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Aerobatic fuel cap redesign. printer friendly version
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jb92563
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Posted Tuesday, April 20, 2010 @ 05:05 PM  

Hi folks,

I am looking for ideas on an aerobatic fuel cap for my RF4D.

I do have a little lathe in the garage and some aluminum that I could perhaps use to learn how to cut my own
threads and make a custom gascap.

Not sure if anyone else needs one but if I get it going on the lathe I could make a few more.

As far as the design goes the parameters that come to mind is that:

- it needs to act as a vent and prevent negative presure in the tank.
- needs to keep the fuel from leaking out in any attitude.
- would be nice to know how much fuel is left if possible.
- needs to be hand operable so you dont need a wrench, that you always seem to misplace anyway.
- Material could be aluminum, brass, nylon or something else entirely...not sure if there are fuel or corrosion issues with some materials.
Aluminum seems like a good option since its light and strong?

Any other wishlist parameters?


[Edit by jb92563 on Tuesday, April 20, 2010 @ 05:09 PM]

--------------------
Ray
RF4D #4057 N-1771 Rectimo 1400cc
http://picasaweb.google.com/jb92563/FournierRF4D
http://www.touringmotorgliders.org

flyingkroeger
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Posted Tuesday, April 20, 2010 @ 07:36 PM  


By flyingkroeger at 2010-04-20
flyingkroeger
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Posted Tuesday, April 20, 2010 @ 07:46 PM  

Hi Ray,

Excellent topic. I have been thinking about this for a little while. Apologies for the low res picture above, for some reason I cannot create a high res jpg from my drawing program.

This could work for fuel caps where you can remove the wire from the float (like mine) or where one would be willing to cut off the bend on the top end of the wire in order to be able to remove the existing float/wire.

The cone is turned from suitable material (brass?) and is attached to the short wire by a thread. The O-ring above the cone provides positive seal when under negative load. The advantage of this would be that you do not need to fabricate another complete fuel cap, but instead you would be able to work with what you already have.

I would be sceptical about any spring loaded check valve thingies. These require pressure drop to operate, which in turn would hinder the fuel flow. Hence my preference for a gravity operated system.

Would this work?

BR

Tim

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Posted Tuesday, April 20, 2010 @ 08:59 PM  

Hi Guys,

I'm not sure if I've already said it here somewhere, but the original Fournier aerobatic cap does not have any contents indication, so for cross-country flights we have to work with watches and hope there are no fuel leaks.

Mine use the float valves from Stromberg CD 150 carburetors. These are self-contained brass valves with a couple of ball-bearings inside. They make a good job of allowing air into the tank and preventing almost all of the fuel getting out when in any other than the upright attitude. The body of one cap is of brass (due ot a misunderstanding) the other is of aluminium.

They work well because the ball-bearings have small mass and move quickly.

I suspect that trying to get something with greater mass (like a float & its wire) to move as fast as petrol/gasoline and seal quickly may prove difficult to acheive, but if you do manage to get one working effectively, please do let us know.

Yours, Bob

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Posted Tuesday, April 20, 2010 @ 11:02 PM  

Hi Bob,

So you are suggesting that the balls move very quickly when the flow reverses, hence the loss of fuel is minimal. Makes sense…

Can you confirm whether there is an initial spill when going inverted? This would support our theory, in that case the system I was proposing earlier today would not be such a good idea.

BR

Tim

Bob Grimstead
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Posted Wednesday, April 21, 2010 @ 03:14 AM  

Hi Tim,

Most of the time there is no spill when either pitching or rolling to the inverted, but sometimes there is a little, tiny puff of fluid. Mostly that evaporates before it gets to the canopy, but sometimes it will leave a very small mark. this is always smaller than my handspan and often smaller, and easy to see through. It shows no sign of damaging the acrylic.

Remember, I only carry fifteen litres of fuel for aerobatics, so the fuel has to go up nearly a foot before it gets to the top of the tank, giving the little balls time to drop into place.

I more often get puffs of fuel from the aerobatic cap when either the tank is full or when taxying over rough ground -- or both, of course, when it's worst.

I hope that's of use. Good luck with your design.

