A performance comparison between a conventional and a ram-wing competition hydroplane

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A PERFORMANCE COMPARISON BETWEEI A CONVENTIONAL AND A RAM-WING COMPETITION HYDROPLANE

Tth SYMPOSIUM ON NAVAL HYDFCDYNAIVflCS PLANING CRAFT PANEL

BY

D. E. CALKINS

JULY 1968

U.S. NAVAL UNDERSEA WARFARE CENTER SAN DIEGO, CALIFORNIA

1.0 The Hydroplane Configuration

The sport of high speed unlimited hydroplane racing provi.des an excellent means of testing new machinery and hull designs. Over the years a variety of

hull designs have evolved to suit the particular requirements of the various classes. However, the design that has achieved the greatest popularity among fans is the three-point hydroplane.

The hull is arranged with two "sponsons" that provide lateral stability and planing support on their aft ectremities and the lift of the "surfacing" propel-ler. The propeller lift results from the vertical component of the thrust times

the sine of the angle that propeller shaft makes with the water surface. The "surfacing" propeller rides so that its hub lies exactly on the water surface with the upper half of the propeller out of the water. This is necessary to avoid propeller blade cavitation by allowing the propeller to break te surface and thus ventilate the negative pressure side of the blade. Figure 1 shows a typical unlimited hydroplane, th MAIRICK.

In addition, since the center portion of the hull i. clear of the water, there is air flow through the tunnel so that an aerodynamic ground effect is experienced. Lift is thus obtained from the hull which. is an airfoil of low aspect ratio. The hull configuration arises from th need to accommodate e pilot and engine in a hull with a flat bottom and a minimal frontal profile. The result is a craft with some degree of positive aerodynamic camber. Airtraps (thin flat plates) are sometim added to the bottom of the hull extending aft of the sponson step to act as endplates and to increase the amou of bgh pressure air trapped under the hull.

lies forward of the center gravity, which means that under certain conditions, the vehicle may be aerodynamically unstable..

2.0 LongitudinalInstability

Unlimited hydroplane design has reached a point in the design evolution cycle where it is at the peak of its attainable performance curve. In addition, it has experienced a serious dnnic pitch stability flaw. Over a period of three years, from

₁₉₆₆

to the present, a rash of accidents have killed five top drivers.

An analysis of the accidents from newspaper accounts and. photographs resulted in the following sequence of events, which are shoi in Figure 2.

Initially,the craft operates with the hydrodynamic forces acting on the sponsons (planing surfaces), the propeller lift force and the aeocynnic

forces acting on the hull at the aerodynamic center. In this condition, the ground effect causes the aerodynamic center to move aft from the position due solely to the hull planform.

At some critical speed, a pitching motion is initiated by rough water conditions which results in 'a divergent oscillation about the aft support point, the propeller. Essentially, the wave encounter frequency is coincidental with the natural frequency of the planing surfaces, approximately 2.5 to 4.O CD; which leads to an amplification of the oscilj.tions rathr than a decay. As th bow

pitches up, the hull moves out of ground effect with a conseq.uent forward shift in the aerodynamic center position along with an increase in aerodynamic lift due to angle of attack.

This motion continues until the amplitude becomes large enough so that the craft rotates about the propeller and becomes airborne with the planing sur-faces clear of the water.

This rotation continues until an aerodynamic stall condition occurs and the restoring moment due to craft weight causes the bow to roll one side, then

pitch down until contact with the water surface occurs. The result is usually complete destruction of the craft. In some cases, dependent on the speed, com-plete rotation of the craft occurs.

3.0 The Formula HuB.

Configuration

-It isclear that the vehicles operate in ground effect and thus are subject to changes in their aerodynanic characteristics. Figure 3 shows a new hull

concept developed jointly by Mr. Bryant of Seattle, Washington and the author. For identification purposes, the design is called the Formula Hull. Dynamic support is provided by the two planing surfaces forward of the CG, the wing in ground effect and the vertical component of the propeller thrust. The purpose of the design is to improve the vehicle .Lift/drag ratio (/D) and: to provide

positive pitch stability.

