Archer II vs. Archer III fuel burn rates - why different?

tshugart3

Taking a look at the Archer II vs Archer III POHs (I own a II), I noticed that the best economy fuel burn rates listed are quite different, and they're quite a bit higher for the Archer III.

Here are the numbers:

% power Archer II Archer III

55 6.3 8.2

65 7.6 9.5

75 8.8 11.0

The Archer III manual doesn't list best power fuel burn, but all the best economy numbers for it are even higher than the best power rates for the Archer II.

Does anyone here know why these numbers would be so different? The two models (which are both PA28-181s) have the same engine, the props are virtually the same (only difference is space length), similar power setting chart numbers, and the cruise mixture adjustment procedures are virtually identical.

Thanks in advance for any info that might be useful in figuring this out. In the meantime, I'm tempted to use the Archer III burn rates for safety's sake!

Scott Sherer

Very observant of you. I'm thinking that this is going to make for a very interesting discussion today. Thanks for posting this. :)

griff390

https://piperowner.org/talk/discussion/155864/archer-ii-vs-archer-iii-fuel-burn-rates-why-different

Tshugart3;

[Major edit of my response]:

Just re-read your original post.

Good question. A 180 hp engine is a 180 hp engine, and it has no knowledge of the airframe attached behind it. Here are a few things to check.

Are both charts from Piper POH's (you mentioned you had a II)? I've found discrepancies from one website source to another even for the same model, so if both sets of numbers are actual Piper POH's, we can compare apples to apples. If not, you may be comparing a POH to a non-authoritative source.

Make sure both charts are comparing burn rates at the same altitudes.

There may be slight differences if carbureted vs injected, but with the same fixed pitch propeller, a static power setting and identical altitudes, the fuel burn should be very similar.

empannin

I would refer to the Lycoming guide for the engines. They usually publish an engine guide with operating points and % power/fuel for all of the engine models. See some details below. The IO-360C1C and C1C6 are same compression ratio and only differ by counterwights (from below)

Looks like the lycoming data table is more consistent with the Archer III

Unit74

The Archer III is fuel injected.

tshugart3

Appreciate the feedback, but the chart above is for the IO-360 (a 200 HP engine), not the O-360.

According to the POH, the Archer III has the same carbureted O-360-A4M as the II.

Unit74

Which model year Archer are you asking about? Archer IIIs we’re sold with injected engines.

jacobsja

I was also curious about fuel delivery method as a possible difference and decided to do a crude survey of the FAA registry based on each generation. Both the Archer II and III came back with some version of the O-360 (not IO-360) although not exactly the same model number. Granted, the sample size was super low, so take that for what it is worth.

Agree with Eric that the Lycoming specs for the engine model (not just a generic O-360) should prove a better source. So with that, what is Lycoming showing for each engine model and do the charts agree with the planes' POH?

One tangible difference I noticed on the II vs III was cowling design. But not sure how that would effect burn rate based on percent power setting. If anything, it would effect range by way of a different parasitic drag.

Unit74

I retract it all. My dinosaur head had Arrow in my brain and Archer in my mouth. The new PA28s do have IO engines, but they are not Designated Archer IIIs or Archer IV.

empannin

Yeah, me too! Archer vs Arrow

Here is some info on the O-360-A4M. The Archer III info is correct and I would fly the Archer III values. You can cross check against EGT's.

Eric

PeteCip

I'm a little late to this game but I didn't see anyone answer the question. I have flown a lot in a Warrior and an Archer II. For the past couple years I've owned an Archer III. The biggest factor to its higher GPH, I believe, is the AC which is standard on the Archer III. As stated in the POH: When the air conditioner is turned off there is normally no measurable difference in climb, cruise or range performance of the airplane. I don't buy it. We're talking at least 65lbs of weight; evaporator, condenser, compressor, blower, switches and temperature controls. Plus, even unloaded the compressor must be consuming power to turn. I'm about to remove the AC which I use once or twice per year. I'll gain an hour of fuel (in useful load).

jacobsja

While not disagreeing, am not completely sold either.

Unless I am missing something, the original question is focusing on burn rate per power setting, and total fuel consumption is outside of this question's scope.

To the tangent of A/C. When Off, agree that the A/C is pretty much dead weight no different than taking a child or pet of the same weight. Am not recalling anybody translating a notable difference in fuel burn based on that amount of weight. Will admit that the total fuel consumption should come in a tad bit higher as compared to flying without that weight as the time of getting to cruise altitude is a bit longer and the pilot is likely to apply a little more power to maintain a desired cruise speed, but the burn rates based on power settings will not care about weight.

Toward the compressor pulley consuming power when Off, it will, but the parasitic drag is nominal and no different than a tensioner / idler pulley on a car engine. Truly would surprise me if the parasitic drag of the pulley alone will translate to something above 0.001 gph. Put another way, when the A/C is Off, the wheel simply spins and the compressor remains still (does not rotate).

Definitely agree that removing the A/C will recapture useful load. Might also help with cruise speed if it shifts the CG aft a bit. But still do not see how this effects burn rate per power setting.

tshugart3

https://piperowner.org/talk/discussion/comment/1444700#Comment_1444700

Interesting, thanks for providing that data point. I'd imagine if the compressor is running, that load would require a higher fuel burn rate to operate at the same prop rpm. I wonder if the power charts are different or the same, as one would presumably still want to limit the engine's power output to 75%, even if the prop is turning slower due to powering the compressor?

jacobsja

Yes, if the goal is to maintain airspeed in the same manner as a car's ground speed with A/C, operations will require adjusting the power to offset the compressor's drag (when the clutch engages). But... Am still not seeing how this changes the burn rate per power setting. Yes, adjusting the power will increase the burn rate, although the new setting should match the POH tables (or at least increase in proportion if there is a difference between observed consumption and the POH).

In practice, I am not aware of people constantly changing the power setting (while in cruise) based on when the compressor engages / disengages as they just accept the temporary loss in speed. As a data point, my folding gear PA-32 loses ~5 knots when the compressor engages and sometimes I add power to compensate (example: when in the arrival phase where I do not want to drop below critical speed / altitudes, and the mixture is already set to Full Rich), and sometimes I just accept the 5 knot loss (example: while in cruise, or descending to arrival) as I do not feel like adjusting the power and re-leaning the engine every few minutes.

So, still not sold that A/C really has any effect on the burn rate per power setting. Yes, the A/C doing its job will increase total fuel consumption as it is either robbing speed (read: increasing the time to destination), or increasing fuel burn rate from the pilot increasing power to avoid loss of speed / altitude.