Advance Ratios and Gear Ratios

[Zipper730]

I was posting a thread on WWII aircraft rate of turns and I started computing up a rate of advance ratios which are helpful in determining thrust levels for sustained turns. That said I was planning on listing a variety of fighter planes. At first I computed everything on the premise of propeller RPM and engine RPM being the same thing, which turns out to be totally wrong in most cases.

I'm not really sure if the gear-ratios are varied in different aircraft with different propellers or just the same across the board. I don't know if there are any rules of thumb used or even where to go and find such esoteric material.

If I can provide the aircraft, can somebody help me clarify these things?

 

[pbehn]

There are a few things that are significant, the prop tip cannot go supersonic, the fewer blades there are the better and the aircraft must be able to take off and land.

 

[Zipper730]

There are a few things that are significant, the prop tip cannot go supersonic, the fewer blades there are the better and the aircraft must be able to take off and land.

This I already know. I guess it's possible to calculate tip-speed based on RPM -- I'd almost swear I saw it come up on this forum somewhere actually, though I'm not sure what the norm was based on the time.

From what it appears K.5083 (prototype) used a Merlin C driving a propeller that is presumably 11.03" (though I could be wrong here). I know nothing about the Merlin-C; the early Mk.I's were using the Merlin II with a twin-pitch prop of 11.03" in diameter

 

[kmccutcheon]

If I can provide the aircraft, can somebody help me clarify these things?

I can help with American engines. Please supply a list of the engine models, including the dash numbers (i.e., R-1820-97), for the aircraft that interest you.

 

[pbehn]

This I already know. I guess it's possible to calculate tip-speed based on RPM -- I'd almost swear I saw it come up on this forum somewhere actually, though I'm not sure what the norm was based on the time.

From what it appears K.5083 (prototype) used a Merlin C driving a propeller that is presumably 11.03" (though I could be wrong here). I know nothing about the Merlin-C; the early Mk.I's were using the Merlin II with a twin-pitch prop of 11.03" in diameter

Not for the first time I don't know what you want, I though I gave you the ratios for the Merlin?

 

[Michael Hope]

Some engine models had both a direct drive, i.e. 1 to 1 engine to propeller RPM, and then a different dash number was geared to a different ratio. Example the P&W 1340 series, as used in the Harvard/Texan/AT6 used a direct drive, but when the engine was later used in DHC-3 Otter it was a geared version with a longer diameter propeller. Even the Merlin had different drive ratios based on the dash of the engine and the propeller it is turning, example the Canadair DC-4M with the early four bladed propeller was one ratio, when they switched to the three bladed propeller they also slightly changed the gear ratio with a different dash number of the engine.

 

[Zipper730]

I can help with American engines. Please supply a list of the engine models, including the dash numbers (i.e., R-1820-97), for the aircraft that interest you.

Sounds great... this is going to be a long list but here it goes

• R-1670
  • XR-1670-5 (long-shot, it was in the early prototype stage for the P-36)
• R-1820
  • R-1820-39/R-1820-G-5
• R-1830
  • R-1830-13
• V-1710
  • V-1710C
  • V-1710-17
  • V-1710-19
  • V-1710-27/29 (YP-38/P-38E)
  • V-1710-33
  • V-1710-35
  • V-1710-37
  • V-1710-39
  • V-1710-49/53
  • V-1710-51/55
  • V-1710-63
  • V-1710-89/91
  • V-1710-111/113

I might have some more to add later on assuming you don't react to this request with "holy shit, you're fucking nuts man!"

Not for the first time I don't know what you want, I though I gave you the ratios for the Merlin?

So regardless of the exact propeller, the gear-ratio is exactly the same? The engines in question were the Merlin C, Merlin II and Merlin III since I'm basically covering the Hurricane Prototype and the early Mk. I at this time.

 

[pbehn]

So regardless of the exact propeller, the gear-ratio is exactly the same? The engines in question were the Merlin C, Merlin II and Merlin III since I'm basically covering the Hurricane Prototype and the early Mk. I at this time.

As far as I can see the ratios weren't changed in the early days. The early Spitfire prototype had its prop changed when it was slower than expected (and I believe slower than the Hurricane).

 

[fubar57]

"holy shit, you're fucking nuts man!".

Nice

 

[MiTasol]

I'm not really sure if the gear-ratios are varied in different aircraft with different propellers or just the same across the board. I don't know if there are any rules of thumb used or even where to go and find such esoteric material.

