Thank you everyone for all the useful tips and for your error estimates! I stand convinced that calibrating my own dipstick is worth the effort.
I went flying yesterday, did this small loop over Sudety mountains with Darek.
For a good half an hour Darek happily served as auto-pilot while I was pushing various buttons and figuring out what they do.
The most important takeaway:
The EDM photo was taken just before shutdown and the bowser of course says how much fuel went in until the tanks were full again. 18,4 gallons = 69.7 l. I like it!
Mooney_Driver:
Can your analyzer feed the GPS? If so, that is a big advantage as you can get fuel estimates for the destination. But then again, the EDM does show you time remaining and you can easily compare that with the flight time remaining by the GPS.
Don’t know if it can feed the GPS, but certainly it can take data from it. This is from EDM-900 manual:
Also some wiring diagrams here: https://www.jpinstruments.com/technical-support/gps-interface-connections/ …
Actually I am still not sure if I have this interconnection already in place. I have two NAV units, GTN650 and GNS430. So far I have been using only the GTN650 and only this one got a current database (I don’t have access to hardware for flashing these fancy GNS430 memory cards). Perhaps someone run these two or three wires between the GNS and EDM.
Other discoveries:
Mateusz wrote:
Actually I am still not sure if I have this interconnection already in place. I have two NAV units, GTN650 and GNS430. So far I have been using only the GTN650 and only this one got a current database (I don’t have access to hardware for flashing these fancy GNS430 memory cards). Perhaps someone run these two or three wires between the GNS and EDM.
It should be pretty easy to find out. Go to the fuel planning page on the GNS430 or the equivalent on the GTN650 and see if the figures there update with an active flight plan and fuel flow.
If it’s not done, i’d recommend doing it asap and I’d connect it to the GTN650.
Mateusz wrote:
The right tank fuel quantity indicator grossly under-reads!
Oops. That needs to be addressed asap.
Yes, TAS is something really nice to have and obviously so is the wind indication. Once you have it, you don’t want to do without it.
GTN software supported only US gallons, not litres, until AFAIK recently…
You will need to be flying to see a computed LFOB, or any indication of flow.
TAS needs airdata (heading and IAS and OAT) going to the GPS. If not provided, I would not worry about it. It makes zero difference to whether you will continue the flight or not. Only LFOB and the destination closing time matter.
Congratulations! Good choice on the Arrow IV 😍 and that’s a nice panel and interior. The 🦊 is really cool. It took me a few landings to get used to the T-tail but it really isn’t an issue. Landing a few knots faster keeps some airflow over the stabilator and limit the thump
Some updates :)
Turns out the GTN650 is properly connected to the EDM and I do have that lauded LFOB function available within a few clicks in the GTN menu.
I did some A-A practice flights, both solo and taking other pilots for a ride. Here the T-tail casts a shade on the wing over the Mala Fatra mountains (Slovakia)
Also I discovered someone took a very nice photo during my familiarization training. It might be the best photo of the Fox Oscar I have so far!
I am wondering where the EDM-900 gets its power % estimate. It says 80% while I feel my MP/RPM settings as well as the fuel flow are much closer to what the POH calls a 65% cruise setting. I tried to confront the actual operating conditions with a graph posted in Lycoming’s 60297-12 (figure 3-21) but so far wasn’t able to make sense of it all. It’s somewhat related to a recent thread started by darkfixer.
On another note, the family already likes the Piper:
Since my last post I burned some 69,8 gallons of Avgas, starting from full tanks. The number comes from summing up what the bowser reported during three refuels. Before the last (bringing the fuel level back to full tanks) EDM-900 indicated REM = 31,5 gal (which means 40,5 to top up the tanks) whereas the bowser claimed 37,9 gal actually went in before the tanks were full. The difference: 2,6 gal divided by 69,8 gal gives an estimate of accuracy on the order of 4%. This is just one data point subject to errors coming from not filling really to the brim, uneven parking, bowser error, temperature variations and what else. So I consider it optimistic for the time being.
Some basic performance metrics averaged over 6.5h of flying in April: fuel burn for planning = 11.2 gal per hour spent in air. Average GS = 120 kt (only A-A flights, so also an estimate of average TAS). Most of these 6.5h was spent at 2000’ – 4000’, with only that one flight over Slovakia reaching 8000’. I was keeping to the 65% recommendations but surely I can still improve my leaning technique.
IT’s good to have the 72usg tanks. 80% may refer to available power at the current PA.
