Paradox of constant speed propeller
Join Date: Dec 2001
Location: what U.S. calls ΄old Europe΄
Posts: 941
Likes: 0
Received 0 Likes
on
0 Posts
An exception, in my experience, is the Bristol Hercules sleeve valve radial engines where we did the opposite and increased boost then RPM.
I think it is not the thermal stress to the exhaust valves (mechanical issue), it is the hot exhaust valves causing detonation (combustion issue) which limits MP at low RPM. No hot exhaust valve - no problem.
No quote or link, I'm afraid, but I remember seeing about 61" MP at takeoff power.
They were supercharged directly so the post take-off power reduction dropped the MP as the RPM was reduced. Throttles were left where they were. Reducing to cruise power (the way they were operated here) involved a large RPM drop - back to about 1800-ish IIRC - and then the MP was fine-tuned using the throttles.
They were supercharged directly so the post take-off power reduction dropped the MP as the RPM was reduced. Throttles were left where they were. Reducing to cruise power (the way they were operated here) involved a large RPM drop - back to about 1800-ish IIRC - and then the MP was fine-tuned using the throttles.
Join Date: Sep 2006
Location: West Coast Canada
Age: 71
Posts: 69
Likes: 0
Received 0 Likes
on
0 Posts
Re - Constant Speed and Over-Speed Control of Sundstrand Ram Air Turbines
Thanks to DaveReidUK for the link to manufacturer.
Here is the link to the Sundstand patent for about 1988
Patent US4743163 - Ram air turbine control system - Google Patents
- this has the speed governor that is referenced in the second patent info regarding the Over-Speed protection.
RAM air turbine over-speed protector using redundant yoke plate linear bearings - Hamilton Sundstrand Corporation
Since I still do not have info regarding the 787 RAT I can not say for sure this will apply - but I would expect it to be the same or similar as a RAT is a RAT is a RAT.
This excerpt from the over-speed patent is probably the biggest reason for at least overspeed control.
I will cross post this in the thread opened on info for the 787 RAT as well.
Thanks to DaveReidUK for the link to manufacturer.
Here is the link to the Sundstand patent for about 1988
Patent US4743163 - Ram air turbine control system - Google Patents
- this has the speed governor that is referenced in the second patent info regarding the Over-Speed protection.
RAM air turbine over-speed protector using redundant yoke plate linear bearings - Hamilton Sundstrand Corporation
Since I still do not have info regarding the 787 RAT I can not say for sure this will apply - but I would expect it to be the same or similar as a RAT is a RAT is a RAT.
This excerpt from the over-speed patent is probably the biggest reason for at least overspeed control.
BACKGROUND OF THE INVENTION
A ram air turbine (RAT) is a device for generating emergency supplemental power in a wide variety of aircraft. A RAT may generate hydraulic power, electric power or both. A RAT incorporates a turbine that extracts power from an air stream proximate the aircraft in flight. A typical RAT in current use is described in U.S. Pat. No. 4,743,163 to Markunas et al., owned by the assignee of this application, and incorporated herein by reference. The turbine is coupled to suitable power generating equipment, such as a hydraulic pump for hydraulic power and an electric generator for electric power.
As described in Markunas et al., the turbine includes a speed governor that changes the turbine blade position to maintain a relatively constant shaft speed to the power generating equipment. Failure of the turbine speed governor can cause an over-speed condition that may ultimately cause the release of a turbine blade at very high speed. Due to the high speed, the wayward blade has very high energy as well. The most common cause of governor failure is due to seizure of the bearing between the governor shaft and the governor yoke plate that controls the pitch of the turbine blades.
The potential release of a high energy blade proximate the aircraft is a concern for both commercial and military RAT applications. Should the wayward blade strike the aircraft fuselage, it may penetrate the skin and cause damage to electric or hydraulic power equipment or control systems. It may also injure passengers or crew. If one of the propulsion engines ingests the wayward blade, the engine may suffer severe damage that results in loss of thrust.
Current methods to minimise hazards caused by turbine over-speed-induced release of a turbine blade have involved strategic placement of key elements or shields to prevent penetration. These methods no longer satisfy increasingly stringent certification and safety requirements promoted by airworthiness authorities.
A ram air turbine (RAT) is a device for generating emergency supplemental power in a wide variety of aircraft. A RAT may generate hydraulic power, electric power or both. A RAT incorporates a turbine that extracts power from an air stream proximate the aircraft in flight. A typical RAT in current use is described in U.S. Pat. No. 4,743,163 to Markunas et al., owned by the assignee of this application, and incorporated herein by reference. The turbine is coupled to suitable power generating equipment, such as a hydraulic pump for hydraulic power and an electric generator for electric power.
