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Thread: Bush Plane Aerodynamics

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    Default Bush Plane Aerodynamics

    When is CG not inside the plane?

    Since we are getting a few hits on this thread.... Here is another one. Since it was not my screw-up, but one I witnessed, I will change it into a teaching moment. The stuff folks usually have to pay for.

    When loading a float-plane, particularly a larger one like a C-185, Beaver or Otter, you can load lots of gear into the aircraft proper and also load more than you should be able to load into the FLOAT COMPARTMENTS.

    Lets say that our daring pilot supervised the loading of a lodge aircraft, but then had to return to the lodge office for a few minutes. The lodge fishing guide then hears a couple of fishing clients snivel about not taking along some more gear. So the guide, decides to load up the FLOAT COMPARTMENTS with as much junk as he can jam inside.

    Now the CG has been moved downward, outside of the fuselage area.
    It may even be only to one side or both... depending on how carried away our guide became with his over-loading.

    Lets say that shortly after a very long take-off run, a turn is required between a couple hills covered with trees. Now the weight acts like a pendulum.

    So when the pilot tries to bank the plane, the weight in the floats wants to pull it back to wings level. This causes a skidding turn at low speed and low altitude.

    In more than one case it has put the plane right into the trees.

    Excessive water in the float compartments from not pumping them out, or from having a disconnected pump-out hose will do the same thing.
    I removed over 15 gallons of water from a local 135 pilot's floats when that pilot could not figure out why the plane took a mile to get off the lake. In that case the hoses had become disconnected and the pilot/owner had never looked into the floats to see why no water EVER came out.

    I used my little Cub for the demo photos, so I don't tick off the Company owners of a wrecked Beaver.

    Well I can't get the computer to let me make them large.... so you will have to click.... dang computer...
    Let me know if you find this post informative.

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    Lets say that our daring pilot supervised the loading of a lodge aircraft, but then had to return to the lodge office for a few minutes. The lodge fishing guide then hears a couple of fishing clients snivel about not taking along some more gear. So the guide, decides to load up the FLOAT COMPARTMENTS with as much junk as he can jam inside.

    Now the CG has been moved downward, outside of the fuselage area.
    It may even be only to one side or both... depending on how carried away our guide became with his over-loading.

    Lets say that shortly after a very long take-off run, a turn is required between a couple hills covered with trees. Now the weight acts like a pendulum.

    Geez! And here I thought that CG was calculated along the longitutdinal axis of the airplane . . . . . So, can you tell us - - - - - how does one calculate the "pendulum" effect, which must relate to both the vertical and the horizontal axes . . . ?

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    So, can you tell us - - - - - how does one calculate the "pendulum" effect, which must relate to both the vertical and the horizontal axes . . .
    ?

    No not yet.
    But after a couple of Beaver drivers put their birds into the trees shortly after take-off, the DPEs and FAA folks have been bringing up the question of weight in one or both floats during every check-ride oral for the past 3 years or so.

    It does make sense when you figure that 100 pounds of weight on the end of a 5 foot long strut, exerts much more than 100 pounds of force on the upper end when you lift it.
    Of course at higher speeds it has much less effect.

    I wonder if the old bomb hangers from WWII had some sort of calculations to use when hanging a bomb on one wing and a drop tank on the other. I guess un-even fuel loads in tanks further out on the wing would equate to the same sort of problem. A B-17 had something like 1,700 gallons of fuel in the wings. 10,200 plus pounds. So uneven fuel flow from the outer tanks had to be a real hazard.

    I had parachutist jumping from my Cub's floats. The extra weight and drag required me to enter a shallow dive with full opposite stick just to stay wings level long enough for them to take some video. That was actually pretty interesting.
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    Quote Originally Posted by Float Pilot View Post
    ?

    No not yet.
    But after a couple of Beaver drivers put their birds into the trees shortly after take-off, the DPEs and FAA folks have been bringing up the question of weight in one or both floats during every check-ride oral for the past 3 years or so.

    It does make sense when you figure that 100 pounds of weight on the end of a 5 foot long strut, exerts much more than 100 pounds of force on the upper end when you lift it.
    Of course at higher speeds it has much less effect.

