David Gladwin

My Air World Hawk has undergone final systems checks before flying. I noticed that during the engine runs there were some absolutely minute air bubbles going to the engine (Wren XL 200 which runs beautifully, cool and QUIET in the fully ducted installation ) which continued to run perfectly at all rpms. . I also noticed a small amout of air had accumulated in the header tank , all this despite checking EVERY component for leakage under pressure and vacuum and ditto the whole system installed. As there is no leakage in the system this air has to be coming from somewhere. As the pickups etc remained totally immersed in fuel theoretically there should be NO air in the system, so where is it coming from ? Could this be as a result of aeration of the fuel due to slight cavitation in the refuelling pump (Kavan).

Components: Airworld 4L glass tank, checked underwater for leaks.
Orbit type clunk.
Header: Jet A1 4 oz centered tank with pleated paper pick up, no leakage.
Festo fittings all OK with square cut tubing, no leakage. (and Festo fittings are contrary to what I frequently hear, designed and approved for use at up to 1 bar vacuum )

I dont think the small amout of air is likely to cause a flameout but I am intrigued that the sytem is not TOTALLY air free. Any theories or explanations, gentlemen ?

Regards, David Gladwin.

Airforce7

My Feedback: (31)

David,

Can you elaborate a little more on the term air? Is it air bubbles (a few mm long one after another) or is it thousands of tiny air bubbles that appear foamish? I know that metal geared pumps can create that foamish type appearance when running, especially if the the pump pumps the fluid faster than it can draw. If you can easily remove the festo fittings and re-test I would start there and then work your way to the pump. Any pictures? Good luck. Ken

Randy M.

My Feedback: (38)

Drain the system and refill with just gravity, see if any difference.

Wayne22

My Feedback: (2)

So the air is appearing on the suction side of the pump?? Are the tank vents free enough to allow air to replace consumed fuel? Are you using an orbit type clunk in any of the tanks? I had one in my fueling jug (completely submerged) and presumeably it would not let fuel flow as fast as the pump wanted it, and it created a big stream of bubbles. Once discovered, I cut the screen off the clunk and problem solved (Although I have one less level of filtering now...)

BTW I had to completely drain my fuel system last fall when transporting my F-16 by air.. when we fueled up, there were (not unexpectedly) huge bubbles passing completely through the pump into the MW-54.. It took about 5 minutes of shaking and bouncing in all attitudes to purge the UAT. During the whole exercise, the little Wren kept running - the bubbles didn't seem to bother it at all!!!
( and no, I would not knowingly fly it with bubbles in the system!!)

747drvr

My Feedback: (5)

I experienced bubbles from my Orbit clunk as well . I would suggest starting there .

Marc

garrett_mcdonal

Hi David,

I had something similar a while back. In my case at least, I found that the line from my fuel can to my filling pump was pulling small amounts of air through a poor seal between a festo fitting and a fuel line when the line was at a certain angle. It must have been pumping tiny little bubbles into my tanks. At full power, I could see them accumulating in the UAT as the fuel drew from the main tank.

If I wiggled the line in the festo on my fuel can while the pump was filling the plane, you could just hear a small difference in the tone of the fuel pump while air was being drawn into the lines.

Not sure if that helps in your case but it had me scratching my head for ages before I found it.

Regards,
Garrett

David Gladwin

Thank you guys for your much appreciated comments.

Firstly I will let the fuel settle for a half hour or so to allow any aeration to clear, then change the Orbit type clunk (which have worked fine on several other models). See how we go. The bubbles are extremely small, and between the header and the fuel pump, so perhaps I am being paranoid and worrying about nothing. That said I am getting rather ratty after the firece storms and weather (250 ml of rain in two days) we have had here in the last week causing our field to be under several feet of water, stopping all flying, and no power for three days !

Thank you again,

David.