Yours, Bob

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Posted Wednesday, April 21, 2010 @ 09:03 AM  

Hi Guys,

I can see one potential problem with the cone idea. If the tank is full and the float presses the cone up into the o-ring seal then the tank will not have an open breather. This could be important during take off when the fuel level is likely to be at its highest, fuel flow is at its highest and the static head from the tank to the carb is low because of the high angle of attack at slow airspeed and the climb attitude. A deafening and sudden silence over the trees, or lake, at 50' agl as a result of fuel starvation would not be much fun.

Best regards,

Dave

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Posted Wednesday, April 21, 2010 @ 11:24 AM  

I like the one way valve with ball bearing idea.

They move quickly due to the low mass and iff there is some suction developing they would easily open to allow air in.

--------------------
Ray
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http://picasaweb.google.com/jb92563/FournierRF4D
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Posted Wednesday, April 21, 2010 @ 02:11 PM  

Hi Guys,
E.I.S. aircraft in Germany still sells Fournier aerobatic fuel caps, I bought one a couple of years ago. The cap is made of white nylon and the breather from brass I think, with a ball valve. Somewhere around 50 Euros I think and works great on XST, I keep it in a small sack behind the backrest.

On the other hand I most often do "spontaneous aerobatics", like when I've thermalled and for some reason need to loose some altitude. On those occasions (with the standard float on wire-cap on) I try to do only positive G maneuvres since negative G's really splash a lot of fuel on the canopy. It would be nice if the standard cap could be "less sploshy" during neg G, but I doubt if it's possible. Please prove me wrong!

May the 4's be wit you/ Jörgen

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Posted Wednesday, April 21, 2010 @ 04:09 PM  

Here is my concept for a possible solution.

There will be a ~ 2-3" sight tube sealed on the top.

A little loose fitting disk will ride in this tube affixed to a wire that
is levered from a float arm in the tank.

There will also be a one way ball stopper valve that will allow air to come into the tank but will prevent fuel from exiting.

If there is negative air pressure in the tank air will be allowed in by the valve even when inverted.

Comments, suggestions?


[Edit by jb92563 on Friday, April 23, 2010 @ 11:40 AM]

--------------------
Ray
RF4D #4057 N-1771 Rectimo 1400cc
http://picasaweb.google.com/jb92563/FournierRF4D
http://www.touringmotorgliders.org

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Posted Wednesday, April 21, 2010 @ 08:17 PM  

Hey, that's really neat! Can't see why that wouldn't work. Might need another link in the pushrod between main float lever and indicator to avoid binding.

BR

Tim

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Posted Wednesday, April 21, 2010 @ 10:34 PM  

Yes Ray,

I think that principle is great. And it would help prevent the wire from blowing back and jamming as it so often does with a full tank.

Remember that both the standard gauge and the aerobatic cap have a 5cm (two-inch) pipe coming up from the top of the cap with at least one hole facing forwards to get pitot pressure to positively pressurise the tank very slightly.

I guess the pitot pipe should be in front for max airflow, and the gauge-in-a-tube at the back, so you can seee it easily.

It is also possible to improve the standard gauge/cap to dribble less fuel when inverted. I've done it on both my Champ and my Fly Baby.

You merely have to replace the original gauge rod/wire with a thicker rod (I used small diameter brass tubing) that almost exactly fits the hole in the cap. Making it a better fit actually improves the gauge because it prevents the rod getting slightly cocked in its guide, which can hinder its operation.
The trick is to get a bunch of pieces of tubing and find one that most precisely fits the hole without binding, drilling out the hole very slightly if necessary. Provided the inside of the hole and the tubing are of different materials (steel/brass or aluminium/brass) there is very little friction or wear.

Nevertheless, the aerobatic cap really does minimise the inverted fuel loss.

Incidentally Jorgen, you will also greatly reduce inverted fuel spillage if you can persuade yourself to do what the proffesionals do. Load only sufficient fuel for your flight and a 45-minute reserve at minimum power (say, three litres). It is extremely rare that I fly with more than 17 litres (two to climb to height, 5 for 20 minutes of aeros and 10 for up to three hours of loitering).

For the Red Bull Air Race and similar city-based displays (half an hour's flying time from my base) I load 20 or 21 litres. 5 or 6 to get there (depending on the wind), giving me 15 for the maximum allowable aerobatic fuel load when I arrive. I then use 3 or so of those in my display and have 12 to get back. That's enough for one hour at 3,050rpm (90nm/105sm and ten litres) plus 2 litres for half an hour of holding or a nearby diversion.

All these number apply to 1400cc motors with my Heliptera propellers, but they will probably be within 10% for any 1400cc motor.

