WO1999052770A1

WO1999052770A1 - Ring-shrouded propeller

Info

Publication number: WO1999052770A1
Application number: PCT/HU1999/000025
Authority: WO
Inventors: Ákos ANDRASEV
Original assignee: Andrasev Akos
Priority date: 1998-04-14
Filing date: 1999-04-14
Publication date: 1999-10-21
Also published as: AU3437299A; HU9800868D0; HUP9800868A1

Abstract

The invention relates to a propeller where owing to the construction the airflow spilling round the blade tip is prevented. The propeller comprises a hub (12) and blades (14, 15, 16, 17) extending from the hub (12). The outer blade tips are enclosured by a ring unit (25) fixed to the blades (14, 15, 16, 17) having an axis aligned with the axis of the propeller and co-rotating with the blades. The subject of the invention is also a device to reduce the spilling of air round the blade tip of conventional propellers. The device comprises a ring unit (32) which can be assembled to existing propellers surrounding the propeller and securing it to the blade tips.

Description

RING-SHROUDED PROPELLER

The invention relates to propeller wherein owing to the construction, the airflow spilling round the blade tip is prevented and also relates to a device for conventional propellers to reduce the airflow spilling round the blade tips. Both the propeller and the device would be utilized mainly in the aircraft industry. The propeller comprises a hub and blades extending from the hub.

It is known that conventional propellers have a hub and various numbers of propeller blades with free blade tips where the individual propeller blades are connected to each other only through the propeller hub. During normal usage, the propeller rotates at a certain angular velocity and the tangential velocity of each blade station is directly proportional to the distance measured from the rotation axis. At the blade tips, it can approach or even exceed the speed of sound. As an effect of this, the aerodynamic characteristics of the air change along the span of the blades. In the vicinity of the hub or blade roots the air is assumed to be incompressible while at the sections where the speed is approaching to the speed of sound, compressible characteristics should be dealt with.

During the rotation of a propeller, the pressure of the air increases on the rear side of the propeller blade, while it decreases on the front side in view of the course of movement. This pressure difference tends to equalize around the blade tip. This effect disturbs the airflow over the whole surface of the blade. The air particles that are flowing parallel before reaching the blade, when moving towards the trailing edge on different -forward and rear -side of the blade, they are moving off each other. The air particles flowing over the front surface of the blade tend to flow inwards (towards the hub), on the rear surface, on the other hand, the air particles flow outwards (towards the tip). The two airflows, having different direction, meeting at the trailing edge of the blade and cause a series of vortices. This is because the decreased pressure over the front surface is less then the pressure outside the blade tip, while the pressure on the rear surface of the blade is greater than that outside the blade tip. This flow distortion is the greatest in the vicinity of the blade tip because most of the pressure differences in front and behind the blade tend to equalize in the vortex forming at the blade tip. All the vortices at one blade tend to join up and form one large vortex called induced vortex which is shed from each blade tip. The presence of the vortices, especially due to the low pressure existing in them, produces a special drag, called induced drag. The amount of this drag depends on, amongst others, the square of the thrust produced by the propeller.

It is also known, that when the velocity of the airflow on the surface of a given blade station reaches the speed of sound, due to the vortices caused by the compressible characteristics of the air, radically increase the drag and decrease the thrust produced by the given station. The ratio of the speed at which this extra turbulence and drag develops is the local speed of sound, and called the critical Mach number, that is characteristic of a given propeller and restricts the efficiency of the given propeller.

Using variable pitch propellers, the optimal angle of attack can be set and continuously fine-tuned, but the spilling of the air round the blade tips cannot be eliminated. By setting the optimal pitch, only the excessively high pressure differences can be dampened, which decreases the airflow round the blade tips, but the vortices can not be eliminated. This effect causes the most adverse effect when using fixed pitch propellers at low forward speed, or whilst accelerating. In both cases the angle of attack of the propeller blade and/or the produced trust is very high. The angle of attack is the angle between the direction of the free-stream and the chord of a given airfoil of the propeller blade. Besides the additional energy required by the propeller to change the direction of the air particles that in the free-stream, and to overcome profile drag and friction, an extra power is required to overcome the drag caused by the vortices produced at the blade tips and behind the trailing edge. The induced drag is a major part of the total drag and it sharply decreases the efficiency of the propeller. It is also a disadvantage that at high blade tip speeds, adding the three velocity vectors of an air particle, namely the velocity vector of the free-stream, the tangential velocity vector resultant of the rotation of the propeller and the velocity vector of the air spilling round the blade tip, the resultant vector can reach or exceed the local speed of sound at lower forward speeds or lower rotation speeds. Due to the supersonic speed and the compressibility of the air the aerodynamical characteristics of the given station of the blade virtually change. When the speed of the air particle decreases again below the speed of sound a shock wave is generated which causes the airflow to separate and causes the profile drag to increase, which in turn, mean an increase in torque and decrease in thrust, and finally, a loss of efficiency. These effects together limit the maximum possible speed of an airplane.

