in "sequential over-yaw", which basically means combined aerodynamic and inertial effects can produce yaw in excess of what is produced by a single full throw of the rudder. Don't pick on Airbus here, as the same condition applies to Boeing or any other manufacturer.

    I'm not sure how (or if) any of this applies to our straight wing Pipers, and I'm not about to test it on my 1941 airframe. When you think about a full slip, I wonder how fast (or slow) it is safe to do so?

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    lower wt.=lower Va

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    Ok, I canít resist posting here. You asked for short posts, so please accept my apologies if I busted the rules.

    Iíve got a buddy that had to drop off a truck in the nearby desert and needed a ride back home.
    I landed on a 700ft patch of dirt, he jumped in and we headed back home to the mountains.

    This first photo is of us entering the pattern at my home port and it was quite turbulent. The photos document some interesting information.
    Note the following from the photo:
    Indicated airspeed is 68mph
    GPS ground speed is 77mph
    VSI documents that Iím caught in a 900fpm downdraft
    GPS altimeter is indicating 8,49x, where x=?
    Clock time is 12:59:20
    RPM is 1700
    The A.H. shows that the wings are level and the nose is slightly pitched down (I was trying to outrun the downdraft)
    Note that the duckbill on my hat is pointed at my flight path and this agrees with the runway alignment (as seen in the upper right side of the photo)
    White knuckles on the fist clasped around the V-brace
    What you canít see is that the stick action looks like Iím churning butter and the foot action looks like Iím in a dance contest.

    Click on image for larger picture.



    In this second photo, note the following:
    The airport is now off to our right
    Indicated airspeed is 54~53mph (14mph reduction)
    GPS ground speed is 74 (3mph reduction)
    GPS altimeter is 8008 (approx 500ft loss, consistent with VSI)
    Clock time is 12:59:55 (35 seconds later)
    RPM is 1700 (same)
    Also, I have added 1 notch of flaps by now
    The fight path is now more aligned with the downwind pattern, but just seconds after this photo was taken, the wings enter a full stall.

    Click on image for larger picture.


    I have to admit that the stall took me by surprise. I applied full throttle, pitched the nose down and recovered.
    Iíve got to tell you that I had to ďcowboy upĒ to point the nose at the pine trees to outrun the new tailwind that hit me.

    So, in conclusion, my published Va is 92mph indicated (if memory serves) but there is no way that Iím going to take on these type of turbulence at that speed.
    70 mph is tops for me.

    Are the published maneuvering speeds realistic? Good question.

  13. #13
    StewartB
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    In the link I posted earlier the formula showed the correlation of maneuvering speed to stall speed. My planes stall at a faster speed when heavy, therefore the maneuvering speed will be higher when heavy.

    In my turbulence story in the previous post, I said the stall horn would squeal intermittently while my normal forward speed was 123mph. That means I was experienceing momentary 70mph wind direction changes, at least at the leading edge of the wing. With that in mind, I'll keep my maneuvering speed near the max allowed to assure I can maintain maximum control in wind shear. Had I been moving 20mph slower I'd have been stalling on a regular basis. I was having enough fun as it was.

    When I'm landing in turbulent surface winds, I keep my speed up a bit to maintain control. There's no difference in flight. Va identifies the safe speed limit.

    Stewart

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    StewardB,

    In a cub I agree Va and turbulent penetration speed may be near or the same.

    However,

    Va and Terbulent penetration speed by definition and the way they are determined are NOT the SAME.

  15. #15
    StewartB
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    Re: "Maneuvering Speed", a discussion.......I hope

    Mike,

    I repeat.......


    Quote Originally Posted by StewartB

    The practical definition is more important to me than the technical one.
    http://selair.selkirk.bc.ca/aerodyna...ft/Page12.html

    Stewart

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    mvivion's Avatar
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    LawnDart,

    Under those circumstances, I'd fly a good bit faster, and keep the flaps up.

    Once you deploy flaps, a lot of structural limits change A LOT. The airframe is NOT certified to 3.8 with flaps deployed.

    Further, the airframe isn't able to sustain 3.8 in curved flight either.

    I'd stack on more speed to penetrate the gusts, and slow the airplane very close to the runway under very rough conditions like that.

