Ground vs Inflight setting

High,

I am interested in your suggestions concerning the following problem or rather your suggestions to solve it. Here it is. My aircraft/engine combo needs a very rich groundsetting to have an acceptable in-flight setting. I aim for a standard 4-2-4 inflight setting but the engine has to run very rich on the ground to the extend that you are really unable to hear when you are in the ballpark. Luckilly the engine has plenty of power so it gets the aircraft airborne even while running at low revs but it is all to hairy and unpredictable. I will give you some information/statistics about my setup.
The aircraft is an OD .35 size F2B ship. Inverted engine, cowled but the cylinder head protrudes through the bottom. No cooling problems. The tank is a standard wedge shaped uniflow tank. It is 2 inches wide, one inch high and 3,5 inches long. Fuel pickup 1 inch from the tank backplate and inline with the NVA. Inflight no big difference in engine run between upright and inverted flight. The tank has a capacity of approx 100cc and is placed directly behind the engine bulkhead. I use a 75% methanol, 20% 50/50 oil and 5% nitro mix.
The engine is a Stalker .40 SE and it turns an 11x6 protech prop. Venturi diameter is already very low at 7mm with a nearly 4mm spraybar through the middle. NVA has a single hole which is pointed downwards inline with the venturi bore. The engine needles well and one extra headshim was added to have an easier 4-2 break. I use an Enya#3 plug. Expect everything else to be OK that is no leaks and no dirt in the fuel system.
There you have it, as standard as can be I think but the difference between the ground and inflightsetting simply is to big. Any advise welcomed.

Have you tried reducing the amount of castor (and/or total oil %)? It may help reduce the difference between ground and air running. Stalker recommends ZERO % castor in the engines, and only 17-22% oil:

See their set-up page:

http://www.modusa.co.uk/engines.htm#setup

.
Kiwi

I've wondered about this too. But when I had particular problems with this it was because I had the wrong NVA in the venturi, so the choke area was too large. Doesn't sound like you have that problem.

If I get this problem with my Leo28 stunt at least it'll mean that its running right again

You never mentioned the type of fuel venting that you have. If you are using uniflow to outside air, tou may have the end of the vent too close to the fuselage and be in a low pressure area that would cause leaning out as airspeed is increased. If muffler pressure is used, sometimesthe small pressure tap holes get slightly plugged and can cause irregular runs. I would check all vents and vent lines for leaks or restrictions. Let us all know what you find when you get it cured.
Bigiron

I am having the exact opposite of this problem. I have to set the motor quite lean on the ground, other wise it goes very rich in the air. Upright/inverted and inside/outside is very even.
Fox 35
Stuffer backplate
Stock head, two shims
Richard Oliver tube muffler
Uniflow tank
15% nitro/29% castor
11x5 Rev-up
40 oz "Sig" Chipmunk, Sig in quotes because the only part from the kit is the canopy.
The power setup worked quite well in my Gieske Nobler and did not exhibit this behaviour. The only big difference I see is that the venturii is buried deep in the cowl so it does not go through the hole like it did in the Nobler. I was also using a Fox muffler on the Nobler.
I plan on trying to pull one of the head shims, and try muffler pressure.

Any other suggestions?
Thanks,
Pat MacKenzie

Pat on the stock head you shouldn't need any head shims . Just the srock .010 gasket should be plenty. Perhaps you are getting some tank pressurization as airspeed is attained. Try putting a short piece of fuel tubing over the end of your uniflow vent and inserting a short piece of 3/32 inch diameter brass tubing in that and wiring it in place. If that works, you may later want to solder it in place.
It worked on my Nobler.
Bigiron

Pat,
Sweet Chipmunk. Where do you have your uniflow vent located? On mine (46 oz with DS 40) I have the vent in the cowl right behind the prop (epoxied to the motor mount and soft wired to the tank). Seems to work real well and solved the "where should I place the vent?" question. You may have a point about the location of the venturi, if the air moving around the cowl creates a zone of low pressure in the cowl, it could effectively increase your draw pressure. Wonder if there is a way to work around that.....

The uniflow vent is routed to the inboard side of the fuse facing forward, to allow filling. Are you filling through one of the cooling air inlets on either side of the spinner? This would place both the uniflow vent and the venturi stack inside the cowl. That is something I could try fairly easily.
I suspect the pressure in the cowl is low due to the cowl flap effect the lip at the bottom would have.
Pat MacKenzie

Pat,
That is exactly where my uniflow vent is located. The hole on the inboard side to be exact. The opening is just inside the cowl. Figured this was the best place for even air and it avoids excessive cutting of the stock cowl.

High Pat,
Interesting to hear you have the opposite problem that I have. If you suspect the venturi because it does not protrude through the cowling you could easilly test this by using a combat style 'Dork tube'on the venturi. Usually a piece of garden hose fits nicely ( it did on my Merco which has a comparable venturi to your Fox).

