Rotol and D.H. Constant Speed Prop Operation

[gecko]

Hi all - I am looking for information about the operation of Rotol and DeHavilland constant speed propellers fitted to Spitfires and Hurricanes. At this point I'm mainly interested in the three blade models, but I assume the operation was similar for the others. Specifically, what provided the force to increase or decrease propeller pitch?

I understand that the D.H. used counterweights and oil pressure, but did the oil pressure work to increase, or decrease pitch? And the same for the counterweights - which direction did they act on the blades?

I know less about the Rotol other than that oil pressure was involved, and that supposedly the governing mechanism was more responsive. But did oil pressure act both to increase and decrease pitch, or were there other mechanisms like a spring or counterweight?

Thanks for any help,

Dan

 

[nuuumannn]

understand that the D.H. used counterweights and oil pressure, but did the oil pressure work to increase, or decrease pitch? And the same for the counterweights - which direction did they act on the blades?

It depends on which props you are talking about. The most commonly discussed DH prop is the 23EX (fitted to Mosquitoes, Lancasters etc), which was a licence built variant of the Hamilton Standard 23E50 Hydromatic prop. For information's sake, 'Hydromatic' is a trademark produced by Ham Std and describes the actuation of the blades, it was via a piston in the hub, which was actuated by differential oil pressure, the piston's movement through a fixed and moving cam altered the blade angle via a serrated edge on the blade base. The DH variant was the same.

I could go into it in more detail but I don't have time. There are knowledgeable engineers on this site who can go into it in more detail.

 

[gecko]

No, not the 23EX - I'm looking for those fitted to single-engine fighters. I believe the early D.H. ones were Type P.3-5-5-7 two-position props which got converted in the field to constant-speed operation, or the D.H. 5-20.

The Rotol I believe would be the RX2 or RX5.

 

[Michael Hope]

No, not the 23EX - I'm looking for those fitted to single-engine fighters. I believe the early D.H. ones were Type P.3-5-5-7 two-position props which got converted in the field to constant-speed operation, or the D.H. 5-20.

The Rotol I believe would be the RX2 or RX5.

The DE Haviland propeller was similar to Hamilton Standard 3D40 type. It used counterweights to increase pitch and oil pressure to decrease pitch. If you lost oil pressure the propeller would go to full coarse pitch.

The Rotol RX5 I believe to be an external cylinder propeller like the one below. If it is a dual acting propeller it uses high pressure governor oil to increase pitch and then engine oil pressure with centrifugal twisting moment on the blades to decrease pitch. The type of blade, i.e. wooden or metal makes no difference to the operation, just differences in the operating pressures required to change the pitch.

 

[gecko]

The DE Haviland propeller was similar to Hamilton Standard 3D40 type. It used counterweights to increase pitch and oil pressure to decrease pitch. If you lost oil pressure the propeller would go to full coarse pitch.

The Rotol RX5 I believe to be an external cylinder propeller like the one below. If it is a dual acting propeller it uses high pressure governor oil to increase pitch and then engine oil pressure with centrifugal twisting moment on the blades to decrease pitch. The type of blade, i.e. wooden or metal makes no difference to the operation, just differences in the operating pressures required to change the pitch.

View attachment 599273

Thanks, that is great info. Yeah, I'm pretty sure I've seen pictures of the external cylinder for the Rotol. If that is the case would that mean it definitely is dual acting, or is that still in question? Would the governor boosted oil pressure cause pitch to increase faster than it decreases? Does it move to fine pitch with a loss of oil pressure due to the twisting moment?

 

[Michael Hope]

Thanks, that is great info. Yeah, I'm pretty sure I've seen pictures of the external cylinder for the Rotol. If that is the case would that mean it definitely is dual acting, or is that still in question? Would the governor boosted oil pressure cause pitch to increase faster than it decreases? Does it move to fine pitch with a loss of oil pressure due to the twisting moment?