    But none of that has anything to do with Maneuvering Speed. You were well below Maneuvering Speed, and all you have to do is be BELOW it for it to work.

    You're better off penetrating gusts at a little (not necessarily a lot) faster speeds. You definitely (as you found out) don't want to be so slow you stall the airplane when low.


    MTV

  17. #17
    StewartB
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    Quote Originally Posted by mghallen
    StewardB,

    In a cub I agree Va and turbulent penetration speed may be near or the same.

    However,

    Va and Terbulent penetration speed by definition and the way they are determined are NOT the SAME.
    Mike,

    I'm interested in your comments. In searching the web I'm unable to find much info about Vb (turbulence penetration speed). This link addresses it a little, about 2/3 down the page. The entire link is pretty good reading.

    http://www.auf.asn.au/groundschool/umodule2.html

    I was taught to use Va as the speed limit in rough air. What little I've found on the subject says that in the absence of a published Vb speed, one should use Va as the turbulence penetration speed. It appears there is no formula for determining Vb, but rather it's a determination of the airplane designer. I've never seen a published or even an implied Vb speed for a Cessna or a Piper.

    Another article.
    http://web.usna.navy.mil/~dfr/flying/turb.htm

    Stewart

  18. #18
    Dave Calkins's Avatar
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    So I'll be the 'dumbass' here. Yep, that'll be me!.

    So when it's really turbulent, I should load 'er up to gross, maybe a bit beyond since it'll stall at a higher speed when overloaded, then go run around at the "speed limit" now to be know as "maneuvering speed"?????

    What about the 'bending loads' on the wings before the 'critical angle of attack' is exceeded (stall)????

    Wouldn't I get lesser 'bending loads' if my a/c were light???? Therefore allowing me to safely fly in turbulence at higher maneuvering speed when lightly-loaded?????

    Too me, it seems like we are deciding pretty important stuff when we correllate a higher stall speed with a higher safe speed limit.

    I'm serious about my first sentence. I am truly putting this stuff up for discussion and have no chip towards anyone here.

    Mike V., do you have any comments on the initial question as it applies to our non-swept Cub or Cessna wings, and their asssociated lift strut, and fuselage attach structure?

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    behindpropellers's Avatar
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    Do VGs or Slots in the wings change the manuvering speed?

    Tim

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    clouds's Avatar
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    Weight vs. maneuvering speed

    The gospel according to the EAA, verified by my old college and Air Force aerodynamics books:

    "Does the maneuvering speed change with weight?

    Yes, the POH usually specifies only one maneuvering speed based on the aircraft loaded to maximum gross weight. When an aircraft is operated at a weight less than maximum gross weight, the maneuvering speed changes. When the total weight is reduced, the maneuvering speed is less; when the total weight is increased, the maneuvering speed increases. A general rule-of-thumb is: "a 2 percent change in weight will result in a 1 percent change in maneuvering speed." Another way of arriving at this figure is to decrease the speed by half the percentage of the decrease in weight. If you had a 20 percent decrease in weight, decrease the speed by 10 percent."

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    manuerving speed

    To add to the fray: The Lear 45 manual defines Va as: The highest speed that full aileron and rudder control can be applied without overstressing the aircraft, or the speed at which the aircraft will stall with a load factor of 2.9 g's at maximum gross weight, whichever is less.

    Note that they did not include the elevator in that definition. I'm not sure if this definition is different for part 23 aircraft vs. part 25, don't have my basic aero. books handy.

    I also have some technical formulas available to calculate Va, etc, with explanations, if someone wants them (some of it actually addresses cubs) but it is rather lengthy.

    Dick

  22. #22
    Dave Calkins's Avatar
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    I'm not up for a lengthy discussion at the moment, so....


    ....If I have a butterfly wafting along on the breeze, and I decide to make him weigh twice what he normally weighs, would it be safe (?) to say that he'll stall first or break his wings first, when a gust hits him at that higher weight??

    I think I'm being told I'm safer at a higher weight in turbulence. I'm having a hard time accepting that.

    Why do we limit how much weight an airplane can legally be loaded to, if it won't have an affect on the bending that occurs before the wing stalls.

    DAVE

  23. #23
    behindpropellers's Avatar
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    Quote Originally Posted by Dave Calkins
    I'm not up for a lengthy discussion at the moment, so....