    I wonder if the old bomb hangers from WWII had some sort of calculations to use when hanging a bomb on one wing and a drop tank on the other. I guess un-even fuel loads in tanks further out on the wing would equate to the same sort of problem. A B-17 had something like 1,700 gallons of fuel in the wings. 10,200 plus pounds. So uneven fuel flow from the outer tanks had to be a real hazard.

    I had parachutist jumping from my Cub's floats. The extra weight and drag required me to enter a shallow dive with full opposite stick just to stay wings level long enough for them to take some video. That was actually pretty interesting.
    Guess the Super Cubs have a lot in common with the ol' B-17s. If ya run one tank dry on a PA-18, it will be sort of out of trim, presuming the plane was originally rigged properly and was most recently trimmed properly. Since all that avgas was just a dab above the pilot's head, couldn't have earmed much of a "pendulum" effect though. Unless the pilot went inverted, that is. Just horizontally out of trim . . .

    As for the Beavers in the bushes, perhaps if the ball had been in the middle . . . . . it wouldn't be there during any pendulum effect, would it? Nor would it in a skid, a slip, or some other uncoordinated maneuver. Or perhaps (a) the Beavers were simply loaded too heavy (and this would include the float lockers), or (b) a turn was started before the plane was ready to be flown out of level, or (c) the pilot had depended upon ground effect to get off, but left it all too soon.

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    perhaps if the ball had been in the middle .
    I agree.

    Supposedly it was the overloaded float compartments that made it difficult top keep it in the center while attempting a low altitude and low speed turn. I know that when it happened to me. Or should I say when I did it to myself. The roll rate at low speed and any angle of attack was much reduced.

    Remember this old diagram from basic physics class?
    "C" the center of mass is moved down and below the primary body when two other suspended bodies create a fulcrum point (P) . Thus making the main body stable in it's location and resistant to movement.
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    Quote Originally Posted by Float Pilot View Post
    I agree.

    Supposedly it was the overloaded float compartments that made it difficult top keep it in the center while attempting a low altitude and low speed turn. I know that when it happened to me. Or should I say when I did it to myself. The roll rate at low speed and any angle of attack was much reduced.

    Remember this old diagram from basic physics class?
    "C" the center of mass is moved down and below the primary body when two other suspended bodies create a fulcrum point (P) . Thus making the main body stable in it's location and resistant to movement.
    There would have to have been a HUGE load in only one float to result in a dramatically eccentric horizontal loading. As to the physics sketch, I find several faults with its application: The "primary body" in that sketch would presumably represent an airplane during a turn. If so, remember that centrifugal force is the primary consideration, and no 'fulcrum' really exists, since a 'fulcrum' more accurately depicts that point upon which a lever will rest. Think teeter-totter. More technically, think of a cantilevered beam.

    In the case of the sketch, and presuming the primary body does represent an airplane, that plane is tethered by an imaginary line to a 'center point' one helluva long distance away. Let's say one-quarter mile, in the case of a turn which traverses a circle one-half mile in diameter. Think swinging a toy airplane on a string.

    Given the relative distance between the plane and its floats when comparing (a) the distance of the floats from the plane and (b) the distance of the plane from the center of its turn, I still con't see water in the compartmented floats as providing any sort of horizontal loading. That load WILL of course add to the gross weight of othe airplane, and hence will untimately have impact upon the plane's stall speed . . . . .

    Water in the floats should in no way affect the needle/ball behavior. The ball relies almost solely upon a coordinated maneuver to keep its assigned place in the center, including every maneuver from cruise flight through all the turns. And loops, too, by the way . . .

    Reduced effectiveness of control surfaces (wings, ailerons, etc...) will me most affected by airspeed, Think stalls and spins.
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    Just because you still don't get the point, or are pretending not to see the point, does not mean we were not friends.
    Since the thread has been sidetracked at this point. I will continue to flog a dead horse.

    A. Initially you responded that W&B calculations were only figured on one plane. Actually center of mass for any object can be calculated in all three dimensions. It is done on heavy cargo carriers every day. The diagram demonstrated how the center of mass could be outside the object in question. You can't change the original question when you don't like the answer.

