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Thread: Rigging floats

  1. #1

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    Rigging floats

    Bushmaster finished, going straight to 2250 floats and I don't know how to get them rigged in their proper place.

    My guess is to find the balance point by rolling them on a pipe and make sure that point is on shoulder line and hopefully little aft cg.

    Any tips would be appreciated.

  2. #2
    C-FIJK's Avatar
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    rigging

    I would say that you want the c of g on the float ahead by 4-7 inches when rigging so when you load the plane it goes aft!!
    Gerry Marcil

    Every day spent flying is a great day !

  3. #3

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    Would love an answer to this as well! Rig/test/re-rig is just not acceptable when putting floats on an exp.(usually no data available from like existing installations) A good base line may be to use the step of the float as a datum in reference to the cg of the aircraft in both forward & aft locations? Or maybe the cg of the float is the best? Any info on this or links would be greatly appreciated.

    Mitch

  4. #4
    mvivion's Avatar
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    I'd get in touch with one or more of the float manufacturers and chat with one of their engineers for a starting point.

    Why re-invent the wheel?

    I'd start with EDO at Kenmore and see if you can get any info from them to start.

    MTV

  5. #5

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    Try Bruce at Iles Aero Services. (902) 245-5277 Digby
    Ron

  6. #6

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    Rigging floats

    Thanks, Ron. Also going to see Paul Easson's W & B, weight and arms.

  7. #7
    skywagon8a's Avatar
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    This is from "Design for flying" chapter 15 by David Thurston (designer of the Colonial Skimmer/Lake, Thurston Teal and others). "To provide stable trim during planing, the hull step should be located on a line running approximately 10 degrees aft from a keel perpendicular through the gross weight center of gravity." He does not address the keel angle to the angle of incidence. You will find that this is generally in the neighborhood of 4 +/- 1 degrees. Any more than this will increase aerodynamic drag reducing cruise speed. Less will increase hydrodynamic drag increasing take off distances.

    Is this what you were looking for?
    N1PA

  8. #8

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    Rigging floats

    It is helpful, skywagon. Yours and others including search is giving me some idea of the process---and for sure more confidence.

  9. #9
    wheat's Avatar
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    This is posted from airwrench in anchorage. I believe he can help you with your question.



    Joined: 16 Oct 2003
    Posts: 63
    Location: Anchorage, AK

    PostPosted: Thu Oct 16, 2003 5:21 pm Post subject: PA-22/20 Producers Reply with quote
    What a great site! Mark from Big Lake convinced me to take the time to look here. Thanks, Mark!
    I'm new here, and in just looking around I started to reply to a few of the posts, but thought that starting a new thread would be better. And to introduce myself, since I was referenced in a few places. I am the son of Jake Bryant, Jake's Aircraft Salvage, of Anchorage, AK and Big Lake, AK. 1950's to date. Jake, passed away in 1986, but his legacy will
    continue to live on in the airplanes we fly. He aquired many STC's during the years, but most significantly, he designed and built the first STC'd stretched Pacer, the "Producer". This was originally STC'd in 1959, as a one-time STC. Many years later we upgraded that to a multiple STC SA3614NM. I am still operating "Jake's Aircraft" at Big Lake.
    To answer a few of the questions that I saw come through, A Pacer is a good airplane, but the "Producer" mod brings it into the SC class. Perfomance will match the SC especially on floats. I have one of them at Lake Hood, and built the other two.
    Mark of Dakota Cub took lots of pics of N24MF ( the Montana Producer ) before he built his version. He did that while I was out flying his slotted wing SC. The Bushmaster is a similar story.
    I normally keep my opinions to myself, but if I feel strongly about it, or feel it is a safety issue, I will toss my .02 in.
    - keep the dirty side down - Steve

  10. #10

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    Thanks Skywagon, this is exactly the type of info I'm looking for! Thanks also for including the source of the info as I'll need this for verification and further research.

    Mitch

  11. #11

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    Just thought I'd post this info I found on float rigging as not much "non-application specific" documentation is easily found when searching this topic.

    Not great at links, plus they tend to disappear with time.

    Note: I paraphrased a bit, this is not all a direct quote although most is.

    Moderators: If long posts like this are unacceptable, PM me to wrap my knuckles.

    Hope this helps in your rigging decisions as it sure has answered a lot of questions for me.


