WO2016022726A1

WO2016022726A1 - Empennage attachment for pusher airframe

Info

Publication number: WO2016022726A1
Application number: PCT/US2015/043874
Authority: WO
Inventors: Izak Van Cruyningen
Original assignee: Izak Van Cruyningen
Priority date: 2014-08-06
Filing date: 2015-08-05
Publication date: 2016-02-11

Abstract

An airframe is provided that includes a forward lifting wing, tail control surfaces, a propeller mounted between the wing and the tail control surfaces, a motor driving the propeller, a fuselage supporting the motor and the wing, and an elongated forward member sloping down from the fuselage when the airframe is in level flight, with an elongated aft member attached to the forward member sloping upward to support the tail control surfaces.

Description

Patent Application of

Izak van Cruyningen

for

EMPENNAGE ATTACHMENT FOR PUSHER AIRFRAME

Cross-Reference to Related Applications

This application claims the benefit of provisional patent application Ser. No. 62033967 filed 2014 Aug 6 by the present inventor.

BACKGROUND— PRIOR ART

[0001] This invention relates to attachment of the tail control surfaces

(empennage) to the aircraft fuselage for a fixed wing pusher airframe.

[0002] The Wright Brother's biplane had a pusher propeller mounted behind the main wings pushing the airframe forward. Many other early manned aircraft were designed this way, but pusher designs fell out of favor as stability became better understood. The tractor configuration could be constructed more easily and be just as stable. A tractor configuration has the propeller mounted in front of the wing, typically in the nose, so it pulls the airframe forward. From the Wikipedia 2014 article entitled "Pusher Configuration", "The main difficulty with this type of pusher design was attaching the tail (empennage); this needed to be in the same general location as on a tractor aircraft but its support structure had to avoid the propeller".

[0003] For model airplanes and unmanned aerial vehicles (UAV) a pusher configuration is desirable for forward view cameras and for safety. A camera mounted in the nose of a UAV for reconnaissance, filming, imaging, or first person view (FPV) provides better images if the propeller is outside its field of view. It is possible to synchronize the camera exposure with the propeller, much like Fokker synchronized a machine gun with propellers. However, it is difficult to accurately trigger and get enough exposure time with consumer-grade cameras given the high rotation rates and variable speed of airplane motors. An efficient pusher configuration is desirable for forward facing cameras.

[0004] One criterion the US Federal Aviation Administration is considering for safely integrating UAVs into the national airspace is frangibility of the airframe— do they fall apart in a collision. A propeller spinning at several thousand rotations per minute is one of the most hazardous components of an UAV. If that propeller is behind the fuselage and wing in a pusher configuration, then it is less likely to be the first part to strike a person or object, so the impact damage will be substantially less. Flights that go off course would be less dangerous with a pusher configuration.

[0005] Larger diameter propellers provide more thrust as long as the engine has enough power, up to the limit where the tip speed approaches Mach 1 and shock waves begin to form. To get thrust, it is more efficient to use a small acceleration of a large amount of air, rather than to use a large acceleration for a small amount of air. However, in the pusher configuration the required support of the tail structure limits the size of the propeller that can be used.

[0006] One approach to allowing a large propeller diameter, while securely supporting the empennage, is twin tail booms mounted on the wings, see for example the Cessna Skymaster or the Aerosonde UAV. This approach requires stronger wing spars and two booms, thus unnecessarily increasing airframe weight. Along the same lines are the Raab Krahe with booms above and below the propeller; the Brditschka HB-3 with one boom on the axis of propeller and a second straight back from the bottom of the fuselage; and the Buselec 2 with a single boom on the axis of the propeller. A boom on the axis of the propeller is a very clean design, but does not protect the propeller from a ground strike during takeoff or landing. No prior art shows a boom extending down below the fuselage and wing to protect against a ground strike and to serve as a landing skid at the same time.

[0007] Another approach to mounting a large diameter pusher propeller used in seaplanes (US 4962978), powered gliders, model airplanes, and UAVs such as the TechPod, Cyclops, Bixler, Super Sky Surfer, etc. is to mount the motor well above the wing. Then the propeller can clear the tail boom attached to the bottom of the fuselage or wing and pointing straight back.