    B. My originally point was over-loading the float compartments. There is a reason they are weight placarded even though they can be temptingly spacious on a Beaver,Otter or a C-185 with Aerocet 3500s for that matter. It is possible to put 300-400 pounds into float compartments which are placarded for 108 pounds. The time I let it happen to me, there was about 30 gallons of water from the floats.
    About 15 gallons each side as said earlier/
    (I believed the owner (blond) who claimed they had been pumped, when the pump-out hoses had long since fallen off the nipples.) So there was @249 pounds of water in the floats. The only thing that plane did well was a forward slip. This was a marginal 4 place aircraft on wheels and a real dog on floats with empty floats.

    C. Yes weight suspended under an aircraft would swing outward via centrifugal force in a coordinated turn. Particularly at speed. But my point was low, slow, following take-off. Take two sledge hammers and hold them by the very ends of the handles. Then try to roll them up to a 35 or 40 degree angle and see how much force it takes. Of course if you slowly start to spin around, they will eventually rise up and orbit your body. But then try to stop them or change directions.

    Simple Physics dictates that hanging excessive weight on struts under your aircraft, particularly one designed as a land plane, will affect the maneuverability in some fashion. If kept within the design limits it is no big deal... but that was not the point of the post.

    Take a C-185 seaplane and load it to gross with all the gear inside the fuselage and within the proper W&B envelope.
    Then take the same plane and load it to gross weight again, but... shift most of that cargo, into the float compartments.
    Now go out and do Dutch Rolls, 360 degree turns, slow flight, stall recovery and so on... Which aircraft will respond better?
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    ummm, since we are getting all scientific here... I assume you guys mean centripetal force..... <grin>

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    Default Bush Plane Aerodynamics

    Centrum-fugere, or fleeing from the center is the basis for the word centrifugal. Like when the Air Force strapped me into a centrifuge to show me what happened if I stopped breathing during high Gs.

    Centrum-petere or aimed at or seeking the center, is the basis of the word centripetal.
    Newton said is was "A centripetal force is that by which bodies are drawn or impelled, or in any way tend, towards a point as to a center."

    You spin weight on a spring over your head. The string tension is the centripetal force which keeps the weight accelerating in a curve. Without that string or force the weight would fly off in a semi straight line. Such as how gravity holds a satellite in orbit.
    The use of the two words is often confused or misused.
    Plus there are those who claim that there is no such thing as centrifugal force. But rather inertia.
    But then again some people think that a hodgepodge of letters like "lol" count as a sentence.
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    Hey Float Pilot,

    I don't mean for you to misunderstand what I'm trying to say. Guess I'm just saying it poorly. I don't find fault with your sketch. I'm just pointing out that it doesn't seem to apply here. I thought I had said AXIS, not plane. Don't want to slip from physics to plane geometry here. At any rate, you're still talking about a simple "overload" rather than a pendulum effect, are you not?

    The value of any centrifugal force (read G-factor here) will depend upon the speed of the aircraft in its turn, modified by its angle of attack (which affects the diameter of the proscribed circle). It's the same as an aircraft in a good old country loop,. except the loop is performed in a vertical plane. In that case, normally somewhere between 2- and 3-Gs, if I remember correctly. The G-factor can be revised by either tightening the turn (loop diameter) or loosening it. Right? Tha same would be the case in a level turn: consider a 15-degree bank angle as opposed to a 60-degree angle of bank.

    Let's put your floatplane in a 90-degree bank. Will the water-filled float compartments induce a higher G-load, or will it simply introduce a pendulum effect? And, if the latter, will that effect be ((a) fore and aft, (b) up and down [that is starboard and port], or (c) directly downward in relation to the aircraft (but horizontally in relation to the earth)? And (careful now . . . ), would that effect be relative to the earth, or only to the airplane and it's pilot?

    Moreover, won't the center of mass be inside the airplane in any event? Your 249-lbs of water in those floats aren't all that much, when played against an A/C's gross weight of maybe 3,500-lbs. That's around 7-percent which, in the overall scheme of things, isn't really mind-boggling. Remember that the FARs allow Alaskans a 15-percent overload at any rate, and they do so with no cautions.

