Juker008

I've been racking my brain with trying to come up with the formula for how long I can fly with x size fuel tank. I determined that my BSFCis .6 (2-stroke), and that a 8.5 hp engine will consume 5.1 lbs/hr, or 85% of a gallon, but I can't seem to get it down to how long I can run with a 24oz tank.

So here is what I've got:

8.5 hp 2-stroke gasser
24oz tank

I'm calculating this on a constant 8.5 hp workload.

MTIA

Juker008

Here is what I have calculated thus far:

Parameters:
8.5 hp (constant)
BSFC.6

8.5 hp * .6 BSFC = 5.1 ( lbs of fuel consumed per hour )
1 gallon ( 6lbs )* .85 = 5.1 lbs

128 oz * .85 = 108.8 oz/hr

108.8 / 60 minutes = 1.813 ( thus I will consume 1.813 oz of fuel per min )

24oz ( fuel tank )/ 1.813 = 13.237 minutes of run time .


Someone tell me if this is correct.

huck1199

If your assumption that the engine will burn 5.1 lbs of fuel/hour is correct the result you got is correct.

pe reivers

Not exactly your question but: A two stroke gas engine at full power will burn about it's displacement in fuel each minute. Double that for glow engines.

Juker008

Pe,

Little confussed, are you saying that a 60cc 2-stroke will bur 60 cc of fuel in a minute?

Tired Old Man

My Feedback: (1)

When engines are left with stock carbs the fuel consumption is quite high which generates a need for a pretty large gas tank for flights of extended duration. Most RC and industrial engines are over carbed and over jetted. In stock form Pe's rough consumption comparison is not far off at WOT.

Lnewqban

Your calculation is correct for full throttle, either using pure gas or E10.

The engine will deliver a little less power in the second case, since the Ethanol in the mix provides about 65% of the net heat/mass than what pure gas produces.
If you compensate opening up the throttle for obtaining similar performance with E10, then, you will get a little smaller flight time out of the 24 oz fuel tank (~4% less).

According to pe reivers' statement, your engine should have a displacement of around 50 cc.

Specific fuel consumption x Gas density = 5.1 lb/hr x 0.026 lb/cu.in. = 196.15 cu.in./hr

And 196.15 cu.in./hr = 3.27 cu.in./min = 53.6 c.c./min

pe reivers

<blockquote class="quote">quote:

ORIGINAL: pe reivers

Not exactly your question but: A two stroke gas engine at full power will burn about it's displacement in fuel each minute. Double that for glow engines.

</blockquote>

Pe,

Little confussed, are you saying that a 60cc 2-stroke will bur 60 cc of fuel in a minute?

<span class="info"> </span>

Yes, that is about it. If you fly less than full throttle, consumption is greatly reduced.
Our 116cc glider tow gets about 10 hauls on a 1.5 liter tank. The full power run is about one minute.

Juker008

I see my error. I was thinking oz when I should have been thinking cc. Idid the calculations based on your theory, and it is roughly the same. Ilike that theory. Many thanks.

pdm52956

My Feedback: (1)

So, I suppose all that has some value but, I'm wondering what purpose it has. I mean no one I know is going to fly full throttle all flight unless they are pylon racing or something so isn't it somewhat pointless to devlop some formula to calculate flight time based on full throttle?

Not trying to be a smart ***** or anything, just curious.

Juker008

On your plane, what's the least amount of time you could fly?

av8tor1977

My Feedback: (6)

Well, if you size your tank based on full throttle useage, then you know you will have sufficient capacity and land with a bit in reserve every time...

AV8TOR

Juker008

But how would you know?

Flyfast1

My Feedback: (1)

Many warbirds are flown at or near full throttle the entire flight.

-Ed B.

Tired Old Man

My Feedback: (1)

Other craft are flown at or near full throttle as well. They may be using minimum sized engines due to weight/payload restrictions while still doing everything possible to maximize flight duration. Normally those are poorly designed in several areas. Others are designed around what I like to call "the law of minimums" so they use the minimum engine size necessary to accomplish the task but in so doing sacrifice other things.

Now how many of you flying 26-30CC engine wish you could fly at 50 to 100% throttle for 20+ hours, pushing a 40+ pound airframe, on 5 litres of gas? That's 1.09 gallons. That should give ya'll some idea of just how innefficiently your 26-30cc engines are currently set up, along with the impact of "dirty" airframes.

The 8.5hp two stroke needs a little attention to be what it needs to be.

pdm52956

My Feedback: (1)

OK, the warbirds and dirty airframes and such make good sense. Appreciate your insight but I guess I like to keep it, what I consider, a little more simplistic.

In other words, if I build a plane and get ready to maiden it, I know how much fuel I put onboard and I know I'm not going to push it's flight envelope. I can calculate reasonable flight time using the displacement per minute theory to get me close and make sure I'm well below that time.

I'll take the plane up and trim it, check stall characteristics and maybe fly it around the patch for a few minutes to see how it handles. I won't be up as long as I would with a proven platform because I'd rather bring it down and go over it with a fine tooth comb after that short first flight. At that time I can check the amount of fuel left onboard against the time of that flight and I can pretty much calculate the safe total flight time available from there.

For me, that just seems easier. Thanks for your answers.

pe reivers

That is how we all do it I suppose. I find it comforting to know I will not have to change my tank size, because my little rule of thumb has helped me decide on the tank size in the first place.

Tired Old Man

My Feedback: (1)

For fun flying it's the same formula I've always used. Displacement = cc's of fuel consumed/minute. Adjust from there after a few flights.

Juker008

+1

Really the starting point to why I wanted to come up with a formula.

Tired Old Man

My Feedback: (1)

When maintaining a pre-determined constant with a known consumption rate the BSFC can be ignored. After that all you're left with are some simple volume and time conversions. More accuracy is obtained when the volumes get broken into kilograms/hr. Flight times from there can be determined by performing a few test flights where fuel in and fuel out are measured and weighed. The results from that information permits the development of some averages that permit relatively predictable consumption behavior. Always add roughly an hour's fuel for "reserve" purposes.

The guy that figures flight time and fuel to the last .001kg is in for an expensive day if the winds aloft are greater, or from a direction different from predicted

pe reivers

some shop talk: (when I was young)
As a ships engineer, at full sea we sometimes performed power, fuel consumption and engine efficiency measurements as follows:
RPM fixed at 96
draw cylinder pressure diagrams.
Measure output shaft torque (twist of length of propeller shaft)
Measure fuel consumption. (consumption time for a fixed volume in a dumbbell tank)
Using data collected and provided with the fuel we could calculate exactly the horsepower, engine efficiency and consumption per BHP/H. This again was a measure of the overall shape the engine was in, and a measure of how good we were at our job.