Calculating Endurance

[Airbusboy]

Hello there.

If the total fuel required is 35.5 USGAL @ 9USG/hr, the Endurance would be 3.9 hours correct?

Is there a way of calculating this with the CRP-1? I've been fiddling with it for a while now cant get any answers.

[mary meagher]

Rule number one, if the book says 4 hours endurance, fill up the tanks, be frugal with the baggage, and stop every 3 hours for a pee. Your passenger will appreciate your thoughtfulness.

[IO540]

The non-wind side of the CRAP-1 is just a standard slide rule, which is a 200 year old multiply/divide tool. It is bent around into a circle; that's all, and has marks at aviation-relevant places e.g. litre/gallon conversions (3.8) etc.

If you google on 'slide rule' you will see how it is used for mult/div.

Once you suss how to use a slide rule to divide 355/9 and then (mentally) put the d.p. in the right place, you are done. This process is in no way aviation (or CRAP-1) related.

I could not go for 3hrs without a pee ;)

[Airbusboy]

lol thanks for the tip. Also I figured out how to do it on the whizz wheel. Thanks for the input IO540.

[Whirlygig]

Or do it in your head? :}

Endurance is easy; about 2 hours or 1 hour 45 mins if I've had two cups of tea.

Cheers

Whirls

[AdamFrisch]

In the POH you will find how much fuel it takes to climd at a certain weight to a certain height at so and so climb speed. Use this number to Top Of Climb. Then derive whatever leaned fuel consumption - at your chosen cruise altitude - for the duration, once again from the POH. Or if you would like to build in an extra safety margin, use the rich mixture fuel burn even though you're planning on flying leaned. Add the reserves on top of this.

[DX Wombat]

Don't forget, it is always a good idea to round down the answer. Eg: if the figure you come up with is 4hrs 25 mins then round it down to 4 hours. It's always better to think you have less fuel than you have rather than more of it.

[Halfbaked_Boy]

If the total fuel required is 35.5 USGAL @ 9USG/hr, the Endurance would be 3.9 hours correct?

Incorrect!

Based on above figures, you cannot work out endurance. I'm sure it's just the way you've worded it, but 'total fuel required' is the amount of fuel you actually require for the planned leg and/or route.

Endurance is based upon the fuel you actually have in your tanks, which will be more than you actually require.

For example, using your numbers above, you require 35.5 USGALS to complete a flight, fair enough. Add on, say, 5% contingency, and an hour reserves at 9 USG/hr (9 USGALS :ok: ), and you get 46.3. Call it 50 USGALS you have in your tanks. This is what you then use to calculate endurance. 50/9 = 5 hrs 33 mins endurance. If this is for test purposes, be sure to use the methods of adding contingency fuel/extra fuel as taught by your instructor, or in your manual. This is just my personal method to produce an absolute minimum.

Just for info, the 3.9 hours figure you came up with would be your enroute time, assuming total fuel required calculation is correct and the 9 USG/hr figure was an average. So (using the method above), deduct 3.9 from 5.55 (5 hrs 33 mins) and it leaves you with about 1 hr 40 mins of fuel above and beyond what you actually need.

I always set one hour reserve as the bare minimum.

All the best. :)

edit: Sorry I can't be of much help on the CRP front - I just cheat and use a calculator!

[Big Pistons Forever]

At the risk of over complicating this there are two definitions of endurance that apply to light aircraft

Flight for endurance means that power setting which will result in the longest time in the air and will always be the minmium amount of power required to sustain level flight for the conditions of that flight.

The other is what the word implies, how long will the aircraft fly with a specified amount of fuel which is 3.9 hrs in this example

Fuel required is the fuel that has to be in the aircraft to complete the flight taking into account all factors, start, taxi, runup, climb, cruise, descent, approach and landign plus reserves plus a suitable extra amount for contingencies.

[englishal]

Use an online flight planning tool...it is far more accurate!

The hardest bit really is to determine exactly how much fuel you burn unless you have a fuel flow computer which physically measures it and sends that data to the GPS to tell you how much fuel you need to the destination.....

That beats the CRAP1!

[Genghis the Engineer]

Unless you are using something published and approved by the aircraft manufacturer, any online tool will just be some curve fits to POH data.

POH data, calculated through on a PLOG, using a CRP supplemented by a pocket calculator, should be as accurate as can normally be achieved.

Some online tool may well be more precise, but that is quite diffferent to accuracy. It also may well contain errors, not having been through the checking process that goes into a POH.

With regard to fuel flow, pretty much all aircraft fuel flow gauges are very innacuraate. I'd uae the POH numbers for primary planning and any cockpit gauge as a check for gross errors, unless I'd done some careful checking first.

G