The lip wing system, an analysis based on channel wing experimental data

Abstract

To increase performance of VTOL aircraft, a system that augments hover lift is needed,but without increasing the rotor diameter or power required, as being detrimental to high speed performance and transport efficiency. The lip wing provides lift in hover, reducing direct rotor lift, consequently the power required, and without the need to increase rotor diameter. This research provides a lip wing system evaluation on hover performance based on channel wing experimental data, as the channel wing could be considered a subset of the lip wing.

Full text

The lip wing system, an analysis based on channel wing
experimental data
Dusan Stan
ALIPTERA Inc.
Ionut Cosmin Oncescu
INCAS - National Institute for Aerospace
Research "Elie Carafoli", Bucharest,
Romania,
Abstract
To increase performance of VTOL aircraft, a system that augments hover lift is needed,
but without increasing the rotor diameter or power required, as being detrimental to high
speed performance and transport efficiency. The lip wing provides lift in hover, reducing
direct rotor lift, consequently the power required, and without the need to increase rotor
diameter. This research provides a lip wing system evaluation on hover performance
based on channel wing experimental data, as the channel wing could be considered a
subset of the lip wing.
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List of Contents
Introduction 3
The VTOL conundrum 3
State of the art 3
Shrouded rotors 4
Channel wing 4
Lip Wing 5
Description 5
Operation 6
Shroud design 8
Lip Wing Validation 8
Experimental data and research 8
Channel wing NACA research 9
Lip wing estimations based on channel wing 10
Conclusion 11
Concluding Remarks 12
Works Cited 12
Nomenclature
Rotor disk areaArot −
W ing surf ace areaS −
W ing lif t coeff icient CL−
otor disk loadingDl−R
otor thrustT −R
Rotor induced air velocityvi−
ir densityρ − A
W ing top surf ace airspeedva−
W ing lif tL −
otor air intake areaAia −R
Resultant thrust vectorR −
T hrust augmentationT Aug −
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1. Introduction
This document describes the lip wing concept, presents research and validation based
on the channel wing experimental data[Pasamanick].
In order to increase performance of VTOL aircraft, a system that augments hover lift is
needed, but without the need to increase the rotor diameter or the power required.
There are a lot of devices that enhance lift generated by a wing at slow speeds as slats,
slots, flaps, but they do not provide enough or any lift at zero aircraft speed. There are
also well known methods for generating lift for VTOL craft, but each of them has certain
disadvantages.
1.1. The VTOL conundrum
A fixed wing aircraft flying at constant altitude and constant speed exerts 4 basic forces:
lift equal to weight and thrust equal to drag. Lift per drag ratio of a typical fixed wing
aircraft is in the range of about 8 to 20. Considering an aircraft having the L/D ratio of
10, its propulsion system needs to provide a cruising thrust that is 10 times less than
weight of the aircraft. Now, in order to provide VTOL capability to that particular aircraft,
the propulsion system needs to provide a VTOL thrust of such magnitude to overcome
weight, or in other words to provide 10 times more thrust than in cruising mode. The
engine needs to be larger, heavier, consuming more fuel, adding even more to the
weight, decreasing transport efficiency. The propeller needs to be larger, at higher
speed having more drag. As stated, the fundamental problems of VTOL design are
balance/control and thrust matching[Raymer]. These are the main reason the VTOL
aircraft occupies such a small aviation niche today.
There is a definite need for improvement, a need of a device that provides control and
augmented thrust at slow or zero speed, yet ensuring low drag at higher speeds.
1.2. State of the art
The most efficient hovering craft to date is the helicopter; it employs an open rotor that
in order to achieve high hover efficiency, the rotor must have a low disc loading,
invariably leading to a large size. The size of the rotor is creating difficulties as the
helicopter speed increases, such as retreating blade stall, high drag and loss of
efficiency, making the helicopters unsuitable to operate at higher speed. A method to
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combat these deficiencies is employed by tilted rotor and tilted wings aircraft such as
Bell Boeing V-22 Osprey and Canadair CL-84. Their design is a compromise between
hovering, having higher disk loading than helicopters, and horizontal flight, having more
propeller disk that they need, and a much larger engine than needed for generating
forward thrust, resulting in an inefficient system, consuming more fuel and having much
more drag than comparable fixed wing aircraft.
1.3. Shrouded rotors
Static thrust generated by a propeller is higher if it is enclosed into a shroud or a duct,
tip losses are reduced, and the shroud intake increases thrust. Although a shrouded
propeller creates more static thrust, as speed increases, the drag created by the shroud
increases too, and above a certain speed, it becomes higher than the shroud created
thrust (break-even velocity), so less efficient than an open air propeller. A shroud
optimized for high static thrust has a big bell shaped inlet, and is inefficient at higher
speed, inherently creating more drag. An example of a craft employed ducted
fan/shrouded propeller to increase static trust is the experimental Bell X-22.
