Hi Gert.
Annie (FF) and I operate Rotax 503's on all our school planes, but we fly many 582's as well.
We have found that the 2 most important factors which HUGELY affect fuel consumption are: ( There are a myriad of other factors, like wing setting -tautness- loose and flapping pannier bags etc., etc., but they only account for small differences in consumption)
1. State of air filters
We have fuel flow monitors on both our main workers (2 Rainbow Safari's), and if our Air Filters have not been cleaned at 50hours then fuel is noted to be flowing at 1.5lt per hour more than with a new or freshly cleaned and oiled filter. Another problem with the conical air filters is that when they are dirty, the suction from the motor is so strong that the filters actually getted 'sucked' in, and become 'concave', instead of the 'convex' shape of a new one. Cleaning a 'concave' filter will not help at all, as once it has become 'concave', it has lost about 40% of its surface area (do some calculus on surface area of spheroid's to get the answer). What this all means is that your engine will be perfoming at very much less than peak for any given throttle position, so to compensate, you add more throttle, which uttimately leads to a huge over-use of petrol - up to as much as 7lt per hour too much. This off course leads to over-rich mixture, which leads to too little oil burn-off during combustion, which leads to carbon formation on cylinder head causing about a 7 - 10% less compression space in the cylinder which means less power which means you add more throttle etc., etc., etc.,
And off course ring sticking due to excess carbon build-up, which leads to loss of compression and therefore loss of power so you add more throttle and use more petrol etc., etc., etc.,
And it all started from bad airfilter management
2. EGT's (exhuast gas temperatures)
We have EGT's fitted to all our 503's. This is the first instrument which tells you without doubt that your air-filters are starting to get dirty.
I have found, even on our brand new engines, that within about 40 hours of use, I need to drop the needles to the groove that the little rubber 'o' ring is in (the top groove), and I reduce the main jets to 155 (from standard 158's).
What this achieves is to maintain EGT's between 570 and 600deg Celcius.
The minute the EGT's are consistently running below 550, it means the airfilters are most probably the first thing to look at as they are getting dirty and restricting air-flow.
What I have achieved by stringent monitoring and usuage of EGT's for fuel consumption purposes (ultimately engine management) is to obtain fule consumption figures as follows:
SEA Level operation - cruise at 5300RPM at 500feet ASL - cruise speed 45 -47mph - half tanks and 2 persons on board
(i) dirty filters and or low EGT 14.5 - 16.5lt per hour
(i) management as above 9.9 - 11 lt per hour
I achieve the same for 582's when I run them. Their RPM is less, but the main jet is bigger. End result is about the same consumption
When flying solo, and I have just cleaned filters and jets and needle jets are new, I achieve between 8 and 9lt per hour.
If I really go out of my way to control the entire flight, from start up till shut-down, including adjusting RPM for climb, planning climb and descent paths, use of ridge lift along the coastline etc., then I can bring consumption right down to craaazy figures
Once flew from Empangeni to La Mercy -solo- in a Safari with 503. Total time 1hr 20min. Fuel used 5.2lts. Had a tail wind, and 'ridge soared the dunes from Mtunzini till Ballito, IAS 44mph (best glide for the Aeros 14.8 wing)
But ja, keep those airfilters clean and make sure the surface area is NOT compromised, adjust main and needle jets to give almost max recommended operating EGT (is usually about 20deg. celcius less than Max permissable) keep the engine free of excessive carbon deposits and fly your wing at it's best L/D speed, which for most local wings is between 44 and 48mph, and you will be amazed at how little fuel you will use........
When FF and I flew up to Ladysmith for the Nationals 2 weeks ago, we were fully loaded with gear, and averaged at IAS of 48mph, flying at FL 085. Our consumption was 10.3lts per hour each. Yup, we both used the same amount of gas.
What is interesting is that our wings are old, really old, and have lots of billow. Therefore we are flying at higher RPM than necessary to achieve 48mph. I am confident that with a new wing, we would have done the same journey with an average usage 9.5l per hour each.
A really interesting topic for me, as I enjoy all aspects of flying, including mastering engine management
If that is possible with these engines and the variable environment we operate in
Hope you enjoyed reading this................
