richrfl

My Feedback: (2)

While looking for a solution to an inverted engine located below the recommended place related to the fuel tank, I found this article in Fly RC magazine: http://www.flyrc.com/optimize-your-f...t-performance/. The proposed system, seems to have a lot of sense, however, I would like to get comments from anyone that have ever used this system in a model.
Thanks a lot.

gerryndennis

G'day Rich,

The main problem with a tank too high or low is that when you fly inverted it becomes the other way round. In your case you can probably tune the engine fine with the high tank, but when you roll inverted (or push to negative 'G' to be more accurate) you now have a low tank. The engine will now be much leaner. The uniflow tank won't change that at all unfortunately.

The uniflow system prevents the engine getting leaner as fuel is burnt ie the fuel pressure is constant from a full tank to an empty one. But it won't help with a low or high tank situation (actually it can see later in the thread). The system works very well where a throttle isn't used. And can be made to work with a throttle with a bit of extra plumbing (you actually need two check valves).

A high or low tank can also give problems with idle or transition, you set the main needle either richer or leaner than it normally would be to allow for the tank position, but this needle setting might not work well with the idle adjustment. If the tank isn't too high and you set the engine rich you might find that everything works acceptably well though.

If if not you will have to either raise the engine, lower the tank, a bit of both, or look into the fuel pump/regulater systems available.

Good luck,

Dave H

richrfl

My Feedback: (2)

thank you for your response gerryndennis. Now I understand better a few things related with the uniflow and CLine fliers. I cannot move neither the tank nor the engine. This is a phoenix PC-9 (very well know in your neighborhood, I guess) as the cowl sets the engine location and the nose gear sets the tank location. No doubt that the plane was designed for electric, although they indicate that you can run a glow engine. Anyhow, in regards with the two check valves, I guessed I would need them anyhow, so I ordered them yesterday. I saw a diagram on the installation of the check valves here:
http://www.rcuniverse.com/forum/glow...ml#post9045226
Is this the installation are you suggesting? because I am not too sure about it.
About the pump/regulator, I was giving it a serious look, but I did not like the idea of tapping a pressure fitting in the crankcase. But if bad goes to worse, I think I will not have another choice than going that direction.

1QwkSport2.5r

I think the best solution is to obtain a fuel pressure regulator of the demand type. These examples include units made by Cline and Iron Bay. These systems pressurize the fuel tank through the use of exhaust pressure and a check valve and the regulator does its job by adjusting fuel flow dependent of the demand at the spraybar. No external pump needed (and pump carb as well). One could also dismantle a walbro gas carburetor from a chainsaw or weedeater for the regulator as it works similar to the Cline/Iron Bay units.

Pumps require regulators, and pumps such as the Perry VP30 really should be used with a corresponding pump carb. It seems rare that they work well with a stock carb unless the stock carb is way too big.

richrfl

My Feedback: (2)

1Q, thank you for your input. I really like the solution althought the challenge is that IronBay seems to be focused in other market than our type of RC. It seems though that its owner may be ready to help using the phone connection. On the other hand, I found no way to access the Cline brand in USA. I will look into your suggestion of using a chainsaw or weedeater carb, but my concern there is if the hardware would work with glow fuel.

1QwkSport2.5r

It is said the newer carburetor kits containing the diaphragms and such are supposed to be more tolerant of alcohol than they were in the past. I do know of a few fellows that have used the pump/regulator section of a Walbro carburetor to pump glow fuel to their engine with no ill effects. The key is to keep the diaphragm wet at all times. If you stop using it, the oil should be rinsed out.

The cline regulators sold today work the same way as the Walbro carburetor's regulator does but requires pressure in the tank for it to work without the pump.

richrfl

My Feedback: (2)

1Q, considering the prices I have seen in some blogs, it seems that savaging a Walbro carb makes a lot of sense. However, I have looked to the weedeater carbs, and cannot figure our the size and connections to the pressure pump/regulator. The whole carb is 2 x 2 in, which is something I cannot afford. About the pressure in the tank, are we talking muffler pressure or something else? (never assume, you know).

1QwkSport2.5r

Muffler pressure with a check valve if using just a regulator. If using a pump and regulator, then muffler pressure won't be used and the nipple plugged in the muffler. Here's a video of a guy using a cut up Walbro carb - he used the pump and regulator it looks like. http://youtu.be/1jjYAX3HhBw

richrfl

My Feedback: (2)

I saw the video, and the guy is using crankcase pressure. That gives me the same creeps than the Perry pump, plus the cut and patches he did, and still seems that the engine is not stable at any position of the tank, although I have to accept that he moved the tank way up or down, more than reality, but generally speaking is not a bad solution. Still there is the size issue, which in the video looks rather as 1.5 x 1.5 in or a bit less. Interesting is that he uses it in a 4.5cc engine, which between the OSs .25 and .35. Other thing I didn't notice before is the carb type. Will try to get more info about the carb size. I also noted in the video about the vent issue. will keep looking into this solution before deciding which way to go.