1.4. Channel wing
Channel (Custer) wing type aircraft, as CCW-5, creates some small lift at zero speed.
NACA tests of a channel winged aircraft showed less than 8% total thrust increase and
lack of control at slow speed[Pasamanick]. It also suffers from vibration problems
because the propeller blades have different loading in the proximity of the channel
versus the open air.
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2. Lip Wing
2.1. Description
A solution for the “VTOL conundrum” is to use a wing capable of providing lift even at
zero forward speed, the lip wing. The lip wing is located at the intake of a shrouded rotor
in order to expose the upper surface of the wing to the rotor induced airstream.
Fig.1 Lip Wing geometry
The wing has a section of its trailing edge that substantially coincides with a fraction of
the duct’s intake. The wing trailing edge has a slanted section, to allow adjacent
placement of the wing and the shroud at a certain angle. The slant angle is
predetermined to maximize the magnitude of a resultant thrust vector. The resultant
thrust vector is determined by vectorial addition of the lift and the thrust vectors.
The wing has such a shape that when placed adjacently to the shroud, creates a
bell-shaped inlet surface, increasing lift. The “lift “, in this context, means the
aerodynamic force created by air passing over surface of an airfoil sectioned object,
roughly perpendicular to the airfoil chord.
The lip wing and the shroud are connected using pivoting articulations. An actuator is
used for adjusting the lip wing and the shroud relative position. Adjusting the relative
position of the lip wing and the shroud, the resultant force is varied, providing pitch
control and augmenting propulsion.
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2.2. Operation
At zero or slow speed, the wing and the shroud are placed adjacently, the intake fluid
stream is accelerated by the rotor, over the upper surface of the wing so it generates lift.
Fig.2 Lip Wing VTOL mode
Adjusting and pivoting the articulation, the lip wing rotates, and opens a slot between
the wing’s trailing edge and the shroud’s leading edge. Through the slot, air is let in, so
the airspeed over the wing is reduced, reducing lip wing lift. The more energetic slot air
passes over the shroud, increasing shroud’s lift, function similar to a conventional
slotted flap. The reduction of the lip wing lift and the increase of the duct lift causes a
pitch-down moment. This adjustable moment augments control in transition, and
opposes transitional pitch-up moment characteristic of ducted fan VTOL aircraft
[US2953321A].
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Fig.3 Lip wing VTOL pitch control
As airspeed increases, the wing-shroud angle is varied to exploit airstream
convergence, so the lip wing has an optimum angle of attack. The angle of attack is
determined for optimizing lift parameters, as increased lift or the best lift per drag ratio.
Varying the wing-shroud angle also modifies the orientation of the lift vector, so it could
determine changes in the pitch moment, used for adjusting craft attitude.
Fig.4 Lip wing follows air convergence
At high speed, the air convergence becomes minimal, so the wing and the shroud are
positioned at an angle of attack to ensure generation of sufficient lift and minimize drag.
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Fig.5 High speed configuration
2.3. Shroud design
The shroud is shielding the rotor, straightening the airflow; the asymmetrical propeller
loading, characteristic to the channel wing, is eliminated.
The change in advance ratio with change in forward velocity is smaller for a shrouded
propeller and may be small enough for the operating range of the aircraft such that a
fixed-pitch propeller could be used instead of a variable-pitch propeller[Roberts].
The shroud has an asymmetrical design, the upper section is designed thicker as it is
more prone to stall during transition, in order to prevent flow separation. The lower
section has a shape to match the lip wing aerodynamically, to form a smooth,
continuous upper surface, and also it acts as a slotted flap to provide efficient pitch
control and to prevent flow separation.
3. Lip Wing Validation
3.1. Experimental data and research
Preliminary experiments were conducted to validate the concept. A prototype for
evaluating static thrust was constructed. Measurements showed a thrust increase of
about 19% for a lip wing and a duct, compared to the ducted rotor alone, for the same
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power input. These results were deemed to be good enough to pursue intellectual
property protection, and as a result, the US 9,845,152B2 patent issued. More details are
presented in chapter 7 of this appendix.
The concept was also investigated at Ryerson University, Toronto, and LARCASE
Laboratory, Ecole de Technologie Supérieure, Montréal. The research is cited in
[Gabor], [Walker] and [Yokota].
3.2. Channel wing NACA research
The channel wing was invented to increase the lift for operation at slow speed. By using
a propeller to provide airflow on the upper airfoil surface, the wing creates more lift, it is
harder to stall, and even provides some lift at zero forward speed.
