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Calculating Fuel Load


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Hey folks-

 

A question for you all in regards to calculating the fuel load for a sortie. In particular, I'm asking for information on this so I can start thinking about adding it to il2missionplanner.com. Don't get your hopes up, because it might be quite a while, but fuel load calculation and wind correction are two features I've wanted to add to the planner for a long time. I understand how wind correction works, but fuel calculation is something I'm less sure about.

 

So, how do you go about calculating the minimum fuel necessary to complete a sortie? Or more simply, the minimum fuel required to complete single leg of a flight plan (since you can just sum all the legs to get a total required for the sortie).

 

A naive implementation would consider only distance, and find the fraction of the plane's total range represented by the flight leg's distance and take that much fuel. For example, if a plane has a range of 1000km on the specs tab, and the route is 100km, you would need 10% fuel. But this calculation ignores speed and altitude and must be terribly inaccurate, right?

 

I would think, for a reasonably accurate calculation, you would need to assume a flight leg consists of four pieces of information: distance, speed, start altitude and end altitude, ignoring wind for the time being. Give those four data points, how would one go about calculating required fuel?

 

For a perfectly accurate calculation, I can think of nearly infinite factors to consider: Ambient temperature, the curvature of the earth, so on and so forth. While I'm also interested in hearing about that stuff, what I'm specifically looking for is a balance between reasonably accurate and reasonable amount of effort to implement.

 

I know we've got plenty of aviation experts and real pilots around, so I'm interested in learning from you all. Your help is greatly appreciated!

Edited by 19//curiousGamblerr
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Another factor I sometimes wonder about is how the CoG change due to fuel consumption really affects the aircraft that are sensible to it in IL.2 Battle of.. ?

 

I'm not even mentioning asymmetric fuel loads on aircraft where wing tanks could induce it, since that's done automatically, but on some aircraft the variation in CoG due to fuel consumption could be rather important.

Edited by jcomm
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I always have the feeling, that the 109s get a little nose heavy after the first about half of fuel is gone, what would be realisti, I think, due to the 'L'-shape of the fuel tank with the vertical part behind the pilots seat, so the first half of the fuel flows out of the rear part of the tank.

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The complex thing is taking the plane condition into account. Loadouts matter, you carry a couple of big bombs externally, you won't get nearly as far as if you're in the same plane, clean condition.

 

In practice, you'd calculate a basic flight schedule like 15 minutes warm up, 15 minutes at climb setting, rest of fuel at cruise setting, plus a 30 minute reserve. No point in going into much more detail, because you don't know every detail beforehand anyway. I'd recommend you try an approach like this here as well, even if the perfectly reproducible computer generated environment does in theory allow you to be very accurate.

 

---

And great work there with getting off topic with the first reply! And on top of that with an issue that has long been answered. :rolleyes:

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Your task is a monumental one.

 

Back in the Jurassic Era when I flew the RF-4 and F-16 we had shortcuts to fuel consumption. We had a fixed fuel burn for STTO (Start, Taxi, Takeoff), a conservative estimate of fuel burned to get to different altitudes, fuel flow rates based upon aircraft weight/drag index (think external stores) and speeds in increments of 60 knots (e.g. 360, 420, 480, 540, 600). So you need the fuel burn to get to the target and the fuel burn to get home with a cushion to hold or divert...any excess is available for air-air. While our flight planning PC would crunch those numbers to the nearest pound of gas, we carried an InFlight Guide (AKA Ladies' Aide) with this info rounded up to a convenient number in "Tab Data" (tabulated format) printed on card stock. We didn't worry about ambient temp, or winds at low altitude. Curvature of the earth? Never enters into the equation.

 

I'm sure you appreciate each airplane in BoX will have a different fuel burn profile. Just like the equation d=rt (distance flown = rate or speed x flight time), fuel burn can be calculated as the rate (or fuel flow) times the flight time. Your challenge is arriving at a nominal fuel flow. How do you get that? Divide total fuel burned by the flight time at one specific speed, at one constant altitude. That's too much work for an extremely small benefit IMHO.

 

How would I tackle the problem? I would fly lots of very methodical test missions. I would start with one airplane (e.g. Bf-110) without external ordnance and 50% fuel. I'd takeoff and fly at low altitude (300 meters) at maximum continuous power (the engines aren't going to fail) over a course of a known distance (use prominent features and use the map to stay on course). I'd note what airspeed this power setting gives me (that's my ground speed). I'd keep track of the number of laps and time or use the mission debrief screen to do this for me. Fly until the engines flame out, then quit the mission. Want to get a little more esoteric? A some point select maximum emergency power BUT be sure to throttle back to max continuous power right before the time limit and you thrash your engines. Comparing the time difference can provide some insight into the fuel flow gulp required by max emergency power.

