Aeroengine
A method of operating a contra-rotating propeller engine that preferably comprises a 12 bladed front and a 9 bladed rear propeller. As is conventional, the engine is operated during at least a take-off phase, a cruise phase and an approach phase; during the cruise phase the engine operates with a generally constant propeller tip speed. The method is characterized by the step of operating the engine such that the tip speed of either or both of the propellers, during at least one of take-off, climb or approach, at least 10% greater than cruise tip speed. With a specific front to rear propeller spacing, increasing the tip speed reduces overall noise generated by the propellers.
1. A method of operating a contra-rotating propeller engine having front and rear propellers, wherein the engine is operated during at least a take-off, a climb, a cruise phase and an approach phase, during the cruise phase the engine operates with a generally constant propeller tip speed, the method comprising a step of operating the tip speed of at least one of the propellers, during at least one of take-off, climb or approach at a greater tip speed than cruise tip speed.
2. A method according to claim 1 wherein the tip speed of both the propeller(s) is greater than cruise tip speed.
3. A method according to claim 1 , wherein the tip speed of the propeller(s), during at least one of take-off, climb or approach, is at least 10% greater than cruise tip speed.
4. A method according to claim 1 , wherein the tip speed of the propellers, during at least one of take-off or approach, is 20%+/−5% greater than cruise tip speed.
5. A method according to claim 1 wherein the propellers comprise variable pitch blades, the method further comprising a step of closing the blades from their cruise pitch to their approach pitch.
6. A method according to claim 1 wherein the propellers comprise variable pitch blades, the method further comprising a step of opening the blades from their take-off pitch to their cruise pitch.
7. A contra-rotating propeller engine having front and rear propellers, the front propeller having at least 9 blades (Nf=9) and a diameter Df, the rear propeller having a maximum number of blades, Nr=Nf minus at least 3, the front and rear propellers are separated by an axial gap x, wherein the ratio x/Df is between 0.15 and 0.4.
8. A contra-rotating propeller engine as claimed in claim 7 , wherein the front propeller has 12 blades and the rear propeller has 9 blades.
9. A contra-rotating propeller engine as claimed in claim 8 , wherein the front propeller has 12 blades and the rear propeller has 7 blades.
10. A contra-rotating propeller engine as claimed in claim 8 , wherein the front propeller has 12 blades and the rear propeller has 5 blades.
11. A contra-rotating propeller engine as claimed in claim 7 , wherein the front propeller has a greater diameter that the rear propeller.
12. A contra-rotating propeller engine as claimed in claim 11 , wherein the diameter of the rear propeller is between 0.05 Df and 0.2 Df less than the front propeller.