Regarding my problem I am going to experiment with different Uniflow vent locations and restricting the size of the venthole because I think this is the area of which we can control the variables the least from aircraft to aircraft.

Pat,
This thread is getting long, and Bram seems to have resumed most of the focus. IMHO, we've looked too closely at minor things, ignoring the main change from ground to air conditions -- prop load.

Again, IMHO, you'd have less audible change with a smaller prop load. For (relatively) inexpensive testing, even a TF Power Point 10-6 --- yes, TEN-six --- will unload the Fox for you. (Balance well, of course.)

Why? Prior to launch the prop is stalled, providing less thrust than at max acceleration during ground roll. Thrust applied to the model likely falls off as the model gains speed. Seem odd? Think a second or two... As the model starts to roll, a speed is reached where flow over the prop again becomes "clean"(er?). That's about max thrust applied to the model.

As the prop cleans up, RPM rises, ---EVEN if the firing mode shifts to 4-cycle (where we hear a pulse every other time). Thje safest conditions to see this are on a test stand. CAREFULLY applying a "brake" as the engine runs in 4-cycle mode. A leather glove squeezed against a spinner will drag RPM down, as a tach will confirm. As the load starts to drag RPM down, the 4-cycling mode shifts to 2-cycling. We hear more combustions, one every time piston crosses TDC, but RPM is lower.

Too much prop load, static, unloads more dramatically in the air. Fox 35s, however modified, have a very small RPM range from 4- to 2-cycling on reasonable prop load. An unusually large prop load limits static RPM more than a smaller prop, and that load drops significantly from zero velocity to flight. Under flight loads, the setting may very well be way too rich. A lesser prop load brings static ground RPM closer to in-air RPM, likely with needle setting better suited to both.

I hope you find this post, and it stimulates some thought...

Bram,
just found your reply, thanks. Steve Helmik suggested the same thing and it is something I will try.
I not using the 11-5 by accident. When I first built my Gieseke Nobler I started out with a 10-6. I could never get it to work right since the break was much larger than I wanted. I tried a lot of things and finally tried the 11-5. The run improved and so did line tension up at 45 degrees.

A few years later I was at a combat contest in Dallas, and Bob Gieseke was practice flying his Nobler. He had an 11-5 on it. He also suggested 15% nitro instead of the traditional 5%. Much better power and smoother runs.

My Nobler did not go rich in the air like the Chipmunk does, and it was less of a load in level flight, so I didn't think it was the prop. It is possible that the cooling on the ground is not as good with the Chipmunk, and this could be the cause of the big change when it gets airborne. A smaller prop would certainly help in that case.
When I get a chance to do some experimenting I will try:
10-6 prop
muffler pressure
uniflow vent in the cowl
baffles in the cowl to improve cooling
Thanks for everyone's suggestions
Pat MacKenzie

Pat,
You may be referring to my remarks...

Bob Gieseke sets up some truly awesome Fox 35s. He is very demanding of perfection; Foxmfg is more interested in getting them out and sold. I have no doubt that one of his own engines, if Bob G flies it, meets his standards. Most other Fox 35s, even the best of the mod'd ones, may not be as good. I was thinking more towards the better end of standard Foxes, including Silver XX's, L&J iron and my own.

Best of luck

Lou,
sorry, that post was of course in response to yours.
Pat MacKenzie

Ron B.
F4Fguy

Don't know if it'll cure it or not,but"straight down" is not the most effective position for the NV jet.It needs to be in the point of highest air velocity which is just below the smallest point of the venturi.In this case that's the side of the NV.The best way to approximate this point is to rotate the NV until the jet opening just disappears when looking directly down the venturi opening.This location will give the best response to flow changes in the airstream.

Hope this helps,

Try Bigiron's suggestion on resticting the uniflow. You just need a pinhole to let air in. I run a one way valve on mine when not running muffler pressure. If you uniflow a tank to get uniform pressure throughout the flight, then why subject the uniflow to changing airpressure as you fly?

Ron,
I had the spraybar hole positioned as you describe but I observed more variations in the engine run on any needle setting then I have now. ( I like to think sometimes the air doesn't know which way to flow and switches to and fro. Hogwash probably but fact is that the engine runs are more constant now I have the hole pointed as I described ) I would like to try a spraybar with two holes though because indeed, located at the narrowest position of the venturi should be best for fueldraw.

Hi Bram,

I am experiencing the opposite.I have to take off almost death lean, to have the engine going richer after 1 lap. Then it settles in a nice 4-2 behaviour. It is a Stalker 61 RE in sidewinder configuration.

I don't tune the engine before a flight, but usually AFTER a flight.

I would assume that the load on the engine is highest when the model is static. Higher load means a leaner run, lower loads mean a richer run. So For me it makes sence that I have to tune it lean, to get a decent 'in flight' behaviour.

So it seems to me that the changing load conditions (static/ in fligth) cannot be the cause of your engines behaviour, unless it has a very different power characteristic compared to my engine.

