Starting my Homemade Hackman

[109jb 181]

I have been thinking about your post but I disagree that you need to control the throat to bowl pressure ratio with the pump system. In a stock setup with no leaning system the bowls are vented to atmosphere and the calibrated needles, and jets take care of the variable fuel delivery. There is no controlling of bowl pressure relative to throat pressure. with the stock system. Therefore I see no reason the leaning system needs to reference the throat pressure as long as a constant pressure in the bowl can be achieved. The pump system reference pressure is ambient, not from within the air cleaner or carb throat. Adding the pump system only changes the atmospheric pressure by the tiny amount of vacuum. So if ambient pressure is 30 inches and you pull 1 inch vacuum you will be at 29 inches in the bowl. Climb up to where the pressure is 29 inches and you will be at 28 because the pump will pull a pretty darn constant 1 inch of vacuum. so essentially the pump system will work just like stock but with the ability to adjust for density altitude. Granted it will still get rich as you climb, but in this way it works very familiar to those used to flying production airplanes with Continentals and Lycomings. Lean as you climb, enrichen as you descend.

 

[nlappos 413]

I waasn't clear enough. The only reason why gas runs from the bowl to the throat is the difference in pressure of the two air spaces. The throat has low pressure ("suction") so it draws up the gas that the needle allows, an amount regulated by the difference in pressure - the suction - and the size of the hole the needle opens. 

The way the Hacman systems work (because the Bing is a 49c carborator with no leaning capability) is allowing you to change that difference in pressure. Less pressure, by leaking vacuum into the bowl, less gas flows to the throat.

[109jb 181]

I understand that. That is basic to the operation of both systems. Essentially both systems are the same except for the source of vacuum. The pump is a constant vacuum source whereas engine vacuum is not.

The vacuum source of the hackman produces more vacuum at low throttle and therefore it seems it would have a better chance of reducing bowl pressure enough to cause starvation with a throttle reduction. I think that is not likely with either system.

[nlappos 413]

109jb,

You have it backwards, the throat pressure DROPS as the throttle is opened and the velocity in the throat increases.  If you put an artificial low pressure in the float bowl, and the throat pressure is higher (at low throttle), the gas will not leave the bowl and go to the engine.

 

http://www.ankn.uaf.edu/publications/VS/carburetors.html

[akflyer 1427]

 

The system works.. it works in 1000s of planes.  Is it worth loosing sleep over or are ya just bored and want to make something simple into something difficult   

If you are messing with the hacman while doing circuits around the patch shooting touch and goes, you are just creating a lot more work for yourself than really needed.  If the temp has a drastic swing from one day to the next, or you are taking off at one altitude, climbing out to more than 3k then you might be playing with it a little, but if you are constantly trying to tweak it to get maximum power/EGT temps your going to drive yourself insane.  These 2 strokes will vary the temps QUICKLY with a little burble of air.  As soon as you hit a pocket and unload the engine your EGTS will go up.  Set it to 1150 and enjoy the flight.  Short trips don't really gain you anything, its the long trips or high elevations where you will really be twiddling with it.

 

[109jb 181]

The system works.. it works in 1000s of planes.  Is it worth loosing sleep over or are ya just bored and want to make something simple into something difficult 

Sure the system works, but does it work how I want it to work? If I have 2 systems to chose from and one may be preferred for whatever reason I am going to take the time to understand them so I can make an informed decision.  It is also a system that if used or installed improperly could cause engine damage and failure.

If you are messing with the hacman while doing circuits around the patch shooting touch and goes, you are just creating a lot more work for yourself than really needed

That is what I'm trying to avoid.  The following is straight from the instructions for the commercial version of the Hacman and it says:

"ALWAYS return the HACman to full rich (clockwise all the way) before adjusting engine throttle settings." 

and 

"NEVER adjust the power / throttle settings of the engine with the HACman in a lean condition. Doing so may make the engine STOP. See ALWAYS DO."

They are pretty emphatic about that and what I am looking for is a system that will allow me to operate in warmer/higher conditions when the base jetting is set up for colder/lower conditions. So if I do want to do circuits on an unusually warm winter day I want to be able to adjust once and then just fly. Based on what the commercial Hacman instructions say that isn't how it works.

You have it backwards, the throat pressure DROPS as the throttle is opened and the velocity in the throat increases.  If you put an artificial low pressure in the float bowl, and the throat pressure is higher (at low throttle), the gas will not leave the bowl and go to the engine.

I think I found part of my confusion. I was under the impression that the port used for vacuum on the Hacman system was a "manifold pressure" port. I now see that it isn't. the 4-strokes use a manifold pressure port but the 2-strokes use a venturi port that is just about smack in the middle of the slide bore. So now what you have been saying is making more sense.