 

You fly that same configuration at maybe 3000 meters or 5000 meters. Bomber pilots would want that information. I only fly down in the weeds (I blame my years of fangs out, hair on fire flying around Germany). I'd repeat with a maximum bomb load. 

 

And in case I'm more than a little opaque, here's how I'd fly my test profile:

 

1) Clean airplane, 50% fuel > takeoff climb to 300 meters > set maximum continuous power > note/monitor indicated airspeed enroute > quit mission when first engine flames out > note flight time and count number of circuits in mission debrief screen.

 

2) Clean airplane, 50% fuel > takeoff climb to 300 meters > set maximum continuous power > note/monitor indicated airspeed enroute > set MAXIMUM EMERGENCY power for just under the time limit > set maximum continuous power > quit mission when first engine flames out > note flight time and count number of circuits in mission debrief screen.

 

3) Max external stores (bombs/rockets), 50% fuel > takeoff climb to 300 meters > set maximum continuous power > note/monitor indicated airspeed enroute > quit mission when first engine flames out > note flight time and count number of circuits in mission debrief screen.

 

4) Max external stores (bombs/rockets), 50% fuel > takeoff climb to 300 meters > set maximum continuous power > note/monitor indicated airspeed enroute > set MAXIMUM EMERGENCY power for just under the time limit > set maximum continuous power > quit mission when first engine flames out > note flight time and count number of circuits in mission debrief screen.

 

I'd collect at least four data sets per airplane. This will give you a ballpark estimate of a worse case fuel burn. Having said that, I'll repeat my first statement. Your task is a monumental one.

 

Our Jurassic Era analog to your il2missionplanner was a template printed on acetate or clear plastic about two inches wide. Along one edge was a distance scale (nautical miles), along the other edge was a time scale for a specific speed (we used 420, 480, and 540) in 10 second increments. Between and parallel to the edges were fuel flow scales (ballpark numbers) based upon that specific ground speed for a relatively clean airplane and another for lots of drag. So with this one piece of clear plastic, I could measure the fuel burn, time and distance of a leg of my low level. If I didn't plan to fly a constant ground speed I'd need to change/swap templates. I carried these plastic templates in my pubs bag for mission planning while on a cross country trip in Base Operations.

 

If I were in my squadron planning a mission I would take a piece of string or long scrap piece of map (that was not needed). I'd mark Zero (start point) and 80 NMs or simply hold my thumbs at those distances. 80 NM is the distance you fly in 10 minutes at 480 KIAS (when low level). Then looking at my map I'd start at home base and measure my planned low level route. This quickly told me roughly my mission time and I'd multiply that times my fuel flow to arrive at my fuel required. No consideration for temperature, density altitude. Easy as pie.

 

Cheers

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Yup.

 

You can never have too much fuel or too much runway.

 

Given the size of the maps to date, you'd be putting in for retirement before you ran out of fuel in any of the twins, or say the P40, if you took off with 100%.

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After learning the hard way, I always (really, always) take a full tank in all Soviet fighters whenever the mission involves venturing into enemy territory because you spend a lot of fuel climbing, maintaining at least corner speed, keeping formation, getting there and back, firewalling it to meet ETA when relevant, firewalling it for long periods during air combat, and finally you can have one to all tanks potentially damaged either in combat or by cheeky flak of all types.

 

In short, unless I'm flying point defence or intercept in friendly territory, 100% is way to assure a safe return at least to friendly lines.

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Maybe when I get better at surviving I'll think of carrying a full fuel load, but for the time being, I prefer being around 200 or 300 Kg lighter.

 

There seem to be too many variables to get an accurate fuel calculation, but as I read in "Red Star against the Swastika", Sturmovik pilots did know how far they could get with their fuel load. My point is: maybe use a generic average for the planner, and leave the pilot the task to estimate if they'll need more or less given the mission.

 

I'd find it helpful myself. Even more than currently, which is already a lot.

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Nice idea, Sergi0.

 

CG, you could try a generic workaround using tiers - missions involving, say, 150km round trip count as short, up to 300km as medium, up to 500km as long, and above that it's extra long. A reference table can be used here to indicate the rough percentage, in game terms, a certain class of aircraft needs to carry for these missions.

 

busdriver gave the most accurate solution, by all means.