    In short, I think your float locker overload amounts to an increase gross load, will change the center of gravity (vertically) only by fractions, and because of it's short distance from the interior CG, won't provide any pendulum effect at all. It will increase stall speeds, of course.

    I enjoy the discourse, but I think we might be wise to finish it via PM or e-mail: mm1235pm@att.net. I'm sure our fellow pilots are ready to tar and feather us by now.

    Regards

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    I have a hard time thinking that FP (that is an acronym for Float Pilot, just like LOL is an acronym for Laugh Out Loud) isn't right on this one. This is a VERY interesting topic!!

    It seems that there are several forces acting on the plane that make it more interesting. Obviously gravity is acting on the plane as a whole, trying to pull it back to earth. Weight is a measurement of gravity pulling on an object with mass. In order to lift something with mass you have to apply enough force to overcome that weight. If the weight is centered between the wings you are not lifting it when turning, the same force that is required to maintain the weight at the current altitude is all that is required to keep it there in a coordinated turn. Now if you have the weight rigidly suspended away from the central axis of the plane then you DO have to move (and lift) that weight. You have to move the floats through the arc length which is substantial, and increases rapidly the further removed from the central axis the weight is held. Here is a great tutorial: http://www.nutshellmath.com/textbook...ing_angle.html

    During level flight gravity is pulling straight down on the plane (to include the floats), you have configured the plane to overcome this with your trim and power settings. Now considering that you have to move the weight in the floats the arc distance to bank the plane you have introduced torque on the plane caused by the gravity acting on the floats. As you try and enter a bank, gravity is trying to pull the floats back to earth. This is amplified by the length of the force vector since ( ). Torque = Radius X Force, Another words the further the weight is from the center the more torque it applies since force is a constant (gravity). In order to overcome this torque you will need to increase the lift on the outside wing.

    The result of the overloaded floats has little to do on if the plane will fly, it has everything to do with how well it will turn. The faster you are going the more lift the wings have and more effective the ailerons will be. So at faster speeds it will be easier to overcome the torque that the heavy floats will induce in a turn. At a slower speed the ailerons are less effective and banking the plan to turn will require more input and if you are going slow enough the torque could overcome the lift (or force) the aileron can produce. If this happens then the torque applied to the floats surpasses that which the ailerons are capable of moving it will produce the "pendulum" effect that FP is talking about.


    Again I am not a pilot, but physics is phun!

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    And here I thought lol was "Lots O' Luck."

    The Examiners think I'm right since it is a test question.
    It is not about how the A/C acts in a coordinated established turn where the centripetal and centrifugal forces are in balance. It is all about changing the direction of the mass.
    Since you won't answer the question about the two loaded airplanes....

    Take Kate Moss and Selma Hyak.
    Place them both on a rotating base in their birthday suits. Spin to 60 rpm.
    Suddenly stop the rotation. Who still has body parts moving after the sudden stop?
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    I see more wrong with this discussion that right. The coordinated turn question is a very good one. Floats have never made my planes harder to turn, loaded or not. Pilots know that turning is accomplished by lift. The floats are still located under the center of lift during a turn, assuming the turn is coordinated. Is the wing's performance different with different weights/wing loading? Absolutely. Does the wing care where the weight is vertically? I've never seen any reference to that in any plane's load charts. As for whether the load is better in the fuselage or in the floats? I like the load centered in the plane rather than shoved aft. That's one reason so many Cubs use belly pods. Float storage space works the same way.

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    Et Tu Pide,,, Et Tu?
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    LuJon & FloatPilot, (I'm still with ya)

    You certainly are lifting the airplane's weight while in a turn. In fact, you must add a little more power in order to maintain your altitude while continuing to lift the weight. That's because the wings are no longer level and have therefore lost a bit of their lifting ability. If power isn't added, one of two things will happen: (1) some altitude will be lost, while the airspeed remains the same, or (2) the airspeed must be reduced to maintain altitude.

    Your reference to arc length has no application here. The floats are rigidly attached to the airplane and are not free to operate independently. If an arc length were to be applied here, that length would apply to the entire airplanle and all its parts, whether hanging down below the pilot's feet or concealed inside the engine cowling. Also, the length of the arc (I don't mean to encourage you here with this arc business . . . ) would be that distance from the airplane's CG to the radius point of the turn the plane is making. In some cases, that distance may be even a mile or more away. Gravity cannot act upon the floats alone, since they are rigidly attached to the plane.

