I thought I put this thread in the open forum, as it applies to the Cruzer, STOL and maybe also to the 650.

 

I understand that Zenith designed the CH750 for engines with a weight of up to 300 lbs. I also understand that Zenith approves a max. gross weight of 1,510 lb, if the max. g-load is reduced to 3.8 G.

The Lycoming O-320 is around 90 - 100 lb heavier than a Rotax 912, but has at least 150 hp and is plentiful available.

 

Most Rotax 912 powered CH 750s are in the range of 750 – 800 lb. Adding 100 lb for the O-320, we’d be at an empty weight of 850  - 900 lb.

 

1,510 lb max. gross – 180 lb full fuel – 900 lb empty weigh =  430 lb payload

 

430 lb payload would still be fine. Since the 900 lb assumption for the empty weight also appears to be a little bit on the high side, I would expect the actual payload to be more in the area of 450 lb.

 

On paper, it sounds as whether an O-320 might be a feasible option, the performance should also be great. Besides of the amphibious 750 on YouTube, I am however not aware of any O-320 powered Zeniths.

Are there other disadvantages, besides of the reduced useful load? Will it become nose heavy? How does it fly?

 

The Titan X340 with 180 hp, a stroked O-320, should also fit. This is the engine which is used in the popular Cubcrafter’s Carbon Cub and the new Kitfox STi. Not sure, if the Zenith can handle the hp and X340 will probably exceed our budget anyway. Still, for discussion’s sake – this should be an awesome engine for a Zenith. Or not?

 

Oliver

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A couple of thoughts off the top of my head...

1) Many 750 builders want the airplane certificated as light sport with a 1320 pound max gw, so the added engine weight would limit the payload for those builders.

2) I haven't studied the 750 weight & balance (I fly a 601XL), but wouldn't be surprised if keeping the 750 W&B inside the CG envelope with such a heavy engine up front would present a challenge.

3) The added fuel consumption of the X340 would limit the range/endurance with standard fuel tanks.

4) I doubt the extra power would result in much of an increase in cruise speed with the 750 airframe.

I could be wrong about all of the above thoughts, but would consider them worthy of further study.

My 750, with the Mattituck-Continental IO-240 on the nose, has eight pounds of lead in the tail, and weighs 895 pounds. It goes up like an elevator at gross, and is usually off in <200 feet. I haven't actually measured the angle of climb, but it's steep. The rate of climb is usually somewhere between 500 and 1,000 fpm. I've got more work to do on measuring performance. I don't know how it stacks up against the auto fuel burners, but in California the lack of fuel on airports is an issue for them, so they end up burning 100LL a lot of the time. Even when you can get auto fuel, it contains alcohol in CA.

WT

I have a friend/builder that has the STOL 750 with an 0-320. He is very happy with it. However, I believe his plane came in arounf 920lbs when complete. Don't forget about the heavier propellor and additional accessories required for the front end.

DALE

Any chance giving out the name of the builder with the O-320. I already have a Lycoming O-320 from another project and am reluctant of spending another $20,000 for another engine

loulacy@gmail.com

Thank you for your responses.

CG and fuel consumption, if the power is pulled back to a reasonable cruise setting, are certainly important factors to consider.

If the empty weight exceeds 900 lb, this would however be a deal killer for us.

I hope that a few people who actually installed an O-320 will chime in and share their experiences.

Every once in a while, 0 hr SMOH O-320s show up on Barnstormers for under $15K. It would be great, if it would turn out that they work well in a Zenith as a powerful, proven, (relatively) low cost option.

I'm currently keeping a close eye on the Viking 130, but am hesitant because of its short track record and would like to have a powerful, not too expensive alternative available in case people start having issues with the Viking.

I'm just guessing, as I can't even spel aireonaughtycal engerneer, but it would seem to me that things like this should be mission driven. If you want to climb out steeply, maybe a heavier engine would be the ticket, but if you want to fly two 200-pounders, full fuel, and max baggage (40 lb. limit), you're probably going to have to settle for a lighter engine. Again, I'm just guessing, but it seems that a heavier engine has got to carry more than its own weight. As the Downeaster once said, "you just can't get there from here." Sport gross and design gross figure in thar somewhar.

All ya'll experts can do the math for me, right?

WT

Oh, yes, the 5.5 gph I quoted was at 75 percent power except for takeoff (and it was a short trip). I liked riding behind the Jabiru 3300 at Mexico and in John's. John whipped the heating "problems."

WT

"I also understand that Zenith approves a max. gross weight of 1,510 lb, if the max. g-load is reduced to 3.8 G."

Wayne, has Zenith put this out in any printed documentation you are aware of...?  Thanks!

Dave

I'm curious about that as well. The most I've seen in Zenith documentation is 1,440 pounds.

He is probably talking about a post and research I did! I got verbal confirmation from Roger as stated in the comments.
http://www.zenith.aero/group/750-cruzer/forum/topics/cruzer-gross-w...

Yes.

The Zenith guys also told me in Oshkosh that an increased gross weight would be no problem. I however do not recall that we discussed specifics.

The information is actually (kind of) already in the table.

Zenith specifies a max. gross weight of 1,440 lbs at an ultimate load factor of +6 G / -3 G.

The FAA requires for normal category aircraft a load factor of +3.8 G / -1.5 G.

http://rgl.faa.gov/Regulatory_and_Guidance_Library%5CrgFAR.nsf/0/D3...

The FAA additionally requires a safety factor of 1.5, what results in an ultimate load of 5.7 G.

https://www.faa.gov/aircraft/air_cert/design_approvals/airships/air...

"Downgrading" the Zenith specs to normal category requirements (like Cessna Piper, etc.), the resulting max. gross weight is 1,440 lbs / 5.7 * 6 = 1,516 lbs

I guess it is out of question that the CH 750 gear can handle the additional weight. Just look at how some people abuse it in some of the backcountry STOL videos on YouTube.

 

The other big concern would be the aircraft's ability to execute a go around at max.gross with the flaps extended. With a 100 hp engine this would certainly have to be tested. If the plane does fine in this regards with a 100 hp engine at 1,440 lbs, 1,516 lbs (5.3% more weight) will however work just fine with 50% stronger engine.

Finally, the 5.3% weight increase will have an almost negligible effect on the the critical speeds.

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