Correction of Model 5 specs

[Mark Goldberg]

Last night Bob remembered something about his original drawings for the Model 5 that we both had forgotten. The main fuel tanks in the Model 5 will be 35 gallons each. Not the same 27.5 gallons like in the Patrol & Model B. And with the aux tanks 89 gallons will be available. LOT of stinking fuel. Mark

[auburntsts]

Mark,
just to make sure I’m understanding what you’re saying is the total capacity with aux tanks 92 gal (35+35+11+11) with 89 gal usable out of the 92?

[Mark Goldberg]

No. The aux tanks on the Model B Bearhawk and the Model 5 hold 9.5 gallons. So 70 gallons in the mains and 19 in the aux tanks. Total 89 gallons. Almost all should be usable. Mark

[auburntsts]

Thanks Mark for the clarification!

[kestrel]

35 gal mains is perfect!!!

[m.mooney]

Agreed! This is a substantial improvement.

[whee]

35 gal mains is perfect!!!

I agree! Might have to put BH5 wings on my plane.

[svyolo]

Mark;
That sounds like exactly one rib bay bigger for the mains. Outboard I am assuming? Hard to do on QB B model wings?

[Mark Goldberg]

That is correct John. One rib bay more. Doing it on an existing Model B wing? Not sure. Probably a Bob Barrows question. You would have a lot of existing rivet holes to deal with. Mark

[Weldingiron]

Is there still the same number of center ribs, just adjusted spacing or do we lose a center rib altogether?

[Weldingiron]

How is the additional volume achieved in the same wing?

[benc]

Is this still the case ? I thought I read somewhere that it's gone back to 55 gal mains, website specs has that also

[Mark Goldberg]

The Model 5 is using the same wing as the Model B. 55 gallon mains and 19 total on the two aux tanks. Mark

[benc]

What was the reason for going back to the 55 mains ? Is it possible to go back to the 70 gal mains in the future ? 74 gals total is a great number for an O-360 powered airplane but not so much for a gas guzzling IO-580 deep the backcountry potentially hours from fuel. 89 gals would have brought it in line with the 88/92 gal of the C185/206.

Also, is there a change in the basic fuel system design assuming this airframe is designed around a larger injected engine being in place ? e.g a header tank ?

Thanks

[svyolo]

Do a forum search on Aux Fuel. All your answers are there, including at least one built with 4 25 gallon tanks.

[zkelley2]

What was the reason for going back to the 55 mains ? Is it possible to go back to the 70 gal mains in the future ? 74 gals total is a great number for an O-360 powered airplane but not so much for a gas guzzling IO-580 deep the backcountry potentially hours from fuel. 89 gals would have brought it in line with the 88/92 gal of the C185/206.

Also, is there a change in the basic fuel system design assuming this airframe is designed around a larger injected engine being in place ? e.g a header tank ?

Thanks

The large HP on these airplanes really is just for takeoff distance, climb and high altitude cruise. Remember takeoff and climb are almost entirely about excess horsepower above what is required for steady flight.
The excess horsepower is dramatic. (and fun), and nothing like most light certified aircraft.

The 4 place with 260hp is 9.6lb/hp and the 5 with the 580 would be slightly better at 9.5lb/hp.
Compared to a 180/185 that's 12.2 and 11.1 respectively.

That said, the bearhawk does have more drag than the Cessnas and since speed is about drag, they're generally not quite as fast. But also, all that excess horsepower isn't really useful in cruise.
The equation for adding cruise speed in HP alone is New Speed = Old Speed * (New Power/Old Power)^1/3. With the cube root in there, you need a crap ton of more power to go any faster. Say like a 180hp BH vs 260, X = 120(kts) * (260/180)^1/3. X = 135. 20kts for 80hp and 8gph more, assuming full power. But if you pull the 260 back to about 55% power like I do on mine, I'm still doing 112kts on about 9.5gph at common weights I fly at. At gross that's closer to 11gph. The 180hp bearhawk on 80% power, which is about the same as a 260 on 55%, burns roughly the same amount of fuel for the same airspeed given the same weight.

Point being in all that is the 580 only gulps fuel in takeoff and climb. In cruise it consumes however much you want it to. If you're flying it at say 2500lbs, and set it to the same fuel burn, say 11gph, I would expect it to do the same speed as my 4 place. If you load it to 3000lbs, I'd expect you to be .5-1gph higher for the same speed. People are used to using something like 75% power because most certified aircraft don't have near the excess power. I've never once cruised at such a high power setting in the BH. It's wasteful.
Since every build is a bit different, through testing you can figure out where your best economy is and where that drag wall is where adding power gives increasingly diminishing returns in speed. For me that's right at 112ktas down low runs into the 120's up above 7000. It'll go faster, but it's uneconomical to do so. For reference I'm on 31s. It was ~10-12kts faster on 8.00s

I do have the Aux tanks on my A model. And I use them all the time, commonly to tanker fuel because you're correct you're not going to buy fuel on a sand bar. But I would expect the 5 with a 580 to cruise at something around 13gph. That's 5h45m flight with an hour reserve and 1000lbs useful remaining. Pretty dang close to a 185 and I think all this is really consistent with Jarred's flight testing.

[jaredyates]

I agree with that and would also speculate that more builders would use a 540 vs a 580, especially when it comes time to write a check.

[zkelley2]

Oh for sure. That 580 is a lot. Used angle valves aren't that terribly expensive since there's no RV that uses them.
It'd be cool to see what the 350HP TIO-540 out of a pa31 would do. Especially for the guys out west.

[svyolo]

I was pretty shocked when I looked up the list price of the 580. There are also a lot of used Conti 470/520/550's around for a very fair price, and because most of the 180-206 Cessnas used them, lots of used CS props for sale for the Conti's as well. The 520/550's are heavier than Bob recommends for the 4 place, but should be OK in the 5, and might save quite a bit if you got a used engine/prop.

[Cardiff Kook]

Last night Bob remembered something about his original drawings for the Model 5 that we both had forgotten. The main fuel tanks in the Model 5 will be 35 gallons each. Not the same 27.5 gallons like in the Patrol & Model B. And with the aux tanks 89 gallons will be available. LOT of stinking fuel. Mark

Not clear from the posts- did it get the 35 gallon tanks?

[schu]

I have also wondered why there isn’t more interest from Bob on running the 520 or 550 continentals. While there are a lot of angle valve 540s out there, they tend to have rear facing sumps and turbo chargers. As I understand it, Bob is working on some intake to turn this around, but wouldn’t a 520 make more sense? There are a bunch of them around with the right intake config and I think you can even run mogas with the injected engine because the fuel pump has a way to deal with vapor.

If I was building a 5 I’d be looking into it.

[Mark Goldberg]

The fuel capacity of the Model 5 is the same as the Model B - 55 gallons in the mains with another 19 in the optional aux tanks. 74 total

Schu - a couple of customers have talked about using a Cont 520. No one has done it yet. It would seem an option on the Model 5 but on the 4 place they are too heavy. So far everyone has gone with angle valve Lyc 540's of different flavors. Mark

[rodsmith]

I think if I was building a Model 5 I might stick with a straight valve 540. They can easily be pushed to 300HP without much effort. I wish I had had an opportunity to dyno mine, it definitely is in that neighborhood and 415#s with all accessories.