    As to adding lift to the outside (high) wing, that only raises that particular wing. Any other result is a byproduct. And none of this has anything to do with torque.

    If you truly bvelieve that the float overload will have little to do with whether or not the airplqne will fly, please do not fly an airplane! Moreover, those floats, loaded or not, have almost nothing to do with the airplane's ability to turn. Torque, even if it did exist, would have nothing to do with ailerons, since they add virtually nothing to the lift of the wing. They are aft of that wing area that produces most of the lift. Once again, there is no torque applied by the rigidly attached floats, loaded or otherwise.

    To FloatPilot: as to that test question, I'd be very interested in seeing it - - - verbatum if possible. I'm sorry if I question your memory about that one, but I'm in a learning mode. If I'm not right, I'd certainly like to know it.

    As to swinging an object on a string, that string has nothing to do with centripetal force. The weight on the end of the string is not attempting to move inward along the string. It is simply the "load", the departure of which is restrained by the string and the anchor to which the string is attached. The object on a string is trying to move farther from the center, not toward it. If the weight should suddenly break loose and fly away, it won't flyalong the string, will it? It will simply fly away, and will do so in accordance with Newton's 3rd, right?

    And, like Mr. Pid, I've never seen a load chart that referenced horizontal loads. And, yes, I've performed W&B calcs for both USAF and scheduled air carrier aircraft. Locations of weights in or on an A/C are static, as far as the A/C is concerned. They do not alter their positions, nor their values, unless acted upon by outside forces. Think crash! Neither do they swing back, forth, oir any other direction.

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    I've been trying to follow you guys...MY BRAIN HURTS!

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    If you truly believe that the float overload will have little to do with whether or not the airplane will fly, please do not fly an airplane! Moreover, those floats, loaded or not, have almost nothing to do with the airplane's ability to turn.
    So yes you agree that overloading the float compartments is bad,,, but then again not really.. I don't get those two sentences being together.


    In fact, you must add a little more power in order to maintain your altitude while continuing to lift the weight.
    You can't, the whole thing is happening during & after take-off. Your Beaver is low, slow an at max power, heading for the gap in the trees. Plus your take-off run on the lake was longer because some noodle head over-loaded you dang floats.

    would have nothing to do with ailerons, since they add virtually nothing to the lift of the wing.
    WHAT??? ... What have you done with Mort and how did you get access to his computer? Do you at least agree that they do cause the wings to roll (bank) when entering a turn? The whole thing started with making an un-coordinated turn entry...

    To FloatPilot: as to that test question,...I'm sorry if I question your memory about that one,
    Hmm, you did not believe my personal observations regarding flight in this situation and now you don't believe me when I say the DPE asks this question.... The DPE asks the same question and makes the same point during every oral for a float rating. Since I observe those orals, I tend to notice when the same thing occurs every couple of weeks. The one I use every couple of weeks, for the last few years is named Dean Eichholtz. There are a few folks here who recently obtained their float ratings with me, so I trust that I am not delusional.


    As to swinging an object on a string, that string has nothing to do with centripetal force
    Oh for the love of Pete.... That basic reference is used as a simple explanation in books and by the internet encyclopedia gizmo...

    I've never seen a load chart that referenced horizontal loads. And, yes, I've performed W&B calcs for both USAF and scheduled air carrier aircraft. Locations of weights in or on an A/C are static, as far as the A/C is concerned.
    Yes the location of the weights stays static. But a combination of them make for a center of mass shift. Try looking up how to figure the CG on a multi-decked ship or double decked AC. Not only do you figure the ships center of mass with the loads, but you also need to figure out the actual center of mass for each item using a calculation like this.

    Of an L-shaped object

    This is a method of determining the center of mass of an L-shaped object.


