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Never-mind, not that simple, the pressure is too high, it would need a regulator....
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Last edited by schu; 04-30-2018, 07:28 PM. -
It's not as if it didn't cross my mind also as the ideal solution. I can't remember where I discussed this, but I did at length, about the potential to suck air in the unported forward pickup. Would the rear pickup and line keep everything primed? When the two lines are Y'd or T'd together, it gets a little more head-scratchin'... This isn't just atmosphere or even an engine-driven fuel pump, it's high pressure, target being 35 psi. Suckage of air could happen fast and a lot of it.Originally posted by Mark Goldberg
I figured the most likely scenario, and the most deadly, for unporting would be in a steep climb, so the rear pickup is the primary.
I'm all ears if someone can figure out how to incorporate the front pickup too without the complication I mentioned.Comment
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If you explained the issue, Zzz, I must have missed it. You mentioned you were using a "bung" near the top to return the fuel, and only using the rear pickup to deliver fuel. It would seem to me that you could use both front and rear bungs (at the bottom of the tanks supplied with the QB kit, and shown in the plans for scratch-builders) for your fuel pickup, and add one near the top to return the fuel. If absolutely necessary, I suppose one COULD even use a "Y" or "T" fitting and use either the front or rear pickup point as the "return" point as well.
To reiterate my concern, there are TWO situations you will encounter on many flights: The first is the high-angle climb-out, where the fuel will shift aft, and (especially in low-fuel situations) the front pickup point may be uncovered for the duration of the high-angle climb. The second is a descent from altitude, where the nose is lowered, and the fuel will shift forward, and (again, especially in low-fuel situations) the aft pickup point may be uncovered for the duration of that descent. The steeper the descent, the greater the likelihood.Jim Parker
Farmersville, TX (NE of Dallas)
RANS S-6ES (E-LSA) with Rotax 912ULS (100 HP)Comment
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Jim, The issue is the amount of fuel being drawn from the tanks by the fuel pump. It is entirely possible that the fuel pump will actually be sucking the fuel out of the tank because head pressure will not deliver fuel fast enough to keep up with the pump. If the pump is pulling fuel from the tank and one of the pickups un-ports then the pump will suck air and not fuel. You’ll be left with whatever fuel gravity can supply through a single pickup which may not be enough.
Also, simply having the return line T’d into one of the supply lines at the tank isn’t sufficient. The returned warm fuel and vapor needs to be cooled and vented in a tank.
Z, we discussed this at length a while back. You also talked to a buddy of yours in AK that suffered a partial power loss because he un-ported one of the pickups on his EFI cub.
The Continental IO360 fuel system flows 30-35gph pretty much constantly, it varies a little with rpm, but the max that is supposed to make it to the cylinders is around 20gph. The remainder is returned. I wanted to use both ports in the tank to supply fuel so I did some testing and decided that I would be comfortable with using both ports if I used 1/2†fuel lines at the rear port. After installing the lines we performed a preliminary flow test. As we hoped the 1/2†line alone will meet the 125% flow requirement. In my system, which is basically what Bob specified except the 1/2†rear fuel lines, gravity should provide more fuel at the fuel pump than required. The pumps should never have to suck fuel from the tanks making un-porting a nonissue.
Scratch Built 4-place Bearhawk. Continental IO-360, 88" C203 McCauley prop.Comment
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I originally was wanting a header tank because it seemed like that was the convention in high wing planes with FI, and I didn't know any better. I still feel that way. After lots of thought, my biggest concern with not having one isOriginally posted by Bcone1381 View Post
1. Low on fuel, landing pattern, or looking for a place to land.
2. Turning in one direction for 15 or 20 minutes.
Because it is a side by side airplane, I will mostly turn left when given a choice for visibility reasons. While the engine may only be burning 7-11 gallons an hour, the FI is using a constant displacement pump, (25 or 35 gph I can't recall which at the moment). So I am feeding out of the high (right) wing at a rate of 35 gph, how long until that tank is empty, assuming I had 7 gallons in it to start?
If I am running the fuel valve in "right", and all the fuel is returned to the right tank, no different than a carb or MFI. If I am running in "both" as recommended, I simply don't know.Assuming that the fuel follows the path of least resistance, I could be feeding 35 gph out of the right tank, and returning 28 gph to the left tank, as it is several feet lower when in a 15-30 AOB turn to the left. I could possibly run the right tank dry in as little as 10 or 12 minutes when running in both. At the moment, this is a concern, but I don't know if that will happen for sure. I could fly for 500 hours with no problem. Then, at 501 hours, I am having a hard time deciding where to land, or am practicing touch and goes, and ......................
Which gets me back to "why a header tank". Some engineers with a lot more experience than me designing high wing airplanes thought it was a good idea. Mostly I am not 100% either way, and am leaning toward following "convention".
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A lot has been written and intimated on this thread and I will throw in a few nickels worth.
Just so happens I design fuel systems and components as my lively hood.
So here are a few points of note:
The system that Bob designed is simple and adequate for carb or fuel injected engines.
It works very well when implemented properly.
I have mocked it up in clear tubing to check flow and line refill, bubble formation and flow stagnation. I did this for two reasons: one to
investigate why A Bearhawk engine went quiet
and would not re establish fuel flow.
Two to make sure that in my installation,
IO-470 that fuel flow would be adequate and
un interupted.
Most general aviation aircraft that have a header tank have one for 1 reason.
to meet certification requirement that after engine fuel starvation of running a tank dry
that the engine can restart and produce power
within 10 seconds after switching to an available source.
The Bearhawk system as designed by Bob will go from a dry tank to fuel at the carb in 3
seconds!