They vary widely

The attachment will provide some of what you want for American engines but it is 1950 so some engines will be missing.

The data you want is in the third row of the third last column.

Some missing P&W data may be in the attached PW page


For other engines I would suggest find a library which has a Janes AWA from the years the engine you want was produced and look in the engine section of that

 

[kmccutcheon]

Ratios for the American engines your requested:
XR-1670-5 = 16:11
R-1820-39/R-1820-G-5 = 16:11
R-1830-13 = 3:2
V-1710C = 2:1
V-1710-17 = 1.8:1
V-1710-19 = 2:1
V-1710-27/29 (YP-38/P-38E) = 2:1
V-1710-33 = 2:1
V-1710-35 = 1.8:1
V-1710-37 = 1.8:1
V-1710-39 = 2:1
V-1710-49/53 = 2:1
V-1710-51/55 = 2:1
V-1710-63 = 2:1
V-1710-89/91 = 2:1
V-1710-111/113 = 2:1

I might have some more to add later ...

Glad to help.
Here are some links that will assist this kind of research:
https://www.enginehistory.org/References/ModDesig/moddesig.shtml
https://www.enginehistory.org/reference.shtml (scroll to page bottom)

 

[Zipper730]

They vary widely

The data you want is in the third row of the third last column.

So the variances are all listed here for aircraft that are listed?
View attachment 522554[/quote]Just to be clear -- these numbers apply for every application of the engine?

For other engines I would suggest find a library which has a Janes AWA from the years the engine you want was produced and look in the engine section of that

That's good to know if I can find it...

 

[MiTasol]

So the variances are all listed here for aircraft that are listed?
View attachment 522554

Just to be clear -- these numbers apply for every application of the engine?
That's good to know if I can find it...[/QUOTE]

Yes - model x-nn engine will always have an x:y reduction gear ratio, regardless of which aircraft it is fitted to

 

[PStickney]

I was posting a thread on WWII aircraft rate of turns and I started computing up a rate of advance ratios which are helpful in determining thrust levels for sustained turns. That said I was planning on listing a variety of fighter planes. At first I computed everything on the premise of propeller RPM and engine RPM being the same thing, which turns out to be totally wrong in most cases.

I'm not really sure if the gear-ratios are varied in different aircraft with different propellers or just the same across the board. I don't know if there are any rules of thumb used or even where to go and find such esoteric material.

If I can provide the aircraft, can somebody help me clarify these things?

There are a few things that are significant, the prop tip cannot go supersonic, the fewer blades there are the better and the aircraft must be able to take off and land.

Actually, prop tips are often supersonic. The irritating snarl of the direct-drive R1340 on a T-6/SNJ is due to the prop tips going supersonic, and the tips encountering the shock wave from the preceding blade.
On a hot day, when th speed of sound is higher than usual, you can hear the sound change during the takeoff roll as the prop tips go supersonic.
Most WW2 era fighters will end up with supersonic tip speeds at the upper right order of their flight envelopes.

I was posting a thread on WWII aircraft rate of turns and I started computing up a rate of advance ratios which are helpful in determining thrust levels for sustained turns. That said I was planning on listing a variety of fighter planes. At first I computed everything on the premise of propeller RPM and engine RPM being the same thing, which turns out to be totally wrong in most cases.

I'm not really sure if the gear-ratios are varied in different aircraft with different propellers or just the same across the board. I don't know if there are any rules of thumb used or even where to go and find such esoteric material.

If I can provide the aircraft, can somebody help me clarify these things?

 

[PStickney]

I was posting a thread on WWII aircraft rate of turns and I started computing up a rate of advance ratios which are helpful in determining thrust levels for sustained turns. That said I was planning on listing a variety of fighter planes. At first I computed everything on the premise of propeller RPM and engine RPM being the same thing, which turns out to be totally wrong in most cases.

I'm not really sure if the gear-ratios are varied in different aircraft with different propellers or just the same across the board. I don't know if there are any rules of thumb used or even where to go and find such esoteric material.

If I can provide the aircraft, can somebody help me clarify these things?