--Since the conventional hull attains a flat wing-like configuration, this suggested designing the Formula Hull with a well defined wing to take full

advntage of the aerodynamic grun. efft. In addition to the sponsons providing

hydrodynaniic planing support, they are extended aft along the wing tip chord to act as wing fences, thus creating what is technically termed a ram-wing hydroplane. The intent is to design a vehicle which has the wing placed so that the center of gravity (CG) lies forward of the wiig aerodynamic center (ac.). This is

ac-complished by designing the wing so that the aft movement of the a.c. in ground effect from its geometrical location allows the proper setup for adequate pitch stability. The pilot, engine power-train, and assorted machinery are contained in a center hull which rides dut of the water at speed. The vehicle configuration is thus composed of three distinct units:

In June of

₁₉₆₆

the design and construction of what is essentially a one-. half scale model of the proposed unlimited design was initiated. Within the limits of the ana1rtical analysis, the design represents the optimum distribution of aerodynamic and hydrodynamic support, approximately

50/50.

The vehicle will be used as a test bed for the basic hull concept by instrumenting it to measure trim

(pitch), speed, thrust, and engine RPM. Construction began in January _196T and should be completed late in

_1968.

11.0 Tow Tank Test Results

As part of the vehicle developmental program, a series of tow-tank tests were conducted in collaboration with Mr. T. Sladeck of the Marine Technolor Center, General Dynamics, to compare the performance characteristics of the conventional hydroplane hull design with those of the proposed Formula Configuration. The models were fre to heave and trim while being constrained in yaw. Speed, drag, and trim measurements were made on the models. The conventional hull had the characteristics shown in Figure 14 Test speeds to approximately 60 FPS were attained. In addition to these data, other model and full-scale performance measurements for various vehicles are presented for comparison.

A rather interesting vehicle, the Hydrodynamic Test System (HTS) was designed ad built by the Boeing Company (i) fo± high-speed hydrodynamic research, Figure

5. Although not iatend&. for racing, it is configured much like the conventional

three-point hydroplane except that it is turbo-jet powered with a third planing ski added at the stern. In addition, the forward area between the sponsons is removed t facilitate attachment of a lift/drag balance mounted to a aralle1ogrm truss arrangement. Model hydrofoils are mounted to this balance and. may be tested to speeds of 80 knots. Its size and similarity to the unlimiteds make full scale thrust and speed measurements valuable for comparison with the model data.

A craft which was specifically designed to generate as little aerodynamic lift as possible in order to minimize the longitudinal instability prb1em is

worthy- of discussion. In 19514 Mr. Donald Campbell commissioned the design of what was probably the most thoroughly engineered craft of its ty-pe for an

as-sault on the world's water speed record (2). The craft which resulted was the jet-powered "BLUEBIRD", Figure

6.

The craft utilizes the forward sponson arrangement much like the unlimited craft except that the sponso are mounted on outriggers in an attempt to arrive at a configuration which is stable in the pitch mode. Although not entirely successful,, wind tunnel tests verified that

an adequate restoring pitch moment due to weight existed at the design speed of 250 mph. Model data for the BLUEBIRD and unpublished tow tank data (3) for

another conventional hydroplane hull (MAVERICK) similar to the tested conventional hull are also presented.

The model performance data obtained were corrected to full scale 'by Froude scaling for speed and weight:

tJdl

U1.1

scale g = 32.2 ft/sec2 (i)

[g Ldl

vg.L1

scale

fifll scale = model

)3 _(L )3

full scale. mode

No attempt was made to correct the data for Reynold's Number as wetted area measurements were not made. Table 1 compares the full scale geometry of. the varioks vehicles.

