4 Main Tanks?

[Zac Weidner]

I'm still doing my homework and trying to decide what airplane I want.
I was wondering if anyone has ever tried installing 4 of the 25 gallon main tanks vs. 2 mains and 2 aux. tanks? Is there any problem with rib spacings, etc. that would conflict with this? Or would it be more viable to install 2 mains and 4 aux. tanks? Or would it be smarter to do a wet wing for something like 90 gallons usable? And what option would be the lightest and lowest maintenance long term?
I'm always a fan of having the ability for extra range if the need arises. I'm coming from a 120 gallon Twin Comanche, so I guess I'm spoiled with range, but it's nice for some types of weather on IFR flights, as well as being able to go there and back many times to an airport with unrealistic fuel prices and not have to fill up there. That can be a big money saver.

Thanks in advance,

Zac

[Battson]

Apart from the question of what purpose would require that much endurance - in a plane which can run on 8 gal/hr at over 110KTAS (with 90 gal that's roughly 1200 nautical miles or 11 hours in the air - that's Mexico to Canada unless I am mistaken), I see a few physical challenges:

First question would be about G loading - up the weight in the wing and you're going to impact the g limits, is that viable?
Then the semi-monocoque considerations, can you cut another big hole in the wing without affecting the structural strength?
Then there's the question of room - most of the inter-rib spaces have something in them, like aileron or flap hardware - can you find room for that extra fuel?

[JimParker256]

I agree with Battson on this one. There would be TONS of engineering work that would be needed to redesign the wings to accommodate 100 gallons of fuel in the wings! And how often would you really NEED that extra range / endurance? With the standard 50 gallons + the 22 gallons of Aux fuel, you'd have over 900 miles of range at 65%, and almost 1300 miles of range at 55% power. Personally, I need a bathroom break after 3-4 hours, max...

If you truly believe you will occasionally need more fuel than the main+aux arrangement allows, you could build (or purchase) a fuselage tank that was easily removable when you don't need the range. Sort of a "ferry tank" if you will... A fuselage tank with a pump that just transfers the fuel to one of the existing mains (or aux) would be much simpler to rig (and to remove when not needed).

[Mark Goldberg]

There was one set of Bearhawk wings built with the two 25 gallon tanks in each wing. The owner said he was going to fly it across the North Atlantic to Europe for the Paris Airshow. It never happened. While it is possible to do - if you look at the range available with 72 gallons and how far you can go if you need to stretch range - I agree with the previous posters that you would never need that much fuel. Mark

[jaredyates]

The inertia of weight in the wings, particularly outboard, might have an unfavorable impact on roll rate.

[whee]

I totally understand the desire to tanker fuel but I would not want to put 25gal tanks that far outboard in the wings. Think about all that extra stress placed on the wing structure by placing 150lbs of fuel out near the end of the wings. Also, like Jared said, the roll rate would be affected.

If this was something I wanted to do I'd be putting the engineering effort into replacing some inboard ribs with hat section stringers and enlarging the main tanks then I'd add the standard aux tanks. The metal wing Luscombes used hat section stringers to shape the wings rather than webbed ribs so it is certainly something that can be done. I've always been amazed that they were able to build such a strong wing but when peering inside it looks like a hollow tube.

[Zzz]

My butt can barely take 3 hours. Really the only reason I'm adding the aux tanks is so that I can get further away from available fuel and have plenty to play around and then return home. Some people have asked "why not just carry extra fuel in a container of some sort like a fuel bag?" I would say while you can potentially have a lighter aircraft for when you're solo, that I'd rather use the cabin for hauling kids and bags when this scenario is afoot. I don't like huffing fuel in the cabin, which is usually the case when I haul a can.

[Mark Moyle]

I'm with Zzz on this....getting further away from fuel. A number of years ago I taxied by Shawn Lunt in Dillingham. At the time he was loading 5 gallon gas jugs into his Supercub. About 30 minutes after landing in Platinum I watched him fly by. 10 minutes later he moose stalled in... Shawn's friend following in another cub described what he witnessed...instant fire ball on impact.....so no gas in jugs is a good call. I'm going with the standard and aux tanks from Avipro. But with a gas guzzling 520 up front I may discover I'm limited on distance from fuel just like my Pacer was... Now installing two 24 gallon tanks in it. That brings up the belly pod possibility from another recent thread. I think it's doable on a bearhawk by passing the landing gear struts through a belly pod....


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[Chris In Milwaukee]

Howard DGA-15 carries 120 gallons in three belly tanks! :-O

[JimParker256]

If you go back and re-read what I wrote, I was not advocating hauling fuel cans in the baggage compartment. I was saying if you absolutely HAVE to have more range, an auxiliary tank in the fuselage might be a good option, and they CAN be made to be removable when not needed. Personally, I would just slow down to Carson speed (roughly 1.32x Vy) and extend the range withe the existing wing-tank fuel. I don't like fuel tanks in the fuselage - at all.