Earlier, some efforts have been made to decrease the above mentioned induced drag and to increase the critical Mach number. These efforts were the geometric and aerodynamic twisting of the propeller blades or the use of winglets on the blade tips, but the air spilling round the blade tips could not be considerably reduced or eliminated.

Another type of propeller is the ducted fan, where the propeller rotates in a duct with only a very narrow gap between the tip of the blades and the inner surface of the duct. This narrow gap prevents the air from freely flowing from the rear side of the blade to the front side. The disadvantage of this construction is that the propeller and the duct should fit very accurately which requires expensive machinery and can not prevent wholly the spilling of the airflow at the blade tips. To decrease the above mentioned spilling of the airflow round the blade tips and the caused vortices, that reduces the efficiency of the propeller, thereby the need for increasing the efficiency of the propeller at the lowest possible price made it our duty to develop a method which prevents the airflow from equalizing round the blade tips and by this means the efficiency of the propeller increases.

This invention is based on the conception that the appropriate solution to this problem is not the various flowdeflecting formations of the individual blade tips or the usage of more complicated mechanical systems involving the propeller itself because their introduction in some cases generate new sources of errors, even new vortices and require expensive and more complicated new technological methods, but that the solution should be reached with a suitably constructed redesign of the whole propeller itself in a more cost effective way.

The invention is based on the recognition that the cause of the airflow spilling round the blade tip is the finite span of the propeller blade. The spilling would not be possible if the span were infinite. Thus the solution is to transform the propeller so that the pattern of the airflow round the blades can be as similar as possible to that surrounding an infinite span propeller blade. So, if the airflow at the blade tip is blocked at the most favorable place, the spilling is limited and its adverse effects are minimized.

The solution to the problem by this invention is a propeller which comprises a hub and blades extending from the hub. The feature of the propeller according to the invention is that the outer blade tips are surrounded by a ring unit which is secured to the blade tips and having a center line aligned with the rotational axis of the propeller and co-rotating with the blades. In a preferred embodiment of the propeller according to the invention the blades are secured in seatings both in the hub and in the ring unit allowing the blades to be turn.

In an other preferred embodiment of the propeller according to the invention the ring unit is aerodynamically shaped.

In a further preferred embodiment of the propeller according to the invention the length of the cross section of the ring unit is longer than the chord length of the airfoil used at the blade tip.

A further subject of the invention is a device for conventional propellers to prevent the spilling of the airflow round the blade tip. The feature of the device according to the invention is that comprises an annular ring which has a form able to attache to blade tips of conventional propellers by surrounding the propeller.

In a further preferred embodiment of the device according to the invention the annular ring is aerodynamically streamlined in the direction of his axis.

In a further preferred embodiment of the device, there are seatings to receive blade tips in the inner surface of the annular ring.

By a preferred embodiment of the device, the inner surface of the annular ring incorporates securing means allowing to be turn of the blade tips in the seatings.

The construction of the hub and/or the blades or the fix or the turnable securing of the blades to the hub and to the ring unit of propeller according to the invention can be any kind. The hub can suitably be either rounded or cone-shaped, the propeller trtades can suitably be geometrically and/or aerodynamically twisted. The propeller according to the invention practically can comprise any number of blades that can hold the ring unit, the shape of the blades can be either narrowing or widening outwards and any combination or variation of the above.

Also, the device according to the invention is applicable for any kind of conventional propellers regardless of the number and/or shape of blades if it can hold the ring.

The advantage of solution according to the invention by the propeller the presence of the ring unit and by the device for conventional propellers the presence of annular ring having both of them having a cross section with longer chord than the chord of the airfoil at the blade tip, the airflow round the blade tip is blocked so the spilling practically cannot develop. Thus the induced drag decreases and the critical Mach number increases so that the speed and the efficiency of the propeller can be increased. Because of the presence of the ring unit, the stresses in the blade roots will be lower. With this lower stress, the number of blades on one propeller hub can be increased. The noise level of the propeller caused by the vortices also decreases. Besides this, it must be emphasized that the operation of the propeller according to the invention or a conventional propeller with the device fitted, is much safer than the use of conventional propellers.

The substance of the invention will become apparent from the description of embodiments in detail referring to the accompanying schematic drawings in which:

Figure 1. shows the front view of a propeller which is constructed according to the invention

Figure 2. shows the cross section of the side view of the propeller shown on figure 1. Figure 3. shows a part of possible propeller construction of the B section marked on Figure 1. in perspective view

Figure 4. shows a side view of an other possible construction of the B section marked on propeller shown on Figure 2.