    Mitch

    ************************************************

    An article by J Frey, a long term Edo guy:


    Seaplanes operate in a complicated environment and present unique design problems.

    I'm sure you have noticed the nose-up trim attitude of a seaplane (floats or hull) as it transitions from the displacement mode (low speed displacement buoyancy) onto the step. As planing speed increases, nose-up trim decreases until a com*promise angle between wing and float lift and drag is reached to permit optimum takeoff distance.

    During climb out, particularly for high powered aircraft, airplane nose-up trim will again increase to provide maximum rate or angle of climb.

    While a seaplane may fly in a level attitude when cruising along, during high speed flight constant altitude is maintained with a nose-down trim attitude.

    Why do these changes in trim angle occur, and what do they indi*cate? What must the designer con*sider when developing the configu*ration of a new seaplane?

    Based upon experience backed by NACA tests referenced at the end of this article, the standard "V" shaped hull bottom with transverse step has an optimum planing angle of +8° in trim as measured at the keel (the float reference line of Fig*ure 1). The term "optimum planing angle" really refers to that hull bot*tom attitude providing maximum dynamic (water impact) lift to drag ratio. A greater angle with the water surface results in increased hull drag, noticeable during seaplane takeoff when the stern drags and so

    increases the water run; and a lower angle of hull trim decreases hull lift with little reduction or possi*bly an increase in hull drag. As a result, the water supported part of any seaplane has a narrow-range trim angle or operation, as every water pilot should be well aware.

    Once we have the hull shape and desired angle of water trim during takeoff established, it is necessary to get the seaplane into the air. Depending upon whether or not flaps are available for takeoff, the wing will provide maximum lift at an angle of attack of about +16° for our standard basic airfoil sections, and at an angle near +12° for wings equipped with single slotted flaps lowered 30°. (Angle of attack is the angle between the wing sec*tion chord line and the relative air*flow over the wing.)

    Let us consider the flapped wing design for this discussion. If the float or hull requires +8° for opti*mum operation while the wing must be set at +12° for maximum lift, it is obvious that the wing chord line must be positioned at +4° to the float reference line. If our wing sec*tion provides the lift required for cruise at a +2° angle of attack, the same angle between the wing chord line and the fuselage reference line would be considered to be a +2° angle of wing incidence as shown by Figure 2. However, as you will recall, we have a 4° angle between the wing chord line and the float or hull keel — which is why seaplanes appear to be flying around on down*hill floats, and why seaplanes with*out flaps appear to be flying more downhill than those with takeoff flap operation.

    As speed is increased beyond the design cruising speed selected for the seaplane configuration, the wing angle of attack will decrease (because as speed increases the required lift coefficient decreases for a given airplane wing area and weight, while angle of attack also decreases with the lift coefficient). This results not only in a nose-down airplane trim attitude, but in in*creased negative trim of the floats or hull bottom as well. I'm sure everyone has seen this effect dur*ing a high speed pass of any sea*plane— it may be flying level, but appears headed for the earth.

    Floatplanes offer the best of two worlds — flying and boating — with the added freedom to go any*where there is unrestricted water. Before a landplane can take to the water, however, serious consider*ation must be addressed that will allow it to perform with acceptance in both environments.

    The floats must be strong, light weight and large enough to provide adequate buoyancy in the displace*ment phase of water operations. The shape of the floats must satisfy both hydrodynamic and aerody*namic considerations and permit the aircraft to move from displace*ment, over the hump, to a planing attitude, where less and less area is in contact with the water surface, allowing the floatplane to attain fly*ing speed and become airborne.

    An aircraft in the displacement mode must show good stability and have acceptable water handling characteristics. The FARs state that you must have a minimum of 80% reserve buoyancy. If we take a set of EDO 89-2000 floats as an example and keep in mind that 2000 is the fresh water displacement of the floats submerged, a pair of these floats is capable of support*ing an aircraft with a gross weight of 2222 Ibs. An aircraft such as the Piper PA-18, which has a gross weight of 1760 pounds would have a reserve buoyancy of 47% (2000 x 2 -:- 1760 = 227% - 180% = 47%).

    Other considerations in the dis*placement condition are the size and location of the water rudders. It is most important that the rudders be located as far aft as possible, in undisturbed, smooth water and be-

    low the after body of the keel.