So what else can be happening? After take-off the pressure in the spraybar might drop because of the way the tank is build and the way outside pressure is applied on the tank. Obviously the tank pressure drops after take-off. What can be the reason for this?

1. In standard vented tanks (The Aldrich system) The static run is very much governed by the horizontal heigth of the fuel in the tank.
In the air, however, the engine run is governed by the centrifugal force on the fuel. This can mean quite a different effect. When you change to a uniflow tank, I would assume this difference in engine behaviour will become smaller.

2. The air close behind the prop in a cowlis very turbulent and unpredictable.
Instead of adding pressure to the system, at a certain speed of the model you might have a suction effect. In this case leaning out would be the consequence. Try putting any pressure in let of the tank at least 15 cm behind the prop.

FWIW,

Regards,
Paul

>2. The air close behind the prop in a cowlis very turbulent
>and unpredictable.
>Instead of adding pressure to the system, at a certain speed
>of the model you might have a suction effect. In this case
>leaning out would be the consequence. Try putting any
>pressure in let of the tank at least 15 cm behind the prop.
>
>FWIW,
>
>Regards,
>Paul

I realize this is something some people have advocated from time to time, but Matt decided two planes ago that he would try putting the uniflow vent (and filler tube) from the tank in an air scoop on top of the nose and directly behind the prop. Since this is connected to the tank by way of a flexible tube, it was something that could be easily changed if it didn't work. OK, everybody said it won't work, we still have to try it. Well, it worked. He did the same on his next plane and it worked. He put it on his next plane. The filler tube and inlet tube to the uniflow tank is hidden in that airscoop right behind the prop. It works. We have also found that there doesn't seem to be as much effect from wind as you get when it is farther back and out in the open.

I don't know. Is this a definitive answer? Certainly not. But in this particular situation it has been working just fine. (Pa-61 running on a pipe)

OK, everybody said it won't work.

Why wouldn't it work?
May be "John Q Everybody" should listen to the experts.

Reminds me last weekend I had someone tell me that the venturi hole has nothing to do with the engine run.... He argued with me. Short story he’s right I’m wrong, no need to waist time and brain cells.

Scott(just out for the fun of it) Riese

Leonard
could you post a picture or drawing of your "hood scoop", I'm having trouble visualizing how refuelling happens.
Andrew Raney

You know how hard we try to shim the tank so upright and inverted run the same lap time. Well, I have two planes where I have to set them very lean on the ground, only to have them go rich in the air. I am experimenting with shims to move the tank inboard/outboard. Same principal as shimming up and down. I'll try to let the group know how this works. Meanwhile, I'm interestd to know if anyone else is doing this.

Hi John,

I like your suggestion very much. Obviously if the fuel pickup is too far outboard the engine would have to suck the fuel against centrifugal force explaining the leaning out during flight. Unfortunately I have only limited space available to trim the tankposition horizontally ( 3 mm ) but I will surely give it a try. Meanwhile I am wondering if a propellor change could do the trick. If, as Paul states, there is a significant difference in engine load between static and inflight conditions ( usually causing a richening effect during flight as I understand it ) then a different propellor with a different ratio of static/inflight efficiency might also help. Lots of advise guys, thanks.

Ron B.
F4Fguy

John:
I've tried this and it does work.Check my "White Profile".

>Ron B.
>F4Fguy
>
>John:
> I've tried this and it does work.Check my "White
>Profile".
>
> Ron B.

Wow! I gotta try this, it also keeps the plane from crashing...

In Christ,

John Wright ,
A long time ago on a profile I had a problem of lean ground,lean inn air. Paul walker told me to move the tank inside ,so after i did I had the reverse,so i went back & forth til it was equal.
John L.

Fat or Lean?

I've been lurking on this one but have some experiences to relate also so here goes. Currently my Super Tucano profile is running a ST-46 Bullring with no mods other then the ST G-51 venturi and needle set up ala factory ST. This is workig well I believe. The tanks is a GRW 4.5 ounce uniflow with all pipes to the front face other then the overflow which exits out the bottom.

Problems: On a as installed no mod run during the models maiden flights and following flights I started to notice that the outboard side of the fuselage was turning pink,..fuel is pink, ...so it's must be fuel blowing out or leaking from somewhere. (Tank pressure check did not turn up any leaks.) As far as I could tell runtime was not showing that this leakage was excessive. On 5% Nitro 20% oil blend, the 4.5 oz's were giving up runtimes in the 8 min. range on a ground set of 9000 rpm's. There was a lean run condition happening toward the end of the tank also, but it was not excessive.

Close inspection during static runs on the ground seem to indicate that a portion of the fuel was being blown/sucked out of the forward facing uniflow vent at the front of the tank. I of course could not tell if this phenom increased during flight, but it sure was turning my white/clear Brodak dope pink.

