Author Topic: freelance radial aero engine  (Read 1485 times)

Offline ackers

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freelance radial aero engine
« on: October 29, 2021, 01:30:23 PM »
posted by ackers.
I describe my engine as 'inspired by' the Bristol poppet-valve radials of the 1920s and 30s, though it is not a model of a particular engine. I live in Bristol U.K. and the engines and airframes of the Bristol company are a big part of our industrial heritage. The engine is to my own design, and the keynotes were simplicity and ease of manufacture; my life was busier then. It is a five-cylinder radial 4-stroke petrol engine, coil ignition, with displacement of 5cc per cylinder giving a grand total of 25cc. It is therefore quite a small engine. When I designed it I did not actually know that the Bristol range of engines included a 5-cylinder radial called the Titan, but it turns out that the bore and stroke of my engine. 18mm and 20mm respectively, are close to 1/8 of the Titan dimensions. The compression ratio is nominally 8:1.

Almost all bearings are plain and oil is supplied at low pressure to the mains, cam sleeve and big end from a gear pump. Everything else in the bottom end is splash lubricated. Between the lower two cylinders is a sump which collects oil from the crankcase and returns it to an external tank. There is no oil feed to the rocker gear but oil galleries in the rocker bearing blocks are filled by oil-can before every run, and together with a drop of oil to each valve guide this has been adequate. There is a ring manifold distributing mixture to the cylinders from a single carb behind and below the crankcase.

In front of the crankcase sits the cambox. The engine uses the traditional cam rings to operate short radial tappets. The cam drive departs from usual radial practice because I could not find a neat gear arrangement that would take the drive from the front mainshaft (partly constrained by available gears) so I used a cross shaft from the rear gearcase instead. I'll do better next time. The engine drives an 18"x10" airscrew directly.

Behind the crankcase is the gear case which provides a half-speed shaft to the rear of the engine. There is also an input shaft for a starter and provision for driving a magneto yet to be tried. The half speed shaft drives the oil pump, contact breaker and distributor. Ignition timing is manually variable from about 40 deg BTDC to 5 deg ATDC by rotating the c.b. and distributor unit on its mounting.

The starter drive has been used to try out various starting means and currently an experimental inertia starter is fitted. This is visible behind the engine mounting plate.

How does it run? it is a mixed bag. When everything is nicely set up and warmed up, c.b. points clean and so on, it starts remarkably easily with a flick of the prop. In fact on a good day it will start pretty reliably on the inertia starter. From cold it needs to be flicked over a few compressions while choked to get some fuel sloshing around the manifold and it wants "fully rich" mixture to run at idling speeds. With the ignition retarded it will idle well at rpms in the high hundreds. With ignition advanced, as the throttle is opened the mixture leans out and the engine comes on song. With the current carb there is a sweet spot of throttle opening at which the engine runs smoothly on all cylinders at around 3500 to 4500 rpm. With other carb arrangements the engine has been run on song at 6000 rpm, the design speed, at which speed it develops a little over one horsepower. But this was difficult to achieve, mostly I think due to mixture distribution problems. I have yet to find a carb design that will allow anything like smooth running from idling to full throttle.

There is another major shortcoming of my engine; the oil control is terrible and it sprays embarrassing quantities of oil from the exhaust pipes. I think this is due mainly to over-supply of oil by the pump, and I am starting some tests to investigate ways to control this. This needs to be done carefully since I know that some positive oil feed is necessary; I have seized the engine solid on one occasion when I did not notice an empty oil tank.

I have some videos of the engine starting and running on Youtube. Just search for "model engine inertia starter". The first is normal speed, the second is half-speed slow motion which makes it sound even better, and the third is 1/10 speed which makes visible some interesting detail such as the reluctance of the engine to go over the first compression when the starter engages.

I will post some more when I have learned a bit more about the mechanics of including images etc - I'm a slow learner.

Additional note 01-01-2021: Thanks for the advice on adding the videos, I'm going to try it now:

full speed   <a href="https://www.youtube.com/watch?v=Gky8z7sTEEs" target="_blank">http://www.youtube.com/watch?v=Gky8z7sTEEs</a>

1/2 speed   <a href="https://www.youtube.com/watch?v=V-6ieebbWko" target="_blank">http://www.youtube.com/watch?v=V-6ieebbWko</a>

1/10 spd    <a href="https://www.youtube.com/watch?v=DlsGvW72jrM" target="_blank">http://www.youtube.com/watch?v=DlsGvW72jrM</a>

« Last Edit: November 01, 2021, 03:24:55 PM by ackers »

Offline RReid

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Re: freelance radial aero engine
« Reply #1 on: October 29, 2021, 02:23:03 PM »
That's a great looking engine. Very impressed! :ThumbsUp:

Is that pushrod/rocker arrangement characteristic of the Bristols?
Regards,
Ron

Offline Vixen

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Re: freelance radial aero engine
« Reply #2 on: October 29, 2021, 03:02:54 PM »
Hello ackers,

Thats a great looking engine. thanks for posting it on the forum. I would appreciate more photos of the inertia starter, when you are ready.