[0008] With a high motor mount the thrust axis is above the drag axis, adding to the nose down pitching moment from the lifting wing. This pitching moment can be counteracted by an additional downward force at the tail, producing additional trim drag. More commonly the motor is pitched towards the center of gravity to minimize throttle- pitch interference. This leads to a downward component of thrust which is undesirable. Either of these corrections forces the lifting wing to carry more load to make up for the downward force from the tail or the motor.

[0009] A better configuration would more closely align the thrust axis with the center of drag to balance the wing moment and minimize the balance force required of the tail. Ideally the centers of lift, drag, thrust, and gravity are all very close to each other to minimize the imbalance of forces. The lift from the wing is used almost entirely to carry the airframe and payload, rather than being used for balancing

SUMMARY

[0010] Figure 1 is a perspective view of an airframe moving in direction indicated by arrow 8 comprised of fuselage 10 with forward lifting wing 12, motor 14, propeller 16, tail fin 18, tail stabilizer 20, and empennage attachment 22. Empennage attachment 22 supports tail fin 18 and tail stabilizer 20 with a downward sloping elongated forward member 24 and an upward sloping elongated aft member 28.

ADVANTAGES

[0011] Pusher airframes with various means of attaching the empennage have been known for more than a hundred years in the prior art. The novel empennage attachment described here allows a large propeller diameter while keeping the thrust axis near the center of drag to provide efficient flight. Various aspects of the embodiments of my empennage attachment are superior because:

• A larger diameter propeller is more efficient at producing thrust.

• Aligning the thrust axis with the center of drag reduces imbalances in flight, thereby minimizing required tail volume and trim drag while maximizing payload capacity.

Additional unanticipated advantages include:

• Base 26 of the empennage attachment can serve as a skid plate during landing, or as a wheel enclosure. It protects the propeller from a ground strike.

• With a folding propeller or a slightly undersize propeller, the empennage attachment can be designed to flex on landing, reducing the impact forces on the airframe and payload.

• When the airframe slows down after landing and the wing no longer produces enough lift, then the airframe will rock forward. It will slide to rest on the edge of the nose, the empennage attachment, and a wingtip thereby protecting cameras in the fuselage; the flaps and ailerons in the wing; and the control surfaces in the tail.

• Angle of upward sloping elongated aft member 28 is designed to be steeper than the stall angle of wing 12, thereby minimizing the risk of a tail strike during landing

• If upward sloping elongated aft member 28 is shaped as an airfoil, it will exert

correcting lift when it is at an angle to the free stream, thereby increasing airframe stability.

• The empennage attachment can be a hollow profile that makes an excellent container for a parachute. To deploy the parachute, the front of the tube is opened so that air blows the parachute out of the tube.

[0012] Other advantages of one or more aspects will be apparent from a consideration of the drawings and ensuing description.

FIGURES

1. Perspective view of an airframe incorporating the empennage attachment.

2. Section A-A of Fig 1 showing streamlined airfoil shape for forward and aft empennage members.

3. Detail of base connection of empennage attachment.

4. Detail of base connection housing a landing gear.

5. Profile of airframe with empennage attachment on touchdown.

6. Profile of airframe with empennage attachment when landing complete.

7. Profile of curved empennage attachment when airframe is landing.

8. Profile of airframe with empennage attachment integrated with tail fin.

DETAILED DESCRIPTION

[0013] This section describes several embodiments of the empennage attachment with reference to Figs. 1-8.

[0014] Figure 1 is a perspective view of an airframe flying to the left in the direction of arrow 8. The airframe comprises a forward lifting wing 12 mounted high on fuselage 10; motor 14 and propeller 16 with axis below the wing pushing the airframe forward; tail fin 18; tail stabilizer 20; and empennage attachment 22. Empennage attachment 22 has a downward sloping elongated forward member 24 and an upward sloping elongated aft member 28 to support tail fin 18 and tail stabilizer 20 around propeller 16. [0015] During flight, the airframe moves forward in the direction of arrow 8 and the air around it is stationary (assuming no wind). For most aerodynamic analysis it easier to change the frame of reference and consider the airframe stationary with a steady freestream wind passing over airframe in the direction opposite to arrow 8, much like in a wind tunnel. The freestream is the uniform, undisturbed flow away from the airframe.

