USAFHummer From United States, joined May 2000, 10685 posts, RR: 63 Posted (6 years 3 weeks 3 days 20 hours ago) and read 7041 times:
Hi,
I was just curious as to the aerodynamics that makes beta thrust act as sort of a "thrust reverser" for propeller aircraft with that capability...can anyone explain this to me???
Thanks,
Greg
Chief A.net college football stadium self-pic guru
FredT From United Kingdom, joined Feb 2002, 2102 posts, RR: 30 Reply 2, posted (6 years 3 weeks 3 days 18 hours ago) and read 7012 times:
From an old post by yours truly, with all the information you want and quite a lot more on turboprops:
Here's an excerpt from my thesis describing how it works in a SAAB 340 with PL + CL: Basic operation
You have two levers for each engine, Power Lever (PL) and Condition Lever (CL).
The range of the CL is divided into
Fuel off where the engine goes to feather (83.5 degrees pitch) and the fuel is cut off
Start, where you are supplying fuel to the engine but the prop is still feathered
UNF, UNFeathered, where the prop is out of feathered and basically in constant speed mode trying to maintain 1180 RPM but without the bottoming governor (more on that later)
Min to max constant speed (CS) range where the prop RPM is controlled to be within 1180 RPM (min) and 1384 RPM (max).
T/M (torque motor) lockout, which will lockout, the engine control unit (ECU, or digital ECU, DECU, in B model a/c) if it malfunctions. Once T/M lockout is activated, you have to shut down the engine (put the CL in fuel off) to reactivate it.
The power lever range goes from full reverse through ground idle (GI) to flight idle (FI) and then on up to full power. Below FI you are operating in the beta range where the PL position (unless the CL is in feather or you feather manually) directly controls the prop pitch from -16.5 to +10 degrees. Above FI there is a minimum pitch stop ranging from +10 (FI) to +25 (full power) degrees pitch. As you go from PL full aft to PL full forward, more and more fuel is added to the engine (naturally) through signals to the Hydro-Mechanical Unit (HMU). At low power settings (below approx 30%), this amount of fuel is not enough to spin the propeller up to the commanded 1180 RPM at the pitch setting commanded by PL in beta range or at the minimum pitch stop.
Why do we have a beta range? Due to the slow response to throttle setting changes in turbo engines it is very impractical to use the throttle to control movement on the ground. You would have to wait for the gas generator to spin up (Ng increase), providing more torque through the power turbine (PT) increasing the prop RPM (Np). The prop CS governor would then tell the pitch control unit (PCU) to increase the prop pitch and then you would get additional power. In beta mode, you change the pitch first instead using the inertia in the propeller system to provide thrust, letting the Ng accelerate or decelerate in response to Np to keep Np constant.
If the amount of fuel burned below 30% won’t keep the prop spinning at 1180 RPM, what keeps it at constant speed in the beta range? This is where the previously mentioned bottoming governor (BG) comes into play. The BG is active when the CL is above UNF and will send a signal to the HMU to add fuel above what the PL setting is dictating to keep the Ng up. The normal reference Np for the BG is 1040 RPM but to give more power in full reverse the BG reference will change to 1200 RPM Np when the pitch goes below –10 degrees (<-10 on both engines on older versions).
CTOT
Early on it was discovered that the torque set in the beginning of the take-off roll would increase as the ram air effect increased with airspeed. To avoid having to stare at the torque (Nq) reading during the entire takeoff roll, decreasing the PL setting to keep it at 100% and not above a CTOT (Constant Torque on Take-OFF) system was added. When active, this system will signal to the HMU through the ECU to add fuel until the preset Nq is reached as soon as you set the PL above a certain position.
AC
If an engine dies there’s an autocoarsen (AC) system, which will detect this. It then proceeds to feather the dead engine automatically. There’s an inbuilt safety making it impossible to feather both engines in flight should this system fail. The AC system continues to monitor a failed engine and will bring it out of AC mode should the engine parameters used to detect a flameout increase above the threshold values again.
