P-40 Fuel tank usage profile

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Hook
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P-40 Fuel tank usage profile

Post by Hook »

After doing a bunch of research on the forums, actual P-40 manuals, the manuals that came with the sim and the real world manuals for some additional aircraft, I think I've come up with a workable profile for what fuel tanks to use and when.

A few of things to remember. First, out P-40 does not have a fuel gauge for the Reserve tank. Second, the engine does not use all the fuel you send it, and overflow goes back to the Reserve tank. This means you need room in the Reserve tank to hold overflow fuel. We have two diagrams for the fuel system on the P-40; one shows the return going to the Reserve tank, the other shows it going to the Fuselage tank. Some information on the P-40 has you using the Fuselage tank first, other info has you using Reserve first. We use the Reserve tank first.

Start on the Reserve tank in our P-40. Run-up, taxi and climb, all on the Reserve tank. Use it for at least a half hour to make room for overflow fuel or the fuel goes overboard and is wasted (this amounts to about 10% of your fuel used). If you intend to fly to the limits of your fuel, empty the Reserve tank first. Without a fuel gauge we don't really care about the first hour or so of the flight, but after that knowing your fuel level becomes important.

Side note: This is something I was introduced to when I was working on a simulated 65 Jaguar XK-E. The fuel gauge shows full until about 80% fuel before it starts going down. I decided it didn't matter.

Next, fly on the Fuselage tank until it is about half full, then switch to the Wing tank until empty. The Fuselage tank becomes your working reserve and a half tank will give you about one hour of flight at cruise. Don't switch back to the Reserve tank until you have emptied both Fuselage and Wing. By this time the overflow has put about 10 gallons into the Reserve tank, enough for 20 to 30 minutes of flight, and this becomes your emergency reserve. There is no gauge, just go by time on that one and land as soon as possible.

It is preferable to use fuel from rear to forward tanks to improve center of gravity. The Fuselage tank is behind the pilot, Wing is in the back part of the wing, Reserve in the front part of the wing. It would probably be better, as it is on some models of the P-40, to have the overflow going to the Fuselage tank, but that would require a fuel gauge on the forward wing tank, called Reserve in our P-40, and we don't have one. In that case you would empty the tanks from rear forward, then switch back to Fuselage.

Even if you intend to do a very short flight, make sure you leave some room in the Reserve tank to accept overflow fuel. If the Reserve tank does not start out full, you can start on the Fuselage tank. Leave at least 10 gallons of free space in the Reserve tank in this case.

If anyone has any good ideas on this, please let me know. I just completed a long flight and had to switch to emergency reserve just before approach.

Note that if you are doing fuel and time calculations using the Shift-2 screen, the Fuel economy shows US gallons per hour even if you are flying an aircraft measured in Imperial gallons. Divide US gallons by 1.2 to get Imperial gallons. As far as I can determine all other numbers in the Estimations are correct.

I have only flown the AVG version of the P-40. I've seen some oddities in the code for other versions but I have not tested those versions yet.

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EDIT:

If you have a belly tank, use half the Reserve tank as above, switch to the belly tank until empty, then resume with the Fuselage tank as above.

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From P-40 Flight Manual.pdf
(P-40 N model)
Section VI
Page 8

Filling of Tanks When Using 91 and 100 Octane Fuel
1. Fuselage tank with 91
2. Wing tanks with 100
3. Belly tank with 91

This is for P-40 models that use the Fuselage tank as overflow:

Use of Tanks with 91 and 100 Octane Fuel
1. Start and taxi - fuselage tank
2. Run-up, take-off, and climb - wing tanks
3. Cruise - drain fuselage tank to 35 gallons for better stability.
4. Cruise - belly tank
5. Land - wing tanks.

For our P-40 this should probably be:
1. Start and taxi - Reserve tank
2. Run-up, take-off, and climb - Reserve tank
3. Cruise - drain Fuselage tank to 35 gallons for better stability.
4. Cruise - belly tank
5. Cruise - Wing tank
6. Land - Reserve tank.

The fuselage tank (or Reserve tank on our P-40) is used first because it is the vented tank for flowback from the carburetor. The normal flow-back is 2-10 gallons per hour.

Hook
Last edited by Hook on 12 Apr 2020, 10:24, edited 1 time in total.