Legnum

Hi David,
Had a similar recently with my ARF Flash, up to half power Jet Cat 120 fuel line started showing bubbles and foaming in the fuel line at half power, removed header tank, blocked filler and vent, pumped to 65 lbs, cap showed leakage, cool, thread tape applied, tested ok, refitted, same problem in the system,really pissed now, changed the the header from a crashed model, no bubbles or foaming, problem solved, have pulled the new header to pieces several times, can not find a fault with it, has been test flown with the second hand had tank, no problems, try changing the header tank, worked for me. !!

Grant NZ

digitech

My Feedback: (10)

David
the "Orbit" clunck (Still chainsaw fuel pickups)
are made for 2 stroke engines with a pickup of 2 liters every 3 hours.
you need to change them every 50 hours
that is roughly 35 liters of Kero

secondly the pickup space on those is very small , and on a larger engine with pump you will run into these problems.
if i am not mistaking your WrenXXL has a Hausl 30020SP.
the engine also accelerates rapidly , your bubbles are 99 % coming from your pump in combination with a "orbit"fuel clunk.
Replace the clucnk with a Tilotson "big"clunk.
Sara at Wren can help you out getting one.

also kavan pumps are not to good for kerosine
if your fuelflow is "milky" When refueling , your kavan is cavitating.
do your self a favour and get one of those jersey pumps.
used them for years still the first one..

Sandor

Craig B.

Hi David,

You probably do not have air in your system....you are observing fuel vapour bubbles created by the vacuum in your fuel system on the suction side. I have had the same problem with the orbit clunk and I agree with Sandor 100%. We are trying to use these pickups in an application for which they were not designed and are not suitable. Some users a may be able to get away with it with engines that don't demand high fuel flow rates. I have sent you a pm. Hope this helps.


Craig.

Kelly W

My Feedback: (2)

As mentioned before, small bubble streams tend to scream either cavitation and / or a festo fitting on the suction side not seating well.

I'll follow up and throw my support on the clunk change. I use to have one in my Hotspot and swore by using it. At close to the 25 flight mark the restriction through the clunk suddenly changed... It was so quick that I thought I had a kinked line or some other blockage. It took ~25 minutes to fill my tank... I could still run the turbine, but the pump voltages were very high. I've seen a few other cases similar to this in on-board fuel tanks and fuel jugs, although not quite this severe a change.

I had no idea these were chain saw filter clunks. We should have seen that coming I guess, but the Sandor's usage statistics tend to back up the general consensus here. They do a great job at limiting the flow of air through the system, but I guess we need to be prepared to change them often if you feel they're an absolute necessity.

Kelly

Robrow

Yepp, does sound like cavation. My personal setup is with a large big bore heavy well tapered brass clunk turned in the lathe mated to big bore tubing.........zero cavitation for even the highest flow engines.

Regards

Rob.

Flying Arrow

My Feedback: (6)

are you using a big filter similar to the one that come with jet cat engines?

If its the case check it out.
regards

David Gladwin

Again guys, MANY thanks for all your very valuable input.

I have used the Orbit clunk with complete success but only on smaller engines such as the RAM 500 and PST 600 but you have convinced me that its not suitable for larger engines so out it comes as the first stage . In fact I will keep the clunk but remove its screen and try again. Even if that is not causing the fuel vapour bubbles I see it as a potential source of problems so thats another reason for its removal.

This clunk problem makes sense as I having been using an Orbit type clunk for the pickup in my refuelling can and on close inspection it is getting gummed up with algae. The Kavan pumps have worked fine for refuelling but now I know why they sometimes change speed abruptly for no apparent reason.

It may interest some to know that on my BVM F4 with an AMTNL Pegasus HPES I use TWO header tanks in series (a flameout on the heavy F4 could be fatal) and the main feed header never gets ANY air, or vapour, in it, consequently never a flameout (just one in flight shut down, finger trouble on my part). My JetCat 160 powered models all use standard clunks with added lead weights and simple geometrically centered pickups in old JPX oil tanks, never a flameout. BVM UATS also give me first class service with the JetCat p70 and PST 600r in my BobCats, again, never a flameout.

My particular thanks to Craig B for his PM which described the Orbit clunk problem in detail, I'm now fully convinced it has no part in the Hawk's fuel system.

