The age-old engine question: 540 vs 390 vs 360 for 4-place

[Zzz]

There are some similar discussions floating around, but they're all incidental to other threads. I thought we could debate what is inarguably the biggest decision a Bearhawk 4-place builder makes.

I've posted in the past about my inclination toward the 540 and its abundance of power and ability to be pulled back for greater efficiency. BUT, one thing I haven't considered is that for truly short landings, a lighter weight aircraft will always prevail. A huge percentage of my flying is just playing around solo and sessioning a landing spot. I don't want to roast my big expensive 540 cylinders just playing.

rstaton commented

I had io540,,,,,was nice and heavy,,,,but nice.i am so use to pa 18s and such I now am flying my bh with io390.i love it.take off good with light mt prop,and angle valve 390 has plenty of cooling fins for these slower planes.we have done many customers hot rod experimental cubs with hopped up 360s,,,200hp+ only to have cooling problems....low and slow.running dual coolers ...same prob amercan chmp had on denali scout.dont get me wrong ,,it wont climp with 310 hp 540,,,but to the untrained eye...it does good.

After months of being mentally committed to a 540 (as I stated here), that statement struck a chord with me. How much would shaving 80-90lbs off the nose (rough difference in weight between 540 and 390 I believe, not accounting for the larger accessories as Battson commented) affect the flight characteristics and stall speed of a 4-place BH on landing? I'm used to flying Cubs and Champs and the like too, and despite a fire-breathing load hauler than can carry the kitchen sink out of a strip with enthusiasm, I dread losing that feeling of a lightweight bird.

As a 220 lb guy who will likely have a young family by the time my airplane is flying, I also dread making the wrong decision in terms of power. Despite the horsepower comparison of a 540 vs 390: Say 260 hp vs 215 hp (both injected motors), a friend recently made the comment that the 390 is running fairly stressed to make that power while the 540 is barely trying.

Propeller options open up as your step down to the 360 and 390 realm: For the 540, your composite choices are pretty much MT. For the 390, you can also choose from Whirlwind (80" CS STOL prop.)

As for price... I think you pay the say either way: A Bob 540 or a Superior XP400? Will Bob build us an IO390 cheaper than Superior? When I asked Mark G once about the IO-390, he said "Zaaaane.. that is a very expensive engine."

I personally don't really consider the 360. I routinely fly a 360-powered 172 and require more takeoff performance than it provides (empty weight 1420 lbs.)

Food for thought. I'm still a year or so out from purchasing my engine and I lay awake at night staring at the ceiling thinking about it.

[alaskabearhawk]

I always said a 390 is the ultimate engine for the BH...if you can afford it. The engine and MT prop combo will be north of 50K. I'll stick with the IO-540.

[Mark Goldberg]

I do not think the weight difference between a O-540 (parrellel valve cylinders) and an IO-390 (angle valve cylinders) will be as much as you think. If you want a 4 cylinder - I like the stroker engines from Titan and Superior. 370's and 375's. Cost would be 10K to 15K cheaper than a IO-390. Just something to consider. If you get up the cost of a new IO-390 - then the 540 starts to look attractive. Mark

[N3UW]

Having power and not using it is now as bad as not having power and needing it. If you were planning to always fly alone in low altitude situations, the smaller engines would be fine. If you want to also be able to load it up and not worry about density altitude than more power is nice. It all depends on your mission and what you are willing to give up or not give up . I do not really think the small amount of weight savings will really make that much difference on landing distances. More power will definitely make a difference on takeoff roll. I currently fly a cherokee 235 while I build my patrol. they offered the cherokee, using the same airframe, with O-320,O-360 and O-540. I have owned all of them and once I had more power I could never go back. The 235 has the O-540_-B4B5 with 3 blade CS prop. It fits my mission because my requirement is a plane for up to 3 people for local and cross country. Also a stable IFR platform and one that will operate in high DA airports. I also wanted a low cost maintence aircraft/engine combination. The O-540 is definitely that.

[7acdriver]

I doubt I have the credibility to talk about this but I have thought about it a lot for WHEN (not if ) I am to build a Bearhawk. I feel a 540 set up to run auto gas is (imho) my best option (pondering the efi/ignition from flyefii.com - supposedly run 9:1's in the 540 on auto gas with that system). I would want the plane to be a truly capable 4 place hauler (what's your mission?), but paying $6.50+/gallon for avgas in an engine that burns 12+gal/hr is just not affordable over the long run (for me). I think I would cringe at my shrinking wallet every time I started it. To get the awesome power/weight performance of the big 4 bangers (my understanding is) auto gas is not advisable. Add to the equation that the 540 can be powered back to burn in the neighborhood of what the 4 bangers will but still be capable of going up no matter what when you push the black knob forward, and burn gas that costs $2.50 less per gallon? Sold. I'll sacrifice the 100 lbs. or whatever it is. Not to mention the engine is cheaper to acquire, and much more proven. Like alaskabearhawk has said, if money is no object then a 390 would be amazing. Unfortunately I don't have that luxury. Just my $.02, thoughts?

