Item 0474_2
FLETTNER 282
-by M. Berry
The progress made in the helicopter field in Germany during World War II was shared by Mr. Anton Flettner, director of the firm bearing his name. Although only twenty-two ships were actually constructed, plans were under way, as the European war ended, for the construction of one thousand.
Known as the F1-282, the Flettner machine is a one or two-place craft having intermeshing two-blade rotors, tricycle landing gear and substantial tail surfaces. Its general characteristics and dimensions are as follows:
Empty weight is 1410 lbs.
Gross weight is 2205 lbs.
Rotor diameter (of each rotor) is 39.3 ft.
Engine set level take-off rated metric hp. is 140.
Engine Siemens Halske Series 4A 7 cyl. air-cooled.
Rotor turns 175 rpm. for best cruising.
Rotor turns 180 rpm. at take-off (Engine 2200 rpm.)
Fuel with one person 27.8 gals.
Fuel with two persons 17.2 gals.
The rotor characteristics are as follows:
Total included angle between axes 24 deg.
Two blades for each rotor.
Blade chord is 11.42 in.
Disc area 1212 sq. ft.
Disc loading at 2205 lbs. is 1.82 lbs/sq. ft.
Solidity .06
Forward feathering 8 deg.
Lateral feathering 3.6 deg.
Aft feathering 4 deg.
Flown more hours and developed further than any other German helicopter projects, the F1-282 has performed in a very satisfactory manner. Being extremely maneuverable, it has been flown blind, in rain and gusty weather without mishap. Many tests have been made of autorotation, altitude, speed, stability, vibrations and vibratory stresses. Hundreds of power-off landings have been made and in an endurance test one ship was flown ninety-five hours without repairs or changes. Permissible CG travel allows the machine to fly with and without passenger without changing the trim, although this procedure changes the vibrations.
With 2200 lbs. gross weight, the vertical climb, at sea level, is given as 300 fpm. The maximum speed in level flight is 90 mph. Hovering ceiling is roughly estimated in the neighborhood of 1000 feet. With disc loading of 1.8 lbs. per sq. ft., the rate of vertical autorotative descent is given as 26 ft. per second, though apparently no systematic tests were made at very low speed to show rate of descen4 as a function of speed. Maximum speed of sideways flight is about 15 mph. Maximum diving speed is about 109 mph.
The F1-282 is said to be neutrally stable in hover. If the stick is given an arbitrary displacement, and then held fixed in the central position, the machine will execute a swing motion, the amplitude of which neither increase or decreases. In forward flight there is a longitudinal in. stability reaching a maximum at about 25 mph, and speeds greater than 37 mph flight is again stable. The period of phugoid instability is about 11 seconds when the stick is held fixed. The machine can be flown hands-off forward flight above 37 mph for indefinite lengths of time by adjusting a bungee in the cockpit to neutralize stick load.
An interesting feature of the phugoid oscillation is that if the stick is suddenly displaced forward and returned the motion is stable, while if the disturbance is caused a sudden rearward displacement, the motion is unstable. The F1-282 is stable in yaw. Oscillations of about 2 per minute have been observed but the amplitude does not increase.
The F1-282 is considered to be rough. The two-bladed rotors (synchronized to be parallel in the 45 deg. position) produce vibrations of the fuselage which appears be torsional about the vertical axis. The blades are provided with flapping and drag hinges, the latter damped by adjustable metal-to-metal friction, and there is no other means of isolating the aerodynamic forces from the pilot. During the rev-up the machine shakes rather badly on its landing gear, and sweetens up before take-off. This vibration is of course sensitive to tire pressure. In flight the vibrations of the control stick were at first severe, but were greatly reduced by the well-known artifice of adding inertia to the system. Movement of either one of the cyclic pitch pushrods drives a sector, engaging a worm to which a small flywheel is attached. Although this device substantially cured the control stick vibration, it has the disadvantage that the stick is sluggish and also tends to a shoot a given displacement. Mr. Flettner emphasizes when the drag hinge friction dampers are very carefully adjusted, the vibrations of the ship are much reduced. (Recommended setting is slippage at 94 lb. ft. applied torque.)
Dr. Hohenemser showed by analysis that a self-excited oscillation could occur in flight at low rpm and high pitch. This did in fact once occur severely at 140 rotor rpm, but disappeared when the ship was put into autorotation and the rpm increased.
Vibratory stresses at critical points were measured by strain gage technique, and found to be excessive in the control links at maximum speed. After this measurement, diving at high speed was prohibited.
The rotor blades are of conventional construction having steel tubular spar, wood ribs attached to steel collars, wood cover and fabric over the plywood. Blades are straight, having a radius of 19.65 ft. and a chord of 11.42 in. The plywood cover .039 in. thick is wrapped over the leading edge. The leading edge is .63 in. wide. The trail-edge is laminated vertically and has a total width of 1.34 in. from 1 to 16 and .98 in. outboard of rib 16. Ribs are uniformly spaced at 3.9 in. along the entire blade. Ribs are built-up construction of two cap strips with plywood webs on either side of the cap strips. A solid block is added between cap strips at the spar for riveting the rib collars.