    I think I'm being told I'm safer at a higher weight in turbulence. I'm having a hard time accepting that.


    DAVE
    Dave-

    Thats right. We need Bill Rusk to give us a "lesson" on this. I used to be able to explain it two years ago but would need some rejuvination of my brain from school 2 years ago.

    Its really a hard thing to explain without the use of a graph. I often get it mixed up myself.

    Tim

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    manuerving speed

    Since I have some info on gust loads, I'll relay some and paraphrase to keep it briefer:
    The effect of horizontal gusts is relatively negligible as far as flight loads are considered, unless the aircraft is near Vne. Vertical gusts however can significantly increase the flight loads, especially in a heavy wing loaded aircraft.
    It is not unusual to encounter vertical gusts of 15 fps (900 fpm) when operating in areas of high thermal turbulence, wind shears, or mts. The formula for computing actual gust loading is complicated and must compensate for many variables, but an aircraft with light wing loading (like cubs) operating at or below the computed Va, and encountering a 15 fps sharp edge gust, may increase the load factor by as much as 1.4. This is not critical and causes nothing more than mild discomfort, Unless the structure is heavily loaded in a steep turn or other maneuver when the gust is encountered. For instance, if you are in a 60 degree bank at 2000 lb GW, the load factor is 2.0 G and the load is 4000 lbs. If you experience a 15 fps vertical, sharp edge gust, the load factor will increase to 2.8 G and the structure load will be 5600 lbs. This is only 525 lbs. under the design maximum structural limit load. However, if the airframe is in good condition with no fatigue or corrosion, no structural damage will result and the airplane will stall when SLL (structural limit load) is reached. BUT, if the airplane is being flown at a speed Above the computed maneuvering speed for the GW, and is loaded heavy in a steep bank or pull out, and encounters the same 15 fps sharp edge gust, serious structural damage can occur...

    Hope that helps a little.

  25. #25
    StewartB
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    Dave,

    Did you read this link? You should.

    http://selair.selkirk.bc.ca/aerodyna...ft/Page12.html

    The Maneuvering Speed is the minimum speed at which the wing can produce lift equal to the design load limit. Below this speed the wing can not produce enough lift to overstress the aircraft, no matter what angle of attack is used.
    The minimum speed of the lighter plane is slower. It makes perfect sense.

    As for a higher weight being safer? I don't know about that. I do know that at a higher weight you'll be more comfortable. Higher wing loading makes for a smoother ride in the bumps.

    Stewart

  26. #26
    mvivion's Avatar
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    Dave,

    No, you are looking at this problem wrong.

    The loads are the same with a heavier aircraft at a higher speed, or a lighter airplane at a lower speed. That's the key, and it has to do with load factors to reach CRITICAL ANGLE OF ATTACK.

    The assumption is that, once the wing reaches critical angle of attack, it will stall, thereby reducing the wing loading. If you are at a higher weight, you will stall at a higher speed, therefore, you will reach Critical AOA at a higher speed, and the airplane will behave the same and experience the same loads as if it were at a lower weight AND at a lower speed.

    Ideally, you want the wing to reach critical AOA BEFORE anything breaks.

    So, would you be any safer at a higher weight, as long as you comply with the manufacturer's Va recommendations? No. Note that Cessna, for example, in the 185 manual, states that Va varies with weight, and that lighter weights require a lower speed for Va.

    Now, the Airbus accident demonstrated that the old saw that you can apply full control application (without further discussion of how many times) at Va is obviously false, at least for Airbus aircraft.

    Maybe Boeing applies the same standards, but I thought Airbus Industrie and NTSB treatment of the AA pilot flying that airplane was terrible. As I understand it, the guy did precisely what AA's program taught him to do, and the FAA and Airbus were totally uninformed as to what they were training their pilots to do in such an event?

    I'm not an airline pilot, never have been, and therefore could well be badly misinformed on this one, but it seems like this guy got hung out to dry.

    MTV

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    Part of the difficulty in understanding the Airbus accident and ruling is that the concept of maneuver speed was developed based upon the wing, angle of attack, and the general plan to have the wing stall before anything breaks, either due to control movements or gusts.

