Author Topic: An engine running bench  (Read 9846 times)

Offline Ramon Wilson

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Re: An engine running bench
« Reply #15 on: August 09, 2014, 11:46:45 AM »
Well whadya know - I've been using 'Tygon' tube for some time now  :o Same stuff as PAW sells - I just had no idea that that's what it is called.

Thanks Bert ( that site is a bit of a nostalgia trip  ;)) and you too Jason for the links.

I remembered this source too - long time since I used it but the tube was clear when I bought it last - look under Sundry Equipment
http://www.freeflightsupplies.co.uk/

Sailing's off today - far too windy - so I've made a tap and now making the gas canister valve ;)

Ramon
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(a very apt phrase - thanks to a well meaning MEM friend)

Offline Ian S C

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Re: An engine running bench
« Reply #16 on: August 09, 2014, 12:53:49 PM »
For my hot air motors I have two tables. They are made with the legs from old folding ironing boards, the top replaced with ply wood about 10 mm thick.  The top is about the same area as the one shown by Ramon, this might be ok for a small IC engine.  Ian S C

Offline Allen Smithee

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Re: An engine running bench
« Reply #17 on: August 09, 2014, 02:00:38 PM »
For ordinary model aero engines your bog-standard engine test stand looks something like the picture below - one side fixed and the other adjustable to cope with different sizes and the sort of thing you could knock up in a mill from whatever gash metal you had lying around in an hour without breaking into a sweat. This is usually screwed to a bit of blockboard which is then gripped in a workmate for use. The blockboard is typically about 12" long by 4"- 6" wide and would usually have four cuphooks screwed into it so that the fuel tank can be put on top of a bit of sponge and held down with elastic bands (this allows the height to be adjusted and reduces frothing of fuel in the tank from the vibration). My motors were mostly piped (ie they used tuned-pipe superchargers) and so th.ere would be a pipe mount as well, but you may not need this. If there is a throttle control the cunning plan is to operate it with a bit of wire (usually 2mm soft iron bike spoke) that is threaded though the elastic bands on the fuel tank for a bit of friction (I always prefer a little bit of friction).

Tank heights aren't that critical on the test stand - most engines will happily suck fuel through a vertical inch or two even without silencer/pipe or crankcase pressure. In an airframe it's a different matter, but the issue is the *variation* in fuel head affecting the mixture rather than the ability to pull the fuel to to carb. On the test stand a lower tank is prefered to a higher one as too much head results in stuff spurting all over the place.

AS
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Offline Jo

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Re: An engine running bench
« Reply #18 on: August 09, 2014, 02:02:17 PM »
 :embarassed: Pete, You forgot your attachment.

Jo
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Offline Allen Smithee

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Re: An engine running bench
« Reply #19 on: August 09, 2014, 02:09:35 PM »
Oh gosh - all that focus on the problems of having too much head and the attachment went clean out of my mind....

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Offline Ramon Wilson

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Re: An engine running bench
« Reply #20 on: August 09, 2014, 03:39:21 PM »

In an airframe it's a different matter, but the issue is the *variation* in fuel head affecting the mixture rather than the ability to pull the fuel to to carb.

Are you not familiar with the 'uniflow' tank set up Allen? Gives a very consistent run without pressure too.

Ramon
"I ain't here for the long time but I am here for a good time"
(a very apt phrase - thanks to a well meaning MEM friend)

Offline Allen Smithee

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Re: An engine running bench
« Reply #21 on: August 09, 2014, 04:17:41 PM »

In an airframe it's a different matter, but the issue is the *variation* in fuel head affecting the mixture rather than the ability to pull the fuel to to carb.

Are you not familiar with the 'uniflow' tank set up Allen? Gives a very consistent run without pressure too.

Yes, it minimises the variation in head as the fuel level drops in the tank but it won't correct for the variation between upright and inverted from a tank that's (say) 1" below spraybar height. Most of my stuff ran on either pipe pressure (around 3psi) or untimed crankcase pressure (up to around 11psi, but varies with mixture setting). Most modellers don't actually realise that the reason for running pressuresed fuel systems isn't to push the fuel into the engine - it's to provide a high pressure "bias" so that the variations due to changes in head when manoeuvering are a small proportion of the overall pressure at the spraybar, thus minimising mixture changes. That's why you never see a CL stunt engine running anything but suction feed - these variations in pressure are actually *used* to kick it from a rich 4-stroke in level flight to a lean 2-stroke though manoeuvers.

AS
 
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Offline Allen Smithee

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Re: An engine running bench
« Reply #22 on: August 09, 2014, 04:22:18 PM »
I should have added - there are refinements on the basic test stand design (around a gazilion and twenty three at last count), but one of the more important ones is to add a squared-off lip at the front of the two engine seating surfaces so that the engine isn't restrained purely by the friction of the two clamps.

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Offline Jasonb

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Re: An engine running bench
« Reply #23 on: August 09, 2014, 05:06:09 PM »
Yes, you don't want this happening :o

<a href="https://www.youtube.com/watch?v=1Pk9y5t1N9Q" target="_blank">http://www.youtube.com/watch?v=1Pk9y5t1N9Q</a>

Offline Ramon Wilson

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Re: An engine running bench
« Reply #24 on: August 09, 2014, 05:10:57 PM »
Yes, it minimises the variation in head as the fuel level drops in the tank but it won't correct for the variation between upright and inverted from a tank that's (say) 1" below spraybar height. Most of my stuff ran on either pipe pressure (around 3psi) or untimed crankcase pressure (up to around 11psi, but varies with mixture setting). Most modellers don't actually realise that the reason for running pressuresed fuel systems isn't to push the fuel into the engine - it's to provide a high pressure "bias" so that the variations due to changes in head when manoeuvering are a small proportion of the overall pressure at the spraybar, thus minimising mixture changes. That's why you never see a CL stunt engine running anything but suction feed - these variations in pressure are actually *used* to kick it from a rich 4-stroke in level flight to a lean 2-stroke though manoeuvers.
 

