AeroInjector Needles (was Guarded Switches)

[sonex1374]

[New topic split from previous thread - viewtopic.php?f=28&t=5501&start=20#p42001]

Ryan,

when switching to a different needle, you'll still tune the engine to the same fuel flow at WOT as the previous needle. In fact, I recommend that before you remove the old needle you reach up thru the carb throat (at full throttle position) and mark a sharpie line on the this-side of the needle where it enters the fuel orifice. Then, when you remove the needle you'll have a very accurate point on the old needle where you can measure the thickness of the needle to determine the "flow area" that your engine requires at WOT. Prior to installing the new needle you locate the point of the previously measure thickness and mark a reference line. This ensures when you install the new needle and adjust its position via the carrier that the same flow area is present (saves lot's of tuning steps, assuming the old needle was in fact well tuned at WOT).

The difference in needle taper comes into play at throttle settings lower than WOT (which after all, you set to exactly the same flow area, even if that thickness was physically in a different spot of the needle). In the case of a #3 needle compared to a #2.5, when both needles are installed with identical flow areas, the #3 has a steeper taper, so the needle thickness at low throttle is thicker and results in less fuel flowing at that low throttle setting.

This is complicated to visualize, and I spend an hour during the Firewall Forward Seminar talking about the AeroInjector and how/why it works and illustrating it with graphics. Once you get it though, it all makes sense.

I hope this helps.

Jeff

[Rynoth]

Actually that made total sense, thanks Jeff.

One additional element to this equation for the Turbo in particular is I never operate at WOT as this would cause overboost. I suppose I could additionally mark my throttle position at "max" power before moving on to marking the needle.

[Onex107]

I’m installing the turbo now and going to change my needle from a 2.5 to 3.0 because from idle to full throttle I’ve always had to enrich the mixture to keep it from dying.

Getting a bit off topic at this point, but when it comes to the 2.5 to 3.0 needle, does it actually change the "curve" of fuel mixture, or just provide more fuel at all power settings? I'm very familiar at this point with needing to lean the mixture at low settings and enrichen at high settings using a 2.5 needle in the turbo, I'm just wondering if the 3.0 needle actually addresses that issue.

When I was setting up my AeroInjector I had the same problem you do. I started measuring the needles and calculating the angle of taper. The angle changes in minutes of angle from the #2 to #3. I determined the sweet spot for wot was where the needle miked about .o80 - .085 thick ( at the entrance to the orifice ) and the difference between the needles was the location/distance of this thickness from the beginning of the taper. The sweet spot for idle is near or at the beginning of the taper and these two sweet spots get closer together as the angle increases. If these two spots are too far apart you have to manage one or the other with the mixture. If you calculate the increasing area of the slide opening from Idle to wot it is pretty linear and the needle taper is flat and linear. I made seven needles with varying/increasing angles and ended up matching the #3. I've been running it for 200 hours and do not touch my mixture from one flight to the next except to make a small richer adjustment to cool #1 EGT on takeoff and back leaner on cruise. It's pretty simple really. The two sweet spots have to match the distance the slide travels from idle to wot.

[jerryhain]

Sorry about getting off topic here. in describing this I’m using 100% as cut off and 0% for full rich. At idle I’m at about 90%, and as I move the throttle forward when I get to 1800 rpm I need 75% or it dies. At 2400 RPM I move the mixture to 45 to 50% and then full throttle 3000 RPM and above I have to go to about 25%. The mixture changes aren’t just to make it run better it’s to keep it running at all.
I’ve done multiple restarts in the air. This was all done normally aspirated so I might wait to change the needle until I finish the turbo install and start testing again. I’ve started from scratch and used the factory install method twice with the same results.
I believe this means I need a steeper ramp on the needle to even out the mixture setting.

[Rynoth]

Sorry about getting off topic here. in describing this I’m using 100% as cut off and 0% for full rich. At idle I’m at about 90%, and as I move the throttle forward when I get to 1800 rpm I need 75% or it dies. At 2400 RPM I move the mixture to 45 to 50% and then full throttle 3000 RPM and above I have to go to about 25%. The mixture changes aren’t just to make it run better it’s to keep it running at all.

This is almost identical to what I do with the mixture using 2.5 needle and the Turbo. It's become second nature to me and the engine is quick to remind me when I add power if I haven't already moved the mixture richer. I think I'm going to try out the 3.0 needle using Jeff's install method. Since I don't use the full length of travel of the throttle (I don't/can't ever go WOT w/out exceeding max MAP) it certainly makes sense to have a steeper taper to find the "sweet spot."

