One prop governor for two propellers

[Onemorechance]

I don't know if I can describe it in detail, but the problem is basically as follows:

We have a gas turbine engine (420 shp turboshaft engine mostly used in helicopters). We want to use it in an experimental aircraft. The engine would drive 2 counter rotating constant speed propellers (probably of diameter ~1800 mm at ~2800 RPM). Propellers are connected to the engine through a transmission system that we want to design ourselves. The propellers are some distance in front and to both sides of the engine (one prop about 1,2 m on Port, the other 1,2m on Starboard).

Now it comes:
We need to have a prop governor to govern both propellers (one governor since they are connected through the same transmission).

How can I find the system I need? Is there somebody who can tell me where I can find expertise and advise (without having to wait for ages)?

 

[Sparweb]

Kind of like this...?

Somewhat more seriously, there was a Piaggio (Italy) military aircraft I once saw in a book that had 4 sets of propellers: Two engines and each engine had props in front and another set behind it. Very strange beast, but they may have had the same challenges as you.

The governor you suggest will not be able to control the speed of the propellers independently. I suppose that with a single engine and an experimental certificate, the need for feathering one prop is drastically reduced.

Types of systems to choose from:

Chains
Pulleys
Driveshafts
Hydraulics
...?

Have you chosen to use two propellers as a way to more efficiently use the power, or for some other issue, such as ground clearance?


Steven Fahey, CET

 

[MikeHalloran]

The front/rear thing works okay with piston engines, less so with turboshafts, with all that hot gas coming out one end.

You might consider a small gearbox on the engine with counter rotating output shafts, and timing belt drives, not unlike the Wright Flyer except for the crossed chains.

The point about the desirability of indpendent feathering is well taken.

Given the relative weight of turboshaft engines and gearboxes, two smaller engines might turn out lighter than the drivetrain you propose.

... and that is about the limit of my expertise in the domain of airplanes. However, I have been designing products for a long time, and I've made an observation: The likelihood of success in building a complex product is considerably reduced by the presence of components that are themselves unproven, because it forces you to put multiple development cycles in series in time. I.e., you can't tie down the airframe design until you've worked the bugs out of the transmission. A lot of projects run out of money before the second development cycle can be completed. A third development cycle, i.e., two unproven components, pretty much dooms the product. Yes, CAD/CAE/CAM shortens development cycles, but it also burns money at a prodigious rate.





Mike Halloran
Pembroke Pines, FL, USA

 

[Onemorechance]

Well Steven and Mike,

First thank you for your reply.

It is a preliminary study that we are doing.
There are several reasons why we chose to have this engine and the 2 props. I don't want to explain all about it except that this small aircraft is supposed to have besides conventional TOL, also VTOL capabilities. Let's say that the aircraft can be seen as a mixture of 3 concepts one is the CONVAIR POGO XFY-1 tail-sitter (or T-Wing tail-sitter), one is the BELL EAGLE EYE and the third is the FREEWING. You can imagine that it is difficult to explain you all of the details.

Yes, the point about feathering one of the two propellers is indeed well taken because we would have two overload couplings, one per propeller.

The transmission comprises 3 single mesh bevel gearboxes, one just behind the engine PTOutput shaft (plus overload couplings) to drive two shafts to port and starboard, then a single mesh bevel reduction-gearbox on port side and one on starboard side to drive the propellers.

Maybe we can use 2 prop governors, one per propeller driven by each reduction gearbox in the propeller nacelle? What do you think?
But then we possibly create other problems, like introduce a new source of vibration excitation.

Very true:
The likelihood of success in building a complex product is considerably reduced by the presence of components that are themselves unproven! That's very true, but then again if you don't try something new … Being a hard headed person, I would like first to learn as much as possible before I decide to let go of this idea.


Regards

OneMoreChance

 

[thruthefence]

It seems to me that not only are you going to have to use two govenors, but you need some way of coupling the govenor system so both props are pulling the same torque,
The reason one would want to feather a prop, is to reduce drag in the event of an engine failure. With one engine, this reason goes away.
And something is going to have to take the thrust loads from the props themselves at the nacelles, as I understand it you will be using one reduction gearbox with two outputs?

 

[Onemorechance]

Let me try show you the possibilities:

1. One governor on the engine

Left Propeller
__________| |__________
| |_____________
| ____________ |
| | | |___________________________________________
| | | _________ ___Hi Pressure to Left & Right Prop__->
__Red Gearbox __ | | __Gearbox__
| ___| |___ | | | | |
| | | |___|____Shaft_______|__|________________|_________|__|___Shaft_
| | | |___|____________________________________________________->
| | | | | ___| |___ | __ __| |
_______________ | ________| Governor |_____|__|___|
| | | |
____________________________________________
| Engine |
|

2. Two governors, one per reduction gearbox


Left Propeller
__________| |__________
| |_____________
| ___________ |
| | | |
| | | |
__Red Gearbox __ | | __Gearbox__
| ___| |___ | | | | |
| | | |___|____Shaft____________________________|__________|__|___Shaft_
| | | |___|_________________________________________________________->
| | | | | | | | __ __| |
__________________ | | |______|__|____|
___| |___ | | | |
|Left |_________| | __________________________________
| Governor_____________| | Engine |
|________| |

It did not succeed very well, but I think you can see enough.

As you can see the reduction gearbox has two outputs.

The reason for using two governors would be if for example in the event of a failure in one of the props, then the overload coupling would uncouple and it could be feathered.

In order to prevent the governors to become a new excitation source (by not pulling the same torque), do you think a syncrophaser (like on twin engine aircraft) would help?

If you have comments, don't hesitate to give them because I don't know much yet, I want to learn as much as possible.

Regards,

OneMoreChance

 

[Onemorechance]

Instead of a syncrophaser, I think I have an easier solution:

When connecting the high-pressure governor line between the left and right propeller with a ball check valve, the problem can be solved (I think).
By doing so, the difference in pressure between left and right can be equalized during normal operation because for small differences in pressure the valve stays open and works equalizing. This will minimize the difference in torque between left and right prop.
Then in a case of failure in one of the props, the overload coupling will open causing that prop and reduction gearbox to stop. The governor on that reduction gearbox will stop and the pressure falls. This high pressure drop causes the ball check valve to close and keeps the pressure high on the other intact propeller which can keep working during an emergency landing procedure.

Let me know what you think!

Regards,

OneMoreChance