Solution: Run muffler pres to stop the flow of fuel from the vent. With the fuel system "tight" no fuel should be getting out right. The use of muffler pressure did make a marked change in the needle position. The now richer mixture had to be dialed down to get the launch rpm's back up to pre-pressure numbers. That should cure everything right? NOT.

With the pres being fed to the uniflow vent erratic engine runs resulted once the model was air borne. Once again static ground runs indicated something was amiss. A close look at the fuel feed line gave up that bubbles were being drawn/pushed into the main fuel feed line and heading toward the NVA. To solve this the muffler pres was connected to the over flow vent, and the uniflow vent capped off,..."Bubbles be gone I say", and they were. Again all seemed to be well, or at least better.,..but the question still begs to be answered,.."Why will it not run on the non-pressure system?"

Well I'm going to try again with standard uniflow venting tonight I hope. Part of the fix is to both move and restrict the inlet side of the uniflow vent as has been noted in replies above. To this end I've fabbed up a little extension for the main uniflow vent. It will move the vent to a place above the cylinder and away from turbulent air that may be working magic at the rear of the cylinder. The inlet to this extension has also been soldered closed and then opened up with a pin vise drill and a extremely small dia. drill bit. I'll let you all know how it turns out. With the tubing connections it will be very easy to re-plumb the system to any of the older configs, so I'll see what else happens if this change fails.

What I've learned so far: I do know this,..muffler pressure will require the needle to be turned in to gain the rpm's I like to launch at. Under pressure the fuel feed system goes rich. At this same new needle (pres.) setting, if the non-pressure system is in place, an extreme lean condition will result prior to the needle being retuned (opened up) due to the natural low feed pressure. With my motor this rpm change can be as much as 1500 to 2000 rpm's for given needle setting between the pres and non-pres feed system.

Something else of note and a guess on my part. Once I was happy with the engine run while under pressure feed I started to notice a step change in rpm (up) at about 6 min's into the engine run. What's going on now I ask myself?. Well close inspection would show that my tank mounting strap at the front of the tank had torn. I believe vides where at work again producing a foamed fuel condition which in turn was acting like the bubbles in the fuel line thing noted earlier. As the tank empties the effects of the lose tank on fuel delivery would become more marked. So make sure there tank is vibe free.

Pictures: My vent extension is held by tubing into the tank factory vent line. As it's of small i.d. it slips into the factory tube no prob. The small collar of 1/8th tubing soldered in place on the extension allows the med. size fuel line to grip it for a secure connection.

Tony,
I am having a similar issue with 2 Tower 40s that I have on a profile Oriental. I too made up a fixture to put the uniflow vent in better air, but did not experience any positive results with standard or smaller tubing. Like you, when the pressure was run to the overflow vent and the uniflow capped off, life was good. That was about as far as I got with my experimentation (if it runs, leave it alone) since there was not much noticeable increase in speed as the tank drained. BTW, I ran 3 different tanks skinny, tall and chicken-hopper with the same results running uniflow.

When I get some time, I plan on trying a plastic tank where I can change the uniflow and pick-up relationship to see if that helps. May need to modify a hard tank if that ends up working. At any rate, let me know how things work out for you. My next "easy" attempts at controlling the problem will be different diameter fuel tubing to see what that does. Good luck and let me know what ends up working for you and I will give it a try here (or may on some Sunday with you guys).

LAST EDITED ON Sep-06-03 AT 12:56 PM (CDT)
 
Well, the results are in,..er sort of. I had four flights last night before the sun went down, last flight started at 20:00 on my watch.

The first three flights I ran the muffler pres. to the overflow vent which negates the uniflow. I'm still working with the idea that the uniflow vent on this GRW tank, as built, will allow bubbles to enter the fuel feed line when under muffler pres. to the uniflow vent line.

Flight #1: I set the ground rpm's to approx the 9800 range. While this was good sounding on the ground it seemed to richen up by degree in the air. Given the weight of the Super T, I did not throw the model around to much,..conditions were rather breezy also, so stunts were minimal,..loops, some invert, outside loops. The richer engine run did not instill confidence to do much of anything. Although on the rich side the run did end with only a slight rpm increase toward the last laps,..then changing to a very marked rpm increase 1 1/2 laps before the tank emptied and the engine quit. This flight, in a general sense, I took as a good sign that things were working normalish.

Flight #2: (pres. to overflow vent/uniflow capped). Dialed up the rpm again on the ground to what I thought was in the 10000 rpm range. This flight again proved to be lack luster for stunts due to high lap times/ low flying speeds. As flight #1, it ended with a small rpm increase near tank end with a large rpm increase for the last lap or lap and 1/2 (which signals near empty) before engine out. This flight was still on the rich side for me.

Flight #3: Again dialed up the rpm's on the ground to try for 10200 rpm. This once again did not produce the flight speed/lap times I was after. But as noted above the same engine run characteristics were present. "...it ended with a small rpm increase near tank end with a large rpm increase for the last lap or lap and 1/2 (which signals near empty) before engine out.".