[0016] With forward lifting wing 12 mounted high on fuselage 10, the wing profile drag and induced drag will act high on fuselage 10. The parasitic drag from fuselage 10 will act on its centerline (for a symmetric fuselage); the parasitic drag due to empennage attachment 22 acts low; and the tail drag centerline depends on its geometry. The center of drag will be partway below wing 12, and it may vary somewhat with airframe speed. Propeller 16 is mounted with thrust axis slightly below the center of drag at cruise speed to balance the drag forces and to partially balance the nose down pitch moment of wing 12 (if it is cambered). This close alignment of thrust axis and center of drag minimizes the tail volume required as well as the trim drag for different flight speeds.

[0017] There is a trade-off between the additional drag created by empennage attachment 22 and the angles of downward sloping elongated forward member 24 and upward sloping elongated aft member 28. The shallower the angles, the less incremental drag, but the smaller the clear diameter for propeller 16.

[0018] A cylinder aligned with the free stream has little profile drag, primarily skin friction drag. Most examples from the prior art cited above use cylinders aligned with the free stream to attach the tail control surfaces.

[0019] A cylinder mounted perpendicular with the free stream has very large profile drag due to flow separation. The biplanes in the early 1900' s had enormous drag from all the bracing wires. Even today, the parasitic drag from landing gear support wires can equal the fuselage drag, see for example R/C Model Aircraft Design by Andy Lennon, p.52.

[0020] Carlos Reyes in Model Airplane Design, p. 31 shows a table of drag of a cylinder due to flow separation for different angles of a cylinder with respect to the free stream. When the cylinder is aligned with the free stream the separation drag is zero and when it is perpendicular then the drag is 100%. At 30 degrees it is 50% and at 20 degrees it is 34%. The downward slope of elongated forward member 24 of empennage attachment 22, and upward slope of elongated aft member 28 reduces separation drag. However, at shallow angles the diameter of propeller 16 must be reduced and that greatly reduces propulsion efficiency. Thus during airframe design a tradeoff has to be made between propeller diameter and angles of the forward 24 and aft 28 elognated sloping members.

[0021] Fig. 2 is a section A— A (in Fig. 1) of elongated aft member 28 for an improved embodiment. In this example the downward sloping elongated forward member 24 and upward sloping elongated aft member 28 are constructed with symmetric airfoil, teardrop, or streamlined shapes instead of cylinders. This minimizes the likelihood of separation and greatly reduces drag from empennage attachment 22, thereby allowing steeper angles and larger propeller 16 diameters. The downward and upward sloping angles of these profiles with respect to the airflow increases the effective chord, thereby increasing the Reynolds Number, thus both reducing drag and increasing lift for the profile.

[0022] Fig. 3 shows a detail view of an embodiment of empennage attachment 22 comprised of downward sloping elongated forward member 24, base 26, and upward sloping elongated aft member 28. Base 26 serves as the transition like a bend in a pipe or hockey stick between the airfoil shapes of downward sloping elongated forward member 24 and upward sloping elongated aft member 28. It is the lowest part of the airframe and the first part of the airframe to touch the ground on landing so it also serves as a landing skid. With streamlined cross-sections for the downward sloping elongated forward member 24 and upward sloping elongated aft member 28, the incremental drag for this landing gear is minimal, so it is a very aerodynamic ally efficient landing skid.

[0023] Fig. 4 shows a detail view where base 26 houses a wheel 42 that serves as the landing gear.