I have touched and worked on these type of propellers but did not get involved with the pitch change oil supply What I read this morning, I believe they are to be the dual acting type, i.e. dual oil source, one the boosted oil pressure from the governor to low pitch, the engine oil pressure on the smaller diameter piston, the one with the rod in the cylinder. If all engine oil is lost, then engine would stop very shortly after, so the pitch would not really mater as it is now a very day for pilot.

The pitch change speed I can not say for sure if there would a difference, because of the very large forces of the twisting moment of the blades toward low pitch.

Below is the test that I have from an Air Ministry document from 1952.

 

[gecko]

I'm a bit confused now. In your first post, you say that boosted oil pressures increases pitch (toward coarse) and engine oil pressure and twisting moment decrease pitch. In your second post you say that boosted oil pressure and twisting moment decrease pitch. The first post makes more sense to me, but there's a lot I don't know.

One other question - the D.H. prop is mentioned in the Pilots Notes for the Spitfire and Hurricane to have a Positive Coarse Pitch setting (airscrew control fully back) where the blades are held in coarse pitch. The same paragraph mentions the Rotol prop, but it apparently does not have this setting. This has always confused me since I thought that having the control all the way back on either prop would effectively command 0 rpm thus ensuring the blades were held in the coarse setting. So how is this special setting on the D.H. different from what the Rotol does? Does the Rotol have a minimum governing rpm where the aft position governs the prop at some higher rpm rather than simply dumping pressure from the fine pitch side of the system?

 

[Michael Hope]

I'm a bit confused now. In your first post, you say that boosted oil pressures increases pitch (toward coarse) and engine oil pressure and twisting moment decrease pitch. In your second post you say that boosted oil pressure and twisting moment decrease pitch. The first post makes more sense to me, but there's a lot I don't know.

One other question - the D.H. prop is mentioned in the Pilots Notes for the Spitfire and Hurricane to have a Positive Coarse Pitch setting (airscrew control fully back) where the blades are held in coarse pitch. The same paragraph mentions the Rotol prop, but it apparently does not have this setting. This has always confused me since I thought that having the control all the way back on either prop would effectively command 0 rpm thus ensuring the blades were held in the coarse setting. So how is this special setting on the D.H. different from what the Rotol does? Does the Rotol have a minimum governing rpm where the aft position governs the prop at some higher rpm rather than simply dumping pressure from the fine pitch side of the system?

Propeller theory for pitch change:
The DHL propeller which uses counterweights to increase the blade pitch, is considered a single acting propeller, so that oil pressure is used to decrease the pitch. The Hamilton Standard equivalent propeller, model 3D40, only requires about 200 PSI maximum governor relief valve oil pressure to decrease the blade pitch. This is the same for almost all propellers that use counterweights to assist in pitch change, the counterweights cause the blades to go toward course blade pitch, and then oil pressure to decrease the blade pitch angle.

The Rotol external cylinder propeller does not have counterweights, it has oil on both sides of the piston, so it is a double acting propeller, the piston side with the larger surface area has the high pressure oil from the propeller governor to increase blade pitch angle, then uses engine oil pressure with the twisting moment of the blade towards low blade pitch. For a Hamilton Standard double acting propeller model 23D40, has a 300 PSI maximum governor relief valve oil pressure to increase the blade pitch and the engine would have a nominal pressure of 50 PSI when testing the propeller governor on the input side of the governor.

Sorry I can not answer your other question. The propeller control is the same for either type of propeller, fully in/forward for low blade pitch/high RPM, and fully out/rearward for high blade pitch/low RPM. The propeller governor is where the propeller pitch change sense is achieved, i.e. either oil pressure to increase or decrease pitch. The both propellers have a high/course pitch stops, in theory to absorb the maximum engine HP when the aircraft is put in a dive, without over speeding the propeller past its structural limits.

 

[gecko]

Thanks for the clarification. I'm pretty familiar with the principles behind a constant speed propeller, but the devil is in the details with these older systems. I'm curious if Positive Coarse Pitch setting on the D.H. prop contributed to the propeller freezing issues that were encountered. Often the cruise power settings involved high boost and low rpm combinations, and at high altitude this meant that the blades were governed pretty coarse. If it rested against the coarse pitch stop long enough it could freeze to it. The Rotols never had this issue, but it might be due to the Rotols allowing much coarser pitch and thus the stop wasn't ever reached.

