Clark, R B

Sparrow, W T

English

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TECHNICAL NOTE 
No. 1804 
EFFECT OF LONGITUDINAL STEPS ON SKIPPING CHARACTERISTICS 
OF PB2Y - 6 FLYING BOAT 
By R. B. Clark and W. T. Sparrow 
Naval Air Test Center 
TNashington 
J an'Jary 1949 
https://ntrs.nasa.gov/search.jsp?R=19930082479 2020-06-17T22:06:05+00:00Z

NATIONAL ADVISORY COMMI'ITEE FOR AERONAillICS 
TECHNICAL NOTE NO. 1804 
EFFECT OF IDNGITUDINAL STEPS ON SKIPPING CHARACTERISTICS 
OF PB2Y -6 FLYING BOAT 
By R. B. Clark and W. T. Sparrow 
SUMMARY 
Landing tests were made with the PB2Y -6 flying boat to detennine 
the effect of production step vents and forebody longitudinal steps on 
skipping. It was fOIDld that skipping occurred at higher landing trims 
without either step vents or longitudinal steps. The step vents alle-
viated skipping somewhat but were not considered large enough to be com-
pletely effective. The longitudinal steps caused a marked decrease 
in skipping which was not further affected by the addition of step 
ventilation. 
INTRODWTION 
Information given on page 18 of reference 1 indicates that external 
longitudinal steps on the forebody of a dynamic model of a flying boat 
constituted a powerful method for reducing the skipping of that particu-
lar model. The discussion in reference 1 indicates that the longitudinal 
steps effectively doubled the step height; a model which had ~uite 
unsatisfactory landing characteristics without them became satisfactory 
after their installation. 
It is generally conceded that the skipping characteristics of the 
PB2Y-6 flying boat are not satisfactory. Reference 2 indicates that the 
airplane would have good hydrodynamic landing stability if the step 
height could be doubled. Since external longitudinal steps on the fore-
body appeared to offer the possibility of effectively doubling the step 
height, the Bureau of Aeronautics re~uested that such steps be installed 
on the forebody of a PB2Y-6 for test. It was thought that useful infor-
mation could be obtained at the same time relative to the influence of 
the production step vents on the skipping characteristics of the PB2Y-6. 
Wind-tunnel tests of longitudinal steps on a similar hull are presented 
in reference 3· 
2 NACA TN No. 1804 
AIRPLANE DESCRIPl'ION 
A PB2Y -6 airplane was used to conduct the tests. The airPlane "Was 
equipped with Pratt & Whitney R-1830-94 engines, with Curtiss electric, 
1 three-blade, 122-foot-diameter propellers, and with two standard produc-
tion step vents. The special internal vent cover plates, mounted flush 
with the hull bottom, could be opened and closed from inside by means of 
a ~-inCh tube with an attached locking device. External longitudinal 
steps were installed on the underside of the hull forebody during the 
latter part of the tests; the location and construction details of these 
steps are shown by figure 1. 
A descriptive arrangement drawing of a PB2Y-6 airplane is given as 
figure 2. 
TEST MEl'HOD 
Tests were conducted at a gross weight of 58,000 pounds with the 
center of gravity located at 28 percent of the mean aerodyriamic chord. 
Flaps were in the fU.ll-down (400 ) position for all landings. 
An NACA trim-angle indicator (see reference 4 for description) was 
mounted directly in front of the copilot and was calibrated to show the 
inclination of the longitudinal axis of the airplane. 
Landings were made with the following four configurations: 
1. Without iongitudinal steps; vents closed 
2. Without longitudinal steps; vents open 
3. With longitudinal steps; vents closed 
4. With longitudinal steps; vents open 
The final approach to each landing "Was made at an engine speed of 
2550 rpm. The manifold pressure was adjusted to obtain a rate of 
descent of less than 50 feet per minute, and the desired trim angle "Was 
obtained by using the NACA trim-angle indicator in conjunction with the 
proper elevator setting. Once contact was made with the water, neither 
the power setting nor the elevator position was changed until the 
lan~ing run was completed. 
At least 10 landings were made at each critical trim angle. The 
average of the wind velocities for all landings was 10 knots. 