    The hump condition is where the floats transition between the dis*placement and planing phase of a takeoff. It is where the maximum overall drag occurs and places the greatest demand on propeller and engine.

    The ability of a floatplane to reach optimum planing attitude, which occurs at approximately 20 mph, represents a great many con*siderations that took place when the floats were being approved. In most cases the floats are installed so that the center of buoyancy is positioned beneath the forward C.G. limits of the aircraft. Dave Thurston carefully reviews the rela*tionship between wing and the floats, based on the assumption that optimum planing angle for the floats is 8 degrees on an undis*turbed water surface (which will give us the minimum drag at the highest speed). The float angle of incidence relative to the aircraft horizontal reference line is approxi*mately 3-5 degrees negative (floats nose down). The smaller angle is used with higher powered aircraft and the larger angle is for low powered aircraft such as the older Taylorcrafts and J-3 Cubs, which do not have flaps. Unfortu*nately, the angle of the float relative to the angle of incidence for the wing, that gives the best takeoff performance, may

    result in greater drag when the aircraft is in level flight.

    Pitch stability and the use of elevator and trim are very important during the planing phase of the takeoff or landing. If pitch limits are exceeded, the aircraft can develop

    a porpoise or oscillation which will increase in amplitude and become so violent the aircraft may be tossed out of the water. The limits of pitch stability are determined by the relationship between the fore-body and afterbody during the plan-ing condition. (The stern post angle can become very critical.)

    Most EDO floats have approxi*mately 8 degrees between high and low angle of porpoise and it should be remembered that as the trim angles decrease, due to higher loads and higher speeds, the force vectors will increase as the wetted area gets larger. Therefore, greater caution should be used when flying heavily loaded aircraft.

    Once a floatplane gains forward speed, the floats have characteris*tics similar to a boat, but since you also want to leave the surface of the water and become airborne the float is designed to run on the deeper stage of its hull, just ahead of the step, to reduce friction and tolerate the increased angle that occurs when the aircraft rotates and breaks free from the water. During rotation the stern post angle must be adequate to allow the aft section of the floats to remain clear of the water surface, otherwise, drag is increased and the aircraft may be prevented from attaining proper angle of attack for the wing. Once airborne, many of the fea*tures that improved the water han*dling characteristics of the floats come back to haunt us. For exam*ple, the large area of the float that is now forward of the C.G. decreases the stability and in most cases, the seaplane must have additional fin area added to the rudder, or a vertical fin, to meet the FAR flight test requirements. (A recent article in a Government of Canada Air Safety Bulletin, traces a number of accidents involving PA-12 and similar model floatplanes to loss of directional control at low speeds due to a failure to install the auxiliary vertical fin.) The floats can also have an adverse affect on climb characteristics and a number of aircraft are limited with regard to maximum flap settings. Cessnas cannot meet the flight test balk landing requirements and are lim*ited to a maximum of 30 degrees of flap. Limiting the flap, on the other hand, can improve loading charac*teristics by preventing a nose down attitude.

    One of the most positive advan*tages we get when floats are installed on an aircraft, apart from added utility, is that it generally lowers the stall speed somewhere between 3 and 5 mph, depending on the gross weight of the aircraft. Lower stall speeds also means lower landing loads, which is why some aircraft can be licensed at higher gross weights when they are on floats.

    As you can see, there are many parameters that must be consid*ered before floats can be attached to a landplane. We recommend that you try to understand the basic prin*ciples at work in this process, in order to appreciate changes in the flight characteristics and handling of your aircraft when it is operated as a floatplane. Of particular impor*tance, is the use of flaps, trim and elevator, as they relate to the above discussion and the manner in which you load your aircraft.

    I trust that this simple explanation of basic seaplane design requirements helps to clarify why seaplanes look and fly as they do. Unfortunately, making them takeoff and fly as desired is usually not quite so simple.

    Reference:

    1. Hydrodynamic Investigation of a Series of Hull Models Suitable for Small Flying Boats and Amphibians; W.C. Hugh, Jr., and W.C. Axt, NACA Technical Note 2503, November 1951

    2. Static Properties and Resistance Char*acteristics ot a Family of Seaplane Hulls Having Varying Length-Beam Ratio; Ar*thur W. Carter and David Woodward, NACA Technical Note 3119, January 1954

    3. Amphibian Aircraft Design, D. B. Thur-ston, October 1976.


    ************************************************


    The following is a comentary by Peter Cowan(an experimental builder). It references the previous article as well as discussions with a former engineer from Edo, Leon Kaplan, who provided much more information.

