But how much time would you guys say is needed for some particular cruise settings to settle in? In my head it’s something like 3 or 5 minutes, but maybe I am plain wrong and I should have waited like half an hour before taking any instrument photos?
Don’t despair. An Arrow IV is generally good for 130-135 knots at moderate power settings and altitudes. Which is what you are seeing. One can get 138 out of them, but with only a lot of fuel.
Anything which you might be missing might be due to a couple of extra antenna, slightly imperfect rigging/fairings, or even a couple of knots of error on the ASI(s). All not too uncommon. Enjoy your plane!
Regarding changing power settings and the time to see everything stabilized: 2-3 minutes should be well enough. I would re-do the tests again. Make sure you are in absolutely still air. I would hazard a guess that when going from peak EGT to 100 ROP, without changing anything else, you should pick up 3-4 knots. But not so much more. After all, that (the fact that going peak of slightly LOP rather than 100 degrees ROP only costs a few knots, whilst saving 1.5 gallons) is why so many people do it….
Brilliant trip! Great to see the aircraft used for a trip with a personal purpose.
Great to see your wife enjoying the trip, and you making sure she’s comfortable 
Will you really get 75% power above 6000ft? I seem to remember that you’re already WOT by then.
For what it’s worth, the Arrow II that I used to fly would give IAS of 125kts for a fuel flow of 10UGS/Hr (Roughly 65% power).
Mateusz wrote:
What could I be doing wrong?
Right now, I don’t know that, but I do know what you are doing very right:
I love seeing someone really taking performance seriously, working the POH and taking tons of real life data to figure out what really happens. That is rare and also your clear understanding how to read this stuff is encouraging.
POH’s do contain some bragging rights, meaning that often enough they are very optimistic. 130-135 KTAS is what I also have heard from many Arrow owners, both 2,3 and 4 variants, so it’s not totally off.
What I would do in your case is to isolate all this data you have and compare it to the POH and also use the POH available settings for your comparisons. I.E. if the POH has data for 2400 and 2700 RPM, those (rather 2400) settings are where you need to run your comparisons. Also make yourself a nice Excel table in which you put all your data and then post flight try to find out what exactly they mean. i.e. what percentage of power you were exactly flying e.t.c. FF works quite nicely to determine that as one denominator.
If you can manage, get an ADC and see that you get TAS properly read out on the G5 or one of the GPS’s. It IS a game changer, even though those ASI’s do not bad if set correctly. Sometimes that is a bit teidious to do and it is easy to mistake something and get a few kts error. Photographic evidence is quite neat in this case.
What also helps with a new to you airplane is to have someone do a RPM check using a stroboscope to make sure that your RPM shows correctly. It’s a few minutes of work (by someone who knows exactly how to do that) but it helps if you find out that the RPM indicator may be off even slightly.
Basically start here:
Then using these settings look at the performance tables:
and
At 6000 ft, 65% corresponds to 2400 RPM and 21.5 MP. At 10.4 GPH for best power this results in approximately 132 KTAS. At Eco Power with 9.2 GPH it results in 125 KTAS.
Those are the values you’d need to compare. If you got a big enough Delta on those, it would be time to ask why. But I’d reckon you’ll find that they might pretty much do.
For any other settings such as 2550 RPM or 2700 RPM (which you are not going to want to keep for the whole flight) you have only partial data or none at all. So you will need to write down your own data points for that. Can be done, is lots of fun and after a while you really know the performance of that particular airplane inside out.
gallois wrote:
IMO it is the IAS you should be building your engine settings around.
No. IAS has nothing to do in this kind of settings.
He has done a remarkable job of documenting his power settings and results. With a bit of organisation he will have a much larger knowledge of his airplane’s true performance than most pilots ever get.
Don’t despair.
Haha, no worries. As you can see throughout the report, I am well enjoying myself at 130kts :)
I would hazard a guess that when going from peak EGT to 100 ROP, without changing anything else, you should pick up 3-4 knots.
So the whole “100 ROP” performance gain is quite overrated…
An Arrow IV is generally good for 130-135 knots at moderate power settings and altitudes
130-135 KTAS is what I also have heard from many Arrow owners
the Arrow II that I used to fly would give IAS of 125kts for a fuel flow of 10UGS/Hr
Thanks guys for these data points!
I would re-do the tests again. Make sure you are in absolutely still air.
So you will need to write down your own data points for that. Can be done, is lots of fun and after a while you really know the performance of that particular airplane inside out.
I will sure continue tinkering! But it will take time. Finding the performance “sweet spot” it’s not nearly as important for me to be making special test flights :) So it is going to happen by the way of next trips.
What also helps with a new to you airplane is to have someone do a RPM check using a stroboscope to make sure that your RPM shows correctly.
Greg checked it with his portable RPM meter on the last (or rather, first) annual and it was within reason. Can’t quite remember but something on the order of +/- 10 or 20.
Time to look for speed mod
Wow, I was sure there are mods like this available – but I didn’t imagine they are so many options!
Sorry Mooney I totally disagree with you. If you don’t know your IAS or better still your CAS based on power/mix/prop settings you will never be able to plan properly because your TAS is based on IAS +Altitude+temperature. You cannot control one of these, whereas you have control of an IAS to build up a database and compare it to the performance charts to see how close your aircraft is..
You then have a choice of methods in calculating best settings for the speed, range, journey ,fuel flow and journey fuel required on your flight. Of course these are targets without wind.
Ground speed is more important to calculating many of those targets accurately but can only be calculated from TAS +/- effective wind which can be cross checked against your GPS ground speed.
If you are testing and building up a database for your particular aircraft you start with a calibrated airspeed. IMO its the only logical starting point.
Mateusz wrote:
Wow, I was sure there are mods like this available – but I didn’t imagine they are so many options!
By the time you’re done with the mods you might just as well have bought a Mooney (lots of the PA28R speed mods are from LoPresti, who optimized the Mooney M20J) ;)
Congratulations, by the way; sorry that I missed your original “coming out” announcement. I know it’s not an exciting journey, but if you end up at EPKP give me a shout.
My old Warrior managed an honest 120 KTAS but it had flap and stabilator seals, and wing root fairings. Very good aircraft and in the UK easy to maintain.
