Tuesday, July 05, 2005

Beta Reverse Valve

I asked what would happen if torque were too low to drive the propeller at the selected rpm. The answer, based on the information I gave you, was that that the governor pilot valve would open fully, maximizing pressure in the propeller hub, driving the piston all the way forward, and the blade angle to -15 degrees. As you can imagine, this would not be desireable. If the pilot reduces the power to land, she wants to descend to the runway going forward, not in reverse. This doesn't happen, and it is the job of the beta system to ensure it doesn't.

Three rods extend from a feedback ring at the rear of the propeller to the outside of the propeller dome. When the propeller dome moves forward past the position corresponding to +21 degrees blade angle, the linkage from the propeller dome begins to pull the feedback ring forward. This movement is transmitted to the beta reverse valve, which begins to close. At less than an 11 degrees blade angle, the beta reverse valve will fully close, not allowing oil pressure through the beta valve. The governor pilot valve will be wide open, but that won't increase oil pressure until the drop in oil pressure allows the springs and counterweights to drive the blades to a coarser pitch, opening the beta reverse valve, and holding the blades at the ideal idle angle of 11 degrees, until torque increases. As a result, when the propeller is in beta range, the power levers control fuel flow and blade angle.

If the pilot really does want to put the propellers into reverse, useful for parallel parking (I am NOT kidding: you really can parallel park these guys), the beta reverse valve can be deactivated. The pilot does this by twisting the handgrips of the power levers, kind of like you may have to push a button or apply downward force on your car gearshift to shift into reverse. Twisting the handgrips allows the beta reverse valve to open, so that blade angles less than 11 degrees can be obtained. As the power levers move aft from that position, they stop controlling the fuel flow and control only the blade angle through a null zone between 11 and -2 degrees blade angle. As the power levers move back past the position that selects -2 degrees, they start increasing fuel flow--normally you push power levers forward for more power--so that pulling the levers further back allows you to increase backwards thrust, either to reverse into your parking space, or to slow the airplane down more rapidly after landing. A blue (or possibly amber, depending on the model number) light turns on to show that the propeller is in beta mode, with a blade angle less than 9 degrees.

The manual says, literally on the first page, even before the table of contents, that you must never ever ever do this in flight under any circumstances. This of course means that loads of people do, but that the company isn't going to be held responsible if bad things happen.


Anonymous said...

So do the props akshully stop befor ereverse thrust? Or am I missing something?

sweavo said...

You're missing something!

The props keep going round the same way, but the individual blades twist in their sockets to push the air the other way

Aviatrix said...

What sweavo said.

dibabear said...

Dang...shoulda said what I was going to last night...that the props move into beta.

Military pilots can use reverse in flight to perform a tactical landing. I've seen a C-130 touchdown and stop using 500 feet of runway during such a maneuver. Likewise, they've got those fancy JATO bottles for those snappy really short-field take-offs too. :-)

Avimentor said...

We're not alowed to use reverse thrust in our aircraft, except during landing when it really is required. Beta alone is very effective in the Caravan for short landings.

Reverse thrust tends to throw any loose debris on the ramp or runway forward, where it can be sucked into the engine.

Using beta in flight is a real gamble. All of the Caravans I've flown have differed widely in how quickly they go from beta to reverse. The saying about old and bold pilots applies here, I think.

Anonymous said...

I've been trying to find a good page summarising what would (does?) happen if beta mode is engaged during flight. As I understand it, an accidental thrust reverser deployment on a jet could dismount the engine from it's pylon, or potentially destroy the wing, but the other comments seem to suggest beta-in-flight is a bearable, possibly even practised phenomenon (on some models of aircraft, presumably). Is this really the case?

Anonymous said...

I've been trying to find a good page summarising what would (does?) happen if beta mode is engaged during flight. As I understand it, an accidental thrust reverser deployment on a jet could dismount the engine from it's pylon, or potentially destroy the wing, but the other comments seem to suggest beta-in-flight is a bearable, possibly even practised phenomenon (on some models of aircraft, presumably). Is this really the case?

Aviatrix said...

1. Beta is not the same as reverse. In the particular airplane I am describing, beta starts at eleven degrees blade angle, still in the forward range.

2. Yes, the result of engaging reverse or beta in flight depends on the aircraft and the phase of flight. It ranges from catastrophic to nifty.

3. Remember that beta/reverse on a propeller aircraft is about changing the angle of the propellers while reverse thrust on a jet is about redirecting the exhaust/bypass thrust as it exits.

Anonymous said...

Clarence: Hey Roger, our wings just blew off.

Roger: Nifty.

Old Blind Dog said...

A very spectacular crash occurred because a Lockheed Electra (not used much after this; you'll recognize it as the P-3 Orion Naval anti-submarine warfare aircraft) pilot elected to use a little "reverse" on approach because he was high on the glidepath. The aircraft basically went straight into the ground.

On the other hand, a Sabreliner can reverse individual engines in flight with no ill effect (the clamshells can, and do, open inadvertantly).

As Aviatrix said, anywhere from catastrophic to nifty. The key is not to experiment. Reverse in PT6 powered aircraft is not approved for use in flight on any installation that I know of. In all cases that I am familiar with it leads to rapid loss of control.

Anoynmous said...

Ugh, terminology drift. I'm insufficiently jargon-literate here, no doubt about it. My understanding of ''beta'' in the context of propellers or turbine blades merely refers to the blade's angle of attack. I suspect there's been some leakage from the idea of a specific propeller pitch being labeled "startup beta" or "idle beta" into a general usage of "beta means a lower pitch than you'd ever use in flight".

My guess about the situation was seriously off the mark. I feel so unsmart sometimes.

Aviatrix said...

Quite the contrary, anoynmous. Your guess was right on the mark. So much so that I thought it would be condescending to mention it. Without the beta reverse valve or beta backup valve (coming soon) an underspeed propeller would continue flattening right through zero blade angle to reverse. The beta system stops that from happening.

You're right that beta is a term for the angle of the blades, to complement alpha representing the angle of attack, when airflow is taken into consideration. I never before connected that beta to the "beta range" but yeah, it's also caled the beta control range.

For this airplane the beta range is defined as the range of operation where the pilot directly controls the blade angle with the power lever.

I'm actually very impressed with my readers. I didn't "get" this stuff on first exposure, and I had far more motivation to learn it.

dibabear said...

Heeeee...they only let me fly those fixed pitched under 200HP planes but I've read enough reviews of PT6 powered doctor killers to know which context of beta you were referring to.

I didn't know, however, that alpha was angle of attack and beta blade angle in general. Learn something new every day.

Anonymous said...

I am writing a paper for my Master Degree and ran across your conversation. This might help. http://www.airdisaster.com/reports/ntsb/AAR94-06.pdf
In short, in a Saab 340, the pilot moved the power levers into the Beta range during flight. The propellers and engines experienced extreme overspeed when propeller and engine governing was lost while operating in the beta range. The engines were substantially damaged during the overspeed and necessitated a power-off emergency landing.
Hope this helps. Jim

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PatBateman said...

you want to know what happens if beta is engaged while flying - here you go :

"The accident appears to result from the Captain's use of the propellers in the reverse beta range, to improve control of his track on short final. A strong thrust asymmetry at the moment when coming out of the reverse beta range would have caused the loss of yaw control, then roll control of the aircraft.