With the light fading I at least wanted to try the open uniflow with my vent pipe extension. The three flights with pres in use at the overflow vent and the uniflow capped did produce consistent engine runs, albeit, lack luster engine performance for stunt work. I must state that moves on the ST NVA were small moves closed for each of the first three flights. I was trying to sneak up on the needle setting rather then grossly over shoot the mark and end up with a blistering high speed run. At anything near 4.5 lap times the Tucano pulls your arm off. I did not need a line failure on the only model I've got to fly.

Flight #4: Uniflow vent extension installed, sans muffler pres. As I'd left the needle were the last pres. run was set I expected to see a major rpm jump once the engine was fired. With a ground set of approx 10200 on the third pres. flight, this fourth run started fine and then the rpm came on to the tune of 11500 or better. I let it run to see if it would settle down but it seemed to initially hunt around a bit. With the rpm's up in that 11000 range I opened the needle again looking for 10200. There was still some rpm movement once dialed into this range. Once in the air the model took on the steady drone as it had done in flights 1, 2 and 3, again at lower air speed then I'd have liked. It was consistent in the run, just not were I'd like it (to slow). Engine did pick up a few r's need the end laps, then came on yet again for the 1 or 1.5 laps before engine out.

So what's it all mean? More flight testing is in order. I need to perhaps get a little more heavy handed with the needle and risk a high rpm run. During Flt. #4 I did note a little cooling effect at the inlet to the uniflow vent. This I again I'm thinking was fuel coming out of this vent line,..I could feel something there but could not see anything in the propwash and failing light. The white dope is stained yet again, so I must believe that fuel is leaving the tank somehow. I do not think it's a case of unburned fuel leaving with the exhaust. I'll have to follow the "blood trail" and find the source of the stains. May get a few flights in tonight. Sundays out John as I'll be working day shifts again. One thing is for sure (I think?), the weird runs of the previous session, a week ago, are no longer present. This I'm still thinking was caused by the micro vides of the tank due to a broken mounting strap. I could not detect this broken strap on the ground btw,..which only made us scratch the head all the more during those flights.

The Quest continues.

Tony

edit: Couple of pic's here of the new vent piping. This moves the uniflow into clean air/propwash, completely away from other structures. Static runs seem to indicate that it will work. Just wonder if the inflight rich condition will return? With the fuel tubing attachment I can see any fuel migration toward the inlet. There was a bit during the front of the static run, but it stopped as the run progressed. Flight testing to follow.

I would just comment that I wouldn't run anyone's metal tank without first opening it up to see what's what inside. Actually I can't remember buying a tank that was vented to suit me in the first place, so I end up opening them up anyway.

Not so much a problem on full fuselage ships, but profiles can experience strange modes of vibration.

I had been successfully using a uniflow 4 ounce metal tank on my profiles, with no muffler pressurization (just uniflow forward "ram"). There was always a ground setting and it changed in the air, but once I found the sweet spot, it worked well. And the engine was always a little richer in the air than on ground. I thought I understood this, it seems like the "weight" of the fuel head is the factor (in flight the ship experiences >1G). Also, all my profiles recess the fuel tank into the fuselage, which I found improved performance greatly by increasing effective fuel head height.)

I'll mention that I could see typical ram air effects on fuel mixture in flight with this setup. I reduced them by pinching or restricting the uniflow vent.

After I installed a new inline fuel filter, one ship suddenly started acting very differently. The fuel mixture setting suddenly had to be much richer to keep the engine running, and as soon as I launched the engine would quit after a lap.

Three or four consistent weird flights like this passed and I was wondering about fuel, prop, glow plug, phases of the moon, whatever?
I was just starting another strange mode flight and suddenly my launch partner stopped the launch and said he knew what was wrong. He observed that there was a resonance - in the FUEL line, with the new filter on it. Start the engine, and it would run fine - no visible vibration. Shortly after launch, the extra Gs on the fuel line moved it to a position where it resonated like crazy, obviously leaning the mixture. Then the engine would lean out and die after a lap. Or it would nearly die, go rich again, and repeat the cycle.

The simple solution was to shorten the fuel line with the filter a little, which stopped this behavior.

I've observed other profile nose resonances which would cycle between rich and lean, slipping in and out of visible vibration modes. A horrible one was seen on an old Flight Streak with a Fox .35 and 10-6 Master Airscrew plastic prop, a Perfect wedge tank, and about 6 washers under the front of the engine for "offset". That nose would turn into a 1" blurr!

This is why I believe so strongly in "overbuilding" the profile noses, and I'll also say that I feel the recessed fuel tank (although it requires effort to cut the hole and modify the tank) is well worth the modification when it comes to profile engine runs. The standard "rotten profile engine run" was never a complaint for my Mo'Bests.

