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Cruise speeds - performance observations

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  • Cruise speeds - performance observations

    Most of my flying involves cross country legs, to reach whatever destination I'm headed to. I spend a fair bit of time watching the ASI and engine instruments on these long cruises..... I thought I would share a few performance observations.


    Cruising lean of peak:

    Running 23" and 2300, we are consistently seeing 115 KTAS burning 37.5 L/hr (10 gal/hr) at around 5000-7000 ft density altitude. At 10,000 ft that goes up to 120 KTAS running around peak EGT for the same 37 L/hr.

    That engine setting puts us around 54% or 57% power, EGTs are about 10*C (roughly 20*F) lean of the peak, depending on the ambient conditions and altitude. That is pretty close to the ideal fuel-air mixture at 10k ft, just slightly lean. Indicated airspeeds is more like 108 KIAS depending on the day.

    With those settings, the engine temps on my install run about 160*C (330*F) in winter, and 175*C (350*F) in summer.

    Incidentally, at sea level we see around 108 KTAS running the same 37 L/hr lean of peak.



    Cruising rich of peak:

    I don't do much of this, but at 23" and 2300 we tend to see around 125 KTAS at the same 5000-7000 ft density altitude. For that extra 10 kts airspeed, I am burning 55 L/hr (14.5 gal/hr). At higher altitudes there is less benefit as the engine runs out of air. For 23" we hit WOT around 7000-8000 from memory, depending on the day. We do get a noticeable ram air effect with our engine's intake system.

    Depending on the day, that is usually high 60's % power. EGTs will be sitting around 50*C rich of peak (100*F).

    The engine temps are much hotter as as a result, about 20*C hotter at around 185*C (360*F). On a really hot summers day, the engine can push 195*C (380*F) and I like to open the cowl flaps around that time, which costs me a few kts airspeed.... I set my redline at 400*F.



    We have been slowly running the engine hard and harder, lean of peak, doing cylinder inspections in between by borescope. We started with a limit of about 21" and gradually moved up to 23". We've seen nothing so far which worries us at all after 270 hours running. I'll probably start trying 24" and 2400 LOP like some other guys are doing (Gavin I think you were?) Of course as we increase the power we need a larger safety margin from peak EGT.

    About the only time I run rich is to keep up with other aircraft who are running rich [e.g. C180] when we're doing formation stuff.

  • #2
    Things that slow you down / speed you up:

    We started out with 8.00 tires, small brake callipers, and no vortex generators. We had no gear fairings or strut fairings.

    We used to see more like 115 KTAS running 35 L/hr LOP in that configuration at 7000ft. That was around 52% power.

    Having added the VGs, we didn't see much airspeed loss at all, and we really looked for it.
    We added the gear fairings and strut fairings and didn't see any meaningful gains either, which was very disappointing.
    Then we added the larger tires and brakes, again not much airspeed loss perceived initially, but then I realised we were burning an extra litre or two per hour after all those changes to get the same or slightly less speed. I wish I had been more scientific.


    So we've increased from around 52% power to around 55% on average, for that same 115 KTAS, lean of peak.
    I would say the VGs cost us very little of that, and the tires were responsible for most of it.


    With all of those comparisons, the plane is "on the step" and cruising efficiently. If the airspeed drops below about 104 KIAS the plane starts to wallow, tail down, and prefers to sit at 100kts. This often happpens flying in mountain wave situations while I try to maintain my ATC nominated altitude. Sometimes it seems to like a little blip of power to get it up and cruising again, otherwise it can wallow around at 100 kts for quite a while.

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    • #3
      A view on the instruments post VGs but before the tires went on:



      And after the addition of the tires, note the different power settings and fuel flow:


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      • #4
        Just thought I should add another data-point to this thread...

        Aggressive Lean of Peak cruise:

        Running 25" and 2300 RPM, we're seeing between 112 and 118 KTAS near sea level, depending on the day and aircraft loading etc, for 40.5 L/hr fuel burn (10.6 GPH). These airspeeds are up to 10 kts faster than the speeds we were seeing running just a few litres per hour less, so there seems to be an obvious economy to be had. This is about 60% power for the IO-540.

        We haven't had a chance to test these settings properly at a mid-level cruise. I will report back, but initially I am hopeful that we'll see a real world LOP cruise at 130 KTAS, based on the first couple of legs at around 4000 ft. Naturally, performance will degrade above about 6000 ft or wherever WOT is necessary to maintain 25".

        We are seeing CHTs stay steady at the same levels, or much colder if you lean slightly more than desired. The EGTs are still around 10*C LOP.

        This behaviour seems to plays into the theory that the 4-place aerofoil can get up onto a "step" above a certain airspeed, whereby it seems to fly more efficiently above a certain power setting. I have certainly noticed that below about 55% power, the cruise airspeed seems to drop away faster than the power does, as throttle is reduced.

        This power setting is also strong enough to do cruise-climbs LOP, although you need to keep a careful eye on the EGTs and keep leaning as you climb.
        Last edited by Battson; 06-14-2015, 08:34 PM.

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        • Battson
          Battson commented
          Editing a comment
          Latest data shows that at mid levels (around 5-6 thousand) we are seeing a steady 125 KTAS with the above power settings. This is hugely efficient compared to ROP for similar airspeeds.

      • #5
        Jonathan,
        Regarding the step, this might be a stupid question but are you running full EFI on your engine?
        Another thought I had was, have you looked at the drag polar of the wing profile and compared with your AoA? Looks like you have an indicator built into your Dynon but it may or may not tell you your actual angle. Would be pretty cool if it did.
        What I'm curious about is if there's a noticeable jump on either of those maps - the drag polar of your EFI Ve table - at the point you notice improved fuel burn.
        Mark
        PS - I couldn't let you be all alone on this thread.
        Mark
        Scratch building Patrol #275
        Hood River, OR

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        • #6
          Hey Mark,

          Good question - we are running the typical dinosaur-vintage mechanical injection system Lycoming engine's often do - ours is a Bendix.