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Jacques
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Re: P-40 Fuel tank usage profile

Post by Jacques »

Thanks for that explanation, Hook! I’ve been trying to understand the fuel switching strategy as well. Another question arose today when I read in one document that, in some models, the “reserve” is actually the same as the fuselage tank, except tapped from the bottom of the tank, while the “fuselage” is tapped from a standpipe that is open a bit off the bottom of the fuselage tank. I’m also still taking baby steps with this aircraft and have, so far, only flown the Tomahawk variant. :D

I should add a “thanks” to A2A for reducing the confusion by not modelling all the variations for fuel tanks!

Hook
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Re: P-40 Fuel tank usage profile

Post by Hook »

Jacques wrote: 08 Apr 2020, 20:38 Thanks for that explanation, Hook! I’ve been trying to understand the fuel switching strategy as well. Another question arose today when I read in one document that, in some models, the “reserve” is actually the same as the fuselage tank, except tapped from the bottom of the tank, while the “fuselage” is tapped from a standpipe that is open a bit off the bottom of the fuselage tank. I’m also still taking baby steps with this aircraft and have, so far, only flown the Tomahawk variant. :D
And thank YOU for reading and taking the time to comment. It is much appreciated.

I'm still trying to decide if it makes more sense to send overflow back to the Fuselage tank. I've coded this but haven't implemented it in the sim and tested it yet. I just realized today that I could use the single fuel gauge on the floor to display either wing tank (Wing or Reserve) by toggling a new switch in the Shift-3 control panel with a few edits here and there. The unused fuel gauge frame for the rear wing tank is in an awkward position so I wouldn't want to have to use it in any case.

I've already rewired the low fuel warning light to only light up if there is about 2 minutes of fuel in the current tank (actually one gallon). This light is typically a low fuel pressure warning (hence its position beside the fuel pressure gauge) and I've also got it lighting if the fuel selector is at Off. Since the warning light is well out of the pilot's normal light of sight I've also wired the "Prestone Warning" light to light up at the same time. This warning light is in the same position as the "Fuel Sig" warning light in the video about flying the P-40. You wouldn't want this to light up if the warning was for an hour of fuel remaining though.

I was flying out to Dutch Harbor from Anchorage and the weather map indicated possible freezing rain near the destination. There is NO WAY I'm gonna fly into freezing rain without my icing indicator which I added to the Shift-2 panel which only appears if there is icing, and even got to test it. "ICE: 0.3%" at one point. On the way back I'd wired the Prestone Warning light to illuminate if there is any icing and got a test of it as well. Normally you'd detect ice by actually seeing it on the windscreen or wing leading edges but we don't have that, and the airspeed indicator will fail by "freezing" in place but that isn't easily noticeable and you'll want to have the pitot heat on anyway.

I figure the Prestone light as a general warning light works fine, as it's only lit for about 2 minutes when a tank is near empty and you can check your fuel tanks and coolant temperature gauge to determine which it is. I doubt you'll get in a condition where you can have both high coolant temperatures and airframe icing at the same time unless you spring a coolant leak and the coolant gauge should let you check for that. It was coming on intermittently for ice and one time it was actually for the Wing tank but it was easy to determine which warning it was for at the time.

I've had four forced landings due to airframe ice (all caused by freezing rain) so I'm very careful with icing.

One interesting thing to note was that my fuel flow (corrected for imperial gallons) was 25 and the wing tank is 50 gallons, therefore exactly two hours. The fuel tank was empty after 110 minutes rather than 120, so the fuel flow is correct for what the engine is using but the tank is drained 10% faster and that fuel goes to overflow. I've tested this twice so far. It always feels good when the math works out. :D

Hook
Last edited by Hook on 08 Apr 2020, 23:31, edited 1 time in total.

Hook
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Re: P-40 Fuel tank usage profile

Post by Hook »

I just realized I needed to mention this.

The idea of having the low fuel warning coming on with one hour of fuel remaining works well for a more casual user. It is also the amount of fuel you'll have in the Reserve tank if you use that tank for a half hour at the beginning of the flight. The light is a useful reminder that you're on your last hour, and our P-40 doesn't have a fuel gauge for that particular tank.

Someone who monitors his fuel usage to the gallon and minute would probably appreciate having a warning light when the selected tank is about to run out. I'm like that and this is why I made the changes.

Hook

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Paughco
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Re: P-40 Fuel tank usage profile

Post by Paughco »

I just hit my Shift+4 when I want to see how my aux tank is doing. I use the aux tank for takeoff and climb, then switch to the fuselage (if full), or the wing tank. The aux tank slowly fills, due to the return from the fuel pump. Usually by the time I get to wherever it is I'm going, the aux tank is the most full tank in the airplane, and I use that for the pattern and landing. The P-40 is such a great airplane!

