Technical Teaser

RE: Technical Teaser

Hi Christer, It’s never been a problem I have encountered very much. Radials slow run around 600-700 RPM, and Max at around the 3,000 RPM area. Power pulses are damped by having a large counterweighted crankshaft with floating steel balls called solomon dampers, these change position with varying centrifugal force. The clever guys who design these things have eliminated most of the problems. All radials since around the mid 1930’s have a bevel epicyclic reduction gear at the front that drops the propshaft speed by about 3:1, but this is to keep the prop tip speed subsonic. Hope this answers your question.
KeithMac.

RE: Technical Teaser

Hi again Keith!
Thanks for the latest information, I´ve got one last question, at least for the time being:

Everything that is rotating has a critical RPM where vibration is the worst due to resonance. E.g. an electrical motor is accelerated to a RPM above the critical value and is operated well above.
Some horisontally opposed engines in general aviation aircraft have a narrow RPM range, say 2.300-2.350 RPM where it must not be run which, I assume, is due to the above reason.
What´s the situation on radials?

Thanks in advance,
Christer

RE: Technical Teaser

Hi Christer, Ouch! Explaining this one is difficult. I always had a model to help. There is a difference, but it’s mostly in ignition and valve timing as opposed to compression ratio. The master rod follows a circular track as the crankshaft revolves, the articulating rods follow an ellipitical path, so there is a few degrees of error. However everything is set up on the master rod and the errors, which are very small, are ignored for all practical purposes. All radial engines have a master rod and articulating rods, it’s the only way to connect them to the crankshaft. Trying to show how it works on a diagram takes a lot of imagination, if I had an animated graphic it would be as clear as day, unfortunately I’m not clever enough to produce one!

RE: Technical Teaser

Keith,
thanks for the tip about the book.
Bill Gunston has been quite busy as an author and I believe that he has written two books on engines, the first less and the second more comprehensive!? If I´m correct I´ll opt for the second one!

Next question:
The system with one master connecting rod and six articulated connecting rods per row (if we stay with the Hercules, that is), do that give a different compression ratio for the cylinder with the master rod compared to the others?
If not, how are the articulated rods arranged to eliminate the difference in throw which seem obvious, at least from the crude drawings that I´ve seen?

Thanks,
Christer

RE: Technical Teaser

Hi Christer. Everything you have on hydraulic lock, or “Hydraullicking” as we called it is basically correct. Normal procedure was to pull the prop through at least 2 revs in the normal direction of rotation, feeling for a lock as you went. If you got an abnormal amount of resistance, then you pulled the lower cylinder plugs to drain any accumulated oil. It was standard procedure on all radials. There was more than one engine which failed because this procedure was not carried out prior to the engines being started. On quite a few occasions the damage was not noticed on start up, then a cylinder would come off in flight. If you go to HTTP://members.tripod.co.uk/esar/accidents.html you’ll see what happened to Pembroke WV737 when the crew failed to carry out the check on a “landaway”. If your interest is in piston engines I can highly recommend “Piston Aero Engines” by Bill Gunston, it’s one of the best books on the subject.

RE: Technical Teaser

Hi Chappies, it’s uncle KeithMac. Christer has pretty much answered Halibags question on the Hercules, so I’ll not steal his thunder. I’ll only add that it’s 1 then 10 because the crankwebs are at 180 deg to one another. As for the Twin Wasp, it works on the same principle. The Gannet however is not a piston engined aircraft! It has an engine called the Double Mamba. It is in fact two turbopops independently feeding power to a gearbox, which output on co-axial shafts. The props rotate in opposite directions to cancel out torque reaction on the aircraft. Both engines are completely independent of one another and can be shut down and the props feathered separately. Hope that’s cleared up that little question. Christer,you talked about Hydraulic Lock, Ill answer that separately on another post.

RE: Technical Teaser

Ok Christer, I have two engines , that I’d like to know the workings of, the First is the Twin Wasp radial and the second it the Gannet ASW a/c, might be time for old uncle Keithmac to step in!

Per Ardua Ad Astra!

RE: Technical Teaser

Halibag,
this is a little trickier due interaction between the two rows but this is how I figured it out and I´m sure Keith will correct me if I´m wrong:

First cylinder numbering, they are numbered in sequence of crankshaft rotation. This means that the rear row will have all odd numbers and the front row all even numbers.
The firing order would be: 1, 10, 5, 14, 9, 4, 13, 8, 3, 12, 7, 2, 11, 6.
The camlobes rotate at 1/8 of engine speed in the same direction as the crankshaft (approximate firures: 51.43º between cylinders * 2 = 102.86º – 1/8 * 102.86º = 90º)

Now it´s time for Keith and the corncob, don´t You think?

