Hi there, I've been reading the "How much fuel" issue for a few days now. There are a few factors I think are missing that might need to be added into the stirring pot.
First some necessary background information. My KR2 is just a standard KR2 and certainly not specifically designed for long distance flights more than 400 NM. Because we use blue foam in the wings, which melts if you get fuel on it, there can be no fuel in the wings, in case of a fuel leak. Blue foam is closed cell and therefore you don't need to spread a micro balloon slurry into the surface before glassing, so the benefit of blue foam is keeping the weight down and with the Jabiru engine, my empty weight is 560 lb. We are limited to 900 lb which is the original build MAUW, but strangely only in the UK? When I take a UK plane out of the UK, I am no longer limited as such, the MAUW is then at the discretion of the country you fly in, but I digress. The Jabiru engine is 140 lb all up with Prop and has to be placed on a long engine frame to place the C of G as far forward as possible for one pilot. This creates a long nose. We did some wind tunnel testing to see what this did to stability, seemingly not a lot. I got the Empty C of G so far forward I had to move the main wheels forward 2 inches to place any weight back on the tailwheel with a full main tank. But then I can fly 'two up' or dual, with no C of G issues at all. If I fly solo, I have to put the spare passengers parachute on the parcel shelf behind me to make the plane fly nicely or the C of G is too far forward. I'm 175 Lb. Because we can not put fuel in the wings, all we can do is fit a fuel tank above your feet. The biggest fuel tank I could fit in this space and still get my feet in past the bottom of the tank to the rudder pedals was 25 US gallons. Fuel burn of the Jabiru is 5US Gallons per hour giving 4 hours plus an hour of reserve. I cruise at 120 mph. So my endurance is 500 miles in a standard KR2 that i can fly solo or dual with a climb rate of 800 ft at MAUW of 900 lb and cruise of 120 mph with the 80hp Jabiru engine at 5 US gallons on hour. That is fact. You want to be careful putting too much fuel in the wings in long tanks. Why? Well research Aircraft Spinning Characteristics and you will find out. If you want to make an aircraft spin better, then go put weights on the wingtips. There is some 'A to B' ratio, 'A' being weight on the longitudinal Axis and 'B' torque created by weight and moment arm, but it was a very long time ago. If I remember correctly, the more weight you place away from the centre axis the more difficult it is to get that weight to stop spinning. Get into a spin with 50 gallons of fuel in long wing fuel tanks and I expect it won't come out. With half tanks, role to the right or left, a bit of side slip and the weight shift could be dramatic. I read I think from Mike that a guy was building a long range KR2 " For flights of shorter length he had removable wing extensions. These also held fuel and the day I was there he was fiddling with the fuel quantity sensors." The idea of fuel in the outer panels makes my eyes pop out. The Lear Jets I used to deal with, they could only put fuel in their tip tanks if the overall fuel weight was above a certain figure due to stability issues. In Russia I had to do 1,000 NM flights between airfields or 1,150 statute miles, so had to carry 220 litres of fuel or a whole barrel of fuel. 58 US gallons. This was done by 25 gallons in the main header tank, 15 gallons in a fiberglass reserve tank that sat on the passenger seat and then 4 fuel bladders carrying 4.5 gallons in each down by the co pilot rudder pedals. Feeding the main header tank from the reserve tank sitting next to me meant the C of G was only going forward. I can fly with a 175 lb passenger with no problems but this fuel weighed about 200 lb. So the C of G was beyond the 6 inch aft limit we use here in the UK, but still within the 8 inch book figure. It was horrible to fly though and had no stability at all, it was truly not nice. "So what!" you may ask? Well all the above is boring. The interesting fact and the one you want to know is, that I couldn't carry efficiently that much fuel in a standard KR2. I took off out of Nome with full power obviously and was requested to climb to FL100. I couldn't do it. The plane stayed on full power for two hours trying to lift 58 gallons up that high and the highest I could get was 8,500 feet and yes that was cold air. I was burning about 7 US gallons at full power just to lift the fuel. It took until I got rid of 20 Gallons of fuel to be able to climb to FL100 and throttle back. The issue is the jabiru's genuine 80 hp is only at sea level. I bet you are down to 50 HP at 8,000 feet and that's not enough to lift a heavy aircraft. So if you have more power, that will help, except more power means more fuel burn. Also, a KR2S is relatively much bigger and able to carry a bigger pay load. For my standard light KR2 with 80HP there is no point putting more than 40 US gallons onboard, or you end up burning fuel so inefficiently just trying to lift it up. Yes I did the 1,000 NM flight, but used 54 us gallons to get there. It was a 9 hour flight so fuel burn averaged 6 gallons an hour which is 1 gallon an hour more just to carry all that fuel. I'm sure if you drew graphs you could find the ultimate maximum fuel for the KR2. You can just 'max it out', as I had to do, but I would say carrying 58 gallons was horrible, something I would never want to do except for the Russians insisted at the time I could only fly to certain airfields that were that far apart. Later they dropped all that and allowed you to fly VFR throughout Russia, so I could have gone back down to my normal nice 400 NM flights. And now of course Russia has shot themselves in the foot and closed their borders to VFR flight again. So please consider that I don't think you can come up with a nice or recommended Max Fuel amount for a KR2 or any plane come to think of it... There comes a point where the more you put in the less efficient the plane will be until a point where the added fuel is not getting you any further down the road. Also, the KR2 rudder is not that big. Before you put a lot of fuel away from the longitudinal axis, I would go and do some spin testing and then slowly place more fuel in the wings and see what the difference in spin characteristics are, before loading up max fuel, heavy, aft C of G with a lot of fuel in long wing tanks... CH.
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