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Outdoor Free Flight Forum => Power => Topic started by: I hate trees on March 29, 2010, 10:44:34 AM



Title: tank size?
Post by: I hate trees on March 29, 2010, 10:44:34 AM
Hi,
I'm building a Dixielander and I'm incorporating a tank into the pylon (moving the engine back a bit). The tank vol is probably around 9cc. I'm going to put an AM 35 in it. How long will the engine run on that amount of fuel? I'm sort of new to power flying and I don't want to have to worry too much about running out of fuel whilst trying to tune the engine and get ready to launch etc.

Is there a rule of thumb for tank sizes for particular engine sizes and run time?

What I should do obviously is run the engine and see how long the tank takes to empty, but where I live I can't really go out and start running engines in the back garden.


Title: Re: tank size?
Post by: blklion on March 29, 2010, 02:50:45 PM
I worked up a chart once to calculate what size tank is needed per cubic inch displacement for a 7 minute run in control line. The AM 35 is approximately .20 ci and would require about a 2.25 ounce tank which calculates to about 66cc tank for the 7 minute run time. If this holds true then you'd get about a 60 second run off a 9cc tank with a 3.5cc displacement engine. This is nitro...not diesel.


Title: Re: tank size?
Post by: applehoney on March 29, 2010, 06:51:38 PM
I have 7.5cc rectangular T/R tanks mounted sideways on my two diesel F/F's (only hard tanks I've used in decades) .. PAW 1.5cc and Webra Mach 1 2.5cc .. and they're quite satisfactory for the thirsty Mach 1 given a quick start and little 'fiddle time' with settings, fuse, etc., and up to a 12 second official run. Your 9cc .. or a 10cc T/R tank .. would likely be fine for the AM35


Title: Re: tank size?
Post by: I hate trees on March 30, 2010, 08:33:24 AM
Thanks for your replies,

What you say is about what I thought, but suddenly I got nervous as the tank will be buried in the pylon.

When I get the model finished I'll let you know exactly how long it does run for.

Adam


Title: Re: tank size?
Post by: applehoney on March 30, 2010, 10:26:08 AM
Way back when I was flying diesels in London/Northern Areas .. 50's/early 60's ... I used to bury tanks in the pylon thinking that in climb attitude it was level with the venturi, good for consistent feed. Maybe so, but then one had to hold the model at an inconvenient angle when starting, to avoid flooding... and there was often a nasty fuel surge 'acceleration blip' at that crucial moment after launch which I figured was partially due to distance and the longer length of fuel tubing required through the timer. I then switched to glow and eventually bladder tanks and never experienced that problem again.

Back to a couple of diesels in recent years for a specific event here and I now place the tank as close to the engine as possible to minimise fuel feed length, and haven't experienced any surge problems - and it's easier to dig out the tank, if ever necessary, than opening up structure. I plug the rear vent after filling to reduce syphoning - and if it's not too late then I do recommend a radial mount instead of bearers, to make thrust adjustments so much easier. With a sidemounted engine on beams you only have sidethrust adjustment .. with a radial mount, any desired angle.


Title: Re: tank size?
Post by: glidermaster on March 30, 2010, 03:33:35 PM
Jim, (off-topic) what is that little protuberance at the back of the pylon?

A Dixielander with AM35 shouldn't need any thrust line adjustments if built true (in my opinion, that is, as a 2 Dixies man). The Dixielander's most successful user back in the 60's was John West of the Brighton club, and he apparently used 5 degs. of RIGHT thrust. Now that sounds a bit scary to me.

John


Title: Re: tank size?
Post by: applehoney on March 30, 2010, 04:46:34 PM
Quote
Jim, (off-topic) what is that little protuberance at the back of the pylon?

John - housing for the tracker transmitter.


Title: Re: tank size?
Post by: I hate trees on March 31, 2010, 03:16:01 PM
Thanks guys for your helpful suggestions.

I have built the bearers into the fuselage. I built a dixie last year for the 50th bash but didn't fly it at Middle Wallop. I have learnt a bit about power from the first one and that doesn't have any side thrust and seems to be safe enough on the few flights I've had with it. I have had a lot of fun acquiring AM35s and am intending to make the second model lighter (moving the engine back should allow less lead to be used at the back!). Hopefully the AMs will have enough power to be respectable and so I can get into SLOP etc. If that works out I will then step up to some OS20fp models, which will have radial mounts (and electronic timers)... but I'm giving my secret plans away!