    The Airbus accident was caused by control inputs to the rudder, which overloaded and broke the vertical stabilizer. So, those wing arguments don't really apply. One could make those arguments for the vertical stab and find some sideslip angle that would stall it before it broke, but the Feds haven't done that. FAR25.351 is the certification requirement for vertical stab strength.

    I agree with mvinion. The AA crew was hung out to dry.

  28. #28
    Dave Calkins's Avatar
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    I'll take some time later to check that link, SB. Thank you. I haven't been able to take the time to check it. I will.

    To all in this 'argument':
    First, thank you! I am enjoying this discussion and............

    I understand the idea of flying into a 'vertical gust' such as a thermal.
    I understand the idea that a wing will stall at a higher airspeed if it is more heavily loaded.
    I agree that a more heavily-loaded a/c will have a nicer ride in turbulence VS. the same a/c a a lighter flying weight.
    I understand it is being said that a more lightly-loaded wing will stall at a lower speed (no argument there), and therefore, one must fly more slowly in turbulence to not break that wing when it is more lightly-loaded. I undertand that is what's being said.

    What I am arguing here is that maybe..........maybe:

    1. maybe not every gust will be vertical
    2. maybe the gust vector will not exceed the critical angle of attack, and thus, may not stall the wing, but rather, may overspeed the Va or Vb, and then break the a/c through overspeeding a heavily-loaded a/c.......because we may have been educated wrongly to believe we are safer to fly in turbulence at a higher weight rather than at a lower weight.

    POINT: maybe a more-lightly loaded wing will bend less in the overspeed than a the same wing at a heavier load.

    There was an article in AOPA or some other mag in the last couple of years about the "Myth of Maneuvering Speed". I didn't read it, but want to find it now.

    This whole topic came to my mind when the owner of my latest refurb. project mentioned the idea of Va decreasing with a decreasing in flight-weight, and I figured it'd be fun to bat it around on SC.ORG

    Anyone else wanna jump-in here?

    Thanks again, everyone DAVE

  29. #29

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    Good points, Dave.

    Just to keep it going, here's a bit more for critical examination.

    Structure wise, it's not only the wing that might break. It's every structure. Consider a battery shelf on the firewall, designed to hold a heavy battery to 3.8Gs for normal category aircraft. A vertical gust beyond limits might cause a vertical acceleration that breaks the shelf.

    Of course, the aircraft's weight (inertia) figures into how it responds to the gust. Here, lighter aircraft are more succeptible, hence you should fly slower in turbulence when you are light.

    I think you are right on with your wind examples. Mother nature has no rules for the direction or strength of wind gusts.

    The lifting loads on the wing are distributed spanwise, whereas the gravity loads are "mostly" in the center. So, from the viewpoint of the spar carry though, for example, you are probably better off light than otherwise. But, from the point of view of the battery shelf, for example, it doesn't care how the plane is loaded.

    Food for thought.

    Hankster

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    Hey Lawndart,

    Who had the nerves of steel to take those pictures? Everything in focus and nicely centered i might add.
    I would have been too busy messing up your back seat to play with a camera....

  31. #31
    RCharles's Avatar
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    speeds

    This is an interesting discussion, but the main reason I assume is to not bend or break your A/C .I have asked this question before but recieved no responce ,maybe it's a dumb one and that's why no one commented, but here it is again . Does anyone know how many Supercubs have had catostrophic failures due to turbulance ? Thanks ....RC

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    My understanding is that the weight of the aircraft is not the real reason Va goes up at a higher weight, but rather because the wing has to produce more lift at higher weights it is already traveling closer to critical AOA therefore the transition from cruise speed AOA to stall AOA is less.

    Mike

    I need to dig out my Aerodynamics for Naval Aviators text.

  33. #33
    Lawn Dart's Avatar
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    This is a pic of my valley, on the day in question, the winds were blowing 50+mph!
    DudesÖ.think about it. Yes Virginia, there are monsters!
    Those are 9,000 to 10,000ft peaks around my airport.

    Click on the image for the larger picture




    When was the last time you were going 111mph in a SuperCub at 1,400 rpm?
    Iíve throttled back from 150mph/gps/gs!

    Click on the image for the larger picture



    I should speed up? Thanks for the advice. Iíll take that under advisement.