I am wondering where the EDM-900 gets its power % estimate.
Lots of people have wondered that 
At peak EGT i.e. approx stochiometric, HP = fuel flow x a constant. The constant is in the region of 12 and this is the same for any petrol engine, plus or minus a factor proportional to the square root of the compression ratio (more CR → More HP at the same fuel flow i.e. free power!).
But once you go ROP then it is basically guesswork.
Another approach is that MP is a proxy for torque and HP = torque x rpm x some constant.
I think it is mostly BS, and anyway you don’t need to know it for any purpose whatsoever.
Agree. Many people, when they first get an engine monitor, are glued to percent power indication. I think that‘s because they got taught to set the power accurately to some exact percent value. Probably because these values (often 55, 65 and 75%) are shown in the manual. I never look at this indication. It’s off anyway. Even if it were accurate, it would still be worthless. Just set the power to whatever airspeed or fuel flow you want. Over time, you get much more expert about the engine and the performance of the aircraft than the book.
Mateusz, you might soon want to try using LOP for higher engine efficiency and longevity or at least peak EGT. Some engines do better than others. Don‘t be put off by very mild roughness at peak or slightly LOP, that’s normal. It will make quite a range demon out of the later Piper Arrows.
Maybe similar to this:
ROP – oxygen controls power:
%power = (MP/27) * (RPM/2700)
HP = %power * 200
LOP – fuel controls power
HP = 14.85 * FF
%power = HP/200
Mateusz wrote:
I am wondering where the EDM-900 gets its power % estimate. It says 80% while I feel my MP/RPM settings as well as the fuel flow are much closer to what the POH calls a 65% cruise setting.
These two pages out of the POH say all there is to know about fuel flow vs %power.
For best economy, which is what you will most likely be using
Best power.
So the figures to look out for are
75% 11.6 65% 10.4 and 55% 9.1 GPH in best economy leaned to peak EGT
75% 10.2 65% 9.2 and 55% 8.0 GPH in best economy leaned to 100° rich.
If you want to do LOP, you will need to figure out the resulting fuel flow by yourself.
If you want to know what TAS this plane does for you vs altitude, the best way to know is to fly it with constant fuel flow at various altitudes and note down the TAS.
Mateusz wrote:
Some basic performance metrics averaged over 6.5h of flying in April: fuel burn for planning = 11.2 gal per hour spent in air. Average GS = 120 kt (only A-A flights, so also an estimate of average TAS). Most of these 6.5h was spent at 2000’ – 4000’, with only that one flight over Slovakia reaching 8000’. I was keeping to the 65% recommendations but surely I can still improve my leaning technique.
Do yourself a favour and look into that performance section and feed what is in there into a decent flight planner. Then, once you verify what your plane actually can do, adapt the profile. That is the only way you can do a proper cruise control and achieve flight planning with any degree of accuracy.
Mateusz wrote:
The difference: 2,6 gal divided by 69,8 gal gives an estimate of accuracy on the order of 4%.
That would be pretty severe off, particularly if the fuel flow is off by that much. The question is if the transducer and the EDM have been programmed correctly. This is something to monitor carefully. If the fuel flow is wrong, then so will the planning be.
boscomantico wrote:
I think that‘s because they got taught to set the power accurately to some exact percent value. Probably because these values (often 55, 65 and 75%) are shown in the manual.
I guess the main reason why those power settings are that well known is because that is how most flight planners are set up. The use of these 3 figures (sometimes augmented by 45%) is not that old actually, but it has become some sort of a convention on how to easiest sort out a performance of our kind of airplanes, by flying a constant percentage of power.
Of course you are very free to create other power setting regimes to your liking. The simple idea of flying constant percentage is that it pretty much also gives a somewhat linear fuel flow. So flying constant power is pretty close to flying a constant fuel flow too.
The most variable thing where most differences to the POH occur is TAS. So the way to verify how a particular airframe performs is to set it up according to PHO with MP/RPM/FF according to the setting tables and then read out TAS over a certain period. This will tell you what bias this airplane has.
boscomantico wrote:
you might soon want to try using LOP for higher engine efficiency and longevity or at least peak EGT. Some engines do better than others. Don‘t be put off by very mild roughness at peak or slightly LOP, that’s normal. It will make quite a range demon out of the later Piper Arrows.
Absolutely. And if it does not yet have GAMI injectors, think of getting some. Of course flying LOP will mean to do the whole performance tables over again. But it is totally worth it.