    1. Divide the shape into two rectangles, as shown in fig 2. Find the center of masses of these two rectangles by drawing the diagonals. Draw a line joining the centers of mass. The center of mass of the shape must lie on this line AB.
    2. Divide the shape into two other rectangles, as shown in fig 3. Find the centers of mass of these two rectangles by drawing the diagonals. Draw a line joining the centers of mass. The center of mass of the L-shape must lie on this line CD.
    3. As the center of mass of the shape must lie along AB and also along CD, it is obvious that it is at the intersection of these two lines, at O. (The point O may or may not lie inside the L-shaped object.)
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    FloatPilot: If an airplane won't fly, any conversation about its turning is purely academic. If it will fly, the added weight will not affect its turning. It will affect stall speed because of the added wing loading. Along with bank angle, the A/C's CG location (inside the plane), and the total weight.

    If the overloaded Beaver had just taken off and was headed for the trees, why are we discussing turns? The driver had best just maintain a wings-level attitude for a bit.

    The ailerons are affected by the airflow's impact loads, rather than by any lift component. The wing's lift component occurs, in the main, much nearer the leading edge of the wing, right? Now, think very slow flight (as when nearing touchdown, for instance). You will have to use the rudder more than the ailerons to handle any wing-low attitude. The ailerons have become almost useless (think stalls and sp[ins). The wing, however, is still flying at that point, and will do so until it, too, stalls out.

    I agree that the "ailerons make the wings roll when entering a turn." If you keep your feet off the rudder pedals at that time, you are already entering an uncoordinated turn. As you recall, the rudder is used primarily to overcome the drag created by the ailerons. That tells me that ailerons add drag, but don't add lift (I know that you're still with me). The ailerons were displaced from a relatively smooth, trailing position intro the airstream flowing across them. That impact (to both the up and the down ailerons) is more noticeable on the "up" aileron - - - - - the one that is pointed down.

    Are you saying that a DPE realy asks about the pendulum effedt in conjunction with floatplane opeprations? If so, can you give me the e-mail address of that DPE? I can see that I still have one helluva lot to learn! I'm not being facetious, and I truly don't mind learning. But, until someone can convince me that floatplane flying has anything at all to do with pendulums (unless one set of float fittings come loose!), I've got to think that some DPE has been running in the sun too long.

    I enjoy your diagrams, with which I find no fault. On the other hand, these diagrams relate only to "resultants", right? Nowhere do they, in any manner, relate to pendulums or to a pendulum effect. Any application of pendulum specifics requires movement. Swinging a weight on a string isn't reflective of a pendulum, unless you swing the string only part-way in each direction. Something of an impossibillity, really. It works with clocks, but only because the weighted end of the pendulum is affixed to the clock's works by a rigid connector, not by a string.

    [Before you assume I'm so dense that simple engineering principles escape me, I'll confess that my last non-flying position was as the Senior Project Engineer for a Trump 55-story development on the beach here in Florida. I wasn't going to admit that . . . . . ]

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    11,415

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    Griz, I think you are taking the term pendulum out of context in this situation. FP isn't using the term to describe a weight swinging freely under the aircraft. The "pendulum" isn't swinging below the plane, it is a reference to the weight being rigidly suspended beneath the roll axis of the plane.

    Lets look at something different to get another point of view. Lets say you had a windmill set up with 6' blades and configured so it will spin when hit with a 10mph wind. Now with the blades of the windmill horizontal you affix a five foot long steel pole to the hub of the windmill pointing straight down. On the distal end of the pole you have a lead weight weighing 20lbs so that the windmill will now have to spin that 20lb weight in order to rotate. Can we agree that it would take much higher wind velocity (force) to be able to spin it?

    Now what if you took the same 20lbs and just added it directly to the hub, can we agree that the windmill would likely spin just fine in that case? Perhaps needing only a little more if any additional wind speed to induce rotation?

    If we can agree on that then it seems perfectly clear that the same principle is in effect when you add weight rigidly suspended below the roll axis of a plane. In this case that weight being in the float lockers. I firmly agree with FP, that that overloaded floats would have a greater affect on slow speed handling than the same weight located inside the fuselage.

  20. #20
    Member
    Join Date
    Apr 2006
    Location
    Anchorage
    Posts
    3,293

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    Go do turning stalls with a load in the floats and again without but at the same total weight in the same CG. Tell me if you can recognize a difference. My guess is you won't.

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