This is the way I see it... Gallons per hour is used to figure out how long you can fly before you run out of fuel. So many gallons in the tanks divided by the GPH give you the time you can fly. Using tach time to calculate GPH would then require using the same tach, instead of a normal clock, to determine if you are running out of fuel.... Seems odd to me, but it works.
IMHO, I think it is simpler just to use clock time to calculate GPH, and then just use clock time to detemine how much fuel has been used. Tach time based GPH will NOT work to determine the DISTANCE you can fly, because none of the other figures you need to use are based on tach time. DISTANCE is calculated using GPH and either: 1. Your airspeed indicator which measures miles or knots per hour, corrected by temperature and altitude to find true airspeed, then corrected with the winds aloft (again in knots per hour) in order to obtain ground speed. 2. A GPS, which directly measures ground speed DISTANCE = Gallons of fuel * (Groundspeed / GPH) Neither the airspeed indicator, nor the published winds aloft, nor the GPS signal are based on your particular (or anybody else's) tach time, but on real time as measured by clocks. Granted, different clocks can have differences, but the differences between them are way less than the inaccuracy of a typical tach. I had a tach with more than 13% error. That's 8 minutes of error per hour! Hope this helps Eliacim > John, > > I need some additional help with your logic for using clock time for your > GPH calculation. I calculate fuel consumption based on tach time for the > very reason that you state. "if you go by tach time your fuel consumption > will be almost the same regardless of engine speed". Or restated; fuel > consumption corresponds with the tach and not the clock. > > My concern is when will my tanks run dry and that is easily calculated > using > tach time. It would be a nightmare using elapsed time unless the aircraft > is > equipped with a Hobbs meter - mine isn't. IMHO > > Andy Anderson > > 415-D with C-90 ( 6 GPH tach time) > > _____ > > From: [email protected] [mailto:[EMAIL PROTECTED] > On > Behalf Of John Cooper > Sent: Thursday, January 31, 2008 9:50 AM > To: 'Randy Hougham'; [email protected] > Subject: RE: [ercoupe-tech] Propeller/Fuel Burn > >> I never get better than 6 gph. If we're doing touch and goes, etc. I >> may > get 7 gph. I'm using tach time, is that right? > > No. The tach "time" passes slower when the engine is running slowly and > faster when the engine is running fast. A typical C85 tach should record > one hour in one hour (i.e. be accurate) at 2500 RPM. Basically, if you go > by tach time your fuel consumption will be almost the same regardless of > engine speed. It will take a very long hour to burn all that fuel at 1800 > rpm, however. > > > > Bottom line, measure gallons used verses actual elapsed time. > > John Cooper > > Skyport Services > > 518 797-3064 > > www.skyportservices.net > > > > _,___ > >
