3 row radials: pros cons

[wuzak]

The Mustang is almost as good as a Spitfire and has the same Merlin so, that could be a contender?

Greg did say that the Merlin powered Mustang Strega is already faster than Rare Bear, but it hasn't done the record runs.

The MC72's 440mph was an average of 3 passes over either 1km or 1 mile (not sure which), and Rare Bear's would be the average of at least two passes over the same distance.

 

[wuzak]

FWIW, 5800hp with standard MC72 airframe (ie no drag reduction) gives approximately 542mph....













....with floats!

 

[Readie]

FWIW, 5800hp with standard MC72 airframe (ie no drag reduction) gives approximately 542mph....













....with floats!

Jeez...that is something.
Imagine trying to control that power !!

My hat off to the pilot

John

 

[wuzak]

Jeez...that is something.
Imagine trying to control that power !!

My hat off to the pilot

John

Sorry, that was just an estimate of performance using 2 V-1710s in an arrangement like the AS6. It hasn't been done.

The MC72 had a "mere" 3100hp when it set its record at 440mph.

 

[Aurum]

Thats quite interesting thread.
I also thought why there were almost non of designs of 3-raw radials (except Deerhound of A-S).

Now I think that such design cannot be balanced properly if it will have 7 cyls. in raw. And 5 cyls are too few because then in 3 raws we get 15 cyls that is almost the same as 14. And again it can no be balanced (I mean in the sense of ignition order).

If we have crankshaft with 120° angle between 3 cranks, we must have either 6 or 9 cyls in each raw to have proper balancing right ignition order. But 3*6 gives 18 which is much better to have in 2 raws with 9 cyls in each. And 9 cyls in 3 raws is definitely too much.

I did not see even example of such design in any of guide-books of piston aeroengine construction.

So 3-raw radial design seems is not convenient in the sense to have good balance right ignition order.

 

[tomo pauk]

Could you please elaborate why a 3x7 cylinder radial was not possible to balance properly? Was the Deerhound encountering any such issues?

 

[Aurum]

Because angle between raws does not correspond to the angle between cylinders

 

[tomo pauk]

What happens when the cylinders are in line?

 

[Aurum]

They can be NOT in-line as 2-raw or 4-raw engines are. But they must correspond to each other. To be in consequence!
Just calculate angles and balancing principle will be clear to you

 

[tomo pauk]

The 2-row and 4-row radials were in staggered layout; the Deerhound's rows were lined up one behind another.

I'll not pretend to be well versed in this issue, hence my question still remain

 

[Siegfried]

The 2-row and 4-row radials were in staggered layout; the Deerhound's rows were lined up one behind another.

I'll not pretend to be well versed in this issue, hence my question still remain

So are modern Lycomming, Continenatal Flat 4s, 6s and 8's and so were the Argus inverted V8's used on the Fiesler Storch. These sea fury's clearly don't have much intake for airflow. I'd say the oil coolers do much of the work.

 

[tomo pauk]

I'm not quite certain how that confirms, or it does not, the claim that it is impossible to properly balance a 3x7 radial.

 

[Shortround6]

Engines usually do better when there are an even number of degrees of crankshaft rotation between cylinders firing. for a 4 stroke you have 720 degrees of crankshaft rotation. A v-12 has a cylinder firing every 60 degrees of ration which works out rather nicely for a 60 degree V. The WW I Liberty used a 45 degree angle but then some of the cylinders fired at 45 degrees apart and other cylinders fired 75 degrees apart which made for some strange vibrations.

A 9 cylinder radial can fire every 80 degrees and the cylinders spaced at 40 degrees. An 18 cylinder doesn't seem to be a problem.
A 7 cylinder radial fires every 102.857.... Degrees and the cylinders are spaced at 51.42857..... degrees. the 14 cylinder two row works out rather well.

The two row radials have to have the crank pins offset to counter act each other.

Trying to get an even firing order AND a even cylinder spacing with the 3 row 21 gets a lot harder.

