Variable pitch propellers

[Luke Mc]

What aircraft have varible pitch propellers and how are they controlled? Do any light twins have them and does the DC3 have them, are they controlled by a prop leaver, and how are they feathred? Also, is there any eqivenlent of reverse thrust on a prop aircraft?

[Dave Incognito]

Many light aircraft have variable pitch props, otherwise known as constant speed units (CSU). Nearly all light twins are CSU equiped.

In the cockpit, you control the CSU with a pitch lever/s, usually located between the mixture and throttle lever/s. The CSU operates by using a governor which regulates the speed of the propeller. Moving the pitch lever sets RPM and does not directly set the pitch of the propeller. The governor then maintains the desired RPM by continually adjusting the propeller pitch.

Not all variable pitch props can be feathered. However, most found on multi engine aeroplanes can be feathered. The prop is driven into the feathered position either by oil pressure or counter weights.

It is almost impossible to explain how all this works without diagrams and a lot of time.

Yes a DC3 has variable pitch props, and I have no idea about reverse on piston engine aeroplanes. I can't think of any that do, but I'm sure there is some whacky Russian machine out there that does have it.

Hope this helps.

[Final 3 Greens]

Luke

Most larger turboprops have a setting that emulates reverse on a jet. The pitch of the props is taken through neutral to a negative setting and effectively pushes the airflow forward instead of backwards.

Even some light turboprops such as the Cessna Corsair have this feature.

[AHORSE]

Luke

The C130 Hercules has reverse thrust. It is achieved by, as decribed earlier, by turning the blade past the neutral to produce a forward flowing air stream. By doing this it can produce up to the equivalant of 60% of its forward thrust in the reverse direction.

This is how it can reverse taxy under its own power, making it an extremely flexible aircraft in the tactical role.

[Airbanda]

Does the DC6 have reversible pitch? Have a vague memory of "a marked increase in noise" after touchdown when an atlantic/atlantique example was displayed at PFA Cranfield 2/3 yrs ago.

Airbanda-Proud to be an Anorak.

[ironbutt57]

All of the large piston twins of u.s. manufacture had reversible propellors to my knowledge spent time on the dc-6/dc-7 and they both had reverse thrust. Most dart-powered torboprops had a nil-thrust blade angle (grnd fine)..twin otter dhc-6 with the pt6 engind had an approach idle setting for steep approaches which allowed selsction of a propellor blade angle lower than that of flight idle..

[thx1138]

Flew lots of Twin Otters. But I have not heard of an approach idle setting which is different from closing the throttles to flight idle. Anybody know what he is talking about? Is this an installation on some Twin Otters?

[OzExpat]

thx1138... Sounds more like the discrete application of Beta to me. Not a practice that I'd recommend but, if the chips are down and you need to use a steep approach, well... suffice to say that I know it's been used.

[PAXboy]

I know that the BAe Jetstreams have reverse as they can 'Power Back' as opposed to 'Push Back'.

However, a captain on the J41 of my acquaintence says this, "Once you are rolling backwards, close the throttles and let her come to rest by herself. DON'T step on the brakes at anything other than a s-l-i-g-h-t roll or she will promptly sit on her @rse!"

I understand that the passengers find this inconvenient and that it also irritates the management. ;)

[thx1138]

Of course reverse on the PT-6 on the Twin Otter can be used on the ground. It could even perhaps get tweaked in the flare on a very short strip but was definitely not supposed to be used(or a bit of so-called beta) on approach. However I have read that the turbo Porter(PT-6) does use at least some beta during those straight down descents after dropping skydivers and that it is legal to do so. Maybe its an airplane specific thing. Any Porter pilots care to comment on that procedure? Any other tuboprops allowed to go below flight idle while airborne?

[FlyingForFun]

To add to all the excellent replies above, not all variable-pitch props have a constant-speed unit. Although most do, and operate as Dave Incognito and others have described, the aircraft I fly doesn't.

I manually control the pitch of my prop using a 3-position switch. The switch is normally in the centre position, but I can push it up to apply power to an electric motor which pushes the blades into a finer pitch, or down to drive the motor the other way and coarsen the blades.

Having said that, though, the majority of variable-pitch props do have a CSU - mine is definitely the exception rather than the rule.

FFF
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[GAMAN]

For a very good run down on piston props pay this site a visit.

http://www.avweb.com/articles/pelperch/pelp0016.html

a very descriptive article.

[777AV8R]

Variable pitch vs Constant speed.

Actually, there are differances between the two propellers. The constant speed prop found on majority of 'propeller bearing' aircraft, use this type of prop speed control. There is a 'goverenor' that looks after the set RPM. That speed, provided enough torque is supplied, will keep the prop turning at the slected RPM.

There IS another prop..and it is the Variable Pitch propeller. In the good old days, we were able to use a handle device, that looks much like a 'window crank' in car windows, to manually change the pitch angle of the propeller. With different power settings, the prop would turn at different speeds and constant manual adjustments had to be made in order to keep from blowing the jugs off the engine or keep the engine turning. I want to say that the aircraft was the old AilonAercoupe that had this option on it, but can't remember exactly.

Beta range is found on many Turbo-prop aircraft. Someone mentioned the C-130. And yes, it does have Beta..the percentage of torque conversion escapes me, but does work very well.

I also believe that the T'wotter has Beta range and can be used for short field work.

Hope it helps. :)

[treadigraph]

Which prompts me to ask: you often see Hercules and smilar tactical transport aircraft demonstrate the "Que San" (sp?) approach at airshows - and I've also seen RAF/RN Jetstreams do it in the past; do they use reverse thrust to keep the speed back during the descent; or is there sufficient drag from the flaps to keep things in hand, or indeed do they just accept an increase in airspeed?

Always wondered...

[777AV8R]

Assault approach on the Herc?

You can't put the props into Beta in the air on the Herc..in fact..if there isn't enough power being generated from the engine to keep the torque positive....the prop will 'decouple' from the engine if the NTS (negative torque system) doesn't correct it.

It's been years since I've done one...I believe that the assault approach was based on flap 50 stall speed...there has to be some other military guys out there that know...I can't remember the speeds!

[Ozgrade3]

With regard to a metro(and some others like the SAAB 340), why is the beta range so much noisier than when the blade have some pitch applied to them as in moving off. As power is increased, it gets quieter. Then the brraap happens as betta is applied.

[thx1138]

O.K. I dug out the old Twin Otter manual. Beta range starts at +17° blade angle, the low end of the constant speed range. Above this angle the propeller lever controls blade angle. Below this angle the throttle(power lever) controls blade angle with the latter being the or part of the definition of beta range. Pitch at idle rpm is +11°(beta light illuminates at +9° in other words after you move the power levers into reverse on the ground). Max reverse angle is -15°. Therefore with this engine you can be in the beta range in flight.
By the way, can anybody tell me why on the DHC-6 it was designed with a prop lever/power lever interlock so that both propeller levers had to be selected at or above 91% in order to get reverse(Max 96%). If you forgot to put the props forward on landing you couldn't get reverse. Yet they didn't seem to have this feature on the King Air. What about other PT-6 aircraft.