Update on soft profile noses and fuel foaming

I finally came up with an easy fix for the problematic Spitfire. If you remember, apparently three pancake crashes had softened up the nose a bit and nothing I tried would get it to run smoothly without fuel foaming. The fuel foamed no matter what.

Today's solution- an LA46 and a 12/4 prop. Flew six flights and they all ran well and had stable predictable settings. The bigger motor could turn the bigger prop up around 10k so the speed is again reasonable- 5.2sec/lap or so on 60 ft lines. The extra horsepower and prop pull the plane with authority and the excess weight( it weighs ~50 oz. instead of the target 45 oz.) is no problem, except it just doesn't turn quite as tight as it should. Apparently the bigger prop and motor don't hit the same vibration frequencies to foam the fuel. I'm keeping my fingers crossed that the nose holds up and doesn't start misbehaving after a few more flights.

If your profile plane runs rough and won't run at a predictable setting on the ground and flight to flight, suspect a soft nose and do something about it. Classic symptom is a motor that goes rich right after launch and won't hold a setting steady enough to tach precisely on the ground. Also, if the motor run changes erratically during flight, sometimes to rich, sometimes too lean.

I once had a strange "resonance" situation on a profile ship, which ultimately turned out to be due to an inline fuel filter. It would resonate, the fuel would foam and it would go lean and speed up, smooth up and then stop resonating. Then it would go rich again, resonate, and repeat the cycle. At higher flight speeds, the G forces would also affect this problem.

Once I relocated the fuel filter in the line just a bit, no more resonance and the engine ran correctly.

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"Basic research is what I am doing when I don't know what I'm doing." - Wernher von Braun

Some folks don't like to do it,but the tiniest bit of Armor-all in the fuel will stop it from foaming,also.
Dale

Yes, but it seems that the silicone in the armorall will build up on the glo plug, reducing its ability to do its thing. I'm pretty sure that's what caused my engine to go south at the Houston contest last year. Don't guess it's a big problem if you don't mind changing plugs, but it caused me to blow an official flight. Bad timing.

>Yes, but it seems that the silicone in the armorall will
>build up on the glo plug, reducing its ability to do its
>thing. I'm pretty sure that's what caused my engine to go
>south at the Houston contest last year. Don't guess it's a
>big problem if you don't mind changing plugs, but it caused
>me to blow an official flight. Bad timing.

Ron B.
F4Fguy

I've seen the plug coating phenomenon with no Armorall,and with straight castor fuel,also with all synthetic.Without better evidence I'd have to say it's not due to either oil,or silicone additives.If I had to guess, and thats all anyone's doing here,I'd say it's simply oxidation of the wire surface,making it inert as a catalyst.

The best cure for foaming is to eliminate the source of the vibration,or to force it to a higher(or lower) harmonic.This can usually be done by stiffening the front end or by soft mounting the tank,or by running at a different RPM.When all else fails,add a few(very few)drops/gallon,of C10 or C8 alchohol.This is a much more effective surfactant than Armorall and burns with the fuel*.

* For all I know it may be what's in Armorall before they add the water.

I use a small amount of armorall, about one squirt in 3 gals of fuel. I decreases the foaming but doesn't eliminate it. I'm afraid to add more due to the water content of armorall.
On the other hand I have a CLC Veco Tomahawk that has the shakes. I've gotten away from it with an Enya 29 on a 9-4. My McCoys won't turn up fast enough to climb out of the resonant frequency. The nose is very stiff. I'm tempted to replace the stiff nose with a rubber nose, but don't have the heart to cut it up. Anybody got a solution for a plane with a severe case of the shakes?

Hi John,
The plastic tank on my test stand foamed fuel like crazy. I put two thicknesses of 1/4" "bubble wrap" under the tank, and no more foaming! I'm not sure you can do this in a Tomahawk . . .

Ralph Wenzel

>
>Today's solution- an LA46 and a 12/4 prop. Flew six flights
>and they all ran well and had stable predictable settings.
>The bigger motor could turn the bigger prop up around 10k so
>the speed is again reasonable- 5.2sec/lap or so on 60 ft
>lines. The extra horsepower and prop pull the plane with
>authority and the excess weight( it weighs ~50 oz. instead
>of the target 45 oz.) is no problem, except it just doesn't
>turn quite as tight as it should. Apparently the bigger
>prop and motor don't hit the same vibration frequencies to

Phil;

How about your setup info, e.g. prop, venturi size, NVA, fuel, and prop make. Also, did you do a squish band reduction similar to what you do on yourr LA 40s?