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Single engine: .5 liters/km

Twin engine:1 liter/km

 

simple and effective :)

 

You know, that's not a bad starting point. :salute:  At least for the brief flight test I did in a 109F-4 with 72 liters (18%) on board with an airstart. I used the Stalingrad map so I could use il2missionplanner.com to check my calculations. 

 

I started at the south spawn point in grid 1726.3 made an immediate turn to the SE to go to the south end of Barmantsak (grid 1927.9) then flew a heading of roughly 357 (just left of north) taking me over the twin bridges  (grid 1227.2) east of the airfield at Tatyanka.

 

So my setup conditions were the F-4 with 72 liters of gas. Start time was 1130, altitude 300 meters.

 

First trial I set the power to maximum combat thrust using technochat. This gave me roughly 82% rpm and an airspeed of 525 km/h (that's 8.75 km/min). Once I headed north I engaged the auto-level autopilot. The first indication of fuel exhaustion happened in grid 1127.4 about 11 mins into the flight. I left the autopilot engaged while the speed decayed approaching 200 km/h then I clicked off the autopilot and tried to hold that speed down to impact. According to the mission debrief the impact was at 1143 at grid 1027.5 (call it 10 km beyond fuel exhaustion). 

 

So what did I get from this sample of one? Well I burned my 72 liters in 11 minutes which is a fuel flow of 6.51 liters/min (call it 6.5). If you could fly at maximum combat thrust indefinitely how long would it take you to burn through your gas? Okay...400 liters of gas divided by 6.5 liters/min gives you...61.5 minutes (call it 62). But if you're limited to 30 minutes maximum combat thrust you burn how much gas? 6.5 liters/min times 30 minutes equals 195 liters burned. Meaning you have 205 liters to be burned. Hold that thought...we'll come back to this.

 

Second trial I set the power to maximum continuous thrust again using technochat. This gave me 64% rpm and an airspeed of 495 km/h (that's 8.25 km/min). I made the hard left turn to the southeast toward Barmantsak and once I turned north engaged the autopilot. The first sign of fuel exhaustion came at about 13 minutes near the first crash site. I repeated the speed decay on autopilot and then deadsticked at 200 km/h to a crash on the north tip of the island at grid 0927.5. So the lower thrust got me about 10 more kms before I was on fumes and about two minutes extra flying time. Looked at from a different perspective, maximum combat thrust up to 30 minutes will cost you less than 6 mins of lost endurance over maximum continuous thrust. That's a small penalty. 

 

In the second trial I burned through my 72 liters in 13 minutes which is a fuel flow of 5.53 liters/min (call it 5.5). That would burn through 400 liters in 72 minutes (400 divided by 5.5). Looking up two paragraphs to the statement about 205 liters remaining after 30 minutes of maximum combat thrust, you would burn that 205 liters up in 37 minutes (205 divided by 5.5) at maximum continuous thrust. 

 

72 minutes endurance at maximum continuous thrust versus 67 minutes endurance (30 max combat + 37 max continuous). And this is at 300 meters.

 

Okay so let's test Nibbio's ROT of .5 liter per km. In the 109F-4 with its 400 liters Nibbio's ROT would yield 800 km range (one way). Recall that max continuous thrust gave me a ground speed of 8.25 km/min (495 km/h) and an endurance of 72 minutes. So our range would roughly be 600 km (8.25 times 72 equals 594) at 300 meters (low altitude). If we used max combat thrust for 30 minutes our range would be about 570 km [(30 times 8.75) plus (37 times 8.25) equals 568]. 

 

But suppose you want to allow for 30 minutes at max combat thrust in the target area (at low altitude). So you eliminate that 30 minutes of fuel (because you need that for maneuvering) leaving you with 37 minutes available at max continuous thrust. I'd round that down to 30 mins available (the 7 minutes or 40 liters reserved for STTO and landing pattern) to get to the target and back. Well that leaves you with 15 minutes one way if you plan to land at the airfield you departed from. 15 minutes at 495 km/h (8.25 km/min) is roughly 125 km or 12 and a half large grid squares on the map.

 

Now that I've bored you to tears  :sleep: what can you do with that bit of gouge? If I'm a 109F-4 guy I'll look at the map and say, "Hmmm every 12 squares is about 15 minutes flying at max continuous thrust at low altitude. I have about 1 hour endurance at low altitude with a full load of gas." 

 

I'm more than happy to discuss this with anybody, here or via PM if you prefer. :salute:

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Another factor I sometimes wonder about is how the CoG change due to fuel consumption really affects the aircraft that are sensible to it in IL.2 Battle of.. ?

