Explaining performance of speed mods

francois57francois57
in General Discussion
Well...

My PA28-180C is now equipped with knots2U wheel fairings and has the madras droop tips.
Knots2U claims 5kts
Madras claimed (hard to find the info since the're out of business) 2 to 3kts
OK: that makes 8kts max.

At 75%, almost sea level (1000ft at 1036QNH), 2450RPM, the plane flies 125kts, that's 10kts over book value.

We were 2 with full tanks, so 300lbs under MTOW.

Is it consistent ?

Francois

Comments

  • 78TARROW78TARROW
    Francois - that seems a bit dramatic to me. I have all the K2U mods on my Turbo Arrow (original wingtips) and they are worth 4-6KTAS in cruise. They have the most benefit, compared to book, at lower cruise power and higher DA. Speed improvement quotes for such things are a bit notorious and they surely do not add arithmetically when multiple mods are applied. On the other hand, someone once opined that "Cherokee" derives from the Greek word for "Drag" (probably a Mooney pilot) so cleaning them up should pay benefits. I have heard that the wheel pants are an area that can pay pretty big dividends.

    Pretty airplane, by the way.
  • francois57francois57
    Read somewhere else that this wing is highly sensitive to load, and that a light airplane (which was the case) does much better than book. Maybe flying light was simply the best of all mods... does the arrow confirm this ?
  • 78TARROW78TARROW
    The Arrow climbs really well at MGW - that is to say a little better than book. I plan my climbs to book values and consistently beat them by a little. The turbo really does help there and the speed mods, particularly the gap seals, may be of some help. This model has the semi-tapered wing, the subject of much controversy regarding performance. That said, my only other comparison is a straight-winged Arrow 180 and it's not fair to compare them due to the engine differences. That airplane was an OK performer, even at MGW, on cool days. It was quite anemic when OAT's got above STD Day.

    The Arrow is sensitive to weight, like any airplane, and I'd much rather fly 300lb below MGW.
  • DLR rentalsDLR rentals
    unless your at standard temp/pressure not much counts. Make the same speed run in August and your numbers will be down.
  • David SheltonDavid Shelton
    At gross weight, your wing will produce about 12% more lift with increased induced drag. I'm going to assume that induced drag is 15% of your total drag in cruise. After chugging a few numbers, I'd expect your cruise speed to be about 3.75% (4.7 knots) slower at gross weight. So yes, your indicated speeds are indeed reasonable.

    About half of your observed speed increase was due to the reduced weight that you were flying at. The remaining increase may have been the result of speed mods. However, it's difficult to tell exactly. Density altitude, airspeed calibration, wing roughness and other factors all play a role. Outside of a wind tunnel or professional test flight environment, our margins for error often exceed the effects of such speed mods.

    In practice, here is the rule-of-thumb for most speed mods:

    Wheel pants: 2-3 knots
    Cowls: 2-3 knots
    Wing Tips: 2-3 knots
    Gap Seals: 2-3 knots
    Speed Wax: 2-3 knots
    All of the above: 2-3 knots

    :D
  • Tim MorrisonTim Morrison
    How does it compare to before the mods is all that really counts. Do you have data from before the mods at similar weight? Also, how are you determining 75% power? If just by RPM then you are probably not accurate since those charts are only at MTOW. I often flow with just me and, setting power by fuel flow my RPMs are higher than the chart.

    It has been said that the best speed mod is to shed weight.
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