1QwkSport2.5r

I feel cutting a Walbro apart for the pump/regulator is a cheaper route for an extreme situation. A Perry pump works at pumping fuel, but the regulator sucks and without a pump carb, your midrange and idle will suck. If I were in your situation, I'd obtain a Cline regulator and check valve and be done with it. Not cheap, but will circumvent the issues you're facing albeit at the expense of adding a little plumbing.

To add, the backplate works fine for a pressure tap. I've done it to use a Perry pump before and had no issues other than the pump would push way too much fuel no matter the regulator setting. Stock carbs are not well suited for the Perry pump.

richrfl

My Feedback: (2)

Got the point 1Q. You are not the only one pointing than the Perry pump "strongly suggest" the use of a Perry Carb, and at the end of the day, it will cost as much (or more), adaptation workout excluded, than the Cline reg, and have the Walbro as Plan B. I do not plan for professional aerobatics or racing, but have safe landings after a few loops or inverted flights.

gerryndennis

Hey Rich,

Back to the uniflow for a sec, yes that diagram you referenced is what I was talking about. The other advantage with that set up is you can go back to fuelling/de fuelling through the carb line. But like I said uniflow isn't the answer to your problem, you are on the right track with Quick.

I understand it may be possible to remove one of the backplate screws, drill through into the crank case, and replace the screw with an ordinary nipple if you need crank case pressure.

Those Aussies across the ditch have some PC 9's we've just bought Texan II's. Similar looking but the Texan's are heavier, more powerful, pressurised, and have bang seats installed as I understand it.

Have you flown the model yet? Is the problem theoretical or real at this stage?

Good luck with it

It's Dave by the way, Cheers,

Dave H

Sport_Pilot

Actually it can help, but only within limits. Obviously if the needle is below the bottom of the tank or above the top of the tank it will not help. If the center line is at say a 1/4 tank above or below the needle it will not help when the tank is full or 1/4 empty but will help when the fuel level is between that.

richrfl

My Feedback: (2)

Gerry, I was not able to fly the airplane because I found myself with the problem that I cannot adjust the engine to run in the floor. Under such conditions, it was almost impossible to even make a runway test to adjust the steering.
Interesting issue about swapping one backplane screw with a nipple (hole considered).
Quick idea although good, is hard to achieve. Cline is no longer in business and the only few regulators left are in Australia, and IronBay will be the shot. Hope I do not have to reach to the Walbro.

richrfl

My Feedback: (2)

Sport, good input. I will take a more critical look to the tank position related to the needle. Flying between 3/4 and 1/4 tank it would not be a big deal. Thanks a lot.

gerryndennis

I'm not quite sure how you figure that Sport. Let's use your example and put the tank 1/4 tank high, the floor of the tank is 1/4 tank below the needle, the roof is 3/4 above. Roll inverted and the 'floor' is now 3/4 below and the 'roof' 1/4 above. The fuel head varies by 1/2 the tank height wether it's a uniflow or standard tank. The amount of fuel in the tank is irrelevant.

Dave H

gerryndennis

Rich,

The PC 9 doesn't look that bad for fuel tank position, it's not perfect but I've seen worse. I would be surprised if you can't get it to work as designed. I'm sure you should be able to get it to run well enough for a taxi test.

How much experience do you have with inverted engines? Is there someone at your club who could help you tune your engine?

My name is Dave, same as my signature below. Gerryndennis is my screen name (Geraldine and Dennis were my cats). There is no Gerry.

Dave H

Sport_Pilot

Because the vent is on a clunk the muffler pressure must overcome the head of the fuel. So the pressure at the pickup is always the same till the fuel level drops below the spraybar. That works if the fuel level drops or the tank position changes. So obviously there is some advantage to the uniflow setup till the tank is completely above or below the spraybar.

gerryndennis

Nope, the pressure is the same from full to empty.

The relative height of the tank and spray bar will change what that pressure is, but the pressure will not vary with fuel level. That's the whole point of the uniflow set up.

There's no change of head pressure when the fuel level reaches the same height as the spray bar.


Dave H

Sport_Pilot

....3

I did not say that right. The fuel pressure at the pickup is always changing, but it is matched by the pressure reduction of the muffler pressure from the vent. When not centered then the pressure changes when inverted, But only by the small difference in head between the fuel level when upright and inverted. Not a big factor unless off by an inch or more.

gerryndennis

OK, you seem to have moved the goal posts once again. I've completely lost track of your argument.

Sport post 1.

A uniflow tank will help between full and 1/4 empty.