[photo not recovered: 3e69bb3870f12ad5.jpg]

"Try not. Do, or do not. There is no try." -Yoda

Tony,
Your pictures look exactly like the set-up that I used on mine except that I chose to use the Prather small tubing to the tank and I also bent my post to bring the vent closer to the center of the airplane. Keep chasing this one and I will start back into my quest to return to standard uniflow venting. At some point we may find an answer that will help.

John, Larry,
I put two flights in on the snorkel vent pictured in the second photo shoot. Prior to my launch team showing up at the flying field (son Ryan 1yr. and wife Lynda) I managed two more static runs. The first one again ran very well. I got the rpm's into the 10200 area as I'd been trying the previous flying session,..heavy fingers on the needle here. It ran the whole tank out at this setting with some rpm increases with nose up attitude I applied. Rpm's returning to set shortly after the return to level attitude. I'm going after the high rpm /low pitch run here, so a 4-2-4 break is not really in the picture, but none the less the rpm came up by degree with attitude change. This I took as an indication that I was not overly lean with the needle setting. There was a minor amount of rpm swing during the this run but all in all I thought it was good. Some small bubbles still show up in the fuel feed line, these could be the cause of the small rpm swings/bumps. The uniflow inlet tubing/piping stayed dry and fuel free during this run.

Second static run produced very similar results. I turned the needle open a bit here to drop the rpm and still had the same consistent engine run. I did notice right off the bat that fuel was trying to exit the uniflow vent system and did blow back on the airframe a small amount. I could see a bit of a turbulent action in the fuel tubing connecting the vent to the snorkel. This stop after a 30 or 40 second time span and the tubing was clear of fuel yet again. Fuel feed line still had the bubbles in it as did run #1. Just extremely small bubbles mind you, now and again, but they where there. I did not see this during muffler pres. system runs yesterday. For both runs the engine shutdown was marked by a rpm increase of magnitude then a clean cut.

The first flight on this set up produced very much the same run as noted above. I do not think that I detected a rpm increase after launch, maybe a small increase if any. This time the flying speed was up and moving the model around was not hard. It was a bit to quick in fact line tension was high. But in keeping with the "keep my one and only safe" game plan, only loops and inverted where flown. The wind was up a bit to so this prompted me to play safe all the more. Inverted runs were the same speed as near as I could tell. I was running the clock to check fuel milage not lap times on this outing. 4.5 oz's got me into 8 min's of run time. Engine shut down was again marked by a step change increase in rpm for approx. a 1/2 lap (not 1.5 laps as I'd stated previously) and then a clean shutdown.

Second flight: You could copy the first flight and paste it here for the second. Very much the same in all respects. I did dial the needle out a bit to slow it down but all else was good. If I could say that this was a good place to be I'd be happy I guess. But I'm still playing with the flying speed. Last week was to slow, tonight was a bit quick. So I'm still looking for the sweet spot. I called it a night at this point as the pit crew had to get home for the nightly bath tub routine. Fine by me,..I just needed to get this thing in the air so I could sleep tonight. Seems much better then before. I still wonder at how or what dynamic is at work that allows the fuel to migrate up the uniflow vent during the start of the run, on the odd occasion? With the vent inlet mounted well above the fuel level I'd not expect that the fuel would migrate up the line, more so given that the engine is pulling vacuum on the system with the fuel draw rate.

I agree 200% with Larry. If the front end is in vibe mode your in for trouble in the run department. I'd like to try the Mo'Best Inlaid tank, but that will have to wait for the next model. Actually, my stalled "Phoenix Twister" (out of the ashes) rebuild might take to the inlaid tank thing?..hmmm. I'll keep playing with the Tucano and see what else I can figure out, but it will be a week or more before next fights,..the work things gona get in da way,..again.

Bram sez he's using an 11-6 Protech prop (what is that, where is it made, and what does one look like, material, etc?) on his Stalker 40. Pat McK. is using an 11-5 RevUp on his Fox .35. Both sound like too much prop, to me. Yes, the oil issue might be a factor on the Stalker. Cooling might also be an issue. It has been extra hot in most parts of the US, Canada and much of Europe. Yup, the uniflow inlet might be the problem also, but I'd vote to put less prop on first, then take a good look at the cooling, clean the fins, etc.
Steve

>Bram sez he's using an 11-6 Protech prop (what is that,
>where is it made, and what does one look like, material,
>etc?) on his Stalker 40.

Steve,

The protech is a European make prop which looks much the same as the standard APC, same material also I think ( fibre reinforced nylon )but it is black. The blades have much the same contour as an APC but for the tip which is 'clipped'. Biggest difference is I think that the blades are much thinner than an APC and it weighs much less ( APC 11x6=40 grs, protech 11x6=30 grs.

As to it being too much prop you must remember the Stalker is in fact an 'old style' loop scavenged engine with modern interiors (ABC, double ball bearings) which does not opperate in the same RPM range as a lot of the other modern engines (8,5K-9K compared to 10K+). It doesn't sound overloaded at all.

