It's hard to find typical flow rates for the Janitrol/Kelly heaters which are common in piston twins, but one google search turned up 2.25 USG/hr which is definitely significant on a long flight.
I've often wondered whether the fuel flow to these heaters gets measured by a fuel totaliser. One owner told me it doesn't, but he doesn't worry about it.
Peter, I suspect that the consumption is 2.25 pounds per hour. The 65Hp Cub I used to fly cruised on just over 3 GPH! From my Aztec days, and that one had two heaters) I recall that we considered heater fuel consumption to be negligible for flight planning purposes. I vaguely recall the Twin Comanche being half a gallon per hour, which seems to correspond with 2.25PPH not too badly.
As I recall the heater fuel supply was before the totalizer in the fuel flow system, but that's recollection, not certainty.
I know the fuel burning heaters in the larger engined diesel cars burn about 250ml of fuel an hour, so Pilot DAR sounds about right.
It always disappoints me how old fashioned technology remains in aircraft, and how the industry doesn't use some of the really reliable stuff developed and tested in the car industry, but we all know why
The manual of the Pa44 (the only janitrol heater equipped plane I currently fly) states „approximately 1/2 gallon per hour“. That would make around two litres per hour compared to 40-50 litres per engine per hour. Which is around two percent of the total fuel consumption, well within the 5 to 15 percent (depending on the type of operation) contingency margin that needs to be applied to the fuel calculation anyway.
On different totaliser-equipped aircraft that I have flown (other Pa44s, Senecas, C421) the heater fuel was not included in the totaliser count, as it is fed from it's own fuel line. It is also unaffected by fuel tank selection valves. For example on the C421, heater fuel is drawn from a crossfeed line.
On my Beech the heater sucks fuel from the left tank and I calculate 1 US GPH. The number from the producer I think is 0.8 gph. Newly tuned up heaters typically run a little lower (as low as 0.5 gph).
Anyway, as the Owner's manual put it: Due to the small amount of fuel burned by the heater, its consumption may be ignored in calculating fuel requirements.
1GPH is 10% of my total fuel flow
On an IO540 twin it would be 5% of the total fuel flow, and I would not say that is insignificant.
It probably is insignificant in the context of how most people do their fuel management - no fuel totaliser, near-useless fuel gauges, so have to be ultra conservative.
The text is actually an accurate quote from the owner’s manual.
The way I plan for fuel it has no insignificance. At 160 KTAS, I have 6 hours endurance which makes the total extra fuel drawn from my left tank 5 gal (with one hour reserve). If I wanted I could level the tanks by cross feeding for a few minutes.
Given that I run the heater from start up to landing on a 5 hour trip, it would maximum shorten my endurance with less than 5-7 minutes. How can that be significant on 6 hours endurance?
Personally I would never go close to the limit where 5 gallons matters.
PS! Just out of curiosity, what speed (TAS) do the TB20 get at 10GPH?
Just out of curiosity, what speed (TAS) do the TB20 get at 10GPH?
About 140 at FL120.
5 USG is half an hour in cruise, for me.
Sure I would not be landing with just half an hour in the tanks, but for me this amount of fuel would be significant.
Perhaps more importantly, the heater consumption would make a mockery of the 1-2%-accurate fuel totaliser. There would be no way to check that it is accurate, with an error of that size creeping in.
And I expect the fuel flow into the heater won't be constant; doesn't it adjust the fuel flow to achieve a given cabin temperature? Or is the cabin temp adjusted by the heater running flat out all the time, and then there is a ram air / heated air mixer valve?
In a SE, one could take care of the heater fuel flow by buying a fuel totaliser for a twin and feeding the heater via the "second engine" flow sensor, but I don't think anybody makes 3-sensor totalisers.
I agree with the principle but practically I have not found it to be of any significance. On a twin, one engine will sip more fuel, the other will use a little bit more oil etc..
Regarding fuel burn it is constant. The system is very simple. You adjust the tempereature (down) by mixing cold air from the outside into the flow from the heater.
At full heat you can bake muffins in the cabin. A bad thing is that spring and summer it can be diffoult to get the temperature down to a comfortable level when on, while to cold to leave it of.
The good thing is that it can run without the engines running so winter time you can heat the cabin before starting up.