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

Discussion in 'Technical Corner' started by T Shugart, Jun 16, 2022.

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  1. T Shugart

    T Shugart Filing Flight Plan

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    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!
     
    T Shugart, Jun 16, 2022
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  2. eman1200

    eman1200 Touchdown! Greaser! PoA Supporter

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    is the III the stretched body? i don't recall, I've been out of pipers for a while...
     
    eman1200, Jun 16, 2022
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  3. T Shugart

    T Shugart Filing Flight Plan

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    It isn't any longer than the II, which also had the 5 inch stretch that came with earlier models (the Challenger, I think).
     
    T Shugart, Jun 16, 2022
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  4. Pilawt

    Pilawt Final Approach

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    The only differences are that the III has the new cowl, smaller windows and new panel and interior. No changes in overall dimensions or power plant.

    Yes, the '73 Cherokee Challenger was the first with the stretched fuselage and larger stabilator.

    That said, having owned a 172 with the same O-360-A4M as the Archers, the numbers quoted for the Archer III sound much more accurate.
     
    Pilawt, Jun 16, 2022
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  5. Magman

    Magman Cleared for Takeoff

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    IIRC the III has a prop with with a few more inches of pitch.

    If both types were run at identical RPM ( not %) the III would produce more power with the slightly higher fuel burn.

    Or they stopped lying?
     
    Magman, Jun 16, 2022
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  6. hindsight2020

    hindsight2020 Final Approach

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    It's just garbage Piper POHs, just like the turbo arrow non-altitude compensating TSIO-360 power tables. OEMS pull that marketing/lazy crap all the time. The Archer III numbers you list are best power (rich) mixture numbers for an O-360, regardless of whatever label they slapped to it. The calculations for brake specific fuel consumption are well known and easy to calculate. You're dealing with best power numbers in your cited example for the III (maybe even richer than 150* ROP).

    POHs may hold some semblance of legal cover for some in this hobby, but they're in many respects false advertisement. Following them with absolutist fiat can lead to undesired outcomes. They are not guaranteed to avoid early wear and/or damage on the powerplants (again, see mixture recommendations on some poh's right at peak CHT, and the aforementioned TSIO-360 turbo arrow power table for such examples).
     
    hindsight2020, Jun 16, 2022
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  7. RyanB

    RyanB Super Administrator Management Council Member PoA Supporter

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    I agree, the POH needs to just be treated as a book of starting points and recommendations. Several procedures written in mine are just flat wrong, for example, start procedures when hot and cold. I’ve tried to operate at ‘book numbers’ and I hardly ever see the performance it says I will.

    As for this question, I really don’t know. The two airplanes are similar enough from what I can tell that their numbers shouldn’t be much different. Maybe the III is tiny bit more ‘draggy’ and that explains the reduced fuel economy. That said, the numbers I see out of the Archer II that I fly, are pretty much the same as the Archer III numbers that I see listed here.
     
    RyanB, Jun 16, 2022
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  8. Salty

    Salty Touchdown! Greaser!

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    Drag has no effect on fuel burn at a given % of horsepower.
     
    Salty, Jun 16, 2022
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  9. hindsight2020

    hindsight2020 Final Approach

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    bingo, neither does propeller application. Hence brake specific fuel consumption. It's already normalized for, which is why these POHs are junk on the powerplant front.
     
    hindsight2020, Jun 16, 2022
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  10. T Shugart

    T Shugart Filing Flight Plan

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    Well, I guess the way to go is to use my FS-450 fuel flow, adjust the K-factor after I have enough data, and go by that plus operating experience and a little cushion.
     
    T Shugart, Jun 16, 2022
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  11. Joe_B1

    Joe_B1 Line Up and Wait

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    I have a 79 archer II and my numbers are closer to the ones you listed for the III.
     
    Joe_B1, Jun 16, 2022
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  12. mondtster

    mondtster En-Route

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    The numbers in post 1 for the archer II look more like warrior numbers. Are you 100% certain that the flight manual being cited is actually for an archer?
     
    mondtster, Jun 16, 2022
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  13. cowman

    cowman Final Approach

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    I have the relevant page from an Archer II POH.
    AB88D1E2-8947-4AA6-9EA8-6FC9CB604272.jpeg

    The Archer III numbers look more like best power in the Archer II. Did they change the leaning procedure maybe?

    IDK when I flew an Archer I just used 10gph as a planning number because it made the math easier and left me some margin for error.

    Also I’ve run into a lot of Piper POH numbers that are…IMO overly optimistic so there’s that.
     
    cowman, Jun 17, 2022
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