Christer

RE: Technical Teaser

[updated:LAST EDITED ON 22-01-02 AT 02:45 PM (GMT)]Keith,
good books on narrow topics are hard to get in Sweden. If specialised book stores should get one or a few copies it´s likely that you miss them.
My reference work has been the internet. Several hours of browsing and mouse clicking ……
The greatest resource on detailed engine description I´ve found, so far, is on the Aircraft Engine Historical Society homepage:
http://www.enginehistory.org/
This site was launched quite recently and my hopes for an abundance of information are high.

A few questions about hydraulic lock:
How was this dealt with in service?
What were the odds for getting in trouble if you just fired up with no precaution taken?
Were the spark plugs pulled on the lower cylinders to drain the oil as a routine measure during pre flight inspection?

I read somewhere that pulling the propeller backwards wasn´t a good idea (with the spark plugs fitted) since the oil would be forced into the induction manifold, promptly returning when the engine was started. Pulling the engine forward wasn´t good either since the force you can apply is very high considering the leverage. It´s easy to bend a con rod that way but, pulling the engine with the starter, ignition off, would only shear off a pin in the gear train if there was hydraulic lock.
Anyhow, this couldn´t be an alternative for the Centaurus or Hercules with no valves in the cylinder heads?

Christer
(I´ll be back …)

(The forum engine seems to sort the replies randomly?, but after this comment I´m sure it will go where it belongs!)

RE: Technical Teaser

As I’m not allowed to have any fun anymore, I might as well add this poser…. What’s the firing order of a Bristol Hercules 14 cylinder double bank sleeve valve radial engine then?

TTFN

RE: Technical Teaser

Hi Christer, yes I do remember or discussion on the relative merits of sleeve and poppet valve engines. You’ve obviously been hard at work on the radial engine. What have you been using as a reference work? If theres anthing I can help with, fire away. That’s what these sites are supposed to be for are they not!

RE: Technical Teaser

Keith,
Your timing was perfect. Do You remember that we had a discussion a while back about the reliability of the Centaurus? I´ve done some home work since then and I´m still having it fresh in my memory. 🙂
There are a few questions though, that I`d like to ask someone so, let me know if You don´t mind!

Christer

RE: Technical Teaser

Christer, You cracked it, well done, go to the top of the class, I can see I’ll have to make them more difficult in future, how about a 28 Cylinder, four bank radial ?. On second thoughts, perhaps not!!!!

RE: Technical Teaser

Hello uncle Keith!
Thanks for an interesting topic.

I´ll try to explain this as simple as I can, with emphasis on try.
The bit about simple, well, if it gets any more complicated I´ll bail out.
Anyway, this is how I believe it works:

Radials could be said to fire and skip and fire ……
to make the firing order 1-3-5-7-9-2-4-6-8 in two revolutions.
With 9 cylinders you have 40º between them. Skipping every second cylinder when firing gives 80º of crankshaft rotation but, the camlobes rotate at 1/8 enginespeed in the opposite direction which gives an additional 10º to make it 90º.

If #1 is at TDC, between compression and combustion strokes then …
#2 is on exhaust
#3 is on compression
#4 is on exhaust
#5 is on compression
#6 is on combustion
#7 is on intake
#8 is on combustion
#9 is on intake

Now it´s up to You to tell me exactly what size of a fool I am!

Regards,
Christer

RE: Technical Teaser

OK Halibag, now your cookin’. You know the four stroke cycle, that’s where to start! Remember it takes TWO revs of the crankshaft to complete your Suck Squeeze Bang Blow cycle. I know you know the answer!!

RE: Technical Teaser

Is it “Suck Squeeze Bang Blow” to the power of nine?
Halibag

RE: Technical Teaser

they go round clockwise, and each is in varying stages of going up until it reaches the top, then the cycle continues.

PURE GUESS so prob wrong!!

coanda

RE: Technical Teaser

Halibag, you little rascal, you don’t escape that easily. What the other engines are doing has no effect you know that! The nationality of the engines makes no difference either, japanese and german pistons know how the engine works. But i’ll concede the point on direction of rotation. So it’s clockwise viewed from the front of the aircraft, now get on and find me an answer!!!!!!!
Uncle keithmac.

RE: Technical Teaser

A-haaa! This is a trick question! it depends upon how many engines the aircraft has, are the others turning, what the rev’s on each engine are & how many blades are fitted to each engine, after that, it would then depend on is it a British aircraft, American aircraft, Japanese aircraft or a German one, and even worse, does it turn clockwise or anti clockwise…. Haa! you can’t trick us THAT easily!!
🙂