Figure 5. shows a front view of a device according to the invention, and

Figure 6. shows in section the side view of the device shown on figure 5.

The propeller (10) shown on figure 1. comprises a central hub (12) and four blades (14,15,16,17) extending from the hub (12). Each blade (4,15,16,17) has a leading edge (18), a trailing edge (19) and a blade root (21) next to the hub. These parts are marked only at blade (14). Further, the propeller (10) has an annular ring unit (25) which surrounds the propeller at the tips of the blades (14,15,16,17) holding the blade tips together. On Figure 1. the direction of rotation of the propeller (10) is marked with an arrow.

The construction of the hub (12) and the blades (14, 15, 16, 17) is known from prior art. The ring unit (25) can be connected to the blade tips either fixed or allowing rotation in seating.

Figure 2. illustrates the cross section of the propeller through the A- A plane marked on Figure 1. The direction of movement of the propeller (10) is marked with an arrow. The ring unit (25) of the propeller (10) is airfoil shaped and the chord length of the airfoil is longer than the longest length of the chord of the blade tip projected in the direction of movement of the propeller. Figure 3. illustrates an enlarged view of part B of the propeller marked on Figure 1. It illustrates that the chord length of the ring unit (25) is longer than that of the blade tip (17) section and also shows the geometric and aerodynamic twist of the blade along its span.

Figure 4. illustrates the construction of part C of the propeller marked on Figure 2. In this case the pitch of the blades (14) is variable. As it can be seen, there is a bearing housing (27) with a bearing (28) embedded in the inner surface of the aerodynamically shaped ring unit (25). The tip of the blade (14) of the propeller is seated in the bearing (28) allowing the blade (14) to rotate in the ring unit (25) as in the hub (12), that is not shown here, without changing the position of the ring unit (25) relative to the hub (12).

Figure 5. illustrates the front view of a device (30) according to the invention which can be assembled on conventional propellers having free blade tips for preventing the spilling of the airflow round the tips. The main part of the device (30) is an annular ring (32) , in which there are seats to receive blade tips of an existing propeller. By the illustrated embodiment there are four seats (34) whose position is marked with broken line and only one of them is numbered.

Figure 6. shows the cross section of the device illustrated in Figure 5. which has streamlined shaping. To facilitate the assembly of the annular ring (32) on an existing conventional propeller, the annular ring (32) may comprise two parts, a front annular ring element (37) and a rear annular ring element (38). This two ring elements are fixed together after they are placed on the blades of the propeller. The two ring elements attach to each other forming seats (34). In the seats (34) there are securing means, clamps which can be fixed to the propeller blade tips, if necessary through bearings in the ring which allow the propeller blades to be turn. The propellers shown on the drawings as examples are simplified versions. The construction of the propeller according to the invention is not limited by no means to these versions and also, the device according to the invention is not limited at all for use with the shown propellers on the drawings. Any variation and/or combination in the number, the narrowing or widening shape, the geometrical and/or aerodynamical twisting of the blades can be suitable besides that the propeller is constructed according to the invention or the conventional propeller is equipped with the air spilling preventing device according to the invention.

Claims

10CLAIMS:

1./ Propeller comprising a hub and extending blades from the hub characterized in that the outer blade tips of the propeller are surrounded by a ring unit (25) secured to the blades (14,15,16,17) having a center line aligned with the rotational axis of the propeller and co-rotating with the blades.

2.1 Propeller according to Claim 1. characterized in that the blades (14,15,16,17) are secured in seatings both in the hub (12) and in the ring unit (25) allowing the blades to be turn.

3./ Propeller according to Claim l.or2. characterized in that the ring unit (25) is aerodynamically streamlined.

4.1 Propeller according to any of Claim 1.-3. characterized in that the chord length of the airfoil of the ring unit (25) is longer than the chord length of the airfoil at the end of the blade or the longest length of the chord of the blade tip projected in the direction of movement of the propeller.

5./ Device for conventional propellers to reduce airflow spilling round the blade tip characterized in that comprises an annular ring (32) which has a form able to attache to blade tips of conventional propellers.

67 Device according to Claim 5. characterized in that the annular ring (32) is aerodynamically streamlined in the direction of his axis. 11

7.1 Device according to Claim 5. or 6. characterized in that there are seatings (34) to secure blade tips in the inner surface of the annular ring (32) .

8./ Device according to any of Claim 5. -7. characterized in that there are securing means in the inner surface of the annular ring (32) for fix blade tips with seatings (34) allowing to be turn.