Most production FI systems utilize an engine driven pump that is positive displacement pump some fixed displacement / some variable displacement. The pumps are sized to provide Approx 1/3 more fuel than needed
to support combustion. Fuel delivery is proportional to engine rpm. Un used fuel is returned to the “ system “
In most cases the header tank provides a means to capture the excess fuel bypassed and immediately make that fuel available for the combustion process.
I am not utilizing a header. Fuel will return to the left tank. Fuel selector will be R, Both, L, Off.
The primary flight mode is Both. Returned fuel
will level as the fuel seeks cross tank equalization. The fullest tank will have the highest head pressure and will drain the fastest.
Aircraft fuel pumps are not designed to
“ Suck “ and operation in that mode will destroy a pump quickly. Pump inlets should always be flooded by free stream supply.
A header tank is not a requirement but a means to meet a requirement.
Mock up and test your system with clear Tyron
tube to know before you build.
Kevin D # 272Comment
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Well, I thought I knew more than I did when I started posting, which happens sometimes, but thankful for Whee and Z pointing out that the big problem with a return to tank fuel system is that the fuel pump can pull fuel faster than gravity, which will suck air into the system if the fuel line in the tank becomes un-ported.
Another thing that is pointed out is that hot fuel lines and mogas (especially winter mogas) can cause vapor lock because the fuel is heated up with under cowl temps, and the engine driven fuel pump heats the fuel in a FI system.
I think Whee's solution to problem #1 is completely reasonable, run 1/2 fuel line in the rear. Makes sense to me.
As for problem #2, that can be mitigated by pressurizing fuel before it goes to the hot side of the firewall, mixing some avgas in, and cooling the engine pump.
I think the take away for me is that I'll need to run some cooling air to the engine driven pump, fire sleeve and reflector tape the fuel lines and/or just run 100% electric pumps on the cold side of the firewall and know that alternator failure needs aux alternator to be turned on before battery dies.
schuComment
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Kevin D;
Awesome info. Thanks. But I still see a problem that I described before.
Whether you return to both tanks, in "Both" or only return to the left tank all the time, I still see draining the high tank very quickly in a continuous, or mostly continuous turn. Assuming a left turn, fuel valve in Both, fuel returned to Both, or in you system, to the left, I see the right tank emptying fairly quickly when low on fuel.
Assuming a 35 gph pump, and 7 gallons in each tank, my high (right) tank is dry in 12 minutes, and the left wing lines are seeing only fumes.
Am I missing something?
I will be flying from the left seat. Given a choice, I will be making left turns in the above scenario for better visibility. If I was going to only return fuel to one tank, I think I would make it the right tank vs the left.
Last edited by svyolo; 05-01-2018, 01:44 AM.Comment
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Bob's system has worked on both carb and injected engines. I am not sure why any modification is needed. I do not see it.
John - if you are in coordinated flight the tanks will draw down equally - even in a turn. At least it would seem that way to me. MarkComment
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With Bob's design and the fuel valve set to both at least one tank port will be supplying fuel in any positive g configuration. Nose up, nose down, slip left or right. Using rear port only, nose down can leave both dry as Jim pointed out.Comment
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As noted, in a positive G coordinated turn the fuel distribution in the wing tanks should be even and level across both tanks.
You tube has several nice Bob Hoover videos that illustrate this point well.
A valid concern is a prolonged slip to landing
in a low fuel condition. Many production planes are placarded against this operating condition. The “saving grace†in this situation is the fuel consumption is down in the 1 to 2 gph range. So 1 quart of fuel should provide
15 min of slipping.
I postulate that if fuel is low beyond reasonable landing reserves, and a half hour
slip to landing is required the issue is not in the fuel system.
As part of Phase 1 testing,fuel unporting tests
and slips to landing, dry tank switchover, should all be conducted at altitude over a suitable landing site.
looking forward to that myself. : )
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Well, I believe I had a disclaimer, "am I missing something"? In any coordinated turn (which is the goal), the lift vector (and load) is vertical with respect to the aircraft vertical axis. Most of us only try something different only in a crosswind landing. So my worrying about fuel being transferred to the low tank in a turn is.............somewhere between stupid, misinformed, or in my case, I think I will go back to stupid. I am an engineer by education, and I "missed something". I started this thread, and I learned a lot from it. Thanks to everybody. I think I will shut up now.Comment
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Don’t do that! Discussions like this are how we learn. There are no stupid questions and there is nothing wrong with being wrong or not knowing something. This is a great discussion, thanks for starting it!Originally posted by svyoloScratch Built 4-place Bearhawk. Continental IO-360, 88" C203 McCauley prop.Comment
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Bob's system works great on carb'd systems, and I agree, just build as planned if running a carb.Originally posted by Mark Goldberg View Post
As for injected systems, there are several different kinds:
1. Systems that have a engine driven pump and boost pump that don't use a return fuel line to tank.
2. Systems that engine driven pump and boost pump that uses a return fuel line to tank.
3. Systems that have dual electric pumps that feed EFI injectors and return excess fuel to a tank.
Add in where the return fuel line goes (duplex valve, or single tank) and which fuel is being used (100LL or mogas) and you end up with a lot of combinations, and I don't think they have been all tested with Bob's design.
Anyone running AFP injection with mogas?
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I must admit that I have never quite understood the "need" for a return line with a FI system. I am currently running an RV-10 with an injected IO-540 with EDP and electric pumps. The electric pump has an idling circuit which deals with any excess fuel. No problems whatsoever...... For my Bearhawk, I have installed the Andair pump which has an internal bypass.
With a low-wing aircraft which generally switches tanks every 30 minutles or so, I see no issue with fitting a return - you just end up feeding off one tank longer that the other. But with a high-wing system designed to run on "Both", it just seems that you are building in potential problems........
As Svyolo said - "Am I missing something?"Comment
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