Regardless, I still think that the pump system won't cause a problem because using adjustments like I'm talking about (just enough to not foul plugs) the vacuum would be very low. At closed throttle it is operating off the pilot jet port which is behind the slide and will have very low pressure there (manifold pressure). Once the throttle comes off idle the pressure in the venturi will drop. Sure I could probably crank up the voltage to the pump and create enough vacuum to do like you are talking, but I don't think normal operation would yield bowl pressures that low. Maybe I'm wrong but 

What I think I will do is I'll plumb up the pump system since it requires absolutely no jetting change from stock and disabling is simply turning the pump off. If testing doesn't work the way I want it to it still got me most of the plumbing needed for the DIY Hacman system anyway.

Edited 19 Oct 2019 by 109jb

[109jb 181]

So I'm still planning on trying the pump setup but remembered something from my carbureted snowmobiling days which got me wondering.

Holtzman Engineering makes temperature and temperature/altitude compensators for snowmobiles. The basic operating principal is the same as the Hacman. A friend of mine had a Tempa Flow on his Polaris years ago and he never worried about temperature and I remember on a trip out west he just adjusted the manual altitude adjustment and rode. The ATACC unit does the temperature and altitude compensating and has a 6-8000 ft range from wherever it is set up., so it sounds like this would fit the bill very well for my use.

This got me wondering if anyone had ever tried this system on any of the Rotax 2-stroke flight engines. 

http://www.holtzmaneng.com/carburetion-products

https://www.youtube.com/watch?v=IOmTh46ff30

Edited 20 Oct 2019 by 109jb

[napierm 2]

I'm a little late for this thread but I thought I would add my 2 cents.  I flew the Hacman on my 582 and had good success with it.

The Bing carb works by metering the fuel flow through the various jets from the float bowl to the venturi.  The fuel flows up because of the venturi vacuum (also known as ported vacuum).  This is different from manifold vacuum.  (I got laughed at once for hooking up a vacuum advance to manifold vacuum rather than ported vacuum so I have this concept down pat)  Ported vacuum generally increases with carb airflow.  The Bing 54 slide carbs increase the venturi area with increasing "throttle" so the ported vacuum is more constant than it normally is with butterfly valve carbs.  Also, the Bing 54 carbs do block off the back of the throat at idle so there the ported vacuum all but disappears.  Then the idle circuit behaves normally. (BTW, I had to replace my "55" idle jets with "45" jets to get it to run well at idle.  Otherwise it would load up and quit on final.  Nothing to do with the Hacman, these jets are too rich from the factory.)

The pressure in the float chamber is vented to the nipple on the side.  So the differential pressure (float to venturi) and the jet sizes are what control fuel flow.  These vents are normally connected into the dual air cleaner so that the float chambers see the same pressure as the intake ports of the carbs.  The Hacman works by slightly lowering this pressure to lean the mixture.  In practical terms hook it up and start with a large orifice from the air cleaner.  Then work down to a smaller one.  You really shouldn't be able (or need to) change the EGTs by more than 200 degrees over the adjustment range.  I don't mind turning the dial quite a bit but most all of the adjustment is in the 1st couple of turns anyway.  As others have said you do wind up needing a larger main jet.  Once tuned up the system works great.  Lets you lean at altitude.  Better yet lets you richen it when you point downhill from a long cross country so you don't get too lean.

I really have to thank Tom Olenik for explaining how to set up and run the 582.  He had is own business, Olenik Aviation, for a while but found a better job as a "Propulsion Engineer" for a corporation that makes big drones.  I gladly bought all my supplies from him at the time.

Greensky also sells this system for the Bing 64 CV carbs.  In this system they use the manifold vacuum.  This vacuum source varies a lot with throttle setting and prop loading.  They say in the online instructions to close the valve (full rich) before pulling to idle, otherwise you could shut off the engine.  A better solution would be to take advantage of the Constant Vacuum (CV) available in that venturi.  Looking at the carb I can see that the venturi is ported to the top of the CV chamber.  I'm still looking at that chamber cap to see if there isn't a good way to secure a nipple there on one of the carbs.  The very top has a thin metal cap that looks to be peened in place and bonded with hi-temp epoxy.  It doesn't look strong enough that I would trust it not to fail with anything attached.  The advantage would be a fairly constant vacuum over most RPMs.  At idle (closed butterfly) this vacuum all but disappears so again the idle circuit would behave normally.

The CV carbs are about 1/2 altitude compensating anyway.  It's only at "high" altitudes (say 7K+ feet cruise) that they could use a bit more leaning.  Not as clear a use case as with the 2-strokes.

FWIW,

Mark Napier

 

 

 

 

 

[109jb 181]

So I've been thinking more and more about what to do and I have come to the conclusion that I was wrong. Did some more examination and now see how the things work.  I'm still contemplating putting on the Holtzmann ATACC system to have an automatic system, but I will probably start off with a DIY Hacman system. That brings up a couple questions.

1. I've read that there is a need to rejet to about 180 mains.  It seems to me that this is because the reference pressure is from inside the air cleaner whereas the stock carbs are vented (referenced) just within the cowl. Has anyone just referenced within the cowl with their Hacman setups?

2. For those that referenced to the air cleaner and the rejetted, did you only have to change the main? What about the needle, needle jet, and clip position? Any change there?

 

Thanks,

 

John B.

[Av8r_Sed 82]

I just changed the mains to .180’s. Works very well.