As described in Markunas et al., the turbine includes a speed governor that changes the turbine blade position to maintain a relatively constant shaft speed to the power generating equipment. Failure of the turbine speed governor can cause an over-speed condition that may ultimately cause the release of a turbine blade at very high speed. Due to the high speed, the wayward blade has very high energy as well. The most common cause of governor failure is due to seizure of the bearing between the governor shaft and the governor yoke plate that controls the pitch of the turbine blades.
The potential release of a high energy blade proximate the aircraft is a concern for both commercial and military RAT applications. Should the wayward blade strike the aircraft fuselage, it may penetrate the skin and cause damage to electric or hydraulic power equipment or control systems. It may also injure passengers or crew. If one of the propulsion engines ingests the wayward blade, the engine may suffer severe damage that results in loss of thrust.
Current methods to minimise hazards caused by turbine over-speed-induced release of a turbine blade have involved strategic placement of key elements or shields to prevent penetration. These methods no longer satisfy increasingly stringent certification and safety requirements promoted by airworthiness authorities.
Pedants Corner, (and off topic):
A Squared; I think you will find that a Turbine only converts airflow into 'work', but Compressors, or Fans only convert 'work' into airflow.
As far as I know there is no such thing as a "compressor turbine"
A Squared; I think you will find that a Turbine only converts airflow into 'work', but Compressors, or Fans only convert 'work' into airflow.
As far as I know there is no such thing as a "compressor turbine"
As far as I know there is no such thing as a "compressor turbine"
"When I use a word," Humpty Dumpty said, in rather a scornful tone, "it means just what I choose it to mean - neither more nor less."
Those constant-speed propellers on sticks that are popping up wherever the wind blows do seem to get called wind turbines. On the other hand, an airplane propeller doesn't become a turbine just because it's windmilling.
Fair points, but a windmilling propeller isn't doing anything other than going round, although I suppose you could argue it might supply some hydraulic pressure via the hyd pump - but that's not what it's designed to do, and the performance is not guaranteed.
DR UK: Can I buy some shares in your perpetual motion machine? I like the sound of it ! Perhaps if you get it going then we can forget all this ridiculous cost cutting to save fuel......
DR UK: Can I buy some shares in your perpetual motion machine? I like the sound of it ! Perhaps if you get it going then we can forget all this ridiculous cost cutting to save fuel......
Join Date: Feb 2005
Location: flyover country USA
Age: 82
Posts: 4,579
Likes: 0
Received 0 Likes
on
0 Posts
Chu Chu:
No, the words in the parts catalog don't change, but functionally it DOES become a turbine(*). Much the same as a starter motor can become a generator once the engine is started.
* I can point to several related accidents.
... an airplane propeller doesn't become a turbine just because it's windmilling
* I can point to several related accidents.
Join Date: Sep 2006
Location: West Coast Canada
Age: 71
Posts: 69
Likes: 0
Received 0 Likes
on
0 Posts
@ volumn
I finally got access to my old Bristol Freighter Pilot Notes - so the following info might help. It's been too many years for me to just go and quote something that I might get wrong. So for what it is worth ??
Re: Bristol Hercules power setting
Quoted-
As per the B170 Mk 31 (734 engines) Pilots Manual the procedure after takeoff and first stage climb is to retard the throttle to the ECB setting first and then adjust the propeller speed as needed.
The throttles have 3 positions Take Off , Cruise, and Shut.
The throttle was connected to the mixture system and it was important to have the position set correctly as a small difference forward would affect the fuel burn upward.
Take off 5 min limit (15 Min emergency) - 2800 RPM @ +13 Psi Boost ( 56.25 inches Hg)
I can remember the crew calling for a Yard and a Half of Boost for the start of the take off run!!
Max Continuous 2500 RPM @ + 8 ½ Psi Boost ( 47 In Hg)
Max Continuous Lean 2400 @ + 3 ½ ( 37 In Hg)
Cruise @ 44,000 Lbs ---- Maximum Weak Mixture Power 2400 RPM @ + 3 ½ Psi Boost (37 In Hg)
At S/L = 149 Knts Ind - 137 Gal Per Hour
At 5000' = 131 Knts - 110 GPH
At 10,000' = 145 Knts 145 GPH
Cruise Recommended Throttle in ECB position engine speed as required
SL = 137 Knts - 111 GPH
5000' = 131 Knts - 110 GPH
10,000' = 125 Knts 114 GPH
Note: Never permit the RPM to exceed 2400 when the throttle lever is at or below the Maximum Weak Mixture ( ECB ) position, or engine damage may result.
