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.
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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,
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 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
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.