(2)

Table 1.0

Craft Weight Length Width

Formula

₇₅₀₀

30' - 0"

15' -

14" Conventional (A,B) (C,D) (E,F)

6150

6350

71400

30' -

0"

-12' -

14" HTS

₁₆₁₇₂

_'

38' - 0"

17' -

_0" BLUEBIRD 14800

25' -

0" _{10' - 5"} MAVERICK '6000

30' - 9"

12'

-The Conventional (A) model and MAVERICK were very similar in detail and hence resulted in similar performance. In both of these tests, no attempt was made to simulate the propeller shaft and boss, nor the thrust moment. Figure

7 compares the lift/drag ratios of the two. In order to be more representative of the full scale configuration, the Conventional model was modified to include the propeller shaft and boss, the thrust moment and sponson spray strips (B,C,D), Figure

8.

A/D is seen to be relatively constant between 70 and 120 mph, and then starts to decrease. The conventional model was then modified to improve per-formance by shortening the sponsons and adding camber to the planing surfaces

(E,F). The resulting improvement is shown in Figure

8.

The Formula hull performance is also shown in Figure

8.

The rather distinct performance curve requires explanation. Essentially three flow regimes occurred:

Ci)

Then the hull was at rest, the waterline lied slightly above the center-line of the wing leading edge. hence, up to the full scale speed of about 60 mph, a sheet of water flowed over the upper surface of the wing resulting in the ex-treznely low /D's.

At 70 mph (f.s.) the hull and wing emerged, although the wing trailing edge and the aft portion of the hull were still wetted. The /D remains constant to approximately 100 mph.

At 100 mph (f.s.) the wing trailing edge and hull emergso that the design performance configuration is obtained. The /D is seen to be approximately twice that of the Conventional design.

The BLUEBIRD and HTS performance curves are shown in F5ure 9. Both show characteristic drag

"humps"

at their respective takeo'f speeds.

From the BLUIRD data it is seen that the other data may 'be approximately extrapolated to higher speeds by the relationship

() =

()

(U1

D2 D1

where.U1 and are known conditions. Figure 10 compares the faired data for each of the hull designs. It is seen that the Formula hail has the highest

over the racing speed range, 100 to l0 mph.

Figure 11 provides an interesting comparison of the trim characteristic3 of the Formula and Conventional hydrooane hulis. The change in trim,

_T,

from

the static trim shows the rather distinst characteristics of the two hull designs. The Conventional hull trim increased with speed up to the point that it literally flew from the water surface. This demonstrated the increase of aerodynamic lift acting forward of the CG which caused the pitch plane

insta-bility. The Formula hull, on the other had displayed rather different characteris-tics. Trim increased slightly up to a point where the wing leading edge emerged from the water. A very rapid increase in trim then took place. The aerodynamic lift then started to increase causing the hull to trim bow don over the remainder of the speed range. it is evident from the data then, that the Formula hull

configuration was successful in displacing the aerodynamic center far enough aft to assure positive pitch stability, although much work remains tooptiutize the configuration.

REFERENCES

Chappelear, D. N. The BoeinjIydrodynmc Test Systen for High Speed

Underwater ReTsearch. Boeing D2-2O438-l.

Norris, L. H. and K. W. Norris. "The Hydroplane BLUKBIRD,'T The Engineer, March29, 1957, p. L71.

Sladeck, T. Unpublished Towtank Model Data-MAVERICK, Marine Tecbnolor Center - Towing Basin, General Dynamics.

(a) . =2.12 ft. m / eg

=0.59

m A,B,C,D

a

= 0.55 APPENDIX Formula Hull A = 2.65 lb.

m

= 2.12 ft. 0.559 (b) Conventional Hull MODEL CONPIGUPATION A Af -lb. A Clean Model 2.16 6150

B A+ Scale Prop Shaft/Boss 2.16 6150

C B+ Thrust Moment 2.214W ₆₃₅₀

D C+ Sponson Spray Strips 2.2I- 6350

E D+ Shortened Sponsons and.

Cambered P'aning Surce 2.6

F E - Without Cambered Planing

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