          I've not looked at the function, the AoA is really just a qualitative readout - we don't get any numbers - just a bar graph.

          Thanks for joining the conversation!

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          • #7
            Mark, Are you using typical Magnetos, or some type of Electronic Ignition?

            Brooks Cone
            Scratch Building Patrol #303
            Brooks Cone
            Southeast Michigan
            Patrol #303, Kit build

            Comment


            • Chewie
              Chewie commented
              Editing a comment
              You mean Jonathan?

            • Battson
              Battson commented
              Editing a comment
              In case it was me, I am just using normal Bendix S6 mags.
              A proper FI and EI system would certainly give better performance.
              I was looking for maximum repair-ability, commonality of spare parts, and simplicity of design - given we're reasonably remote down here in NZ.

          • #8
            Can you comment on the usefulness of the AOA indicator on the Dynon panel? It seems like changing flap settings would throw off the indication enough to make it unreliable but I would like to hear from pilots who have used it.

            Sent from my SCH-I535 using Tapatalk

            Comment


            • Battson
              Battson commented
              Editing a comment
              Short answer - I cannot overemphasize how much of a game-changer the audible AoA alarm is.

              Any pilot who is regularly flying close to the limits, or in high workload conditions, at low altitude - will get huge added safety and value from the system. It makes every high g turn and landing safer, more accurate, and more predictable. It even gives you insights into variable wind conditions during slow flight. I could sing it's praises all day long.

              You calibrate the AoA alarm to trigger specifically for your aircraft, typically you reach a higher AoA with flaps retracted, so the calibration process leaves makes it conservative with flaps extended. More safety margin. I would say it errs on the side of caution, rather than being unreliable. It's very reliable.
              You can always calibrate it with flaps extended, if you want more accuracy in that regime.

          • #9
            Just another data point here,

            We are still on 26" tires.

            Cruise lean of peak at 42 L/hr now for lower altitude work, making 65% power with 25-26" MAP and 2350 RPM. I am seeing an honest / steady 118 KIAS, and often 120 - 125 KTAS at lower altitudes. Of course once we get higher than about 6000ft the MAP drops below, and we slow down a little by IAS, but TAS stays about the same, as high as we go.

            Comment


            • Bcone1381
              Bcone1381 commented
              Editing a comment
              42L=11.1 u.s. gallons

          • #10
            Yet another data point:

            31" Alaskan Bushwheels

            With the addition of 31" bushwheels, we are still able to cruise lean of peak EGT at about 118 KIAS (~= 120 to 125 KTAS).

            This is with the same power settings as above, making 65% power with 25-26" MAP and 2350 RPM. Maintaining this airspeed with the added drag demands an increased fuel burn of about 42.5 to 43 L/hr, up between 0.5 to 1.0 L/hr compared to the 26". That's about 0.2 GPH increase. Next to nothing.

            At high altitudes above 7000ft, where I cannot increase the power to offset the added drag, I am seeing reduced airspeed of about 4 to 5 KTAS in the worst cases. It seems to vary depending on the conditions - I am still doing testing.

            We have noticed that at higher speeds, such as cruise decent or a steeper dive, that the aircraft is considerably slower. It takes quite a steep decent to pass Vno and fly in the yellow arch.

            Comment


            • Bcone1381
              Bcone1381 commented
              Editing a comment
              43L=11.35 u.s. gallons. I add these comments not to be critical, but to assist those of us from the U.S.

            • JimParker256
              JimParker256 commented
              Editing a comment
              Also, one of the great things about the Dynon displays is that they can display IAS, TAS, and GS (usually located where the clock is in this picture). The TAS shows at 123 kts (look just to the left of the altitude tape), given 118.5 IAS, 2*C OAT, and 3210' altitude (2800' DA).

          • #11
            Those speeds are pretty awesome. I've got 29's with an IO-390. sweet spot seems to be around 112 KTAS burning 41-42 lph. That's at around 6,500 DA. I guess that's another advantage of the 540. being able to cruise LOP down low and still make good power.

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            • #12
              I am researching speeds and VG’s and came across this old thread. As always great into from Jonathan!

              N678C
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              • #13
                Maybe it’s just the cool spring weather, but we are seeing some very consistent and rewarding performance lately. I thought I would share a couple of examples.
                I am very happy with the cruise speeds, given we have 31” Alaskan Bushwheels equipped and extended hoerner wingtips. The Bearhawk still makes me smile after all these years!

                You can see the altitude, temperature, etc in case anyone is interested in detail. Fuel burn is lean of peak.

                These are taken on two different days:

                Day 1, fully loaded at full aft CG, with more power at 43 L/hr (11.3 Gal/hr) and seeing cruise speeds in the 123 KTAS to 126 KTAS.

                IMG_20231008_095019.jpg

                IMG_20231008_095027.jpg


                Day 2, lighter weight – about 1/3 loaded with 40% fuel and just a couple of occupants. Lower power settings at 38 L/hr (10 Gal/hr) and seeing cruise speeds in the 120 KTAS range.

                IMG_20231008_115457.jpg
                IMG_20231008_115459.jpg

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                • #14
                  Those are indeed impressive cruise numbers with the big tires. In addition, the best cruise numbers are usually obtained at higher altitudes. On those days I bet 3-4 more kts at 7,000 - 8,500 ft msl. Mark

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