Part of me also wonders if the Positive Coarse Pitch setting wasn't just a bit of PR on D.H.'s part to try to find some benefit from its shorter pitch travel vs. Rotol - "it's not a bug, it's a feature!"

The relevant paragraph from the Spitfire Mk.V pilot's notes:

 

[Tom Fey]

Rotol Propellers
de Havilland Propellers

Also, the natural blade twisting moment is towards flat pitch, not course pitch. Single action propellers use this natural moment to return passively to lower pitch. If the natural moment was towards course pitch there would be no such thing as a runaway prop (uncontrolled movement to low pitch and engine overspeed when control is lost; very high propeller drag; low thrust). The counter weights on the blades simply reduce the amount of power it takes to move the blades to course pitch.

 

[gecko]

Yes, I think that's what was said re the Rotol.

Everything I've read on counterweight props like the D.H. 5/20 indicates that the counterweights where the primary force moving towards coarse pitch when the governor releases oil from the piston. Are you saying the oil was also pushing towards coarse instead?

AIRCRAFT PROPELLER CONTROL AND OPERATION

I'm pretty sure runaway prop scenarios are usually due to governor failures, since if it was due to a loss of oil pressure and the prop naturally went to fine pitch, the lack of oil would cause the engine to seize in pretty short order.

 

[Tom Fey]

Edited. See below.

 

[Michael Hope]

Thanks. Yes, the governor has its own pump which boosts engine oil (50-70 psi) to approx. 300 psi and ports it to the prop dome/cylinder to move and hold the blades in a selected course pitch. Runaway props are indeed most often governor failures, often a broken drive, in which case the system no longer provides the proper control or high pressure oil to the dome and the blades, counter weighted or not, decay to flat pitch by centrifugal force.

A Hamilton Standard/De Havilland Counterweight type propeller will blow the cylinder cap if you put 300 PSI oil pressure into it, and the leather seals on the piston will probably leak also. Your statement only works for NON Counterweight blades in single acting propellers. In counterweight blades loose of oil pressure is always to coarse pitch.

 

[MiTasol]

This may help
I thought I still had the HS CW prop handbook but cannot find it. If I do I will scan and add

 

[Michael Hope]

Here are three figures of how the Hamilton Standard counterweight type propellers function. The De Havilland counterweight propeller functions in a similar manner.

 

[Tom Fey]

I stand corrected. Thanks.

 

[MiTasol]

I stand corrected. Thanks.

Hi Tom, maybe you have been caught by working on the Hartzel counterweight props on some turbine engines. The mantra on them is oil IN TO drive the blade to COARSE. It is easy to forget that the HS props are the opposite because they are pretty thin on the ground these days.

 

[Michael Hope]

Hi Tom, maybe you have been caught by working on the Hartzel counterweight props on some turbine engines. The mantra on them is oil IN TO drive the blade to COARSE. It is easy to forget that the HS props are the opposite because they are pretty thin on the ground these days.

The Hartzell as well as most other manufacturers, single acting type, with counterweights uses oil pressure to low/fine pitch, regardless of engine type, example is any aircraft with a P&WC PT-6A series engine when the propeller is stopped it is in a feathered position in most cases

 

[XBe02Drvr]

with a P&WC PT-6A series engine when the propeller is stopped it is in a feathered position in most cases

This is generally the case with a "free turbine" engine where the power turbine turns independently of the gas generator spool. The starter has an easy job if it only has to spin the gas generator shaft. Direct coupled engines like Garrett TPE731s or the big Allisons can't hope to swing a feathered prop on the starter, so have to shut down and start up in a low pitch setting.

 

[nuuumannn]

example is any aircraft with a P&WC PT-6A series engine when the propeller is stopped it is in a feathered position in most cases

Same on the other P&WC turbines, the PW100 series for example, all free turbine engines.