XJet

The interesting thing is that *neither* the SK50 nor the SK90 I received (from China) came with any instructions at all.

Objective testing means that, regardless of what other knowledge you might have, you must treat the engine just as any other modeler would.

The SK50 was run in just as you'd run in any regular ABC engine and the SK90 was done as I'd expect a chrome-bored and ringed engine to be run in.

I repeat that the particular SK50 I received was so tight as to be way beyond what anyone would consider to be "tight" and is the only ABC I've ever had to take a gas-torch to just in order to get it to turn over. It's also the only engine I've ever had where bystanders took several steps back when it started because the mechanical noise from all that pinch and thrashing of the conrod was so loud that we thought it was going to grenade on us.

If someone can come up with a plausible explanation as to how fitting a 12x5 prop would magically reduce the pinch to manageable proportions I'd love to hear it. This engine was broken in on a prop that produced about the same load as a 12x5 so it wasn't under-propped to start with.

This almost certainly was an out-of-spec engine which explains why it doesn't perform as well as others have reported.

Which leads me back to the assertion that (like so many Chinese manufacturers) SK are happy to ship anything that even remotely looks like an engine, regardless of how poorly it's been machined or assembled.

I'm quite prepared to look at an engine that's been QC'd by Kange in the USA who (I suspect and certainly hope) are tossing the duds before they make it to customers' hands.

If someone can explain where I've lacked objectivity or been unreasonably harsh in my observations my ears are open.

rainedave

My Feedback: (1)

XJet, sorry if it sounded like I was targeting your review in particular.

It's clear from your posts that you gave your .50 a careful break-in and a fair test.

I don't think there's any question that the QC in China is not what it needs to be. Your review points out the fact that there's no guarantee every customer is going to get the same level of product. As long as there is this demand for very inexpensive engines they're going to be produced cheaply enough to meet it. I just bought a new O.S. AX .46 and you can feel and see the quality.

David

XJet

Hopefully that's where the likes of Kange come in, providing that missing QC check that doesn't happen at the factory.

I'm seeing a growing number of Chinese products gain an envious reputation for quality and performance and this is primarily due to the immense amount of work the US distributor is doing to sort the wheat from the chaff, thus making sure that customers get a usable product every time.

This was the reason I challenged Kange to provide an engine from *their* stock so I could compare a "direct from China" engine with one that had been through their own QC process. We've seen how *bad* an SK50 can be, I wonder how good they might be :-)

Turk1

Hi XJet,I am told by Ray Nano that all engines of Kangke origin are having test run before delivery and customer can see the mount tracks on carb neck though being in a separete heat sealed nylon bag.None of my 4 SK s neither a many of my friend s were tight like yours.BTW SK ringed engines are not like ordinary ringed engines because they have tapered,chromed liners and instructions say not to richen to 4 stroking.

rustypep

My Feedback: (6)

Mine had the mount tracks you mentioned. My SK 50 was fairly tight (tighter than the TT 46 I broke in at the same time) but not anything like what XJET experienced. Ray Nano did mention at one point that the chrome was harder than normal which if true should mean a longer breakin and hopefully longer life. So far I have experience both. That would still be a seperate issue from the fit of the parts. I wonder if XJET has access to a parts source or warranty service? Sounds like he deserves a new piston and liner or maybe a replacement engine. I had a good experience with Kangke service over here but not sure about what he can expect overseas.

XJet

the TT46Pro engines have very little pinch by most ABC/ABN standards and I've never had any problems with them being tight, even on exceptionally cold days.

That's silly. *all* chrome is *much* harder than the aluminum used to make the piston. When an ABC breaks in it's the piston, not the liner that wears to size -- so it makes absolutely *no* difference how hard the chrome is because it just ain't going to wear either way.

I'd question Ray's understanding of this.

I guess I could chase up some replacement parts but I bought this engine solely for review purposes. I've got .46-sized engines coming out my ears right now (that's a sight to see eh?) so I don't actually *need* this engine and don't have a plane for it. I usually buy stuff to review and then sell it when the review is done (complete with warnings or praise as applicable) so that I can reinvest the money in more review products.