Adam


Title: Re: tank size?
Post by: RobinB on April 01, 2010, 06:08:59 AM
Adam, I use PAW 19 diesels and they're thirsty so I find that a 15 - 18 cc tank works best. It's extra weight, but when I had a model with a smaller tank I got the occasional 'miss' just after launch. I was test running a couple of AM35s last year and I seem to remember that they were a bit more frugal than the PAW, so your tank should be ok. Good luck.

Robin


Title: Re: tank size?
Post by: applehoney on April 01, 2010, 09:24:01 AM
Quote
I got the occasional 'miss' just after launch

I've always put that symptom down to fuel surge - a very common occurrence with a hard tank that's not pressurised.  Nothing like having the engine quit momentarily at launch and then come in again after the model's nose has dropped....  :(& Is there any reason that a bladder can't be used for a diesel? Have never thought about that aspect before.


Title: Re: tank size?
Post by: glidermaster on April 01, 2010, 10:49:57 AM
No reason at all, Jim, diesels can use crankcase pressure too. However, these guys are building/flying SLOP - no pressure fuel system allowed.

A tank with a bit of extra volume can help - so can a baffle.

John


Title: Re: tank size?
Post by: blklion on April 01, 2010, 04:53:11 PM
Have you tried using a wedge tank on edge (wedge down) to alleviate the leaning out at launch?


Title: Re: tank size?
Post by: Pit on April 01, 2010, 08:53:49 PM
One of the power FF guys I met in Monterrey, Mex. back in the early '60s used wedge tanks on all his FF models, mainly 'cause that was all that was available at the time. He advised me to mount the tank, wedge down, in my RamRod 600 - worked, how consistently I never found out. Lost the model on the final full runtime/30 sec DT trim flight (third flight), to a monster thermal headed toward Saltillo with DT deployed :(.


Title: Re: tank size?
Post by: gossie on April 01, 2010, 10:33:35 PM
No doubt pressure tanks either bladder or hard tank keep the engine running hard and fast with a heave ho launch straight up without a miss.

Have found on a hard tank without pressure a gentle launch is much more desirable so as not to get the miss.


Title: Re: tank size?
Post by: RobinB on April 02, 2010, 01:31:46 PM
Wedge tanks like this one? Looks to be 10cc.

Robin


Title: Re: tank size?
Post by: I hate trees on May 16, 2010, 04:20:37 PM
Ahhhh disaster!

I finished the model and tried it out a few days ago. The engine will only run for 15-20 secs. You can see the bubbles coming down the fuel pipe as it runs out. The tank has two vents, one to fuel and I fill the tank until the fuel spurts out of the second vent. You can also see it going down the fuel line to the engine, so I don't think its a blockage. Is it possible that the tank is not filling completely? or could it be siphoned off in large amounts if the vents were just in wrong position etc. I have run the engine and it gets 2minute runs on an 18cc tank so it really should do a bit more than it is doing. Obviously its a strange shaped tank to fit into the model but the feed line to the engine should be at the best point in the tank to get all the fuel. Holding the model with the nose up gives a few more seconds run but not too much.

Any suggestions before I take a knife to the pylon?


Title: Re: tank size?
Post by: Pit on May 16, 2010, 05:41:20 PM
Bubbles in the line while running is usually a sign of a leak somewhere - possibly in the tubing from tank to engine (from your description, a hole or cut where the brass tubing ends inside the plastic tube). Is there still fuel in the tank after it quits? You might try plugging the fill line after filling - could very well be siphoning from there.


Title: Re: tank size?
Post by: blklion on May 17, 2010, 06:33:31 AM
The pickup tube on a wedge tank runs to the rear corner of the wedge. If you've got the wedge on edge and level then the fuel may not be flowing back to the pickup end once the tank gets near empty. You'd need to mount it at a slight angle to get the pickup end below the level of the output end.

Wedge tanks were/are designed for U/C flying in that the centrifugal force pushes the fuel into the wedge. Even square tanks are designed similarly with the pickup positioned at the bottom, rear of the tank.

Another thing that helps/may help is to either cut the fill and overflow tubes at a 45 degree angle or place some short lengths of fuel line on them that are cut at an angle. Position that angle so it faces into the prop wash. This adds a bit or pressure into the tank and eliminates fuel siphoning by the airflow over those tubes.


Title: Re: tank size?
Post by: KDus on April 03, 2017, 02:52:33 PM
I'm looking for input on mounting a wedge tank for control line flying.
Am I interpreting the instructions correctly? The tank will be on the outside and fly counter clockwise.


Title: Re: tank size?
Post by: faif2d on April 03, 2017, 04:07:48 PM
You have it correct.  I found pacifier tanks 40 years ago and have only used a few metal tanks since on diesels.