Sport post 2.

Pressure at the pick up is always the same until the level of the spray bar so there is 'obviousely' some advantage.

Sport post 3.

Pressure at the pick up is always changing and there is a difference in 'head between fuel level'? Not a big factor unless off by an inch or more.

So the same as a normal tank then?

A normal tank mounted 1/2 inch low or high will have a difference in head pressure of 1 inch from upright to inverted. Not true because when inverted the vent is under the fuel, the tank becomes a uniflow tank. So will a uniflow tank, which is why I said right at the start (incorrectly) that uniflow would make no difference to the tank height problem.

I think you are now agreeing with me?

I'll say it again for clarity. A uniflow tank will have constant pressure at the clunk/spray bar from full to empty. The pressure at the spray bar will change if the tank is raised or lowered relative to the spray bar

Dave H

Sport_Pilot

No argument, just explaining my initial comment. As I said I did not explain it correctly, and I got the issues confused. So how can I be changing the goal posts when admitting I committed a foul? What you say is correct. However if the uniflow is off center the change is not as much because the pressure is reduced by the fuel above the pickup. The pressure at the pickup is always changing, but the pressure at the needle stays the same from full to empty.

Also the correct neutral (as far as pressure change from upright to inverted) position for a standard tank is for the centerline below the spraybar, it varies a bit because of tank size but it is always a small distance, 1/4" to 3/8" is common. But the neutral position for a uniflow tank is for the centerline of the tank to be even with the spray bar.

dspeers

My Feedback: (22)

Fascinating discussion. Thank you all. I do not get how the needle pressure is the same but the pick up pressure varies, should they not be equal at all times? Also the siphon effect in a tank high situation can be alleviated by routing the carburetor feed line with a loop from the tank down to the level of the spray bar and then back up to the level of the tank centerline, then to the carburetor. Unfortunately this would do nothing for inverted (tank low) flight and the "anti-siphon" effect would be the same.


What really confuses me is that regardless of whether the tank is set up 2 or 3 line, the engine continues to run both in vertical up and down orientation and the length of the fuel column, which is what determines the siphon or anti-siphon pressure differential, is determined by the distance between the tank and the carb. A 3 line system will smooth out the variations, but they will be present regardless. Since the distance between the tank and the carburetor probably exceeds the up or down distance of the "misplaced" tank described above shouldn't the system be able to tolerate the pressure differential induced by the tank position. Or is the point to also eliminate the leaning out effect of a vertical up position and the richer mixture resulting from a vertical down position? Seems the only answer in that case would be to use a regulator which as long as the required minimal pressure was provided to the regulator intake would provide consistent pressure to the carb regardless of flight orientation or tank position.

My last question refers back to the prior statement about a modified uniflow "can be made to work with a throttle with a bit of extra plumbing (you actually need two check valves)." See picture. Wouldn't this flood the muffler with fuel if the throttle is cut in a nose down orientation?

gerryndennis

G'day dspeers,

Yep with a uniflow tank the pressure at the pick up clunk and therefore the spray bar remains constant from a full tank to empty. The pressure at the spray bar is obviousely different to the pick up pressure and will change if the tank is raised or lowered.

You are right about the change in head pressure in a vertical climb, it's worst in a 3D type hover and not as bad in a vertical climb with reducing airspeed (less than 1:1 power to weight ratio). A loop is not bad at all because the force is mainly centripetal, the head pressure changes very little in this case.

Regarding your last question, yes this would flood the muffler when throttling down on a vertical descent... exactly the same as a normal two or three line tank would. However the rest of the time it would avoid flooding the tank when throttling down which a uniflow tank would otherwise do.

Welcome aboard,

Dave H

gerryndennis

Hey Sport,

No problem like I say I wasn't fully understanding your posts.

Ok let's imagine we have a uniflow tank 2" high, mounted with the centre of the tank 1" above the spray bar. the tank is half full. Fuel flows because of the exhaust pressure, the carb pressure (negative), and any head pressure. As you quite correctly say the pressure due to the fuel in the tank is cancelled out because the exhaust is at the same height as the pick up clunk. In this case the head pressure is zero because the floor of the tank is the same height as the spray bar. Now invert the set up. The clunks are now 2" below the spray bar. The difference is 2".

Now let's change the tank to a standard two line set up, but otherwise the same 2" tank mounted 1" high, half full. Now in the upright case the fuel level is 1" above the spray bar so the head pressure is 1". Inverted the fuel level is now 1" below the spray bar, but the tank now acts as a uniflow tank, so the head is 2" below the spray bar. A reduction of fuel head pressure of 3", 1" more than the uniflow tank.

So I still can't understand (now I do) why you would say that a uniflow tank would have less change. If that is what you are saying?

My apologies if I have misunderstood your post again.

Dave H