[Helidesigner]

A significant consideration is CG. I did some CG calculations before selecting an IO540. An average 540 powered BH is 1520 lbs. I came out at 1522 lbs. I found the average by looking at all the 540 BH empty weights in two years of Bearhawk calenders. I looked up the weight of IO390 and an IO540. The IO390 is almost exactly 100 lbs less. I figure a lighter muffler and a couple quarts of oil is another 10 lbs. A 1520 lb BH with four 175 lb adults and full fuel is 2520 lbs, cg at 18.3, 65% aft of the fwd limit. An IO390 with four adults, full fuel, and 90 lbs of baggage is 2500 lbs, with a CG at 22.0, 95% aft of the fwd limit. Remove the baggage and you are at 20.0 in, 79% aft. Given the choice I would prefer to fly a utility category plane slightly over gross with a more favorable CG than a lighter one with the CG near the aft limit. A number of people have reported constant attention required when flying with an aft CG. I plan on talking extended trips and landing on runways that are short but generally used by aircraft. My suggestion is that if you plan on flying mostly as a two person plane on rough strips take out the back seat and keep it light with an IO390. If you are often taking a lot of baggage or people and filling to gross weight, you might want the 540. If you want the power and lightness go bare bones on interior, instruments, and no aux tanks, no skylight, fabric covered doors, no strut or gear fairings, and all those other weight saving tricks.

Hopefully this helps. I tried to upload my spreadsheet calculations but it would not. I'll sent it to anyone who wants it.

Mark

[Helidesigner]

One more thought regarding my plan..... If I have four people or lots of baggage and heavy I'm only going to fly off well prepared surfaces. If I'm going to rougher strips I am 2400 lbs or less.

Mark

[Matt K.]

A few thoughts:

1. How much would using Oratex help move the CG forward and give a wider envelope with a lighter engine? Has anyone calculated the moment of fabric cover? (Counterweights might be able to be reduced also)

2. XP400 is very interesting. It's my understanding Barrett (who developed the 390) helped bring the XP400 out of hibernation b/c the Lyc price has become too high.

3. The HP range of those Whirlwind props cover the lower HP 540's (up to 265HP). I asked about it at OSH a couple of years ago and they seemed ok with the STOL/200 series prop on a low compression 540 and might even make something up for it if necessary. There appear to be more blade options now than 2 years ago. WW does make a 540 prop for the -10 so it wouldn't be too far for them to stretch.

4. ECI 409 is even more expensive than the 390, not sure how many are in service. Hopefully they're well supported come overhaul time.

5. The intake tubes are lower/wider on the IO-390 and don't fit under the cowl. There were some pics from Larry Sullivan over on the yahoo group that showed where some blisters were needed. I haven't had a chance to measure the XP400 yet, I'm curious if it shares intake dims with the 390.

I still have the 540 engine mount but keep looking at these higher HP 4 cylinders.

-Matt

[DesertBearhawk]

Reading all this has me thinking this subject might be overthought. My head is starting to hurt....

[Wayne Massey]

A lot of times in the discussion for the smaller vrs bigger engine the decision is made for the smaller engine because home base and primary flying is over “flat lands”. One thing that doesn’t get talked about much in the 0-360 vrs 0-540 discussion is the convenience to get to a higher cruse altitude quickly - doing so to take advantage of smoother air or tailwinds. Nothing worse than bouncing around below a scattered cloud layer when you could jump up quickly and have a better ride. Then once at altitude you have the higher power available to set up a nice fast cruse and leaner/lower fuel burn. The smaller engine is going to be more sluggish in the climb and cruse. One of the best features of the 4-place Bearhawk is it’s a GREAT cross country airplane. Having the extra power is a huge benefit even if you don’t plan on mountain flying. Climbing up to 11,500' or higher in an 0-540 BH is no big deal and it doesn't take long and could prove useful even for short cross country flights over flat lands. Your don't feel like you are struggling to get to altitude and then when your get there you are moving right along!

[Zzz]

I guess I was just looking to have my 540 decision affirmed. Haha. Fantastic points and arguments all around.

We recently had a discussion on BCP about 320 vs 360 powered Super Cubs, and there was a lot of talk about the pleasure of flying a lightweight Cub that's well-balanced. I suppose the main argument is the same here: A well balanced airplane is a pleasure to fly whereas one with an aft CG, even if lighter, can be very unpleasant.

My argument that landing short is made so much easier by being lighter still stands though. I have flown with Greg Miller in his modified M5 with hotrod O-360 and the landing distances are impressive. It's a bare bones machine though with a fixed pitch prop.I'm curious why it seems to balance ok and the Bearhawk has trouble with aft CG on lighter engines.

I do love power though, and climbing out with authority.

[bestbearhawk1231]

Another option is an intercooled turbo normalized 360, split the difference in weight between the 360 and 540 and have full sea level hp up to critical altitude. My field altitude is 4000' and density altitude is in the neighborhood of 7000' +- 300' or so pretty much all summer. Most of the places I want to go are all uphill from here so an NA 360 is pretty much out of the question since I want to be able to get off the ground in anyplace I can get in and not take half the trip to get up to an altitude that has favorable tail winds.

