Source: 2020 Q3 Beartracks
The original gravity-only Bearhawk fuel system design is very robust when it can be used. This is important, because lots of homebuilts suffer fuel flow problems. However, some engine installations require fuel pumps, and when using pumps, there are some extra considerations. In August Bob issued an “Operation Notice” as follows:
The Bearhawk Fuel System as shown in the Bearhawk Book is designed for use without a fuel pump. If a fuel pump is used, extra care in flying is required so that neither main tanks become unported, as a fuel pump would rather suck air than fuel. If one tank is very low and the other is not very low, set the fuel selector on the fullest tank and fly the plane as not to unport that tank. A header tank (3 gal) could be used to solve this problem, but fuel in the cockpit can be a safety issue. (Vent the header tank to both main tanks).
There has been an extensive discussion about this topic at bearhawkforums.com, including a discussion about venting one tank to the other, header tanks, and much more. I think it is fair to say that there is not a consensus about fuel management in cases where the system includes a fuel pump. One data point from Ed Meyer in Idaho is below:
As some of you know, we have a Patrol with the EFII ignition and injection system which includes fuel pumps (main and secondary) to boost fuel pressure up for the injectors. Also, return lines were installed to return the excess fuel from the continuously running pump. All the details of our system installation in the Patrol are in a Bear-tracks article I wrote a while back.
Not knowing positively whether un-porting of the fuel outlets on one tank would allow air to enter the fuel system and starve the injection system,a concern expressed in this thread; I decided to do a test:
I flew up over my home airport about 4500 AGL with about 9 gallons of fuel in each side. I was using a cruise pow-er setting with about 8GPH fuel flow showing. I turned the fuel valve to the right tank (I usually run both) and then put the right wing low in a pretty hard slip/skid to intentionally un-port the fuel outlets. I expected that fuel pres-sure would quickly drop and the engine would quit. After about 10 seconds or so, the fuel pressure did drop and the automatic switching to the secondary fuel pump occurred but the pressure stayed low. At this point I knew I had succeeded in un-porting the fuel outlets on that side. I then went back to normal coordinated flight and the fuel pressure slowly recovered in about 10 - 15 seconds. Much to my surprise, the engine never missed a beat through this.
Following this, I switched the fuel selector to both and again did the right wing low slip/skid. I held this for probably about two minutes monitoring the fuel pressure closely. It never wavered in the slightest.
My conclusion from this is that there is apparently enough fuel flow capacity feeding from only one tank via 3/8" lines to supply all the fuel the engine needs plus whatever amount the fuel regulator returns to the tank even with the other tank feed open to the vent. I recognize that this is not an exhaustive test but it was enough to boost my confidence in the fuel system as installed.
You are not yet authorized to view this page. Learn more here.
(0 votes, average: 0.00 out of 5) You need to be a registered member to rate this post.