[0024] Figure 5 is a side profile of another embodiment where the airframe is just touching down on ground 30 in a landing. In this example, fuselage 10 has wing 12 mounted near its vertical centerline and propeller 16 lines up almost on the same axis. Propeller 16 is a folding propeller that closes down when motor 14 slows down. Vertical tail fin 18 has tail stabilizer 20 mounted on top in a T-tail configuration. Empennage attachment 22, has a downward sloping elongated forward member 24, base 26 with a hard chine, and upward sloping elongated aft member 28 to hold tail surfaces 18 and 20. Center of mass 32 is on the wing and fuselage centerline in the forward part of the forward wing 12 chord. [0025] On landing, wing 12 has a very high angle of attack to maximize lift and minimize speed. Upward sloping elongated aft member 28 is angled more steeply than the stall angle for wing 12 so that tail surfaces 18 and 20 are in no danger of striking ground 30.

[0026] During a preflight check, center of mass 30 location is verified. For stability it must be in front of the neutral point and for responsiveness it should not be too far forward of the mean aerodynamic chord of wing 12. With the location shown in Fig 2, it would be very close to the center of drag. Propeller 16 is mounted just below this axis so the propeller thrust balances the drag and the wing moment at most airspeeds.

[0027] Figure 6 is a profile view of the airframe when it has stopped moving after a landing. In this embodiment, fuselage 10 has high wing 12, motor 14 with thrust axis near the center of drag, and folding propeller 16. The VTail has port fin 17 and starboard fin 19 mounted on empennage attachment 22. Sensor and battery 33 is connected to servo 34 which in turn controls cover 36 over downward sloping elongated forward member 24 of empennage attachment 22. Parachute 40 is folded inside upward sloping elongated aft member 28 of empennage attachment 22. Parachute attachment and shock cord 38 feeds through empennage attachment to attach to fuselage 10.

[0028] When the airframe comes to rest after a landing it will balance on an edge of the fuselage 10 nose, the bottom 26 of empennage attachment 22, and a tip of wing 12. Cameras pointing forward in the nose or pointing down in the fuselage are protected from ground contact. Likewise motor 14, propeller 16, and VTail surfaces 17 and 19 are protected from ground contact. Flaps and ailerons in wing 12 are also well off the ground.

[0029] Parachute 40 is a useful backup device for an emergency or if the landing area is too obstructed for a reasonable glide path for a normal landing. The parachute could be deployed automatically on loss of main power, loss of GPS signal, if the airframe flies outside a geofence by too large a margin, if the altitude is too high, if the altitude is too low, if the speed is too high or too low, on loss of radio signal, etc. Sensor 33 might be a barometer, a power watchdog, a radio link check, a pitot tube to sense speed, the autopilot, or several of these connected in parallel to cover multiple emergency situations. If sensor(s) 33 detects a fault or is commanded by the autopilot, then it activates servo 34 to open cover 36 over downward sloping elongated forward member 24 of empennage attachment 22. If the airframe is flying, this admits air into empennage attachment 22 that blows folded parachute 40 out the back of empennage attachment 22. The inflating parachute 40 pulls shock cord and attachment 38 tight with its attachment to fuselage 10 and slows down the airframe for a parachute landing.

[0030] Figure 7 is a side profile of another embodiment where the airframe is just above the ground 30 in a landing. In this example, the components normally mounted in the fuselage are housed in a much thicker forward lifting wing 12, much like in a flying wing. Motor 14 and propeller 16 are mounted at the trailing edge of forward lifting wing 12. Vertical tail fin 18 has tail stabilizer 20 mounted on top in a T-tail configuration. Empennage attachment 22, has a curved downward sloping elongated forward member 24 mounted below forward lifting wing 12, a smoothly curved base 26, and upward curved and sloping elongated aft member 28 to hold vertical tail fin 18, stabilizer 20, and rudder 21. The faired, gradual curves of the empennage attachment provide a large surface area for landings at different angles of attack for forward lifting wing 12.

[0031] Fig. 8 illustrates yet another embodiment of empennage attachment 22 where upward sloping elongated aft member 28 has been curved to fair into tail fin 18. Forward lifting wing 12 is mounted on fuselage 10 with motor 14 and propeller 16 at the rear of fuselage 10 and center of gravity 32 near the mean aerodynamic chord of forward wing 12. Empennage attachment 22 supports tail fin 18 with rudder 21 and tail stabilizer 20. Both downward sloping elongated forward member 24 and upward sloping elongated aft member 28 have streamlined sections with wide chords and upward sloping elongated aft member 28 blends in seamlessly with tail fin 18.

