GB2093152A

GB2093152A - Boundary Layer Control

Info

Publication number: GB2093152A
Application number: GB8104340A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-02-12
Filing date: 1981-02-12
Publication date: 1982-08-25

Abstract

To allow an aerofoil such as a wing to operate at a large angle of attack, especially at low Reynolds Number when the boundary layer is laminar, some of the free stream air is ducted into the inside of the wing by an intake slot 2 at the wing leading edge and ejected from exit slots or holes 3 along the wing upper aft surface so that the boundary layer on the wing upper outer surface always flows aft and does not separate. The invention does not require any additional power sources or mechanical devices in order to operate and it need not reduce the fuel carrying capacity of the wing. It can operate in wet, icy or dusty conditions. Engine bleed air may be used to prevent intake and exit hole icing. The exit holes may be closable by a sliding plate. <IMAGE>

Description

SPECIFICATION Wing Boundary Layer Control for Aircraft Flying at Low Reynolds Numbers or High Angles of Attack Aircraft flying in low density atmospheres, such as high altitudes on Earth or low altitudes on Mars, or model aircraft flying at low altitudes on Earth, operate at low Reynolds Number.

The characteristics of such flight is that the boundary layer is laminar. For flight at high Reynolds Number the boundary layer is turbulent.

A turbulent boundary layer can remain attached to a wing even for a thick cambered aerofoil at a large angle of attack. A laminar boundary layer can normally only remain attached to a thin cambered aerofoils at low angles of attack. At large angles of attack the laminar boundary layer separates from the wing surface causing a large drag and a small lift.

The invention to be described allows an aerofoil to operate at a large angle of attack at low Reynolds Number without laminar boundary layer separation from the upper surface of the wing, and without the use of any additional power supplies such as blowers or engines, or the use of mechanical devices such as flaps.

The same invention also allows aircraft with turbulent boundary layers also to operate at much higher angles of attack before the turbulent boundary layer separates from the upper surface of the wing. Thus the angle of stall can be increased.

Figure 1 gives a cross section through the aerofoil.

Figure 2 gives a cross section through a wing containing a fuel tank.

Figure 3 gives a cross section through a wing showing the internal airflow control.

With conventional aircraft, all the air flows over the external surface of the wing, and any internal air flow only occurs through the engines. With this invention, part of the free stream air flows over the external surfaces of the wing and part of the free stream air flows over the internal surfaces of the wing.

Along the leading edge of the wing (1) is an intake slot (2) which allows some air to flow through the inside of the wing (4). Because the intake is at the leading edge of the wing then the pressure inside the wing is slightly below the total pressure of the air at the nose i.e. it is higher than the static pressure by almost the full dynamic pressure The pressure on the upper surface of the wing is slightly below the static pressure in order for the wing to develop lift, and is therefore at a lower pressure than the inside of the wing.

The laminar boundary layer has a lower average velocity than a turbulent boundary layer, and an adverse pressure gradient on the aft upper surface of the wing can bring some of the air in the boundary layer to rest, or even flow forward, which leads to separation of the wing boundary layer from the upper surface of the wing. The laminar boundary layer separates at a lower adverse pressure gradient than a turbulent boundary layer.

The boundary layer flow is a relatively small proportion of the air flowing over the wing and hence separation can be delayed to higher angles of attack by allowing the high pressure air inside the wing to be discharged aft through holes (3) in the wing upper skin, parallel to the wing surface thus increasing the aft velocity of the boundary layer air. As the adverse pressure gradient is retarding the air continuously along the wing upper aft regions of the wing then the air should be discharged continuously through a number of holes or slots in the wing upper skin.

An actual aircraft requires fuel to propel it over large distances and usually the fuel is stored in the wing tank (8). When the wing is also used for internal air flow then the fuel tank must not be allowed to occupy the full depth of the wing as shown in Figure 2. There is an air passage (6) between the top of the fuel tank and the wing upper skin which connects the air (5) foward of the front spar (11) to air (7) aft of the rear spar (1 2). When the internal velocities are relatively high, the internal flow should be streamlined in order to reduce the internal drag.

Actual aircraft usually are designed for the pilot to have complete control of any device and hence, if required, the internal air flow through the wing can be shut off as shown in Figure 3. The intake slot is shut off by lowering a plate (9) over the slot and the exit slots can be shut off by sliding a plate (10) across the slots. These plates contain holes which can be moved into coincidence with the wing skin holes when the holes are to be open.

If the air flow is still laminar at the wing trailing edge then control of the aircraft in roll can be achieved by using all-moving tips or differential operation of the tailplane; control of the aircraft in pitch can be achieved by symmetric operation of an all-moving tailplane; and lateral control can be achieved by using an all-moving fin. The wing tip, tailplane and fin would all be designed similar to the wing, except that lift could be required from either surface of the aerofoil and hence the exit slots would have to be cut in both surfaces. This would also apply to a wing which carried out upside down manoeuvres.

However, only high upward lift force is required during the cruise and hence the wing may only require exit slots on the upper surface.

The positive static pressures on the wing lower surface do not lead to separation of the boundary layer there, and the lower wing surface is not a problem and can be made to operate normally.

Hence the lift generated by the previously described wing develops between half and one times the lift from a wing operating perfectly at the same angle of attack, but it can keep on increasing its angle of attack and lift beyond the angle at which the conventional aerofoil has stalled.

When the internsTair flow passes through the front and rear spars, the spars must be of the non continuous type above the fuel tank region i.e.

they must be of the open lattice girder type with the verticals and diagonals having a streamlined shape, or they can be of the continuous type with holes cut through them in order to pass air ducts. The addition of air ducts (1 3) from the leading edge intake and through the front spar and possibly the rear spar would give the lowest internal drag, but the highest weight.

In order to prevent large dust particles from entering the internal duct then filters should be provided at the intake and relatively larger exit holes (14) provided at the trailing edge to allow for the escape of particles which manage to pass through the intake filter.

For conventional aircraft on Earth operating at high Reynolds Number then the boundary layer will be turbulent and conventional controls including ailerons or flaps along the wing trailing edge can be used. However an extra long slot would be required to speed up the boundary layer over a highly deflected flap if separation of the boundary layer there is to be prevented.

The new stalling angle of the aerofoil depends on the amount of air required to prevent separation of the boundary layer on the wing upper outer surface and whether this quantity of air can be supplied by the slot at the leading edge.

This investigation is applicable to aerofoil shapes which have favourable pressure gradients over the forward part of the aerofoil for the operating range of angles of attack of the aerofoil.

The area over which exit slots or holes are required is therefore reduced and more air can be supplied to the holes where it is required.

During flight in icing conditions, a hot air bleed from the engine can be directed at the intake and exit holes in order to de-ice the system. During flight in wet conditions any excess water which accumulates in the lowest region of the wing can be extracted by a water pump. Small amounts of water vapour will travel straight through the internal flow path in a similar manner to the air.

The holes should be inspected regularly to find out whether any holes have become blocked.

Claims

Claims It is claimed that the invention:

1. Allows an aerofoil such as a wing to operate at a large angle of attack for flight especially at a low Reynolds Number when the boundary layer is laminar by ducting some of the free stream air into the inside of the wing by an intake slot at the wing leading edge and exit slots or holes along the wing upper aft surface so that the boundary layer on the upper outer surface always flows aft and does not separate regardless of whether it is laminar or turbulent.

2. Does not require an additional power source or mechanical or electrical device in order to operate.

3. Does not appreciably reduce the fuel carrying capacity of the wing.

4. Can operate in wet, icy or dusty conditions.