Single row radials tend to have the main rod assembly and the crankcase/cylinders moving in opposite directions at any given time. While the two row smooths that out it introduces a rocking couple. if the crankpins are 180 degrees out the front row of the engine is trying to move down while the rear row is trying to move up and vice versa. Trying a three row radial at 120 degree crank pin spacing may not work that well.

 

[tomo pauk]

Many thanks for the explanation

Back to the Deerhound :did it experienced any issues, apart the cooling (those were fixed, according to the Wikipedia)?

 

[Shortround6]

It seems to be one of those quirks of history that most engines, with just a few prototypes made, have ALL their problems solved JUST as the short sighted government officials cancel them while the engines chosen for production (and after passing type tests) require months, if not a year or more, of work, thousands of hours of testing and test engines by the handfuls in order to sort out problems that DID NOT show up in initial testing.

The R-2800 had 3000 hours of ground running before they ever stuck it in a test Mule aircraft.

 

[wuzak]

Thats quite interesting thread.
I also thought why there were almost non of designs of 3-raw radials (except Deerhound of A-S).

Now I think that such design cannot be balanced properly if it will have 7 cyls. in raw. And 5 cyls are too few because then in 3 raws we get 15 cyls that is almost the same as 14. And again it can no be balanced (I mean in the sense of ignition order).

If we have crankshaft with 120° angle between 3 cranks, we must have either 6 or 9 cyls in each raw to have proper balancing right ignition order. But 3*6 gives 18 which is much better to have in 2 raws with 9 cyls in each. And 9 cyls in 3 raws is definitely too much.

I did not see even example of such design in any of guide-books of piston aeroengine construction.

So 3-raw radial design seems is not convenient in the sense to have good balance right ignition order.

If you have 3 rows of 7 with the cylinders in line then the crankpins need to be at 0°, 137.14° and 274.29° for even firing.
If you have 3 rows of 9 cylinders with in-line cylinders you need them at 0°, 106.67° and 213.33°.
If you have 3 rows of 5 cylinders with in-line cylinders you need them at 0°, 96° and 192°.

If you use 120° crank you will get 3 sets of firing close together followed by a wider space before the next firing - for 5 and 7 cylinders per row. For 9 cylinders per row you have one cylinder from the front row and one from the back firing at the same time, and one from the middle row evenly spaced between them.

For a two row radial if the cylinder are in line you can use a 180° crank to get even firing. But if you stagger them to maximise cooling (for an air-cooled engine) you have to offset the rear crankpin, otherwise you'll end up with 2 cylinders firing at once - one from each row.

 

[wuzak]

It seems to be one of those quirks of history that most engines, with just a few prototypes made, have ALL their problems solved JUST as the short sighted government officials cancel them while the engines chosen for production (and after passing type tests) require months, if not a year or more, of work, thousands of hours of testing and test engines by the handfuls in order to sort out problems that DID NOT show up in initial testing.

The R-2800 had 3000 hours of ground running before they ever stuck it in a test Mule aircraft.

To be fair, Tomo said that the cooling problems had been fixed - after a major rework and a total redesign. He did not say all the problems were solved - only a handful of Deerhounds were ever made, so it is unlikely, given that they wer erestricted by the cooling problems, that they discovered all the problems.

 

[Shortround6]

I am not picking on Tomo, and we don't KNOW that the problems had been fixed, we know that they CLAIMED the problems were fixed. How many hours of testing in the air did the Deerhound get before the crash, which was NOT a fault of the engines?

Bristol Taurus suffered from overheating as the did the Hercules in some installations and certain flight conditions. Certain liquid cooled British engines were judged as being acceptable for use in England but not for use in the tropics. These are engines that went into service.

I am picking on a number of both allied and axis engines that were canceled however.

 

[wuzak]

I believe only the Deerhound I flew. The II and III never did. The one and only III aparently survived until the '70s before being scrapped (bastards!).

 

[wuzak]

btw, if you stagger the cylinders of teh 3 row radial evenly, you can use the 120° crank and get even firing intervals.