    I think the point that Iím trying to make is, SLOW DOWN!
    Let the airplane waddle through the lumps, regardless what the published penetration speeds are.
    Iíve heard this thing bang so hard that I was sure that something was going to disconnect.
    Slow down and let it ride the waves.

  34. #34
    mvivion's Avatar
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    Lawndart,

    Yes, there can be a lot of turbulence in mountainous terrain.

    I wasn't suggesting that you speed up to 150 mph for turbulence penetration, but rather fly just a bit faster than the numbers you stated in that first post, WHEN close to the ground.

    It's one thing to encounter a full stall event at 1000 feet, quite another to do so at 100 feet.

    You can land the airplane at 65 mph or more if you need to and that speed or 70 offers plenty of gust protection from the standpoint of load factors.

    My point was simply don't fly so slow in that stuff close to the ground that you lose control of the airplane down there. That's a quite different problem than structural damage, and just as deadly.

    And, yes, I have flown in some turbulence at times. Kodiak has a tendency to be a little bumpy at times, with vertical terrain for the most part.

    The other consideration is: under what wind conditions should one be going somewhere else? I don't have the answer to that, but most every place has the capability of producing conditions that simply shouldn't be flown in. In other words, you get into a corner aerodynamically where the forces and bumps are so severe that either the wing will break or the pilot doesn't have full control of the airplane, with virtually no room in between.

    At that point, it's time to find another place to park.

    MTV

  35. #35
    cubflier's Avatar
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    Quote Originally Posted by behindpropellers
    Quote Originally Posted by Dave Calkins
    I'm not up for a lengthy discussion at the moment, so....



    I think I'm being told I'm safer at a higher weight in turbulence. I'm having a hard time accepting that.


    DAVE
    Dave-

    Thats right. We need Bill Rusk to give us a "lesson" on this. I used to be able to explain it two years ago but would need some rejuvenation of my brain from school 2 years ago.

    Its really a hard thing to explain without the use of a graph. I often get it mixed up myself.

    Tim
    Seems to be a leap of logic to conclude that there is a direct correlation between increased weight of an aircraft and safety in turbulence.

    I understand that Va is a speed that at or below it is difficult (not impossible) to exceed the 3.8g design standard of an aircraft.

    Aircraft structural limits are designed around a g limit instead of a load limit because not all systems in an aircraft are subject to the forces exerted by a heavier payload. The battery box mounting stucture is a perfect example of this reasoning.

    So we get from this a counter intuitive Va graph/chart that shows that a light loaded plane should be opertated at a slower Va since, even though the wings can handle the forces of a higher g load (when loaded at less than gross), the battery box (rated to take 3.8 g) may disconnect and slap you in the back of the head during flight.

    How do we then jump to the conclusion that more load weight = safer ride in turbulence.

    Exceeding the design limits in turbulence is just one way to die. I had a friend got rolled inverted at 1000 ft agl in a supercub. He followed with control inputs that completed an aileron roll. He was very light loaded and is alive because of it.

    My guess is that more turbulence related deaths come from "loss of control" than structural failure. There are plenty of aircraft that have failed to out climb terrain in down draft conditions and I would guess that more weight would not have helped them climb.

    Jerry
    If it looks smooth...it might be

    If it looks rough...it is!!

  36. #36
    Gordon Misch's Avatar
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    mghallen wrote: I need to dig out my Aerodynamics for Naval Aviators text.

    Me too! Pg 331.
    Gordon

    N4328M KTDO
    My SPOT: tinyurl.com/N4328M (case sensitive)

  37. #37

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    RCharles, I have never seen stats on supercub breakups either. Anyone else ?

    All,

    Suppose we run a little "thought" experiment.

    Case 1: We fly an aircraft at a given airspeed, straight and level, into a vertical gust. That gust results in an additional (to that required for straight and level flight) force F. From the loading of the plane, we can compute the resulting acceleration from F = m x a , or a = F / m, where m is mass and a is acceleration.

    Case 2. We do exactly the same as before, holding everything the same except that the mass is now larger because of additional weight loaded in the plane. Now, a2 = F / m2. The force is the same, because of the same airspeed and wind gust. Mass 2 is bigger than before, so acceleration 2 is smaller.