[builderflyer]

[New topic split from previous thread - viewtopic.php?f=28&t=5501&start=20#p42001]

Ryan,

In the case of a #3 needle compared to a #2.5, when both needles are installed with identical flow areas, the #3 has a steeper taper, so the needle thickness at low throttle is thicker and results in less fuel flowing at that low throttle setting.

f

Hi Jeff,

Take another look at what you said in the above paragraph........."the #3 has a steeper taper".............which is correct but some may be confused by what followed in that it applies to the #2.5 needle, not the #3 needle as it might appear. So for clarification, what it could say is "the #2.5 needle thickness at low throttle is thicker and results in less fuel flowing at that low throttle setting".

I hope I haven't confused this any more,

Art,,,,,,,,,,,,,,,,,,,,,Sonex taildragger #95,,,,,,,,,,,,,,,,,,,Jabiru 3300 #261

[Rynoth]

[New topic split from previous thread - http://www.sonexbuilders.net/viewtopic. ... =20#p42001]

Ryan,

In the case of a #3 needle compared to a #2.5, when both needles are installed with identical flow areas, the #3 has a steeper taper, so the needle thickness at low throttle is thicker and results in less fuel flowing at that low throttle setting.

f

Hi Jeff,

Take another look at what you said in the above paragraph........."the #3 has a steeper taper".............which is correct but some may be confused by what followed in that it applies to the #2.5 needle, not the #3 needle as it might appear. So for clarification, what it could say is "the #2.5 needle thickness at low throttle is thicker and results in less fuel flowing at that low throttle setting".

I hope I haven't confused this any more,

Art,,,,,,,,,,,,,,,,,,,,,Sonex taildragger #95,,,,,,,,,,,,,,,,,,,Jabiru 3300 #261

Hmm, I have to disagree, I think what Jeff said and meant were correct. He was suggesting that if either needle is set to the same thickness at WOT, then the throttle is pulled back to idle, the #3 needle would have a leaner(thicker needle) mixture with less fuel flow. In other words, steeper taper means a larger variance in thickness/fuel flow as the throttle is moved. So if they are equal at one end (WOT), the #3 needle will have a larger variance at the other end (less fuel at idle.)

[sonex1374]

Art,

I understand what you're saying, and this is confusing stuff. I'm including a picture from the FWF Seminar that I use to explain needle selection and adjustment. This picture is somewhat out of context and is missing most of the information that helps explain it, but it illustrates the central concept. The red lines are the area required (eg thickness of the needle) to flow enough fuel for that throttle position.

Jeff

[builderflyer]

Jeff, Ryan,

Of course, you fellows are correct. If I'd only gone back to my notes where I had measured the thickness of each needle at each station I'd have noticed my error in thinking. What sent me down the wrong path is that from an operational point of view it appears to me that when I've set a #2.5 needle where it wants to be at WOT, it runs quite a bit leaner at idle vs where the #3 needle would be at idle. This is backwards from the way it outta be based on needle thickness at idle. In any event, my 3300 has always preferred the #2.5 needle over the #3 needle, unlike the experience of many others.

Sorry to add to the confusion,

Art,,,,,,,,,,,,,,,,,,,,,Sonex taildragger #95,,,,,,,,,,,,,,,,,,,,,,Jabiru 3300 #261 (Aeroinjector/#2.5 needle)

[sonex1374]

Art,

I recently measure my supply of various needles (old brass and new stainless) and reviewed the results. As I've mentioned before, the brass needles were generally smaller in diameter by 0.02 mm, and varied in diameter by as much as 0.05mm. By contrast, the stainless needles were generally very consistent in diameter (measuring all identically with my particular calipers and imperfect technique). The one notable exception to this was my #4 needle (I only had this one to try), which was slightly smaller than the others. This particular needle was ordered 3 years after the others, and could have simply been machined from slightly different round stock. In any case, the smaller diameter stock will flow more fuel at idle even when adjusted to the same needle thickness. Again, individual needles can have differences, and individual carb fuel orifices can as well. This explains why each engine seems to prefer something slightly different than the next engine.

I wish it was as simple as selecting a needle # and adjustment position from a table, but it's not. The good news is that with a bit of trial and experimentation, and a few tuning sessions, each builder will find what works for their particular need.

Jeff