Having less than full fuel also gives you a better sink rate for your gliding!

Yours, Bob

[Edit by Bob Grimstead on Wednesday, April 21, 2010 @ 10:35 PM]

jb92563
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Posted Friday, April 23, 2010 @ 10:58 AM  

I think I am going to go ahead and build one and see how it works.

Is there anyone else that might like one?

I will buy enough material to build a few since the cost is only a bit more to get extra materials.

The only challenge I anticipate will be for me to learn how to cut threads with my lathe and duplicate the existing cap threads.

That will likely be a bit of trial and error, but a fun new skill to learn.

--------------------
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Posted Friday, April 23, 2010 @ 01:41 PM  

Hi Ray,
I would definetly like one if you make a couple of extras, I think your design looks neat. In my opinion the "AeRay" fuel cap increase the already incredible versatility of the RF 4, nice to be able to do "spontaneous aerobatics" without spilling gas all over the canopy.

Bob, you are of course utterly correct, it´s pointless to fly around with 30 l of fuel if you know you will only use 10. It's also a good point that you get better minimum sink when lighter- glide angle (which everyone quotes) is much less interesting if you just want to thermal for a bit. If the next area of climb is too far away just dive and jerk the handle and you've got a better "glide" than even the ETA (http://www.leichtwerk.de/eta/en/project_eta/index.html).

I'm probably too lazy, but I find myself a little reluctant to splosh fuel around, tapping it off, spilling it back in again etc. I have however become more used to it after I started using MOGAS last year. I generally just use a jerrycan with an extension cap on, but that doesn't help when you tap it off. Since you are probably the most meticulous Fournieteer out there, would you care to share some tricks on how you spill your fuel in and out in another thread, Bob?

May the 4's be with you/ Jörgen

[Edit by Jorgen on Friday, April 23, 2010 @ 01:43 PM]

Bob Grimstead
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Posted Saturday, April 24, 2010 @ 07:54 PM  

Hello again Ray,

If you can make a prototype work OK, I would love to have a couple of your new caps, and I suspect Matthew would like one, too.

One problem I can foresee is that with both the current standard fuel caps and the current aerobatic caps, it is possible to turn around the central vent pipe within the cap, to ensure it faces forwards to allow for variations in the thickness of the fuel cap gasket or O-ring.

On your cap this would not be possible, so the thinkness of the gasket or O-ring would have to be very precise and fixed somehow.

Not sure how you can do this, but I look forward to seeing the results.

Yours, Bob

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Posted Sunday, April 25, 2010 @ 04:12 PM  

Hi guys,
I have a variation on a theme.
It is so simple...........an original cap and tube with the wire removed and the top soldered in. On the underside is a big ball bearing that is held in place by a split pin. Turn upside down and it falls to top of tube and seals. The bottom is drilled out so there is an big air gap for the positive pressure.
10-15 mins work and if it doesn't work for you just take the ball and split pin out, de-solder the top and put the old float back.
Best of luck.
Matthew

--------------------

jb92563
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Posted Monday, May 3, 2010 @ 11:51 AM  

My materials to make 3 caps have arrived and I have started on the 3" diameter chunk of aluminum machining it down and cutting threads on the lathe.

My first attempt at the threading operation produced 16 Threads per Inch but later discovered that my cap is really 13 Threads per inch since I found a Thread guage in my tap & Die set to confirm it.

It may really be a metric thread but matches the 13 tpi perfectly, so hopefully everyone has the same thread pitch on their tank caps.

I'll have to get an impression of each caps threads to confirm it just in case. (Put some graphite pencil lead on your cap threads and roll it on some paper to make an impression of the threads so the pitch can be measured)

So now that Ive learned a bit I am ready to start a real one.

It was a good exercise that took 5 hours as I adjusted my lathe to take out any play, and figured out how to set up the screw cutting gear drive ratios and refine my skills and technique on the first 16 TPI sample.

I did manage to bust the only plastic part on the lathe and wondered aloud in the garage with plenty of expletives why a manufacturer would do such a dumb thing, but I figured that I could also use the lathe itself to make a metal replacement part.
After my project is done I'll used the 16 tpi test peice to make a replacement.

I'll post some photos of the progress of the fuel cap along the way for everyones amusement.

