Series 7 vs. CH-750 (again?)

[SkyPirate]

OK on a more serious note I have been to the Zenith factory n Mexico Missouri ..actually just went up there last week and spent a $1000.00 on some aluminum for a project,..while I was there ..I got a tour ..they have a nice set up,..I am also a member of an EAA chapter that is building a 601,..what got me asking myself questions about the CH's after seeing the wing construction ..some of the bays between the ribs are 22 inches apart ,.and from I was told ...020 6061 T-6 is used for the skins and basically the whole wing construction,..some that own CH's have said the wing beer cans,..meaning the surface actually moves in and out like a beer can if you had squeezed it ..in flight due to thermals and or rough air etc..
The CH that was parked out on the ramp actually showed sign of this with the top flying surface looking like a dented can between the ribs,..I'm sure in flight while the wing is creating lift ..the surface is pulled out so the wing is uniform,..and 6061 T-6 is pretty resilient ,..the 601's actually use the top skin of the wing as the aeleron hinge,..advantage no need for gap seals on top or hinges for the control surface,..the CH's I believe actually use hinges.

I'm not trying to force judgement..just giving a heads up on what I did see concerning the wing construction..you can make your own analysis,.
along with the strut design,.it not being a solid strut ..but split on both leading and trailing struts at the jury strut location and not having an adjustment on either end of the strut.

This evidently is all fine,..or the FAA would ground the CH's as they did the 601's concerning wing construction until the AD's where completed.
the 601's AD was the bellcrank support inside the wing for the aeleron.

you might also want to research recent aircraft accidents in Eldon MO 2 with in 2 weeks of each other,..1 for sure was a CH


Chase

[DesertFox4]

Time to get back on topic guys. Don't make me put my Administrator hat on. It cuts off my circulation.



To address the original question in this thread: I have not flown a CH-750 (nor do I really care to). I have a friend that loves to fly in my Kitfox anytime he gets the chance and he'd been talking up the great STOL capabilities he'd seen in videos of the CH for two years but had never flown one. My brother arranged to get him a flight in a brand new one with the 912S Rotax and same prop as I fly with so the comparison was spot on. When he returned after a 45 minute flight he didn't say much. I think it was out of respect for the owner/builder. Nothing was said the whole hour we flew back to our airport. I finally had to ask him what he thought after we landed and tied down. He said he was very disappointed. It didn't take off much quicker than my Kitfox and climbed to altitude slower and was noticeably slower in cruise. I think he said it was extremely noisy inside but don't hold me to that one.
Not much of a decision in his book or mine for that matter. A beautiful aircraft that does everything very well or a not so pretty one trick pony.
Anyone that flies their Kitfox anywhere near max performance knows it does STOL as well as any aircraft in that market.
Go Kitfox and you'll never ever have to feel like you made the wrong decision. Also people will actually compliment you on its good looks and ask you if they can get a ride in it.

[Av8r3400]

STOL must be a consideration to you, judging by this thread. Remember, if you are thinking STOL performance, a 3300 rpm engine will not get you there.

I have a buddy with a Jabaru 2200 on his Avid mark IV, normally an awesome STOL performing design. The Jab requires such a short prop for the high RPM (3400) it's take-off and climb performance is terrible. Easily triple my take off distance and half my climb from my 80 HP Rotax powered KF-IV. He should be able to to much better than me...

The 900 series Rotax is perfect for these planes. LIGHT weight and high HP turning the prop at a relatively slow RPM.

Variety is the spice of life, so I say go for the UL motor. I will pretty much guarantee you will not get the performance you want and will be unhappy. Just don't blame the Kitfox design, though.


Out of curiosity, what do you consider to be a "high" oil temp on your ULS?

[Av8r3400]

Rotax says the redline temp for oil is 260° + with synthetic oil... I run 220-230° as a norm.

[RobS]

Granted, I do not own a Kitfox or a Rotax or ULS engine, but if I understand the issues correctly, it's torque, horsepower, and engine/propeller rpm that will determine overall performance (in the same airframe).

If I'm reading the ULS350i torque/horsepower crossover graph (see below) correctly it will produce 110hp at 290Nm torque at 2700 rpm. At 2500 rpm (an acceptable prop speed) it will produce 105hp and 300Nm torque and at 2300 rpm (a speed where the ULS and Rotax props could be identical) 100hp and 308Nm torque.