I have a 5 cylinder Seidel radial engine. It has no pumped oil system. Instead it uses 10% oil mixed with the fuel, there is enough piston blow-by to ensure the bottom end is adequately lubricated.

Your push rod and floating rocker arrangement are almost identical to those on my Bristol Mercury and Bristol Jupiter engines.

Mike
It is the journey that matters, not the destination

Sometimes, it can be a long and winding road

Offline ackers

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Re: freelance radial aero engine
« Reply #3 on: October 29, 2021, 05:18:52 PM »
That's a great looking engine. Very impressed! :ThumbsUp:

Is that pushrod/rocker arrangement characteristic of the Bristols?

"based on an idea by" is probably the best way to put it. It is certainly meant to look similar to the Bristol designs, though the best known of the poppet valve engines are probably ones like the Mercury with four valve heads. In detail, the Bristol designs had the rocker bearing blocks secured to the head at the rear, but located at the front by the pushrod tunnels which attached to the cam/ tappet casing. This compensated for thermal expansion of the cylinders (I think). Very clever; without that feature the tappet clearances opened up as the engines warmed up. I hope I have that right! I haven't bothered with pushrod tunnels but have used the general layout.
Regards, ackers

Online Kim

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Re: freelance radial aero engine
« Reply #4 on: October 29, 2021, 05:19:48 PM »
Neat-looking engine! 

You can embed videos by simply pasting the youtube link directly in your post.  No tags required.  Try it!  We'd love to see the videos.

Kim

Offline Admiral_dk

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Re: freelance radial aero engine
« Reply #5 on: October 29, 2021, 09:16:36 PM »
I found it - but not with quotes ...!

<a href="https://www.youtube.com/watch?v=Gky8z7sTEEs" target="_blank">http://www.youtube.com/watch?v=Gky8z7sTEEs</a>

Sounds and runs very nicely  :ThumbsUp:

Offline Zephyrin

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Re: freelance radial aero engine
« Reply #6 on: October 29, 2021, 11:11:18 PM »
Wow, a very impressive demo, beautiful engine, and fine runner !
congratulation for the personal design too.

Offline Craig DeShong

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Re: freelance radial aero engine
« Reply #7 on: October 30, 2021, 01:05:19 AM »
Very nice !  if my LeRhône runs half that well I'll be thrilled.
Craig
The destination motivates us toward excellence, the journey entertains us, and along the way we meet so many interesting people.

Offline Trevorc

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Re: freelance radial aero engine
« Reply #8 on: October 31, 2021, 07:36:03 PM »
Fantastic! That is all I can say.
Trevorc

Offline Roger B

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Re: freelance radial aero engine
« Reply #9 on: October 31, 2021, 07:51:36 PM »
The engine looks excellent but I can't see the video (usually my problem  ::) )
Best regards

Roger

Online Kim

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Re: freelance radial aero engine
« Reply #10 on: October 31, 2021, 08:37:45 PM »
Some browsers don't seem to like the "[ youtube1][ /youtube1]" tags.
The way that seems to work for all browsers is to just include the link WITHOUT any tags, like so:

<a href="https://www.youtube.com/watch?v=Gky8z7sTEEs" target="_blank">http://www.youtube.com/watch?v=Gky8z7sTEEs</a>

This is the same link that Per posted for ackers above (but with the tags removed).

Kim

Offline Admiral_dk

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Re: freelance radial aero engine
« Reply #11 on: October 31, 2021, 10:02:56 PM »
 .... and I never noticed the Youtube Icons before they were mentioned - so I had to try them .... oh well ....

I hope that Kim just have rescued the enjoyment for some of you  :ThumbsUp:

Offline 90LX_Notch

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Re: freelance radial aero engine
« Reply #12 on: October 31, 2021, 10:07:21 PM »
askers-

That is magnificent.  I love this engine.  Nicely done.

-Bob
Proud Member of MEM

My Engine Videos on YouTube-
http://www.youtube.com/user/Notch90usa/videos

 

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