Thank you all again, RCU at its very best.

Regards,

David.

Craig B.

You are welcome David. Glad to be of assistance. Let us know how the hawk test flight goes. Would you like us to send over some floats?

BTW, are you still writing for RCJI?

Regards,

Craig.

David Gladwin

Thanks Craig, looks like test/trim flights will be on practice days in NI. Field still badly flooded, and more gales and wind this wekend, another 60mm last night, rain forecast all week !


Regards,

David.

Wayne22

My Feedback: (2)

Glad to hear that, collectively, we nailed down the source of the problem....

David Gladwin

Yep, I think you guys who went for the Orbit type clunk were right on target. I have removed the tank and clunk and there was considerable restriction when sucking through the clunk line with the clunk attached. The clunk was opened up, ye gods, its amazing the engine ran at anywhere near full power. No wonder no dirt can get through the screens they are amazingly restrictive. The four "windows" have very fine screen mesh which when removed considerably eased the flow restriction. Inside the clunk there is another filter made of a sort of porous plastic. When that was removed all the restriction cleared. With THAT plastic removed the two feed holes were revealed and they are TINY, about 1.5mm square, totally inadequate for our purpose, certainly for a 36 pound thrust engine.

I have attached the pictures and if the clunk is NOT the cause of the vapour bubbles it certainly WAS a potentially serious problem now, thanks to you guys, removed.

The other picture is of some of the final details going onto the Hawk, this morning it was the inner widscreen and MDC.

Thank you ALL again SO much for your input.

Regards, David.

digitech

My Feedback: (10)

if you use a UAT
just go for felt clucnks , the large one,s
your UAT wil filter anything out.
the felt clunks are perfect and do the same jobs..

ravill

My Feedback: (11)

In my SM F-4, I found that I was getting minute airbubbles from the CAP (not the blue nut) of the UAT! These little bubbles just kept pilling up until the UAT was completely empty. Granted this was at Full throttle for 3 minutes (that's 3.8 Volts to the fuel pump), the UAT was just almost puckered flat from the suction and my festo connection NEVER LEAKED on the suction side.

I redid my fuel set up and the bubbles dissappaered and the UAT puckered less after that.

I'm with David, my festo's have always worked great on the suction side.

Raf

Kelly W

My Feedback: (2)

Ok, understood but my comments regarding 'Festo' fittings are geared towards o-ring sealed union fitting, some most people in these forums tend to refer to them as Festos. I did not mean the fitting on the outlet of the UAT, even though it is a Festo product. If presented with a barbed fitting, particularly a captured barb like the UAT has, it is a far superior connection vs. that of an O-rind sealed and clamped around the outer surface of a hose. There is even a UAT alternative now that uses a press in union style connector to feed the pump, not a well thought out fitting choice in my opinion.

On the o-ring sealed unions, plenty will say they have worked fine. Extrapolating that statement into an expectation that they will always work fine is a dangerous assumption.

Ironically, the term Festo as a description of a particular fitting type is incorrect in itself, since Festo buys the clear majority of its fittings from another manufacturer.

Kelly

patf

My Feedback: (4)

david

what clunk did you end up using?

David Gladwin

I used a standard Dubro clunk with a wrap of lead sheet around the pipe as a sinker, a system I have used in many other jets.
Haven't yet run the engine but will tomorrow.

Not bothered about loss of filtering as my fuel is extremeley well filtered BEFORE it gets into the tanks using an orbit clunk in the refuelling tank then a Festo fliter and after the pump a Jet Cat filter. THEN it goes into the model. There is a Festo filter before the fuel pump and of course the last chance filter in the engine cowl.

Regards, David Gladwin.

digitech

My Feedback: (10)

david you will have the same problem soon
you are filling your tanks with bubbles in your fuel , the clunk for refilling will do the same and pump oxygen in the fuel
and later settles down wiht air in your uat.
the dubro clunks ARE the ones i mean , i guess they shop at the same adres as most of us do..