Roger Mellema, Jost Walliman
Newsletter #16
With all three wheels on the ground, there is no fuel flow by means of gravity feed below 12 gallons in each tank. With the help of the booster pump, fuel flow starts above 6 gallons in each tank. The angle with all three wheels on the ground is
equivalent to a 10 deg climb.
FUELFEED FLM W,3 ATTM DE 10 DF CLIMB Gravfty only 29 gph 24 gph Booster pump 37 gph 37 gph
UNUSABLE FUEL 1 gal each 6.6 gal each
(booster on!)
Jost Wallinian
ed note:
We are really sorry to hear about your BD. As the pictures show, the airplane appears very well done and I believe the only one operational in Europe.
When Ray Ward flew the CAFE 400 two years ago he also had a problem with fuel flow. We had drained the tanks so that he would know exactly how much fuel was in the airplane for the Td-Aviathon. As we were doing ft, he said that after refueling we should open the quick drain on the gascolator and blow on the fuel cap openings to be sure there was no air in the lines. We forgot and on his take-off the engine quit cold. He wisely closed the throttle and landed on the end of the runway. Ray has his fuel lines along the back of the door just as Jost has and even this didn't prevent the problem. After Ray brought the airplane back to the ramp, we blew into the tanks and that solved the problem.
After I rebuilt my BD recently, I flushed my entire fuel system to make sure I wouldn't have the problem again. After cleaning everything, I decided to do a fuel flow check. I left the tail on the ground and put 12 gallons of gas in each wing. The fuel would not flow by itself (gascolator drain open). I blew on the caps and immediately got the fuel to flow. The flow rate was about 20 gph through the small drain in the gascolator. The gas continued to flow until there was about I gallon left in each tank.
I had only one problem in that one of the tanks flowed for awhile and then quit when there was still 5 to 7 gallons left in the tank. Apparently I had not cleared the air out of both down tubes from that tank. I guess the only one that was flowing was the forward one. This would explain why ft flowed for awhile and then quit when the fuel got below that outlet. After blowing on the tank again, the rest of the fuel flowed out.
If you have a header tank, ft is really important that you be able to see how much fuel is in ft. Jost probably did some ground running before he tried to take-off and he was just cutting his margin of safety given by the header tank as he was using fuel out of ft and the tanks were not flowing into ft.
Fuel pumps don't always pump the air out of lines and suck the fuel through. However, sometimes they do. I can set a can of gas on the ground and as long as the engine will start, ft will suck all the air out of the line and start the fuel flowing. This is with the plastic line connected to the wing root fuel fittings.
Roger D Mellema and Jim Huber
Newsletter #17
I have been promising for some time that we would do a real BD-4 fuel flow test. We finally got it done (I have been delaying the newsletter for it). Jim had an old style end rib from a BD Panel Rib wing so we used it to build a small quantity fuel cell with the outlets all in the right places. We hooked everything up with just the right size of clear plastic tubing. All the tubes were run in the same pattern as the BD-4. We hooked an electric fuel pump up just about where the engine driven pump would be.
We first set the wing fuel cell at 10° , closed off the fuel line at the carburetor, filled the tank with gas, and watched the lines. We found that the diameter of the lines will allow the air to bleed out and the lines to fill with fuel. This is a very slow process and any reverse slope of any line will cause an air bubble to stay in the line. We had to really work at getting all the air out. You cannot build the airplane so that all the air will automatically bleed out. Without clear plastic tubes, you cannot guarantee that there will be no air. If fuel is flowing in the lines at a 10 gal per hour rate, the bubbles will not migrate upward.
If the fuel is higher than the highest point of the gas lines, and the fuel line is opened at the carburetor, the lines will fill fully with fuel (it helps to blow on the fuel filler hole, especially if you only open the gascolator which doesn't let the fuel run as fast).