Seeya
ATB
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Hook
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Re: P-40 Fuel tank usage profile

Post by Hook »

I use the Shift-4 panel to check fuel but usually only for testing. I'd prefer having it in the cockpit.

I switched to running the overflow to the Fuselage tank and did a long flight, intentionally running everything else dry. When I switched back to Fus I had about 20 minutes of fuel, about 8 imperial gallons. It doesn't register much on the gauge, but it does register and gives you a good idea of what is left and it uses the best and easiest to see gauge to do it.

I'm not sure that's much better than using half the Fuselage fuel with overflow going to the Reserve tank. Draining the rearmost tank first does move the center of gravity forward which is intended to make the plane more stable. Is it better to drain the Fuselage tank to improve center of gravity and let overflow go there as your emergency reserve, or better to use about half the fuel no matter where the overflow goes? Or send overflow to the Res tank and use it for a half hour first as suggested in the first post? More thought is needed. In the absence of period documentation I'll have to defer to the local experts.

Next flight I'll leave the overflow going to the Fuselage tank, take off, climb and fly until the Fus tank gets down to about 20 imp gallons which should take about an hour, then use the other tanks. I think it should be Wing first, then Reserve to keep moving CoG forward. This still needs a rear wing tank gauge for the Res. I may get ambitious and implement that.

Hook

Hook
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Re: P-40 Fuel tank usage profile

Post by Hook »

It turned out the Reserve fuel gauge was extremely easy. Kudos to the A2A designer and coder who made the system so flexible. I've temporarily used the Fuel Gage[sic] light switch to switch between tanks. If it had been spelled Gauge it would have worked the first time. :D

So I did a 1000 nautical mile flight with the new system. Start, taxi, take off on Fuselage, keep flying on Fuselage until the gauge shows 20 imperial gallons. Switch to Wing, then Reserve when Wing is empty. Overflow goes back to Fuselage. Having a fuel gauge for Reserve helped a lot. By the time the other two tanks were empty the Fuselage gauge was up to 28. (Mathematically that's 8.4 imperial gallons of overflow and Fus was actually down to 19.5 by the time I caught it. That's 10 US gallons.)

With this system, basically you have about two hours on Fuselage, two hours on Wing and about an hour and 20 minutes on Reserve. This was with a fuel flow of 25 imperial gallons per hour. I could run lower power and get fuel flow down to 20 or even 18 gph if necessary, and I did that after the Fuselage tank got below 20. I landed with just over 10 imperial gallons remaining, enough for 24 minutes at cruise power or 33 minutes at lowest suggested power (21" MP, 1800 RPM, auto lean, 18 imp gph). And I could monitor it on an easy-to-read gauge! :D The gauge is very sensitive between 20 and 30 which makes it even better for keeping track of overflow.

I find this system to be intellectually satisfying. It just seems right. Interesting enough it's all done in one .xml file too.

Thanks A2A for making this aircraft available. I didn't expect to like it nearly as much as I do.

Hook

Hook
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Re: P-40 Fuel tank usage profile

Post by Hook »

I added an EDIT at the end of the original post detailing use of the belly tank if installed and notes on using a mixed load of 91 octane and 100 octane fuel.

Hook

Hook
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Re: P-40 Fuel tank usage profile

Post by Hook »

Lots of real world P-40 manuals here:

https://ww2aircraft.net/forum/threads/p ... nual.7478/

Hook

Hook
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Re: P-40 Fuel tank usage profile

Post by Hook »

Excerpts from some of the manuals:

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P-40D E
(P-40 D and E models)
Page 20
(f) Coolant and Fuel Pressure Test Switch: This is a double throw switch with two "ON" positions and a neutral "OFF" position. The coolant test "ON" position is upward, and the fuel pressure test "ON" position is downward. Both positions of this test switch are momentary and are used to test the operation of their respective warning systems.

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P-40 Flight Manual
(P-40 N model) [Overflow to Fuselage]
Section VI
Page 8

A vent line connects the carburetor fuel chamber to the fuselage tank; this vent allows air to be expelled from the top of the carburetor fuel chamber when it is being filled and prevents vapor lock. This passage is closed by a check valve when the chamber is full. This carburetor vent line and the boost pump are the two aids for the prevention of vapor lock in the fuel system.

Use of Tanks with 100 Octane Fuel
1. Start and Take-off - fuselage tank, and burn all but 35 gallons, which is the reserve.
2. Drain belly tank.
3. Cruise Wing tanks.
4. Land on fuselage tank.