If Kange are certain that their SK engines are tested to a higher standard I'm more than happy to spend the time, fuel and video-tape to test that claim and perhaps get an idea of what a "good" SK50 runs like -- but in the meantime I can't in all honesty recommend them based on the experiences with this one.


Interestingly, my SK90 has *zero* pinch (and had none even when brand new) -- so where does this taper come into play?

If the piston/ring/bore interface is perfectly good when it's cold, then a tapered bore serves little purpose. I suspect they may have just used some legacy tooling (from an ABC version) an decided to leave the taper in the bore rather than spend money making a conventional (untapered) bore. Remember that these are a *very* low-cost engine and it's decisions like this that help keep the prices down.

And -- if the piston/ring/liner are a good fit when cold, what harm can possibly be done by running such an engine rich?

Since it's a brass liner and a hypereutectic aluminum piston, the clearances will only increase as it warms up so if it's not tight to start with it sure as heck isn't going to get any tighter even when run filthy rich -- is it?

OUTCAST

People should read all of Xjet's posts on SK engines epically in this thread, http://www.rcuniverse.com/forum/m_45..._1/key_/tm.htm
It makes his perspective clear.

Clarence Lee from RC Modeler wrote "It's and undeniable truth the way an engine is broken in has a direct effect on both performance and longevity."
And he forgot more than XJet will ever learn.

Xjet you were putting down these engines before you ever owned one, I don't know if the problem is the engine or you.
If I were an manufacture you are the very last person I would send a product to for a review.

You are not impartial, you are biased, you think you have all the answers, your right and the factory is wrong, you pretend to be much more than you are, and you spout paragraphs of BS to anyone who doesn't agree with you.

Oh and just as a point of interest we just sent back an OS91FX with a crank that broke on it's third flight, now I'm sure you would be spouting crap about why would anyone spend more than $200 on and engine that has a serious lack of Q.C. when you can get a TT for halve the price and it's even good in bed.

one more thing, just so you know I have a electro/mech engineering degrees from Purdue University "in the corn", { private joke for alum}.
I would like to know where you got yours?

I don't profess to know all about glow engines. It's not my field just my hobbie, and thing change too fast to keep up unless you do it every day.
I am however sure you don't either.

Turk1

Hi XJet, I totally agree with you on this point that I just checked (my very early Kangke offer SK 80 ringed engine which having max. 2 workhours) and no pinch too(but great compression).So its meaningless to warn about no 4 stroking.I wish Ray Nano would be still Kangke staff to debate on this.

OUTCAST

Turk
“Pinch" is an interference fit between the piston and the cylinder in non ringed engines. You can have taper without an interference fit, they do it on motorcycles all the time. If you take the plug out there is no “pinch" felt at TDC.
Also keep in mind that aluminum expands more than steel per BTU absorbed, that’s why a piston will seize in a cylinder if it overheats.

This is a direct quote from Clarence Lee, taken from the June 2002 RC Modeler, page 40 column 3.
As we have explained quite a few times in previous columns, the ABC and ABN pistons are fit tight at the top to allow for differential expansion rates between the piston and sleeve. At operating temperature the proper running fit is achieved. If the engine is run overly rich it does not come up to operating temperature and the fit remains tight. This more or less results in the piston being jammed up and yanked down, stressing the rod and wrist pin, especially when the engine is misfiring and running ragged in a 4-cycle. This, in turn, can lead to a broken rod or crank pin.

Turk1

Hi OUTCAST, SK ringed engines have all chromed brass liners.If there is no pinch then no risk to squeeze and damage piston and /or connecting rod.So I can make it run cool because no need to straight taper in order to release of piston.This makes meaningless that break in instructions of ringed SK s not to run 4 stroking..