APR
340B a/c has something called automatic power reserve (APR) which when one engine goes into AC during CTOT operation automatically adds 7 percent units of torque to the other engine to compensate for the loss of thrust.
If you find three levers in a cockpit you're probably looking at a power lever, a prop lever to set RPM and a condition lever to control the fuel (fuel off, idle, run).
Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
XFSUgimpLB41X From United States, joined Aug 2000, 3315 posts, RR: 39 Reply 3, posted (6 years 3 weeks 3 days 15 hours ago) and read 6966 times:
In simple terms..think of beta range as taking the angle of the propellers that were there to produce forward thrust, and changing the pitch on them so they blow air forward instead of backward.
FredT From United Kingdom, joined Feb 2002, 2102 posts, RR: 30 Reply 4, posted (6 years 3 weeks 3 days 13 hours ago) and read 6942 times:
Actually beta range works both for forward and reverse thrust.
Normally, you set the power and the propeller governor adjust the propeller pitch to give the selected RPM. In beta, you set the prop pitch and the power is adjusted to give the selected RPM.
For the why's, read above.
Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
AJ From Australia, joined Nov 1999, 2287 posts, RR: 41 Reply 9, posted (6 years 3 weeks 3 days 4 hours ago) and read 6830 times:
To add to the technical information already given, beta range is what causes that lovely growl as turboprops taxi around. It also gives fantastic taxi speed control without the need to ride the brakes.
411A From United States, joined Nov 2001, 1596 posts, RR: 9 Reply 10, posted (6 years 3 weeks 3 days 3 hours ago) and read 6820 times:
FredT
With the Allison 501 engine, RPM remains the same thru-out the flight regime...ie: push the throttle forward (or back), more (or less) fuel is fed to the engine.
Prop pitch changes, RPM remains the same.
Called...'instant power', has to be experienced to be believed.
A very good design, especially with AeroProducts propellers.
T prop From United States, joined Apr 2001, 910 posts, RR: 2 Reply 11, posted (6 years 3 weeks 2 days 18 hours ago) and read 6769 times:
For the PW100 and PT6 you are in Beta Range in flight when the power levers are set at the flight idle gate and very slightly above. The gate divides beta operationaly into 2 ranges, with settings below the gate for ground use only.
In flight, having some manual control (beta) of prop blade angles enables pilots to better control drag for deceleration and sink rate. On the ground with power levers below the gate, the crew is then able to control blade angles all the way to a maximum negative setting, to provide flat pitch (high drag) or reverse thrust.
On Garrett's TPE331, Beta Mode as they term it, operates similarly except in flight beta is used only on single engine applications .
About engine speed on single shaft turboprops; Garrett TPE331 engine speed is controlled by the speed lever. High rpm (100%) for take off, Cruise (96-97%) for climb/ cruise/ descent and Low (65-73%) for starting, ground operation and taxying.
411A From United States, joined Nov 2001, 1596 posts, RR: 9 Reply 13, posted (6 years 3 weeks 2 days 2 hours ago) and read 6719 times:
FredT,
Allison 501 (on the Lockheed L188 Electra), the prop (and the engine) turns at a constant RPM in all flight regimes, takeoff, climb, cruise, descent, landing, and during reverse after landing. Only during taxi is the engine (and prop) RPM slightly lower.
Normally, with most constant speed prop designs, maximum RPM is used for takeoff, another for climb, still another (possibly) for cruise.
The Rolls Royce Dart 7 engine series for example , uses 15,000 RPM for takeoff, 14,500 for climb, 14,200 for cruise. (Some operators use 14,200 for climb as well).
With this design, there is no separate prop control, throttle and prop are combined in one lever (power lever). For FCU setting for different altitudes/temperatures, a separate (electric) fuel trimmer is incorporated.
L-188 From United States, joined Jul 1999, 28610 posts, RR: 73 Reply 14, posted (6 years 3 weeks 2 days ago) and read 6708 times:
Allison 501 (on the Lockheed L188 Electra), the prop (and the engine) turns at a constant RPM in all flight regimes, takeoff, climb, cruise, descent, landing, and during reverse after landing. Only during taxi is the engine (and prop) RPM slightly lower.