 

I'm not even mentioning asymmetric fuel loads on aircraft where wing tanks could induce it, since that's done automatically, but on some aircraft the variation in CoG due to fuel consumption could be rather important.

Yeah-

 

I'd also like to see a thread on this with some testing on ac with wing tanks:

Rate of roll full vs empty wing tanks.

 

On ac with large fuse tanks:

I'd like to see stall speed testing to determine CLmax changes (fore aft cg delta) with full vs empty tanks.

 

Maybe someone could start a thread on this.

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busdriver, thanks so much (and everyone else too!)! I really appreciate you taking the time to write up this info, and extra thanks for taking the time to run some tests! Like you said in your first post, my task definitely seems monumental, but the info you've provided is a great starting point. I'm going to take some time this weekend to work through some tests and examples based on your write up and see if I can come up with a practical system.

 

A small aside, you said "I used the Stalingrad map so I could use il2missionplanner.com to check my calculations." Just so you know, il2missionplanner has Moscow and V. Luki maps as well, and will have Kuban shortly after its released. You can change the map by selecting the settings button on the bottom right (looks like a gear). People miss this all the time, so I definitely need to do something about it when I get back into working on the planner.

 

Back to the topic at hand, we'll see what I come up with this weekend. But as pointed out by folks in this thread, the differences between planes and weapon loads are going to be so important to these calculations, this feature might be too tricky for the planner. At the very least, each flight route will need a setting for plane type, which is something I haven't even thought about. Perhaps I'll just give distance and time totals and leave fuel calculations up to the pilot. But we'll see what I come up with this weekend.

 

In any case, this has definitely been educational!

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Interesting. So my estimate of .5 l/km is actually closer to .7 l/km for a single engine fighter, at low altitude. Good to know, thanks!

 

I just ran a quick pair of trials with an FW-190-A5. Same map, spawn point, and spawn altitude (300 meters) with fuel set at 20% or 104 liters. The Cliff Notes (AKA Ballpark) numbers as follows. 

 

First trial, maximum combat thrust set to 86% rpm according to technochat. Airspeed settled at approximately 540 km/h (9 km/min). Fuel exhaustion at bit over 13 minutes. Fuel burn at maximum combat thrust 104 liters divided by 13 mins equals 8 liters/min. I'm not a FW guy, so I'm assuming it has a 30 minute limit for max combat thrust. If so, 30 mins times 8 liters/min equals 240 liters of fuel required (for max combat thrust use).

 

520 liters minus 240 liters (for max combat thrust) leaves 280 liters for STTO, transit to and from battle, and landing.

 

Second trial, maximum continuous thrust set to 81% according to technochat. Airspeed settled around 520 km/h (8.7 km/min). Fuel exhaustion just under 14 minutes. Fuel burn at maximum continuous thrust 104 liters divided by 14 mins equals 7.4 liters/min. I rounded to 7.5 just for the math.

 

Referring to the 280 liters available for STTO, transit and landing, 280 liters divided by 7.5 liters/min equals 37 minutes available. I subtracted 7 minutes like I did with the 109F-4 to say that is fuel used for STTO/landing, leaving (conveniently enough) 30 minutes or 225 liters of gas for low level transit to and from the fight. Again assuming you plan to land at the field you departed, only 15 of those minutes are available for the outbound part of the flight.

 

15 minutes of flying at 8.7km/min gives you a radius of 130 km, and you will burn 112.5 liters (15 min times 7.5 liters/min). 

 

So how does the FW fit into Nibbio's ROT? 112.5 liters divided by 130 km equals .86 liters/km. And just so there's no confusion, I think it's pretty impressive Nibbio came up with his initial estimate. It is a great point of reference. My trials are all at low level and at maximum thrust parameters. You can probably get close to his initial hack if you pull the power back a little and cruise at a comfortable 420 km/h then push up the power when you engage, but obviously the weight and drag from external stores (that expression is an artifact of my previous RL gig) will push the liters per km fraction up.

 

Edit: Sorry I forgot to add, what's the takeaway for an intrepid FW pilot? Well, now 13 large grid squares represent 15 minutes of flying at maximum continuous thrust whilst at low latitude provided you're not hauling iron. And just like our heros in their 109s, if you take off with full fuel tanks you'll have about an hour of "playtime" down in the weeds.

 

[sidenote to cruiousGamblerr, I did in fact know your great app has other maps. What I meant to write was I was using your app to validate my profile. Mea culpa.]

Edited by busdriver
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