[Edit by jb92563 on Friday, May 7, 2010 @ 04:18 PM]

--------------------
Ray
RF4D #4057 N-1771 Rectimo 1400cc
http://picasaweb.google.com/jb92563/FournierRF4D
http://www.touringmotorgliders.org

jb92563
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Posted Tuesday, May 11, 2010 @ 10:36 AM  

The beginings of a leakless aerobatic fuel cap, with fuel level indicator, for the RF4D.

The First task is to duplicate the threads, at 13 Threads Per Inch.

I'll check that it screws into the tank properly before going further as the thread cutting exercise is a new learning experience for me with my little mini lathe from Harbor Freight.

I have already learned the "measure dozens of times and cut once" rule as I turn down the diameter to the correct size.

Using the caliper is better than only using the graduations on the feed screews, although the feed screws are actually
pretty accurate so far, but I like to double check my progress often because once the material is shaved off you cant get it back.

I have been thinking about how to make sure the cap is oriented correctly and stops in the proper position.

I figure a compressible gasket such as rubber would allow tightening the cap to the correct position with the least amount of
fuss.

[Edit by jb92563 on Friday, May 14, 2010 @ 04:08 PM]

--------------------
Ray
RF4D #4057 N-1771 Rectimo 1400cc
http://picasaweb.google.com/jb92563/FournierRF4D
http://www.touringmotorgliders.org

Bob Grimstead
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Posted Sunday, May 16, 2010 @ 09:49 PM  

Hi Ray,

I have a good friend, Bob Hart, who reads out pasts although he is not a subscriber.
It was Bob who shot most of the footage of my blue Fournier on YouTube.
Reding your thread, he suggested:

For the person building the petrol cap published in the CFIAmerica post
If he encounters stuttering or inconsistent motor speed control with the little lathe he is using, it may be that one of the motor brushes has hung and is arcing. If they hang, they get hot.

Once heated, the sleeve/brush holders which support the brushes in the motor casing may slip partially out under brush spring pressure. On my lathe, there was no positive retention of the brusholder, interference press fit only.

If the brush holder slips out far enough, there may be a short to the metal motor case and the power controller will fail. With lesser spring pressure from a partial move of the brush holder, the brush will be even more inclined to hang, heat and allow the brush holder to move out furthur.

A version of this little lathe was sold into Australia as the CH250. The solid state devices on the motor control board in the 250VAC version seemed inclined to burn out. The 110VAC US version may be less inclined to do this.

An updated board was sold to me to replace the burned out one. The devices also failed in that board. Additionally the replacement board was wired in the reverse direction.

I ended up replacing the failed transistors?? with higher rated ones I got from Atkins. In meantime, I discovered the hung brush and short to case problem. The hung brushes thing needs to be watched as the commutator may also cook and begin to separate.

For precision on mine in cutting threads ( I've been cutting 0.7mm pitch camera filter theads, ) I have been setting the gears to neutral and manually rotating the chuck as the drive pulls the tool along, winding the tool clear, never disengaging the drive, manually rotating the chuck backwards until the tool is at least five turns clear of the workpiece, then winding the tool in furthur for the next cut and rotating the chuck again.

To get exact precision in the cut, he may find it necessary to make sure the engage lever on the traveller is weighted into engagement with something like a pair of lockjaw pliers or a piece of heavy tube about eight inches long, as the lever tends to rise and partially disengage and allow the cut of the thread to drift into error.

For longer thread cuts than lens rings, I have used the motor drive, but I still do not ever disengage the traveller lever. The thread cut on the cap may not need to be as precise as my camera threads but I thought I should mention it anyway.

The lathe gearbox on my machine is only fastened to the bed by three setscrews and it tends to cut a taper, which may cause his cap to bind part way into the tank if his clearances are snug.

He should take care not to force the cap into the tank to "dress" the thread as aluminium which is not surface hardened may pick up and weld to the tank threads, a problem I have had with my camera threads. Some of your molyslip might be a good thing to use on the threads as they settle.

This all might be a red-herring but there is nothing worse than doing all the machining then have the job fail on the very last detail, the thread cutting.

His machine might be a revised design, in which case I am barking at shadows and being a nuisance. He also may be an experienced machinist in which case the nuisance word will apply once more.

I hope this is of use Ray,

Yours, Bob

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Posted Tuesday, May 18, 2010 @ 11:20 AM  

Thanks Bob, I'll keep that info in mind while working along.

So far the last thread cutting went OK except that I used the wrong thread pitch, but I have fortunately not experienced all those problems
with the motor and stuttering.