[0032] In this embodiment, parachute 40 is packed into the larger volume of tail fin 18. It is connected to fuselage 10 by attachment and shock cord 38. Sensor 33 is connected to servo 34 that opens parachute cover 44 in the side of tail fin 18 to deploy parachute 40.

[0033] The wider chords of downward sloping elongated forward member 24 and upward sloping elongated aft member 28 increase the Reynolds Number for these sections, thereby reducing drag and increasing lift. Using a symmetric airfoil section, for example a NACA 0024 airfoil near base 26 and a NACA 0012 at the upper ends, makes the profiles contribute to the airframe stability. If the airframe yaws to one side or the other, the flow over the profiles will generate lift to push the airframe back into alignment with the free stream. The wider profiles also increase the lateral area below center of gravity 32. [0034] Upward sloping elongated aft member 28 has bottom edge at a larger angle than the stall angle for forward wing 12 to reduce chance of tail strikes. It extends underneath rudder 21 to also protect it. The top edge of upward sloping elongated aft member 28 integrates smoothly with tail fin 18 to avoid a junction that creates vortices.

[0035] In this embodiment, empennage attachment 22 serves as landing skid, propeller protection, rudder protection, yaw stability, and parachute enclosure. Serving that many purposes is a very efficient use of material leading to lighter airframes and increased payload and endurance.

[0036] This section illustrated details of specific embodiments, but persons skilled in the art can readily make modifications and changes that are still within the scope. For example fuselage 10 can be different shapes; motor 14 can drive propeller 16 through gears or chains; or the tail surfaces may have other configurations.

Claims

CLAIMS I claim:

1. An airframe comprising:

a forward lifting wing,

tail control surfaces,

a propeller mounted between said wing and said tail control surfaces,

a motor to drive said propeller,

a fuselage to support said motor and said wing,

an elongated forward member sloping down from said fuselage when said

airframe is in level flight,

an elongated aft member attached to said forward member sloping upward to support said tail control surfaces.

2. The apparatus of claim 1 wherein said forward member and said aft member have a streamlined cross-sectional shape whereby drag is reduced and stability of said airframe is enhanced.

3. The apparatus of claim 1 further comprising a base at the lowest point of said airframe connecting said forward member to said aft member whereby said base provides a landing skid for said airframe.

4. The apparatus of claim 3 wherein said base is a faired transition between said forward member and said aft member whereby said landing skid provides a large landing area at different angles of attack while minimizing drag during flight.

5. The apparatus of claim 3 wherein said base supports landing gear.

6. The apparatus of claim 1 wherein said aft member extends under said tail control surfaces whereby said aft member protects said tail control surfaces from a ground strike during landing.

7. The apparatus of claim 3 wherein said aft member is curved up and faired into said tail control surfaces whereby drag is reduced and airframe stability is enhanced.

8. The apparatus of claim 3 wherein said aft member and said tail control surfaces

enclose a parachute.

9. An airframe comprising:

a forward lifting wing,

tail control surfaces,

a propeller mounted between said wing and said tail control surfaces, a motor mounted on said wing to drive said propeller,

an elongated forward member sloping down from said wing when said airframe is in level flight,

10. The apparatus of claim 9 wherein said forward member and said aft member have a streamlined cross-sectional shape whereby drag is reduced and stability of said airframe is enhanced.

11. The apparatus of claim 9 further comprising a base at the lowest point of said airframe connecting said forward member to said aft member whereby said base provides a landing skid for said airframe.

12. The apparatus of claim 11 wherein said base is a faired transition between said

forward member and said aft member whereby said landing skid provides a large landing area while minimizing drag during flight.

13. The apparatus of claim 11 wherein said base supports landing gear.

14. The apparatus of claim 8 wherein said aft member extends under said tail control surfaces whereby said aft member protects said tail control surfaces from a ground strike during landing.

15. The apparatus of claim 11 wherein said aft member is curved up and faired into said tail control surfaces whereby drag is reduced and airframe stability is enhanced.

16. The apparatus of claim 11 wherein said aft member and said tail control surfaces enclose a parachute.