[Battson]

Wow this has generated some discussion!

You know my thoughts Z, but seeing as they are free - feel free to read some more!!

Balance, Balance, Balance:
Mark (Helidesigner) has made excellent points about CG.
I was flying a hunting trip Sat-Monday, 3 'real size' men, and the cargo area and 4th seat LITERALLY full to the headliner with gear. We organised the load and were inside CG, "just". At full gross there is too much weight on the tail for "serious" off-airport ops, you need 150ft of smooth surface to get the tail up. I concur with Mark's calculated findings, 4 men and a long weekend's luggage would not be possible with 100lbs missing from the nose.

I can also affirm Mark's suggestion about flying aft CG. If you are measurably OUTSIDE the aft limit, the plane wants to bite you at any moment. It goes into runaway dives or climbs if you're not looking, it cannot be trimmed to stability, and locks you into high-G turns without any continued stick force required. It's not "uncontrollable", but "constant attention" is about an understatement. I guess that any further aft than that point, and you'd be killed trying to take off. This isn't a plane with 'soft' limits. Stay within the limits for safe results.

I agree that you NEED the -540 to get the best load-hauling capability, for reasons other than raw power. With the large elevators [and correct technique], the aircraft is very manageable at forward CG. Installing VGs under the elevator would make up for poor technique but could result in more tailwheel-first landings. I still haven't fitted the set I have in my toolbox.

Climb:
The IO-540 machine goes from sea level to 10,000 in 6 minutes. That only comes from power:weight.

Landing:
This is where you need to be honest, weight makes all the difference to a good off-airport aircraft. If you will be flying 90% of your time in the playground, I would seriously consider an IO-390 and a long list of other mods (longer wings, slotted flaps, slats - in fact, am I describing an SQ4...?). To me, that doesn't make sense. It's good to land short, but I couldn't sacrifice all the other Bearhawk performance areas just to subtract 50ft from my landing distance. Besides, it would only be a realisable benefit those few times I am dead empty on fuel+gear+people AND landing on gravel/rock (which is not 90% of my time). The limiting factor on other surfaces is the friction coefficient of the bushwheel or the extra weight in the machine!

[Zzz]

I guess when feeling doubtful about the 540, I should recall a ride I took with Mike Creek out of Stehekin. Me, and a third in the back seat, both of us probably 225 lbs, Mike weighing somewhat less. He pointed it at the moon and we climbed out about 60 mph at a very impressive angle.

I'm sort of conditioned to not carry any backseat passengers after owning a 145hp C-170 so I can't see myself carrying any in the Bearhawk; it's just not my mission profile, though some munchkins may be on the horizon which would turn mama into a backseat passenger. I do enjoy packing a good load of gear though.

Interested to hear from some 360 pilots on loading issues, just for the sake of knowledge. Are you out there, Gavin Chester?

[Zzz]

Another option is an intercooled turbo normalized 360, split the difference in weight between the 360 and 540 and have full sea level hp up to critical altitude. My field altitude is 4000' and density altitude is in the neighborhood of 7000' +- 300' or so pretty much all summer. Most of the places I want to go are all uphill from here so an NA 360 is pretty much out of the question since I want to be able to get off the ground in anyplace I can get in and not take half the trip to get up to an altitude that has favorable tail winds.

Joe, that has to be the most expensive option of all...right? I love the idea of turbo normalizing for high altitude mountain ops but the addition of the turbo seems like it adds quite a bit of $$.

[bestbearhawk1231]

Z, if you would buy a certified TN engine or use certified parts on it I would agree with you. However I am confident that "rolling your own" turbo system would come out somewhere between the price of an overhauled 360 and 540. Both Garrett and Turbonetics have viable solutions turbo wise that run in the neighborhood of an engine cylinder. Add an external wastegate, electronic wastegate controller, intercooler, and blowoff valve for over boost protection and you would be well on your way. Electronic wastegate controllers have come a long way since I was in high school and the dependability has increased immensely...to the point I would have more trust in an electronic controller than in an oil pressure controller. In addition, Turbonetics offers a ceramic ball bearing option which will allow the bearing to operate without oil in a loss of oil situation...in other words, if you loose oil pressure your engine will seize before the turbo. One big downside is dropping a valve. If you have a turbo the remnants of the valve are going through your hot section and will damage the fins. If you are NA and are lucky, the remnants will fall out your pipe and not harm anything...but lets face it, if you drop a valve you are going to have a bad day regardless. Since you wouldn't be building boost the engine would never know it has left sea level until you reach critical altitude so minimal if any changes would be required to the engine, especially if you are carburated. Maybe I am way off in left field on this one but it seams like a viable option. I have not sat down and ran the weight numbers or prices yet but I have done enough research to know that it is still an option on the table. That being said, if I came across a smoking good deal on a 540 that still has life left in it I would pick it up...but if I come across a smoking good deal on a 360, I will pick it up and roll my own turbo system.