1 __ .. 
NACA TN No. 1804 3 
Deformation of the steps, resulting from water loads imposed during 
the first series of landings, is shown in figures 3 and 4. Rather than 
restore the steps to their original shape, reinforcing patches were 
attached, and the remainder of the test was conducted with the steps in 
the deformed condition. 
DISCUSSION OF RESULTS 
The results of the skipping tests of the PB2Y -6 airplane are 
presented in graphical form in figure 5 and in sUIlllIlru'Y form in the 
following table: 
Vent position Trim angle for skipping 
Without Abo-ve 4.50 longitudinal Open 
steps Closed Abo-ve 30 
With 
9° longitudinal Open Abo-ve 
steps Closed Abo-ve 90 
Indicated airspeeds varied from 76 knots at a trim angle of 20 
° to 60 knots at a trim angle of 10~ . 
approximately 66 knots to 50 knots at 
True water speeds -varied from 
10 trim angles of from 20 to 102 ' 
respectively. 
Since the primary purpose of the test was to determine the trim 
angle at which the initial skip occurred and since severe skipping devel-
oped at the higher trim angles, the airplane was not landed above a trim 
angle of 70 without longitudinal steps installed. However, with longi-
tudinal steps installed, landings free from skipping were possible up 
to and including a trim angle of 9°. Upon contact with the water above 
a trim angle of 90 , the nose-down pitching moment was sufficiently 
strong to cause porpoising or a pitching oscillation which resulted in 
skipping. At a trim angle of approxiJnately 8.50 , the fore body and 
afterbody of the airplane hull c,ontacted the water simultaneously. 
The critical trim angles for skipping were slightly lower during 
calm water landings. 
4 NACA TN No. 1804 
CONCLUDING REMARKS 
Landing tests were made with the PB2Y -6 flying boat to determine 
the effect of production step vents and forebody longitudinal steps on 
skipping. 
The standard production step vents had only a small effect on 
increasing the range of trim. angles free from skipping. 
Longitudinal steps on the forebody constituted a powerful method 
for improving the landing stability of the PB2Y -6 airplane used in the 
tests. It was found that they are subject to rather high water loads 
and hence care should be exercised in their design should they be used 
on a production airplane. 
Naval Air Tes t Center 
Pat uxent Ri ver, Md., May 27, 1946 
NACA TN No. 1804 5 
REB'ERENCES 
1. Benson, James M., and Bidwell, Jerold M.: Bibliography and Review of 
Infonnation Relating to the Hydrodynamics of Seaplanes. N.ACA.ACR 
No. L5G28, 1945· 
2. Locke, F. W. S., Jr.: An Analysis of the Skipping Characteristics of 
Some Full-8ize Flying Boats. NACA.ARR 5J24, 1946. 
3. Fullmer, Felicien F., Jr.: Wind-Tunnel Investigation of' a Systematic 
Series of Modifications to a Flying-Boat Hull. NACA TN No. 1576, 
1948. 
4. Gough, Melvin N.: The Use of the Trim-Angle Indicator for Seaplane 
Take-Off. Jour. Aero. Sci., vol. 4, no. 7, May 1937, pp. 288-291. 
6 
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NAeA TN No. 1804 
Vertical edge 
Figure 1. - Longitudinal steps for PB2Y -6 airplane. 
- I 
NACA TN No. 1804 7 
~------38' 10"------~ 
. J 12' 2" 12' 6"diam. 
If;;,,; 3260 lbs displacement diam. ,9, s~'~l~~ f~~'t lAO L; , 9~"; 4160 Ibs displacement 1 ~3~'~7=tit~~----- 49' 11~ " ________ ...l 
~------------------79' 3" 
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Figure 2. - Descriptive arrangement of PB2Y -6 airplane. 
,,~O.25" 
~--------------------- T --------------
(a) Before deformation. (b) After deformation. 
Figure 3. - Sketch showing permanent deformation of transverse section of longitudinal steps. 
(a) General view aft. (b) Detail of starboard longitudinal step; aft end. 
Figure 4.- Photographs showing deformation of longitudinal steps caused by water loads. 
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Figure 5. - Skipping characteristics of PB2Y -6 airplane . Velocity and 
trim -angle measurements were made the instant airplane contacted 
water. 
9 


Effect of Longitudinal Steps on Skipping Characteristics of PB2Y-6 Flying Boat

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