[Zac Weidner]

I appreciate all of the input from you guys. It sounds like there isn't really room anywhere in the wing for extra fuel, which is what I was actually wanting to know. I just had the thought that maybe one could add an extra set of aux tanks with no hassle at all, but it sound like it's not practical. I would think that the extra fuel being out in the wing wouldn't place hardly any stress on the wing spar. I know it's a different design, and I'm no engineer, so take what I'm about to say with a grain of salt. It was determined by one of the experts of Comanches (Maurice Taylor, I believe) that tip tanks on the Comanche models actually lowered the stress loads on the wing spars. This is why most of the models benefit from a gross weight increase when the extra weight is in the tips. This is why I thought extra fuel would be nice to have on occasion, but not really add any extra stress.
I don't think more than 72 gallons is really necessary, but (please correct me if I'm wrong anywhere here) an IO-540 at 75% is most likely going to burn something like 14-15gph, which only gives a 3.8 hour range with a 1 hour IFR reserve, or 640nm at 140kt cruise. I'm not a big fan of the idea of flying 110 knots for the extra range very often, because I'm usually going to want at least 60-65% anytime I'm wanting to get some place.

Thanks for all the input.

[Battson]

I don't think more than 72 gallons is really necessary, but (please correct me if I'm wrong anywhere here) an IO-540 at 75% is most likely going to burn something like 14-15gph, which only gives a 3.8 hour range with a 1 hour IFR reserve, or 640nm at 140kt cruise. I'm not a big fan of the idea of flying 110 knots for the extra range very often, because I'm usually going to want at least 60-65% anytime I'm wanting to get some place.

Thanks for all the input.

I would suggest you won't like cruising at 75% with an IO-540.

You can go to 65% lean of peak, and only lose a few knots in the process. But >= 75% is a whole other ball game. It's loud, energetic, you can't run LOP safely, the deck angle is really nose-low, and the plane takes more proactive controlling / steering. It's just not as comfortable and the plane doesn't feel as natural there. Plus, remember drag is the square of airspeed, so 75% doesn't buy you 140 kts, more like 130-137KTAS.

Also, you can only get that much power at low level where air density isn't a factor (unless you go turbo, which = $$$$$ and fuel cost $$$$).

In my view, it's better to cruise up high at 55% and get the same airspeeds (125-130 TAS - heck, maybe 135 with a slick ship) lean of peak, save the fuel, get the range and comfort advantages, and accept the slightly lower airspeed vs 75% cruise down low.

[Battson]

I mean to say - in a Bearhawk you might not like 75% for long distance cruising. That's been my experience anyway. In certain other aircraft with different design, I am sure it's fine to run about with the hammer down.

[Zac Weidner]

I generally like to cruise at 7-10 thousand feet WOT in what I fly now, which is around 75% at 7,000 and more like 65% at 10K. I'm assuming the 540 would perform the same at those altitudes.
I've read on here that reflexing the flaps a few degrees might help with the deck angle issue and possibly even help slightly with cruise speed. I do like the sound of LOP and higher altitudes though.


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[Battson]

I generally like to cruise at 7-10 thousand feet WOT in what I fly now, which is around 75% at 7,000 and more like 65% at 10K. I'm assuming the 540 would perform the same at those altitudes.
I've read on here that reflexing the flaps a few degrees might help with the deck angle issue and possibly even help slightly with cruise speed. I do like the sound of LOP and higher altitudes though.

Those numbers seem a little high perhaps....? I could well be mistaken, but I am not sure how you would get that much power from a normally aspirated engine. Perhaps by running an unusually high RPM?

Here's the instrumentation readouts for an IO-540 running at WOT and peak EGT (maximum power) at the stated altitudes (10,100ft DA). You can see it's producing 57% power. Increasing to 2350 or 2400 RPM doesn't make much extra power, less than 60% from memory.




Note she was going a little slow when I took the photo - she normally sits about 125 KTAS.

By the by, Lycoming advises no leaning at 75% or greater power output. Leaning above 65% needs to be done carefully to avoid overheating the exhaust valves and damaging the engine, as test runs have shown on the IO-550 in a test stand. For that reason, I like to stay =< 65%.

[kestrel]

I own the bearhawk with 4 "main" tanks. I expect to fill all four of them next weekend for our trip from NH to GA. If your usage patterns can utilize it, it works great! Bob did the engineering for it. Most flying won't require that much fuel. I have it, so I sometimes use it. I wouldn't say it is a "must have" for most people. The aircraft also has a T-fitting to connect an aux tank in the fuselage.

I have flow from southern NH to WV and back without refueling. Is was about 835 NM and I had 2 hrs of fuel remaining.

It is worth noting that not everyone gets the performance that Battson gets. I don't have fuel injection and my bird seems to be a bit more dirty than some. At 10-11 gph I get about 110-125 kts, depending on altitude.

[whee]

Pretty cool that you ended up with that plane kestrel. If you have any details about what was done inside the wings it would be interesting to hear. Also a pic or something about the tee fitting would be interesting.