If you now start to tilt up the front of the wing tank (no pump but the carburetor running at 1 0 gal per hour), the gas runs (siphons) in a good fashion until the angle of attack is such that the top surface of the gas is level with the 'T" by your left knee where the two lines from the tank are tied together. At this time the siphon is broken in the rear feed line and the flow stops. The angle of attack that this takes is really extreme and cannot be reached in any type of normal climb-out unless you have 345 hp or so. When pilots tell you that the angle of attack causes the fuel to quit flowing, it usually really is a skid or slip condition (pushes the fuel to the outboard side of the tank) that does them in. This force is much more important than the angle of attack. If the "T" were lowered to the floor as suggested by some, the angle of attack would have to be greater to break siphon.
Now for those of you who believe that the fuel pump (it was off in the above test) will suck air rather than gas - it just ain't so. We put the wing tank in a very high angle of attack where the front fuel port was not covered with gas - in fact the gas was two feet down the line but absolutely level with the top of the fuel in the tank. Then we energized the fuel pump to see if it would suck the fuel in the front port line down to the "'P'. It did not, it only lowered the fuel in the half filled line by about 1/2 inch. So you see the fuel pump did have some effect but a very small one.
If either (not both) outlet port was uncovered for a short time and then recovered, fuel flow continued without a problem.
A header tank is a good idea but there are some problems that should be considered. Unless the header has a "fuel low" warning light that grabs your attention, it really doesn't really do what it should. If a "both" position is used on the fuel selector, one side or the other should feed even if the rudder is misused. However, fuel can be accidently cross-fed and sometimes one side will not feed as quickly as the other due to venting differences. If you get really low on fuel and the low fuel light comes on you will not know which tank still has some gas in it.
If you expect the fuel pump to pull gas from the wing tanks when a header tank is used, it will not, it will only pull air from the header tank air vent. If the siphon flow stops, it will have to be restarted by blowing on the tanks or slipping to push the fuel inboard (it may start just by filling the tanks and then opening the gascolator to let the system purge itself.
Jim is considering a single point feed system. The following figure shows what happens with the standard system for different angles of attack and what the unuseable fuel levels are. If the Oll line is followed to its lowest point (this is the position of the forward port), you will see that about 5% of the fuel will be unuseable. The lowest dashed line indicates that there will be 14% fuel remaining if the attitude of of the wing is such that gas is 1/2 inch
higher than both ports.
If you are low on fuel, it would be best to fly slow (high angle of attack) so that all the fuel goes to the rear of the wing. This is shown by the lowest point of the 121, line (this is the three-point angle of the tail-dragger).
The upper dashed line shows the 1/4 tank mark that the BD-4 is placarded for. I just talked with Jim today at lunch and he has another plan for how to build his fuel system. He seems quite sure that this is the right way as he has already started building the wing. The new method is to put a sealed bulkhead a short distance behind the spar that goes from the top to the bottom of the wing. He now has two tanks in each wing, one
foreward and one aft. There are two outlet lines, one for each tank. The outlets are close together about where the forward one normally is located. The front tank will be used for take-off and the rear is for cruise or auxilliary. By having only one outlet in each tank, Jim has gotten around any problems with one outlet breaking the siphon of the other. He will run the two outlet lines (from one side) to a selector, for: main, auxilliary, or off. Now when the airplane is flown in a skid the outlet line can still suck air but when the skid is stopped, the fuel pump will quickly purge the line and suck gas. There is a separate electric pump for
each side. To get rid of the 'skid' problem both of the gas valves could be set to main (left
and right) and one of the tanks will always be feeding (check valves will be needed to keep from pumping fuel from one tank to another). Each front tank will hold 22 gallons and each rear tank will hold 27.5 gallons. There will be a total of 7 bays on each side that can hold
fuel. Jim has some plans to fly from the US to Australia in the future. Of course now there are a couple of stops in the Soviet Union planned.
This fuel arrangement has not been tried yet so use your discretion!!