End of Quoted information.
As per the note above I would take that to mean that an increase above 2400 RPM would require the throttle to be moved first which moves the mixture out of the maximum weak mixture position before the propellers are selected to give a higher speed.
This makes sense to me as the mixture needs to be addressed first so as to prevent detonation at the higher power settings. It is a funny way of doing things but it works very well and it got rid of the possible addition of power by the flight crew and the omission of setting the mixture rich first.
In a nutshell:
The reduction of power from Maximum is by throttles first and then set desired engine speed.
The addition of engine power below 2400 RPM is not addressed but the notes do say that you adjust speed and boost to get the desired target airspeed usually using the charts supplied.
To go above 2400 RPM throttle is moved first which affects the mixture as well and then propeller speed.
There are no mixture controls on this aircraft there is a Carburetor Fuel Cutoff which has 2 positions OFF and RUN. Mixture control is done automatically with the position of the throttle.
Hope that helps.
Can anybody quote a Bristol manual, just like the P&W some posts ago?
Re: Bristol Hercules power setting
Quoted-
As per the B170 Mk 31 (734 engines) Pilots Manual the procedure after takeoff and first stage climb is to retard the throttle to the ECB setting first and then adjust the propeller speed as needed.
The throttles have 3 positions Take Off , Cruise, and Shut.
The throttle was connected to the mixture system and it was important to have the position set correctly as a small difference forward would affect the fuel burn upward.
Take off 5 min limit (15 Min emergency) - 2800 RPM @ +13 Psi Boost ( 56.25 inches Hg)
I can remember the crew calling for a Yard and a Half of Boost for the start of the take off run!!
Max Continuous 2500 RPM @ + 8 ½ Psi Boost ( 47 In Hg)
Max Continuous Lean 2400 @ + 3 ½ ( 37 In Hg)
Cruise @ 44,000 Lbs ---- Maximum Weak Mixture Power 2400 RPM @ + 3 ½ Psi Boost (37 In Hg)
At S/L = 149 Knts Ind - 137 Gal Per Hour
At 5000' = 131 Knts - 110 GPH
At 10,000' = 145 Knts 145 GPH
Cruise Recommended Throttle in ECB position engine speed as required
SL = 137 Knts - 111 GPH
5000' = 131 Knts - 110 GPH
10,000' = 125 Knts 114 GPH
Note: Never permit the RPM to exceed 2400 when the throttle lever is at or below the Maximum Weak Mixture ( ECB ) position, or engine damage may result.
End of Quoted information.
As per the note above I would take that to mean that an increase above 2400 RPM would require the throttle to be moved first which moves the mixture out of the maximum weak mixture position before the propellers are selected to give a higher speed.
This makes sense to me as the mixture needs to be addressed first so as to prevent detonation at the higher power settings. It is a funny way of doing things but it works very well and it got rid of the possible addition of power by the flight crew and the omission of setting the mixture rich first.
In a nutshell:
The reduction of power from Maximum is by throttles first and then set desired engine speed.
The addition of engine power below 2400 RPM is not addressed but the notes do say that you adjust speed and boost to get the desired target airspeed usually using the charts supplied.
To go above 2400 RPM throttle is moved first which affects the mixture as well and then propeller speed.
There are no mixture controls on this aircraft there is a Carburetor Fuel Cutoff which has 2 positions OFF and RUN. Mixture control is done automatically with the position of the throttle.
Hope that helps.
Join Date: Sep 2006
Location: West Coast Canada
Age: 71
Posts: 69
Likes: 0
Received 0 Likes
on
0 Posts
Information on the B787 Ram Air Turbine:
I have 3 different sources of information relating to the speed control of the RAT on the 787 - all 3 confirm that the blades change pitch as a function of the speed limiter - which at the higher aircraft airspeeds or the offloading of the RAT will maintain a maximum constant speed of the turbine.
Quick link to the manufacturers site with just a quick overview of their product line;
Aircraft Ram Air Turbine (RAT) | UTC Aerospace Systems
I have 3 different sources of information relating to the speed control of the RAT on the 787 - all 3 confirm that the blades change pitch as a function of the speed limiter - which at the higher aircraft airspeeds or the offloading of the RAT will maintain a maximum constant speed of the turbine.
Quick link to the manufacturers site with just a quick overview of their product line;
Aircraft Ram Air Turbine (RAT) | UTC Aerospace Systems