V/r

I tried some fuel with Armorall in it, but it didn't solve the problem. I can give you all the details about the engine setup, but as Larry says, the key was figuring out that it was a vibration problem and doing something that appears to have fixed it. I wouldn't be surprised if the problem comes back after some more flying. Then I'll have to try some fixes on the nose. It's easy to see the foaming fuel in the silicon fuel line. No foam, perfect run, foam, nothing but trouble.

The engine is Ken's LA46, with three head gaskets. No head work. OS 1a needle valve into a .187 spigot venturi(roughly equivalent to the standard OS NVA in a .262 or so venturi), Sig 10/20(50%syn oil) plus 7 oz. of castor per gallon. Thinned Zinger 12/4. The 46 will 4 stroke at 9800 with the 12 in. prop and pull the plane at 50 mph. on 60 ft lines(5.1sec lap). Same prop on the 40 wouldn't go much over 8800. The plane would barely make 44 mph on the same lines(6 sec lap). Tons of time in the maneuvers, but unflyable in anything over a 5 mph breeze. But, it ran just fine.

That is what finally tipped me off to the vibration. When we tried smaller props/more compression to get the hp and speed up the motor started running erratically. A little bigger motor, more torque, less vibration at the critical frequencies, fixed the hp/speed problem and appears to be avoiding the vibration problem.

Of course the "forklift upgrade" would be as I did: strip the ply, replace(possibly extend) the engine bearors and any oily balsa. But I could have built another fuse as quickly as all that. If the ship is considered toast anyway, try Boatlife "Gitrot". Boaters with soft transoms drill long cross grain holes and pour the watery epoxy in. It migrates along the soft grain and solidifies things. Probably adds a lot of weight, but if you're going to trash the thing anyway, it may be worth a shot. Don

I've been trying to avoid any real work to solve the problem. Quick fix is more my speed. I'll fiddle with the 46 for awhile. If it continues to work, then I may try going back to the 40(I like how the plane flys on thinner lines better.)and do some more experimenting with some sort of tuned exhaust to see if I can make the 40 turn the 12 in. prop.

Beefing the nose will take some work. The construction is a bit different- no ply involved. The front end is 3/4 in. contest balsa. No wing cutout, the wings butt up against the sides of the fuse like a Detroiter. The motor mounts are just that, short pieces of 3/8x3/4. The whole thing is stiffened by two layers of 2 oz. glass on each side, back into the wing centersection. Quite light(5 oz. before finishing) and very stiff. But maybe just a bit too light. It was fine before the forced landings so maybe just a little bit more wood up front will make it hold up better.

>I've been trying to avoid any real work to solve the
>problem. Quick fix is more my speed. I'll fiddle with the
>46 for awhile. If it continues to work, then I may try
>going back to the 40(I like how the plane flys on thinner
>lines better.)and do some more experimenting with some sort
>of tuned exhaust to see if I can make the 40 turn the 12 in.
>prop.
>
>Beefing the nose will take some work. The construction is a
>bit different- no ply involved. The front end is 3/4 in.
>contest balsa. No wing cutout, the wings butt up against
>the sides of the fuse like a Detroiter. The motor mounts
>are just that, short pieces of 3/8x3/4. The whole thing is
>stiffened by two layers of 2 oz. glass on each side, back
>into the wing centersection. Quite light(5 oz. before
>finishing) and very stiff. But maybe just a bit too light.
>It was fine before the forced landings so maybe just a
>little bit more wood up front will make it hold up better.

Ron B.
F4Fguy

Phil:

A couple of thoughts:

It sounds as if the problem is,as suspected,a sympathetic vibration.You might try increasing the frequency by wrapping on another layer of glass,or,better yet,bias laid carbon cloth.

Another approach would be to isolate the tank,by using a ply tank mount silicone cemented to the fuse,with the tank rubber banded to the mount.You could go another step by putting a piece of viscoelastic foam between the tank and mount.