The blade is built with a twist of 4 deg., the angle of incidence decreasing from inboard to tip. A turned-up trailing edge tab of 17 in. length and extending .79 in. to the rear of the trailing edge is installed on the blade just inboard of the tip. The tab is fixed but can presumably be bent to change the moment coefficient of the blade.
The blade section appears to he of the NACA 23000 series with 17% maximum thickness. The tubular spar is 1.5 in. OD x .138 in. wall thickness.
The blades were originally built with chordwise balance weights only on the outer half of the blade. This was found to be unsatisfactory and chordwise balance weights were added on the inner half of the blade. These weights are attached to the aft side of the leading edge with screws.
The total weight of the blade is 88 lbs. Lacquer was used the fabric cover because it gave a very smooth surface. It was also contended that it gave a more uniform distribution and hence less apt to disturb the balance of the blade. It was claimed that protracted flights in rain and snow were possible without damage to the leading edge.
The spar is threaded on its inboard end for attachment to a sleeve having a forked end which forms part of the horizontal hinge. The sleeve telescopes over the spar approximately 6.3 in. so that the threaded section of the spar is some distance from the point of maximum moment the spar. A pin through the sleeve and spar prevents relative rotation between the two.
The two two-bladed rotors are mounted on shafts which ye an included angle of 24 deg. and are spaced 23.2 in. apart at the intersection of the planes of the horizontal pins and the centerlines of rotation. The horizontal and vertical hinges of the blades are coincident and are located at a point 12.8 in. outboard of the centerline of rotation. The location of the horizontal hinges so far outboard of centerline rotation allowed the hubs to be brought together and gave better blade clearance. It has the added advantages of producing greater control moment for a given flapping. It has the disadvantage of greater roughness with divergence of the tip path plane from the plane normal to the mechanical axis. This divergence with forward speed and center of gravity position. Hence, it is possible to have considerable roughness in hovering if the center of gravity is such that the tip path is not perpendicular to the axis.
The bearings for the pitch axis of the blade are located inboard of the horizontal and vertical hinges and the axis is inclined up at an angle of 6 deg. to the centerline of rotation. The control, being located inboard of the hinges, is not affected by movement of the blade about the hinges. The vertical hinge is parallel to the centerline of rotation and hence is 6 deg. from a normal to the pitch axis.
The vertical and horizontal hinges as well as the pitch axis have needle bearings. The thrust bearing for the pitch change axis is also a needle bearing. The loading on the bearings of the vertical and horizontal hinges is of the order of magnitude of 5000 psi based on the nominal projected area of the bearing. No trouble has been experienced with these bearings under this loading.
The blade dampers are mounted on the vertical hinge. They are of the friction type with multiple steel and brass plates. The load is applied through a rubber disc, the pressure being adjusted by turning the cap of the damper. The usual setting of the damper gives a torque of 94 lb. ft.
Centrifugally operated droop stops are installed on the blades which allow them to he revved up at a coning angle of plus 1 deg. and still allow the blades to flap down angle of minus 5 deg. in flight. The stop is a cam which is moved by a hinged weight, which flies out due to centrifugal force. At 110-120 rpm the cam moves and allows the blade to fall to an angle of minus 5 deg. To set the stop for a coning angle of plus 1 deg. the blade must he raised by hand until a spring moves the cam into proper position, or this will he automatically accomplished if the collective pitch is at a large enough setting as the rotor slows down.
The upper transmission, which provides the mounting for the two hub shafts, is supported on a steel tubular structure similar to an engine mount. This structure is attached to the fuselage at the upper longerons.
The two hub shafts are mounted in roller bearings with an additional ball bearing used to take the thrust. The hub shafts are geared to a short cross shaft upon which is mounted another gear meshing with a central pinion shaft. The design of the cross shaft is rather unusual and, while heavy', should give a very rigid mounting for the pinion on either end. In order to decrease the diameter of the bearings for mounting this shaft, the shaft itself has been used as the inner race.
On the central pinion shaft is mounted the freewheeling unit which disconnects the engine and upper transmission. A rotor brake is also mounted on the shaft.
The upper and lower transmission units are connected through a short drive shaft having a universal joint at each end. The lower transmission is mounted on the forward end of the engine crankcase and the pinion of this unit is mounted on the crankshaft using the usual propeller splines and cones. This unit changes the direction of drive, upward and aft 650 to line up with the upper transmission. On the nearly vertical shaft is mounted a multiple disc clutch and a dog type coupling. To rev up the rotor, the friction clutch is engaged by moving a lever in the cockpit. When the engine and rotor tachometer indicate that there is no slippage of the clutch, the dog type coupling is engaged by a further movement of the clutch lever. Because the propeller shaft was not long enough, an extension was added to allow the mounting of the fan forward of the transmission.
The two transmission units, by means of three sets of gears, effect a total reduction of 12.2 to 1 between the engine and the rotors. When the engine turns at 2200 rpm the rotor turns at 180 rpm. All gears are spiral level. Considerable use of roller bearings has been made, ball bearings being used only where thrusts or combined thrust and radial loads required them. As the transmission cases are made of magnesium, heavy steel sleeves have been inserted in the cases wherever bearings are located.