Just want to add that on the number 4 engine there is a two speed generator, which enables it to be used for electric power during ground turns where a ground power isn't available.
Don't remember the exact numbers, I never noticed a slower speed when I was parking a forklift with a bin next to the thing when it was running to access the forward bag pit.
OBAMA-WORST PRESIDENT EVER....Even SKOORB would be better.
FredT From United Kingdom, joined Feb 2002, 2102 posts, RR: 30 Reply 15, posted (6 years 3 weeks 1 day 15 hours ago) and read 6681 times:
So, in short, there's only a fixed RPM CSP mode and no beta on the L188.
I like the Saab 2000, only 1100 and 950 RPM depending on which mode out of four you are in. Also certified to fly the approach at 950 RPM, does the noise footprint a whole lot of good.
Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
411A From United States, joined Nov 2001, 1596 posts, RR: 9 Reply 16, posted (6 years 3 weeks 1 day 12 hours ago) and read 6667 times:
FredT,
There is a beta blade angle on the AeroProducts propeller fitted to the L188. However, the prop (and engine) are turning at a constant speed, all the time.
Scootertrash From United States, joined Aug 2001, 569 posts, RR: 11 Reply 17, posted (6 years 2 weeks 5 days 2 hours ago) and read 6566 times:
411A...
15,000 RPM on the Dart? And that is prop RPM? I didn't realize you could spin metal (or should spin metal) that fast. And the prop tips don't go supersonic? Makes 1200 RPM on the Dash 8 on takeoff look like slow motion. Holy Smokes!
411A From United States, joined Nov 2001, 1596 posts, RR: 9 Reply 18, posted (6 years 2 weeks 4 days 21 hours ago) and read 6549 times:
Scottertrash,
The 15,000 rpm refered to was the engine rpm, not the propeller.
With the rpm indications on RR Dart powered aircraft, it always refered to the engine.
Cannot offhand remember the prop rpm on the RR Dart RDa7 series engines, but suspect it was approximately 1200 or so, as the props were very large.
Also, as the Dart was an English engine, the prop turned left hand.
The Dart was the very first successful turboprop...and very reliable.
The Brits got it right...absolutely.
Scootertrash From United States, joined Aug 2001, 569 posts, RR: 11 Reply 19, posted (6 years 2 weeks 4 days 2 hours ago) and read 6500 times:
Alright, that makes more sense... When I was a kid, I used to love that sound the Dart equipped Fokkers would make coming onto the ramp at my hometown field. Whistlepigs!
411A From United States, joined Nov 2001, 1596 posts, RR: 9 Reply 20, posted (6 years 2 weeks 3 days 11 hours ago) and read 6468 times:
Scootertrash,
Yes indeed, the Darts were noisy on the ground.
And if the pilot(s) failed to select ground fine pitch after touchdown, the engines rpm would reduce, and very expensive damage would result, due to overtemp.
Personally operated RR Dart engines on a variety of aircraft for more than 5000 hours...and never had a problem.
EMBQA From United States, joined Oct 2003, 8424 posts, RR: 19 Reply 22, posted (6 years 2 weeks 3 days 5 hours ago) and read 6450 times:
I guess in the simplest of form, think of it this way......... drive down the runway at 150 miles an hour, then open two round 12' car doors..!! When the props go into 'Beta' they actually go into 'Ground Fine' and have a zero degree blade angle. So they actually become 2 big 'Speed Brakes'. Easy on the engine, low noise in the cabin, no dirt being kicked up and injested and no stone damage to the props.
"It's not the size of the dog in the fight, but the size of the fight in the dog"
411A From United States, joined Nov 2001, 1596 posts, RR: 9 Reply 23, posted (6 years 2 weeks 3 days 1 hour ago) and read 6436 times:
Sovietjet,
No, not 180 degrees.
Picture a propeller at fine pitch. This would be (for example) 20 degrees.
If it went to ground fine, the blade angle would be zero.
If it went to beta...-5 degrees.
Reverse...-15 degrees. (examples only)
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