I'm not sure I would quite see it that way Allen - no self respecting stunt flier would ever put a tank on anything but the centre line  of tank to centre line of spray-bar give or take a millimeter or two of shimming to get an even run from upright to inverted level flight.

As for the variations in fuel head/pressure as the means to kick the run from a four stroke to two stroke I think you may find this is very much down to the timing of the engine. Dedicated C/L aerobatic engines excepted very few engines will run in this fashion 'out of the box' The Fox35 and OS Max 35S being the predominant ones but many others require the timing, along with other tweaks such as head shimming and venturi size, to be modified to create this in flight characteristic. Most R/C motors simply converted by using a standard venturi will not run in this fashion until the blowdown period is altered.  Well, it was in my day which ended in 2006 - perhaps things have changed since  :shrug:

The uniflow set up has been used for many years in stunt circles, sometimes on pressure - though personally I have never had any consistent results going down that path - but mainly without. As the vent pipe uncovers toward the end of the run that's just when the extra power as the engine increases in revs is helpful to get round that last manoeuver - the four loops in the clover leaf. There are a lot of factors that go into getting a good four two four run besides the tank though.

Though I have no personal experience of them in aerobatics, tuned pipes have been used too though not so much to extract power but to give that 'consistent' run so desired of stunt fliers - the difference is quite distinct, the higher rev on a fine pitch prop the speed almost constant no matter at what attitude the aircraft the pipe acting as a 'brake' to prevent an increase in revs as the model goes 'downhill'. I believe that these set ups do usually run on pressurised feed as opposed to plain suction.

All of this must be a bit academic for 'poor old Jo' - all she wanted to know is what kind of bench to get make - you're very quiet Jo have you come to a decision yet  ;)

Re the sound advice to have lips at the front to prevent a wayward engine - all well and good until you have an engine that has to come forward to clear any protrusion at the back. 'Touch wood' I've never experienced it as yet and that old stand has seen a few runs in it's time but two pieces of 1/32 ply under the clamp bars on top of the lugs will hold most things - providing you tighten the bolts of course  :o

Regards - Ramon

"I ain't here for the long time but I am here for a good time"
(a very apt phrase - thanks to a well meaning MEM friend)

Offline Allen Smithee

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Re: An engine running bench
« Reply #25 on: August 09, 2014, 05:34:59 PM »
Yes, you don't want this happening :o

Indeed - next time you're in the office remind me to show you the video of a J85 (engine from the Northrop F5) bench run that follos a pretty similar script...

AS
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Offline Marinus

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Re: An engine running bench
« Reply #26 on: August 09, 2014, 05:53:07 PM »
I had to build one a few years ago for my ABC-12 glow engine , just made it from a few scraps and the throttle was kinda like a Boeing's.
Kind regards

Marinus Kruger

Offline Jo

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Re: An engine running bench
« Reply #27 on: August 10, 2014, 09:05:03 AM »
A lot All of this is going over my head  :insane:.

What I am interested in is the dimensions of that engine clamping arrangement. Is it about 150 to 200 mm wide? What is the height of the clamping blocks?

Looking at my Kiwi, it needs 30mm clearance under the mounting lugs and is 75mm wide. So I would guess the block need to be 35mm tall.

Width wise the Stentor has 25mm wide mounting lugs the Kiwi nearer 50mm. So some sort of tool maker's clamping arrangement of say 40mm wide?

Jo
Enjoyment is more important than achievement.

Offline Ramon Wilson

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Re: An engine running bench
« Reply #28 on: August 10, 2014, 09:40:13 AM »
As I said Jo quite academic to your original point  ;) .

Without measuring it my engine mount is about 180mm long and 50 or so wide. The blocks are about 30 mm high. Ignore the slot you can see in the outer side - that was for an (unsuccessful, wear wise) adjustable clamp support to ensure the clamp itself was parallel depending on engine lug thickness. It now has a pan head screw fitted from the top which does the same but that really does need improving too to be honest. The right hand block slides through about 50mm to adjust for width. Like clamping on the mill table the clamps should be near parallel - it's when they are not that an engine could work its way loose

It was made around 1982  :o so has worn well. I've never had an engine pull itself out though as said that is not to say it couldn't. Throughout it's day its handled from .5cc to some quite powerful 60's but bear in mind this is a 'running stand' not an all out test bench

So to that end then I guess you just need to take the basic principal and make the dimensions to suit your ideas.

Will you have your Stentor for next years Guildford ? - will I have my Stentor for ... How longs a piece of fuel tube :D
Keep on with that Centaur - at least you've got a handle  ;) just remember you'll have to develop a well practiced flick to start these little beggars ;)

Ramon
"I ain't here for the long time but I am here for a good time"
(a very apt phrase - thanks to a well meaning MEM friend)

Offline Allen Smithee

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Re: An engine running bench
« Reply #29 on: August 10, 2014, 10:16:32 AM »
What I am interested in is the dimensions of that engine clamping arrangement. Is it about 150 to 200 mm wide? What is the height of the clamping blocks?

What size engines are you likely to be building? If they're all in the range (say) 1.5-10cc then you will be able to cover the whole range with one stand like the one above. If you want to do "small stuff" under 1.5cc then you might benefit from making a smaller version and above 10cc it's usually better to make a dedicated mount from a but of 4mm dural plate screwed to a bit of wood.

If you're not likely to do very many of these standard beam/lug mounted engines then don't bother making anything and just borrow my test stand when you need it...

AS
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