In addition to that data, if you wish to find propeller efficiency and net thrust without diving into the Mathematical Rabbit Hole, download and check out "Propeller Performance Charts for Transport Airplanes", NACA-TN-2966
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930084064.pdf
Using this, you'll also be factoring in Activity Factor, Number of Blades, Single or Dual Rotation, and Mach Number effects.
Note that this only really works for infinitely variable (Constanwt Speed props, basically) which can provide peak efficiency over a wide range of altitudes and Advance Ratios. Fixed Pitch or 2-Position props are most efficient at a single combination of Airspeed, Altitude and Power Setting. (Well, 2 points for a 2 position prop.)The efficiency falls off sharply in other conditions.
This will have a big effect on Early Spitfires and Hurricanes, Most Ki-43s and others of that ilk.

 

[Shortround6]

I would also note that many times the two position prop was used as follows.
Put in fine pitch, start take-off run, clear runway and retract landing gear, shift propeller to coarse pitch leave it there until the plane is back on the ground!!!!

Spitfires for instance switched to coarse pitch once they hit 140 IAS right after take-off, this is below best climb speed. Climb was done in course pitch.

I would also note that you need to find out the actual pitch of of the props, some British constant speed props only had 20 degrees of travel while some others (and many american props) had 30 degrees of pitch adjustment.

You had a real good propeller if you were getting 80% efficiency from it in most flight regimes so spending a crap load of time figuring out some of this stuff might not actually tell you mutch.

 

[Michael Hope]

I would also note that many times the two position prop was used as follows.
Put in fine pitch, start take-off run, clear runway and retract landing gear, shift propeller to coarse pitch leave it there until the plane is back on the ground!!!!

Spitfires for instance switched to coarse pitch once they hit 140 IAS right after take-off, this is below best climb speed. Climb was done in course pitch.

I would also note that you need to find out the actual pitch of of the props, some British constant speed props only had 20 degrees of travel while some others (and many american props) had 30 degrees of pitch adjustment.

You had a real good propeller if you were getting 80% efficiency from it in most flight regimes so spending a crap load of time figuring out some of this stuff might not actually tell you mutch.

The counterweight type propellers both British and US had similar ranges, about 20 degrees of travel. When they went to the Hydraulic or Electric pitch change is when they got a larger travel range, for both British and US propellers.

 

[Zipper730]

In addition to that data, if you wish to find propeller efficiency and net thrust without diving into the Mathematical Rabbit Hole, download and check out "Propeller Performance Charts for Transport Airplanes", NACA-TN-2966
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930084064.pdf
Using this, you'll also be factoring in Activity Factor, Number of Blades, Single or Dual Rotation, and Mach Number effects.

WITHOUT diving into the mathematical rabbit hole? If that's without diving into it, I shudder to think what is !

This will have a big effect on Early Spitfires and Hurricanes, Most Ki-43s and others of that ilk.

Yeah but the WW2 Aircraft Performance page largely has the TAS/IAS figures and the RPM data. I figured I was just going to compute the RPM for the engine if I could find the reduction gear ratio for the Merlin C and Merlin II; then simply use the regular advance-ratio formula.

 

[PStickney]

WITHOUT diving into the mathematical rabbit hole? If that's without diving into it, I shudder to think what is !
Yeah but the WW2 Aircraft Performance page largely has the TAS/IAS figures and the RPM data. I figured I was just going to compute the RPM for the engine if I could find the reduction gear ratio for the Merlin C and Merlin II; then simply use the regular advance-ratio formula.

It's not that bad - there are some really useful graphs in there that make things fairly painless.
As to the simple computation - for a Constant Speed Prop, that's feasible, since it's continuously changing pitch to maintain its set RPM over as wide a range of conditions as possible.
Consider that Horsepower is, basically, the product of Torque (Manifold Pressure/Boost) and Rotational Speed (RPM).
With a fixed-pitch or 2 pitch (Which can be considered 2 fixed-pitch props) if you're not at the Sweet Spot of altitude and airspeed, you run into either the upper limit of Manifold Pressure or RPM, and in either case, you're not developing the power (and thus thrust) listed in the engine specs.
Take a look at the Flight Test Reports for the Spitfire Mk I on World War II Aircraft Performance WWII Aircraft Performance
It has reports for Fixed-Pitch (Watts), 2-Pitch, and Constant Speed props for the same airframe and angina.
You can see how the engine-propeller combinations interact.

 

[Zipper730]

It seems the Merlin C and Merlin II's gear ratios were 0.477. There was an earlier engine, the Kestrel which was 0.55