NACA performed static thrust experiments on a channel wing prototype in
1953[Pasamanick]. Some data from the channel wing experimental study:
Propeller diameter: 1.82m, Rotor disk area: .60 m2 Arot = 2
Wing chord: 0.90m, Wing area: , Wing lift coefficient:
2.57 m2 S= 2.2 CL=
Fig.6 Channel wing, static condition, from [Pasamanick]
The NACA report shows that the channel wing, static condition, generates 340 lbf of
lift(1513N), while the propeller is producing thrust of 800 lbs(3559N). The lift and thrust
vectors are perpendicular, as the propeller disk is perpendicular to the wing chord. As a
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consequence, the resultant thrust vector is only 69 lbs more than the propeller thrust,
less than 8% increase. These results, and the fact that the aircraft does not have any
control at slow speed, deemed the channel wing to be ill suited for VTOL operation.
3.3. Lip wing estimations based on channel wing
Based on NACA’s channel wing report, calculations are performed to determine the
propeller induced airspeed and the wing top surface airspeed. These results are further
used to validate and determine lip wing thrust augmentation, or increase of the resultant
thrust. The channel wing can be considered as a subset of the lip wing concept, having
a zero degrees slant angle.
The propeller disk loading is calculated: /A; Dl=Trot 367 N/m2 Dl= 1
Induced airspeed: ;
vi=√D/(2ρ)
l3.62 m/s vi= 2
Airspeed on wing top surface: ;
va=√2 L
C ρ S
L
0.88 m/s va= 2
To increase the addition resultant of two perpendicular vectors, the vectors can be
pivoted to a more acute angle. In order to increase the resultant thrust, the wing needs
to be positioned so that the wing lift and rotor thrust vectors are at a more acute angle.
This also has the effect of decreasing the airspeed on the upper wing surface, and
consequently the lift generated.
Fig.7 Lip wing air intake area
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The following calculations are made to determine variation of lift augmentation versus
the lip wing angle, and also to determine an optimum designed lip wing angle, alpha,
geometrically equal to the trailing edge slant angle. The slant angle is optimized for
maximum thrust augmentation.
The air can be considered incompressible at speeds under 150m/s[Kermode], the air
flow rate can be considered constant, so the airspeed on the upper wing surface is
calculated using the air intake area which varies with the slant angle (same as
) (Aia
alpha).
As the air flow rate, v onstant Aia a=c
The wing lift is calculated using the relation: ρv SC L=2
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aL
Doing the calculations for a range value of alpha, and considering augmentation defined
as , the following graph is plotted :
TAug =R
T− 1
Fig.8 Lip wing thrust augmentation vs. slant angle
3.4. Conclusion
The maximum value of 12.5% is found at about 40 degrees slant angle. These results,
as based on the channel wing, do not take into consideration a ducted rotor. Adding a
duct, the induced velocity is increased[Pereira], and so the lip wing lift could be higher.
These numerical values are comparable to theoretical and experimental results
previously obtained[Gabor][Walker].
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4. Concluding Remarks
The lip wing system is a good solution to a VTOL aircraft. In hover, it creates lift and
eliminates the “dead weight” wing problem. It augments total generated lift, so the rotor
diameter and power plant size can be reduced, increasing the aircraft effectiveness. It
creates pitch control as the magnitude and orientation of the lift is adjustable, improving
control at slow speed, a definite need for VTOL aircraft. In transition, the wing can
generate maximum lift as it is positioned to exploit changes of air convergence,
reducing the thrust needed to be generated by the rotor. At high speed, the lip wing
generates conventional lift, enabling the aircraft to have similar performance to a fixed
wing aircraft.
5. Works Cited
Gabor, Oliviu Sugar, et al. Preliminary Analysis of the Lip Wing System for the ALIPTERA VTOL
Concept Aircraft. Conference: Sustainability 2015 - An International Conference on
Environmental Sustainability in Air Vehicle Design and Operations of Helicopters and Airplanes.
Kermode, A.C, et al. Mechanics of Flight. Pearson Education Limited, 2012.
Pasamanick, Jerome. Langley Full-Scale-Tunnel Tests of the Custer Channel Wing Airplane.
National Advisory Committee for Aeronautics, 1953.
Pereira, Jason. Hover and Wind-Tunnel Testing of Shrouded Rotors for Improved Micro Air
Vehicle Design. Dissertation submitted to the Faculty of the Graduate School of the. University
of Maryland, 2008.
Raymer, Daniel P. Aircraft Design: a Conceptual Approach. American Institute of Aeronautics
and Astronautics, 2006.
Roberts S.C. Trecom Technical Report 64-41. The. Marvel projects. MSU, USA, 1964.
US2953321A. Vertical take-off flying platform. Hiller Helicopters Patent, 1960.
Walker, Peter. Preliminary Analysis Of Lip-Wing Concept Using Computational Fluid Dynamics
And Actuator Disc Theory. Ryerson University Thesis, 2016.
Yokota, Jeffrey. Walker, Peter. A Preliminary Study Of Potential Advantages Of Non-Uniform
Extended Fan Ducts Using Computational Fluid Dynamics. International Journal of Mechanical
And Production Engineering, ISSN: 2320-2092, Volume- 4, Issue-5, May.-2016.
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