As noted b4, lots of info and experiments abound:
1) Change uniflow inlet position and/or diameter.
2) Lower oil content.
3) Horizontal position of fuel tank.
4) Use muffler pressure.
5) Different prop.
I will keep you guys posted on the final solution.

Guys,

Some results. Tried muffler pressure today and it certainly is an improvement. But note that the original effect-ground setting rich, inflight setting more lean (in other words the engine revs up/runs more lean inflight) remains the same. This would suggest that there was no problem with siffoning via or a low pressure area near the uniflow vent. I suspect that the positive fuelpressure and the resulting leaner needle setting (half a turn) brought me in the finer needling range of the engine enabling me to better dial in a groundsetting and better able to reproduce that. (I aim for a fat 2 stroke with the aircraft on its tail).

I tried both pressure to the uniflow vent and pressure to the overflow and got the best result with pressure on the uniflow. When I fed pressure to the overflow the engine started leaning out during the second half of the 6min. flight needing lightning responses in the cloverleaf.

Unfortunately I can't use a much less wider tank to see if the horizontal position could be brought -much- more to the centerline of the aircraft/engine. The current tank (100cc) being minimum to fly the entier pattern and I have very limited space available to make it langer and thinner.

I will try another day relocating the uniflow vent but think after above experience that it won't solve the problem. Keep you posted.

Bram,

The pressurized uniflow is usually an excellent setup, but can suffer from a "bubble" problem - that is, the extra pressure lets a bubble form, which is close enough to the fuel inlet that it gets picked up.

Sometimes this does not happen until the tank has partially emptied. Anyway, the symptom of going lean at some point can be an indicator of this problem. The best solution is probably to relocate the uniflow vent terminus inside the tank, so that it is just another 1/16" or so further from the fuel line end. But sometimes restricting the uniflow inlet can help, so you might want to try that first.

A proper uniflow setup, pressurized or not, should not go lean until the uniflow vent inside the tank gets uncovered, at the very end of the flight.

[photo not recovered: 3e69bb3870f12ad5.jpg]

"Don't wait for your ship to come; swim out to it."

Centrifical force has a lot of effect on fuel flow. Once the airplane is in the air centrifical force can increase the fuel pressure and make the engine run rich. Centrifical force can also reduce fuel pressure and make the engine run lean. Try moving the fuel tank or relocating the fuel line connection to the fuel tank.

Also forward speed of the airplane produces ram air like a turbo charger on a race car. As the airplane travels 65 mph through the air it increased the intake air pressure causing the engine to run lean. If you have a cowling of something over the intake or infront of the intake it can create a vacuum on the intake making the engine run rich.

Some comments on Gary's post. I think you got it backward on effects of positive vs negative pressure making the engine run rich vs lean.

With a uniflow tank, the engine thinks it is sucking fuel from the end of the uniflow submerged in the fuel. So the relative position of the uniflow vs the spraybar is different when the airplane is sitting on the ground, vs when it is flying. The position of the uniflow is much more important than the position of the fuel pickup, in my experience.

Larry is right on about solid noses on profiles (or anything). I've put tanks all the way through the body, recessed them, or just banded them on to the side; they all worked fine. I do think a recessed tank looks neat.

To round things off. After a number of flights changing a number of things suggested in this thread I settled for the solution using muffler pressure on the uniflow vent. Relocating the position of the uniflow vent changed nothing to the original problem ( very rich ground setting ). I had 3 mm play inside the fuselage and used it to move the tank more inboard to eliminate the possible negative effects of centrifugal force as much as possible (Less distance for the engine to suck the fuel against the centrifugal force but honestly I am not quite convinced that it is of real significance to ground vs inflight setting using the uniflow setup especially because my problem is more often than not opposite to what others in this thread experience undoubtedly with comparable tank location.Why? To answer that question could be the subject of a new thread).
After a preliminary test with muffler pressure showing promising but still not convincing results I opened my tank to check and if need be alter the position of the uniflow inside the tank. As it turned out the uniflow came out at the ouboard front of the tank, 5 centimeters from the fuelpickup point but the end was loose, solder joint broken possibly giving cause for vibration/foaming inside the tank. This was repaired and after re-installing I test flew the plane with satisfying results. So much so that I could install a slightly bigger venturi ( 7,2 mm iso 7mm ).
As stated before, the original behaviour remains unchanged ie. ground setting rich, leaning out inflight but I am better able to dial in a reproducable ground setting caused by (I think) the higher fuelpressure allowing to use the finer needlingrange of the NVA. Going further through the steps of trimming this aircraft I will monitor closely the behaviour when I use a different propellor which is more resistant to unwinding inflight.

For now SSW again proved to be invaluable in helping to solve this problem. Thanks to all.

Bram,

I've been lurking on this thread for a few days wondering if anyone would run with Larry's comments about vibration.