    This builder has first hand experience rigging floats on experimental aircraft(mostly Rans S7 but also an exp 170 with EDO 2425'S) and has had great sucsess using this data.



    Centre to Centre width.


    If you look at successful float installations it is reasonable to conclude that the range of float center to center distance varies of from 40 to 50% of float length, with the norm usually in the 44% to 48% range.

    A wide stance will improve crosswind and taxiing stability but will make standing near the side of a tandem fuselage more difficult.

    Once you have decided on your maximum and minimum width in regaurds to stabilitiy issues then you can consider cabin access and astetics as long as you do not sacrifice the stability minimums you previously set.



    Height of fuselage above floats.


    This measurement also seems to be highly related to personal preference.

    You need to keep the prop away from the water but most installations result in the prop tip being much further off the water than the minimum 12”.

    There is also some correlation between height above the floats and step position when you adopt one approach to positioning the step(keep reading).



    Angle between datum and floats.


    Frey points out that for best take off performance we want the wing at the angle of attack for maximum lift while the floats are riding in the water at an angle for minimum drag. He suggests that a flapped wing needs 14 degrees (this is not true for all airfoils but close) so the geometry has to provide this. So, how do we achieve that 14* angle of attack?

    First we need to use the horizontal datum line of the aircraft as the reference line for rigging the floats. Next, most designers have built in a positive angle of incidence of the wing center line to the datum line of 2 to 3 degrees. Let’s use three for now.

    Frey points out that early studies showed floats need to ride at 8 degrees for minimum resistance while planing. Thus, if we mounted the floats parallel to the datum line we would have an angle of attack of 8 + 3 or 11 degrees when the aircraft is on the step. So, we need to mount the floats at 3 degrees negative to the datum line to get our 14 degrees.

    A knowledgeable friend determined from studies of similar airfoils that my Rans S7 actually achieves max lift at 18 degrees. This would require not 3 degrees between float and datum but 7. I have mounted the floats a little more than 3 degrees and takeoff, cruise and landing performance is excellent; the best performance I’ve had over four different S7 float planes.

    I would predict that cruise speed and landing characteristics would suffer with more angle between float and datum. The compromise here is that we don’t want the nose of the floats too low while in level flight to increase drag or to make it difficult to achieve a slight nose up position of the floats on landing. While I will experiment with this in the future, for now, 6 degrees between float and wing CL works well.

    When I was installing 2425 Edo’s on the 170 homebuilt a seasoned FBO operator, Jim Leggat, told me to put 6 degrees between wing center line and top of float. That worked just fine. So given that the 170 is double the gross of the Rans, it looks like this is a pretty solid rule of thumb.

    Finally, where to position the airframe on the floats (Usually looked at as the fore/aft location of the step)?

    Based on what Edo shows on their drawings one could argue that this measurement relates more to the fluid dynamics of the system rather than aerodynamics. Edo does it this way: the weight of the bare airframe is positioned vertically above the centre of buoyancy of the float when the float is resting in the water. (And Noel L, you are the only other person I’ve come across that also considered the C of B.)

    While most float mounting instructions have the airframe positioned with the datum line level (an in-flight attitude), the Edo drawing brings out the concept of looking at the fluid dynamics of the package while resting in the water. I like this approach.

    The discussion of angles above deals with the transition to flight, in cruise and landing attitude, none of which has much bearing on the position of the step. The loading of the floats by setting an airplane on them could be compared to loading a boat. The small outboard sitting at the dock rests at a specific, more or less level, probably a bit nose up, attitude. If we are loading several people into the boat, we position them not all at the front or all at the back but more or less evenly distributed to retain that level attitude. I suggest we are loading the boat by distributing the weight equally around the C of B.

    Most floats sit in the water with about a 3 degree nose high attitude (although the two sets I’ve installed are at 4.5* to water). The centre of buoyancy is close to the C of G of the float and both are a few inches ahead of the step. Frey says we draw a VERTICAL line up from this point ahead of the step while the float is in this floating attitude (not level, not with the 3 degree nose down to datum that we know we need). It is on this vertical line that we position the cg of the bare aircraft minus landing gear.

