Article by J Frey


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 com­promise 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 indi­cate? What must the designer con­sider when developing the configu­ration 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 Fig­ure 1). The term "optimum planing angle" really refers to that hull bot­tom 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

oC     Angle of Attach °^i-     Angle of Incidence } Degrees T'     Float Trim Angle T.L.  Thrust Line



increases the water run; and a lower angle of hull trim decreases hull lift with little reduction or possi­bly 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 sec­tion chord line and the relative air­flow over the wing.)

Let us consider the flapped wing design for this discussion. If the float or hull requires +8° for opti­mum 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 sec­tion 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 down­hill floats, and why seaplanes with­out flaps appear to be flying more downhill than those with takeoff flap operation.

46  •   1990 Water Flying Annual


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 in­creased negative trim of the floats or hull bottom as well. I'm sure everyone has seen this effect dur­ing a high speed pass of any sea­plane— it may be flying level, but appears headed for the earth.

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,


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 Char­acteristics ot a Family of Seaplane Hulls Having Varying Length-Beam Ratio; Ar­thur W. Carter and David Woodward, NACA Technical Note 3119, January 1954


Fig. 2B Cruise Flight Trim

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



Floatplanes offer the best of two worlds — flying and boating — with the added freedom to go any­where there is unrestricted water. Before a landplane can take to the water, however, serious consider­ation 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 displace­ment phase of water operations. The shape of the floats must satisfy both hydrodynamic and aerody­namic considerations and permit the aircraft to move from displace­ment, over the hump, to a planing attitude, where less and less area is in contact with the water surface, allowing the floatplane to attain fly­ing 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 support­ing 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 dis­placement 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 dis­placement 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 con­siderations 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 rela­tionship between wing and the floats, based on the assumption that optimum planing angle for the floats is 8 degrees on an undis­turbed 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 approxi­mately 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. Unfortu­nately, 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 approxi­mately 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 characteris­tics 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 fea­tures that improved the water han­dling characteristics of the floats come back to haunt us. For exam­ple, 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 lim­ited to a maximum of 30 degrees of flap. Limiting the flap, on the other hand, can improve loading charac­teristics by preventing a nose down attitude.

One of the most positive advan­tages 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 consid­ered before floats can be attached to a landplane. We recommend that you try to understand the basic prin­ciples 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 impor­tance, is the use of flaps, trim and elevator, as they relate to the above discussion and the manner in which you load your aircraft.