Longitudinal and lateral control is accomplished by' joint cyclic pitch change of the two rotors i.e. by tilting both gimbal rings in the same direction.
Steering control is obtained by a combination of rudder and differential collective pitch change on the two rotors. The large rudder is necessary to give sufficient directional control in autorotational condition as the differential collective pitch is ineffective in this condition. It has been stated that in power operation sufficient control in yaw is available to make all maneuvers even when hovering in winds of 20 mph. Some trouble is experienced by new pilots when operating near the ground, since as they come around into the wind they lose lift and strike the ground.
An adjustable stabilizer is provided, but a setting of plus 4 deg. is found to be satisfactory for all forward speeds.
The collective pitch control is located adjacent to the throttle so that both may he moved simultaneously if desired. The collective pitch lever does not actuate the collective pitch directly but operates it through a blade pitch governor.
The blade pitch governor holds the rotor rpm within certain limits. The mechanism controlling the pitch consists of springs acting against levers which are moved by centrifugal force, the springs tending to decrease pitch and the levers to increase the pitch. The initial load of the spring can be adjusted on the ground to govern at any desired rpm, and also the minimum angle can be set, below which the governor cannot further reduce the pitch. This latter setting is chosen to give positive autorotational characteristics in case of power failure.
In early tests of the machine, the governor was set for a minimum of 140 rpm, and it appeared that in one case of power-off flight, autorotation actually stopped and the machine dropped and hit the ground very hard. Also on another occasion self-excited vibrations were encountered in flight. After these episodes the governor was set at a minimum of 160 rpm.
The collective pitch lever allows the pilot to override the governor and adjust time pitch. It has been stated that the pilot can override the governor only to increase the rpm, but cannot reduce the rpm below that of the governor setting.
The governor is power operated through two oppositely rotating clutches driven from the upper transmission.
Dr. Hohenemser had planned a barometrically controlled governor to take care of large ranges of altitude, but it was never constructed as the ceiling of the present machines was not sufficiently high to justify it.
Specific values of pitch angles used in the controls are as follows:
The engine is a Siemens Halske. Series IA, 7-cylinder air-cooled, radial, mounted with crankshaft horizontal. It is rated at 140 hp. at 2200 rpm. for take-off at sea level. Cruising rpm. is 2050. The engine is reported to run 400 hours between overhauls.
An eight-bladed wooden fan of 26.8 in. diameter is mounted on an extension to the propeller shaft. The air enters at the bottom of the fuselage, passes through the fan to the engine and passes down and out through another opening in the bottom of the fuselage. No means of controlling the airflow is provided. Normal engine temperature is 2 25 deg C (437 deg. F).
Fuel capacity is 105 liters (27.8 gals) in two tanks, but if two people are carried, the fuel load is limited to 65 liters (17.2 gals).
An oil sump, located under the nose section of the engine, holds approximately two gallons of oil and serves as both sump and tank. An oil filter is used which cleans the oil every two hours. Oil consumption is approximately one quart per hour.
Two carburetors are used opening into the same manifold. No mixture control is provided and no means of pre-heating the air is used.
The engine is started by means of a high-pressure air tank which is connected to time cylinders through a distributor. With the ignition 'on,'' fuel is pumped into the intake manifold. 200-psi air pressure is sufficient for warm weather and 400 psi for winter.
A conventional steel tubular engine mount is used, bolted directly to the intake manifold with no vibration absorber.
The landing gear is of the tricycle type. Shock struts are of the spring oil type and are said to work very freely. On the main gear the shock strut is inclined inward at an angle of 35 deg. to 10 deg. from vertical and is subject to bending due to the offset of the wheels. The total strut travel is 2.36 in., while the vertical wheel travel is 1.33 in. Tires are 18.32 in. diameter by 6.50 in. wide. Wheel tread is 81.5 in., with shock strut extended and 84.3 in. with shock strut compressed. The nose wheel strut travel is 6.30 in. and the vertical wheel travel is 4.92 in. The tire is 13.80 in. diameter by 5. 2 in. wide. The nose wheel is steerable, being connected to the rudder pedals.
It may be of interest that Flettner ships have been landed from vertical descents in the manner characteristic of autogiros, (by pulling back on the control stick after nosing down from vertical descent). Collective pitch was not used. In one such landing the tail was damaged by hitting the ground. Dr. Hohenenser feels that a ground angle of approximately 20 deg. between wheels and tail is desirable for this type of landing.
The fuselage is of welded steel tubes. The cockpit enclosure, bottom front and sides is of transparent material with the top open. The center section enclosing the engine is metal covered and the rear section is fabric covered. The body shape is far from being aerodynamically desirable, as tuft tests have indicated that the flow separation or turbulence is so great that it makes much of the tail surface ineffective.
Article from American Helicopter, June 1947 ~ Thanks to DS.
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Last Revised: June 10, 2002