Based on the information found at the Rotax website, the 912ULS will produce 95hp at 5300 rpm (100hp at 5800 rpm but for only 1 minute) and somewhere in the neighborhood of 128Nm torque (128 at 5100 rpm - no torque info found for 5300 rpm). With a 1:2.273 gearbox that equates to 2330 rpm at the prop and with a 1:2.43 box 2180 rpm.

With those numbers in mind, I don't understand why the Rotax would perform so much better than the ULS. The Rotax weighs in at 150lbs - the ULS at 173 so I can't believe 23lbs is the difference. The horsepower of both engines are close - and in fact 5hp greater for the ULS at the 2300 engine/prop rpm when compared to the gearbox reduced 2330 prop rpm for the Rotax.

So the question is, what am I missing?


ULS350i

[DesertFox4]

So the question is, what am I missing?

Maybe real world experience? We know to a certain how the Kitfox performs with a Rotax. It performs great! No idea if the claims on the UL hold water. You want to gamble they do or don't, have at it. Wish you all the luck in the world as the stakes are quite high. Most can't afford to loose that bet. Though we do call big time gamblers pioneers when the bet pays off. Hoping it does pay off if you roll those dice. Another viable engine choice is always good i guess and the world still needs pioneers.

P.S. Doesn't that fuel injection system look nice? Rotax!?

[Av8r3400]

One word answer, Rob: Gearbox.

The numbers you are quoting from Rotax are Crankshaft Power. The propeller runs off a 2.43:1 gearbox which multiplies the power by reducing the RPM. At the 5800 rpm redline (max for 5 minutes) the Prop is only turning 2400 rpm. At 5500 max continuous rpm the prop is turning 2260 rpm.

This makes a huge difference in static and slow speed thrust, ie the 120 mph and slower speed envelope of our planes.



The 60 hp direct drive VW installations go like crazy in a Sonex or other high speed planes that can utilize the short prop's speed efficiency.

[RobS]

Av8r3400, your points are well taken, but I think I took them into consideration in my thought process, although I may not have clearly stated them in my first post. For example, using the numbers you provided from your direct experience (5500 rpm = 2260 prop speed) and numbers from the Rotax website and plugging them into the following formula to calculate torque - "Torque (Nm) = HP x 5252 / rpm" – where the HP numbers are for the Rotax 912ULS and the RPM are those of the output shaft of a 2.43 gearbox based on the engine crankshaft RPM for the given HP - you get the results contained in the table below.

An engine produces power by rotating a shaft which can exert a given amount of torque on a load at a given RPM. And if torque is defined as a force around a given point, applied at a radius from that point, then wouldn’t I want a motor that produces more torque to drive the prop at any given rpm. If so, I think the table suggests the UL350i is more than comparable to the Rotax 912ULS. I understand the issues of the tried and true Rotax, 2000 hour TBO and thousands upon thousands installed in aircraft. But from a pure performance standpoint, I think the numbers indicate it is a good engine. Even if its weight to HP ratio is slightly higher than the 912.

I think at equal prop speeds – which means the ability to run the same props – the two engines are very good performers.

Shaft/Prop RPM--------Rotax912 Torque--------UL350i Torque
---1650------------------217---------------------290
---1850------------------227---------------------295
---2050------------------230---------------------300
---2260------------------225---------------------300
---2390------------------220---------------------300

I agree with your statement the "gearbox multiplies the power". In fact, the engine shaft torque for the 912 at 5800 RPM is about 91 Nm. So the gearbox is critical, because the resulting gearbox RPM (2390) yields 220 Nm.

[BajaCat]

Thanks RobS for your thorough power analysis. One additional point--it doesn't appear that the installed weight of the UL350i would be that much more than a Rotax. A builder in Europe compared the installed weight of the Rotax 912S with the UL260i in the same aircraft. He found that the UL260i was 4.4 lbs LIGHTER than the Rotax. The installed weight of the UL350i is listed as 13.5 lbs. more than the UL260i. I calculate \the UL350i installed weight to be 9.1 lbs more than the Rotax.

As I said at the beginning of this discussion, I don't have any desire to be the first to try this installation and will probably stick with the Rotax to be conservative. But the UL Power engines sure look good to me.

BTW RobS, you say at the bottom of your post that you're considering a Kitfox. What else are you considering?

Jim

[RobS]

Jim,
I'm considering how I can sell my 172A in today's plane market . Seriously, I do need to sell the Cessna before I consider anything else, but a Kitfox is very high on the list followed by a Highlander.

Not only do I want something that qualifies as Light-Sport, but is experimental so I can maintain it myself.