Use of Tanks with 91 and 100 Octane Fuel
1. Start and taxi - fuselage tank
2. Run-up, take-off, and climb - wing tanks
3. Cruise - drain fuselage tank to 35 gallons for better stability.
4. Cruise - belly tank
5. Land - wing tanks.

The fuselage tank is used first because it is the vented tank for flowback from the carburetor. The normal flow-back is 2-10 gallons per hour.

Filling of Tanks When Using 91 and 100 Octane Fuel
1. Fuselage tank with 91
2. Wing tanks with 100
3. Belly tank with 91

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P-40 pilot training manual
(various models) [Overflow to Fuselage]
Page 19
Unless absolutely necessary, don't run any tank lower than 5 gallons indicated.
Maintain your fuel pressure between 16 and 18 psi. When the pressure falls below 10 psi, a red warning light on the left side of the dashboard flashes on (except on P-40 N which has no light). Don't hesitate when you see that red light. Change tanks immediately.
VENTED TANK
The fuselage tank is called the vented tank, which means that there is an overflow fuel line between this tank and the carburetor. The fuel line carries excess gas from the carburetor back to the fuselage tank. At high engine rpm, the fuel overfly may run as high as 10 or 15 gallons per hour.
Always use the fuselage tank for takeoff. During takeoff the chance of fuel overflow is especially high, and unless you are using the fuselage tank -- the only one built to handle overflow gas -- the overflow is lost.
After takeoff, always use at least 15 gallons from the fuselage tank before switching to another fuel tank.
The fuselage tank accumulates possible overflows from the other tanks during flight. Always turn the selector handle to the fuselage tank if it appears that you have run out of gas. The fuselage tank probably contains a few gallons of overflow fuel.

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P-40 Tomahawk
AVG model [Overflow to Reserve]
Page 8/24
[This concerns the schematic fuel diagram that shows overflow going to RES tank.]
[ES: Fuel tanks were not as shown. The fuel selector had five positions. 1. Res. 2. Main. 3. Aux. 4. Fuse. 5. Off. The reserve and main tanks were one and the same tank . The main tank was feed by a stand pipe in the main tank and the reserve fed from the bottom of the same main tank. The reserve was used for all take offs and landings. When reserve was selected, fuel was taken from the bottom of the main tank and fed fuel to the engine. In other words the stand pipe used all of the fuel in the main tank, but when the main was selected, it used all, except I believe, a positive amount of fuel giving 20 minutes at METO power. This was a positive indication of exactly how much fuel remained, eliminating the pilot having to rely upon a fuel gage which at best was an unreliable.]

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Hook

Zacke
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Re: P-40 Fuel tank usage profile

Post by Zacke »

Hey guys,

when running try on the selected tank with mid or high power setting does it harm the engine when it suddenly loses power because of fuel starvation and then power comes back very rapidly when fuel comes back online through switching to an other tank?

Regards,
Zacke

Zacke
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Re: P-40 Fuel tank usage profile

Post by Zacke »

I can give the answer to myself: You run in a prop overspeed quite fast when switching from an empty tank to a full one. Should have been clear! :shock:

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MkIV Hvd
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Re: P-40 Fuel tank usage profile

Post by MkIV Hvd »

I've always felt it best to not run tanks dry...for that exact reason... :wink:
Rob Wilkinson
A2A: Civilian Mustang, T-6, Bonanza, Comanche, Cub, C182, Spitfire, P-40, Cherokee, P-51 - VATSIM P4 and some other stuff...

Hook
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Re: P-40 Fuel tank usage profile

Post by Hook »

After putting 142 hours on the P-40 and averaging over 4 hours per day, I haven't flown in two days.

My modifications are tested and are actually ready to publish, but I'm giving A2A right of first refusal.

Besides a few bug fixes and cosmetic changes I've got it selectable for fuel overflow to go to reserve or fuselage tanks, a switch to have the fuel gauge on the floor display either wing or reserve tank, and have wired the low fuel light to light up with 30 seconds of fuel in the selected tank with the Prestone Warning light echoing the fuel warning light. I've also got the fuel pressure gauge dropping when fuel in the selected tank starts getting low and the fuel warning light coming on with low fuel pressure rather than when there is an hour total remaining. The low fuel pressure warning light now works across a variety of situations besides low fuel.

I suspect next time I get the urge to fly it will be a different aircraft.

Hook
Last edited by Hook on 02 May 2020, 03:18, edited 1 time in total.

maddz
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Re: P-40 Fuel tank usage profile

Post by maddz »

Sounds interesting Hook, look forward to maybe seeing it.

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