    You need to make a drawing to see the impact of this:


    (IF DRAWING DOES NOT APPEAR I'LL ADD IT LATER AS AN EDIT)



    This drawing illustrates several points.

    First, it shows that the horizontal distance of the step from the cg depends on how high the plane is above the floats when we do the rigging with the datum horizontal.

    Further it illustrates that if we go back to the more traditional set of rigging instructions which do everything with the datum horizontal (try rotating the picture counter clockwise to get the datum level), we see that the cg is well ahead of the step. But what cg is this? Edo is using the cg of the bare airplane without landing gear. Some other rigging instructions for the level attitude method use the most aft CG limit.

    What this all boils down to is that to simplify rigging instructions in an environment where minor variations are not very critical, people have adopted methods that are easy to use and generally work. It is simpler for us to deal with a level datum and put the step at or aft of some cg point.

    My S7 above is a good example. The bare cg – gear is 74” (aft of prop hub); the max rearward is 81” a range of 7”. In fact, I put the step at 1” aft of the aft cg. (datum level) or 8” aft of the forward cg. Using the above diagram and the height of the S7 cg above the approximate C of B of the floats, that 6 degree angle line puts the C of B about 4.5“ aft of the forward cg. Let’s say the C of B is 3” ahead of the step making the step 7.5” aft of the forward cg. This means that I’m within ˝” of the Edo method. ** see note below re CG range on the Rans S7

    So, it is easy to see that the Edo method with the forward cg is likely not much different from the pragmatic method with the aft cg.

    Perhaps a better translation of the Edo method is: With the aircraft datum level drop a vertical line from the CG of the airframe without landing gear; run another string from the CG angled rearward by the number of degrees between the float datum and the centerline of the fuselage + the float at rest nose up angle to the water (3 to 5 degrees). This sum will frequently be 3 + 4 =7 degrees. Locate the Center of Buoyancy of the float on this line. The problem is finding the C of B of the float. A reasonable estimate would be to place the C of B about 3” ahead of the step and about 40% of the depth of the float. (See below in discussion of 1450 floats for method of approximating the C of B position).

    Here is another hypothesis: the center of buoyancy is probably pretty close to the C of G which is easily determined by weighing the floats using two scales under the spreader bars. The CG of the Murphy’s is 4.5” ahead of the step; the CG of the 1350’s is 8” ahead of step.

    Here is another curve. After posting this site to the Matronics Seaplane list, Hagen Heckel from Germany pointed out that German regs REQUIRE the step to be 100 mm or 4” aft of the most aft CG.

    With this in mind, I moved the step on the 1350 floats to 5” aft of CG. This also seems to work fine. With a 220lb person in the back seat, the heal of the float submerges slightly when I also stand on the float beside the rear seat, so I am going to move the fuselage another inch forward. Why not if fluid dynamics is the only issue? Yes overall CG is still fine.

    What I don’t know is the symptom of the aircraft placed too far forward on the floats. I’m guessing it could be a nose over tendency on landing, something I’m not seeing yet.

    These floats have a unique M shaped bottom forward of step. They appear to accelerate more quickly as they get on the step but ride noticeably harder on waves than a straight V bottom.

    Lotus floats have less of a rise from the step aft so the S-7S below is mounted at 4.5 degrees to the float top so that little rotation is required at lift off. The wide angle is noticeable but they are still faster in cruise than a set of Murphy 1500’s that were on the plane previously.

    The step is also further aft to provide more rearward flotation when loading because these floats tend to have minimal rear end flotation.

  12. #12
    skywagon8a's Avatar
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    That is good information Mitch. JJ Frey ran the floats division of EDO for many years and of course had access to all the EDO data which had been developed since the 1920s.

    I will add this: As the step is moved forward in relation to the loaded CG it will require more horsepower to get on the step. A low horsepower plane needs the step further aft. If the step is too far aft it will promote porpoising tendencies. An airplane with higher stall speeds would be happier with a more forward than aft step.

    I have a friend who installed a set of Greenwood fiberglass floats on his J-3. These floats had flat bottoms and no step. They planed easily and were extremely difficult to keep from porpoising. It was nearly impossible to take off and land without porpoising. The bottoms eventually tore out on a landing and the floats were totally demolished. Yes they were STCd. I can not figure out why the FAA approved them. They were horrible.
    N1PA

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