All the effort on tanks is worth investigating but a tank is a pretty simple mechanism that seldom gives or solves problems as long as it is free of leaks and doesn't vibrate.

I'd look at a couple of simple things.

I noted in your first message that you had "added a shim for a better 4-2-4 run". That was probably a bit premature since you apparently didn't have any kind of run yet. Couple of problems with that.

When you reduce compression by adding shims you also retard the ignition timing of the engine. Our glow ignition engines start combustion through a combination of three basic things: the volitility of the fuel; the ignition spark of the glow plug and the heat of compression that is a substantial part of the ignition process. Almost everyone has experienced the engine that starts up while being choked with no battery attached. All the heat to start that ignition came merely from squeezing the fuel air mixture...just like a diesel.

When these three factors are neatly balanced the engine runs clean and powerfully. Alter any of the three and the run will "change". It might get more powerful, less powerful or even quite if the change is extreme enough. Altering the head compression ratio through shim changes should "only" be done as a fine tuning mechanism when you want to make a subtle change in the run...specifically, to alter the point at which the load on the engine causes it to change from a four to a two stroke for instance. Changing nothing else, adding head shims will delay the break from a four to a two stroke and can, other conditions being right, allow an engine to run strongly and never break into a two stroke.

The key phrase is "other conditions being right" and that's what you've got to determine before you start to fiddle with it.

Run this test the next time you go flying. Start the engine up and leave the battery on until you get the setting you think you want. Ideally, use a tach to monitor RPM. When satisfied with the setting, remove the battery and listen carefully to see if the engine loses revs. Check the tachometer again.

If you've lost revs when the battery is removed you're looking at an ignition timing problem that could be the root of your problem. This could be the result of too little compression reducing the heat of compression and retarding the spark so that under load its got to be running pretty lean to get the revs you want but, when in the air the prop unloads and allow the engine to drop back into the four stroke.

First thing I'd do is take out the additional head shim (by the way, do you have any ideal how thick that shim is?). Put the head back on and run the test again. If you don't lose revs you might be right back in the ball game.

If it still loses revs, make sure you are running a hot glow plug like a Thunderbolt RC Long or four cycle plug.

Finally, you could use a higher nitro fuel but this shouldn't be necessary since the European engines are intended for use with no or low nitro.

Vibration can cause all sorts of funny engine run problems. Check the standard items such as prop and spinner balance. Add a couple of squirts of ArmorAll to a gallon of fuel to reduce the foaming. Very important, make sure the needle valve isn't vibrating. If you're using a Super Tiger type of collet supported needle with no spring retainer make very sure that the collet is tightened to the point it takes some effort to turn it. If the tip end of the needle vibrates you will *never* get a decent run.

Finally, if there is not perceptible vibration you could still have a harmonic situation that will cause a very short amplitude high rate vibration that might not be noticable but will still cause fuel foaming. Trying props that require significantly different RPMs will alter the frequency of the front end. Brett Buck suggests adding mass to the tank...maybe a half oz of stick on lead, for instance.

Bottom line, I doubt very much that plumbing is causing the sorts of problems you've experienced. I'd look hard at engine timing and vibration.

Ted,
Good thoughts on onset of ignition. There IS another factor that helps a "diesel-like" kick -- catalysis. Methanol and ethanol, and some other flammables, heat in the presence of platinum. There's either platinum or a metal with similar effect in our glow plugs. Good compression, a sharp flip, the right mix in the chamber, AND this catalytic effect can surprise us.

Ted,

After the experience of this weekends contest I think you might be exactly right with your analysis. I did notice a considerable drop in RPM when the battery was disconnected so obviously, the shim has got to go. You are very right, I have been too hasty. Things are improving I must say especially after I changed from an 11x6 to an 11x5 prop. The engine behaved much more 'stuntlike' with a reasonable 4-2 break and maybe more important, sounded much happier.
I also think you might be on to something regarding the NVA. The needle is a rather loose fit in the spraybar and might very well be vibrating a little. Also it is very critical, one or two clicks is enough to change an accepatble engine run in to screaming dissaster. Room for experiment here.

One thing keeps puzzling me though. I watched a lot of airplanes taking off this weekend and the general engine behaviour was; a fourstroking static run leaning out somewhat when the aircraft accelarates and you notice the engine picking up revs as it unloads. Once airborne a return to a solid fourstroking run. In my case the original situation remains, it has to run very rich static to prevent it from totally running away in the air. During accelartion a gradual leaning out to ( hopefully ) the desired fourstroking run. I checked my tanksetup with others and it is absolutely comparable. As such I don't mind this behaviour but my problem is that it is very difficult to reproduce the right setting from flight to flight.

Indeed I think I will stop fiddling with the tank and concentrate from now on on props, elimination of vibration/foaming (if any, the engine runs very smooth), NVA and try to be patient enough to change one thing at a time ( but that is solely my problem, can't blame the plane)