I'll take it easy on this little machine though and see how the threads fit the tank before doing the remainder of the machining.

The threads are really the only critical lathe work on this and once I get that done Its likely clear sailing.

--------------------
Ray
RF4D #4057 N-1771 Rectimo 1400cc
http://picasaweb.google.com/jb92563/FournierRF4D
http://www.touringmotorgliders.org

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Posted Wednesday, June 2, 2010 @ 10:50 AM  

I have been carefully working the thread dimensions but on my first try last evening to see if I could screw my cap onto the tank I found the diameter slightly oversized so I need to turn down the diameter a tad and deepen the threads a bit.

Once the threading is accomplished then the easy stuff is all thats left to do.

Time to get my butt in gear and get it done, I have been fiddling with my CHT sensors hookup and wondered why the engine was running so cool 200F.

Then I realized that the sensors are just laying on top of the cylinders and have not been installed under the spark plugs yet......EGT probes are next.

[Edit by jb92563 on Wednesday, June 2, 2010 @ 11:00 AM]

--------------------
Ray
RF4D #4057 N-1771 Rectimo 1400cc
http://picasaweb.google.com/jb92563/FournierRF4D
http://www.touringmotorgliders.org

jb92563
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Posted Thursday, September 2, 2010 @ 12:29 PM  

An update/revision on the leakless Fuel cap.

I finished the basic aluminum cap shape and have parted it off with the lathe.

I made some measurements of the tank internal shape (Since its empty for my tank leak repair) to verify clearances required.

I drew up a template for the aluminum linkage support, which will be solid riveted.

I also figured out how the linkage segments will be connected.

As always its designed light, strong, simple and therefore hopefully fool proof, and loose enough that binding should not occur.

I was wondering if I needed to put a grounding braid between all the segments, but I cant imagine that wire to wire
linkages would not be self grounding by the constant contact and movement.

I do like to err on the side of safety however and think I may just ground everything regardless, no point in tempting Murphy.

More details and pictures to follow.

[Edit by jb92563 on Thursday, September 2, 2010 @ 01:08 PM]

--------------------
Ray
RF4D #4057 N-1771 Rectimo 1400cc
http://picasaweb.google.com/jb92563/FournierRF4D
http://www.touringmotorgliders.org

jb92563
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Posted Tuesday, September 7, 2010 @ 11:13 AM  

Here is the latest progress on the cap prototype.

The design has evolved a bit.....simplified really to remove redundant weight and add simplicity.

The pictures below show what is done so far.

The protruding screws, which run through threaded holes and hold the linkage framework in place, get trimmed later after
I finalize the attachment of the clear tube.

The tube is just resting in place and not aligned or attached yet for the photos.

The white disk in the tube is teflon so the tube does not get scratched over time and to keep a frictionless somewhat loose fit in the tube.

There are some holes in the disk to equalize presure on both sides.

The action of the linkage is very smooth and free and have not been able to get it to bind up in any attitude.

I think the one way valve with a forward facing inlet tube will be mounted on top of the tube. since it needs to be sealed in any case,
and again simplifies the device, and provides some positive preasure to the fuel tank by the ram air effect on the tube in theory.

The linkage attachment to the float arm is temporary so that I could adjust the attach point easily.

I will solder a proper tab in place later once the position is finalized.



[Edit by jb92563 on Tuesday, September 7, 2010 @ 11:16 AM]

[Edit by jb92563 on Tuesday, September 7, 2010 @ 11:18 AM]

--------------------
Ray
RF4D #4057 N-1771 Rectimo 1400cc
http://picasaweb.google.com/jb92563/FournierRF4D
http://www.touringmotorgliders.org

Jorgen
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Posted Tuesday, September 7, 2010 @ 04:50 PM  

Very neat Ray,
nice update and nice pictures! Hope you find a small enough ball valve, Go Ray Go!

May the 4's be with you/ Jörgen

jb92563
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Posted Wednesday, September 8, 2010 @ 10:55 AM  

I found valves that are the same diameter exactly as the plexi tube, so I was thinking I could use the valve to seal off the top of the tube opening.

I would mount a forward facing inlet on the valve barb.

--------------------
Ray
RF4D #4057 N-1771 Rectimo 1400cc
http://picasaweb.google.com/jb92563/FournierRF4D
http://www.touringmotorgliders.org

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