    This logic leads to the "heavier is better in turbulence" theory. But there is a flaw in the logic. Everything is not the same in Case 2 as Case 1. In order to fly straight and level at the same airspeed with more weight (case 2), you need more angle of attack for case 2. So, the action of the gust on the wing will be different. Maybe to a pretty good approximation you can say they are the same, but I'm not sure about that.

    Food for thought.

    Thanks to all, folks, for having such an enlightening discussion.

    Hank

  38. #38
    mvivion's Avatar
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    One thing to keep in mind:

    We can all relatively easily demonstrate loss of control due to acceleration.

    It is really hard (and probably extremely painful after) for us to demonstrate exceedance of structural limits.

    Critical angle of attack is something we don't want to exceed close to the ground.

    Structural testing requires that all aircraft structures exhibit the ability to sustain 150 % of the limit load on the airframe. (I know, I know--its a lot more complex than that).

    So, the airplane may reach it's certified limit at 3.8, but theoretically, at least, it won't break until you achieve a MUCH higher load, like maybe 5.7 G. That's a LOT, by the way. Believe me, your lips gonna be down around the stick in a 5.7 load.

    That's not to say you can't achieve destruction in an airplane in turbulence.

    The thing concerns me is getting the thing on the ground safely, and the best way to do that is NOT to exceed the critical angle of attack whilst close to the ground.

    I'll fly a little faster in turbulence, thank you very much. I DO NOT want any stall events close to the ground. I trust that the airframe is well contstructed, and in fact that the factory did a LOT of testing to verify the structural integrity of the airframe.

    I may be disappointed one day, but I've been in some turbulence that qualified as "extreme" per the AIM and nothing broke.

    MTV

  39. #39

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    Dave,

    The reason for the decrease in Va with an decrease in gross weight is to protect secondary structures, not primanry structures. In other words, it's not to keep the wings from ripping off, it's to keep your battery from falling off the firewall. Cubflier touched on this in his post. Here's a different way of explaining it.

    Say you had a 2500 lb MGTOW airplane. let's say Va is 120 knots. That means that at 120 knots, if you apply full nose up elevator, the wing will produce a maximum of 9500 lb (2500 x 3.8 ) of lift, then stall as it exceeds critical AOA .

    Now, the gross weight doesn't matter to the wing, all the wing sees is 120 knots of airspeed. if it was just you and miunimum fuel in in that same plane at 1700 lb gross weight, the wing would still produce only 9500 lb before stalling, and the structire of the wing would be no more llikely to fail than at MGTOW of 2500 lb. It will withstand 9500 lb, and 9500 lb is all you can get at 120 knots, period.

    WHere it *does* make a difference is in the secondary structures. Take the battery box. It's designed for a 15 lb battery, therefore the battery box can withstand 57 lb (15 X 3.8 ) However if you're flying at 1700 lb and 120 knots and apply full up elevator, you get more than 3.8 g of acceleration. Remeber, the wing produces 9500 lb, before stalling at 120 knots. 9500 lb / 1700 lb = 5.59 g, which is a lot more than 3.8 g. so if you are flying at 1700 lb and 120 knots and apply full up elevator, you accelerate at 5.59g Accelerating the 15 lb battery at 5.59 g generates 83 lb of force on the battery support, which is considerably more than the 57 lf it was designed for.

    In a nutshell, the decrease in Va at ligher weight is not to keep you from ripping hte wings off. If you can't rip the wings off at 120 knot and MGTOW, you won't be able to rip the wings off at 120 knots and minimum load and fueal ... but, you might damage some other part of the plane.

  40. #40
    Dave Calkins's Avatar
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    Andrew, Thank You. Thank You to everyone, actually!!

    I think Andrew gives the most-convincing argument. I can believe that secondary structures such as the battery box example need protection.

    What I could not understand was how one would get the idea that one could not "rip the wings off her" with a heavier VS lighter a/c.

    Mike V., "...I'll fly a little faster in turbulence, thank you very much.."

    I understand this to mean on-approach or near terrain. Certainly we all have been taught, and it makes sense, that slowing the a/c in turbulence will increase break-up margin........or did I just open another can of mythological worms?

    Thanks again, guys.!

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