OUTCAST

I don't know the answer to that Turk.
I do know that pistons expand more than the cylinder with heat and if you over heat an engine the piston will seize in the cylinder until the engine cools.
I also remember from my old race car days that piston to wall clearance gets smaller at engines heat up, the forged pistons in my Nova would rattle until they got warm.
Maybe the piston gets hot but the cylinder stays cool and the interference starts then, not when it's cold.
Again I'm only guessing, this is not my area of expertise.

spad4mebaby

Turk1 I would like to propose that an Sk 50 has a solid aluminum piston moving up and down in the crome plated brass liner. On the top of this setup is a small controlled explosion. Which heats up the top of the crome plated brass liner, and the alumilmum piston. Aluminum Brass and Chrome is supposed to be the best combination of materials for an air cooled alchocol burning engine. They work as one unit.

The pinch in a Sk50 is solid metal to metal which needs the uneven-heating of the top of the cylinder to get round. The pinch is done on purpose to make the engine last longer,make power seal better.

On a ringed engine the piston is a much looser fit. NOT touching the cylinder wall. But relying on the piston ring to seal. which should be the major thing touching the liner.
The pinch of an SK80 means the ring is loose on the bottom of the stroke and tight on the top of the stroke.
You may feel less resistance when turning the ringed engine over. But the piston ring is being compressed and loosened with every revolution.

Not following the break in instructions will probably shorten the life of the rings.

Charlie P.

I'm going to give it one last try. I dug out my SK50, tossed the carb in a parts can, miked the intake opening and ordered a Perry Carb from Bob Boumstein. May be tossing good money after bad, but I have had excellent results with a Perry on a TT Pro-46 I snapped the needle sleeve casting off of on the original carb during a dirt nap and hopefully this will also redeem the SK50. It still has lots of compression (yes, I broke it in following the instructions - always do with ANY engine), and these aren't rocket science engines. Air + Fuel + Compression = Bzzzzzzzzz.

spad4mebaby

No warranty?? How much is the carb?

Use a new glo plug. Let it heat up outside of the engine where you can see it glo red.
Let it cool.

Install then prime , apply glo power try to start.

I switched from an os8 to an a3 all of my stuttering stopped. I use Omega with extra oil added.

Blow out the pressure tap on the muffler too.
I use a home made bladder tank with a windex one way valve for fuel flow at any angle . Like a jett bladder tank but much cheaper.
But I fly in 115 degree temps in the summer.

From now until July is the sweet time for outdoor activities for me. No Snow!!

Charlie P.

Warranty probably expired when I used JB Weld to re-attach the throttle arm that had pulled off. By then and dozens of dead sticks I was just flat disgusted and wrote it off as a bad experiment at the labs of Hard Knocks U. and it went back into the box and into the "Cabinet of Death" in the back of my workshop where salvaged parts from obliterated aircraft wait for rebirth or just rust away. I just happen to have my checkbook and the purchase date was 5/04/2005. I paid $49.77 for it new and was not surprised, so I never pursued the warranty.

On a happier note. The S/K-50 Fun Fly model itself I consider a howling success.

The Perry Carbs from Bob run from $25 to $50 depending. The SK 50 size (0.510") he just charged me $29 plus $3 S&H from Omaha. I understand the price on the latest models is higher. Progress. Great guy: Bob Boumstein [[email protected]] He deals in used engines and parts and has quite a collection.

They're weird little carbs. The low idle is a dial that is near impossible to move, and that's good since you make 0.001" steps in tuning it. No kidding. (Using a narrow tip screwdriver in the slot is easiest). The High-speed needle you back out just one turn. Once you get the high end set and the motor is warm you tip it nose down. If it stalls lean the low end. If it doesn't after 30 seconds, you tip it up. (Blip the engine in between to keep the fuel moving). If it stalls you richen it. When this process is complete it will low idle for minutes at a time! Takes a tankfull of futzing to get it set, but from then on it's just back off the needle 1/8 turn, start, lean it up and back off a tiny bit to prevent an over-lean run and fly. I have not had to make a low-end adjustment in two years flying four-seasons in 15º to 95º temps. Just amazing.

XJet

Go back to your textbooks and compare the COTE of brass with the COTE of hypereutectic aluminum and you'll see that in an ABC engine (ringed or not) the piston will never expand more than the cylinder.

The thermal characteristics of metals is a very important part of *my* work :-)

XJet

Well let's see -- my day job involves designing and building rather powerful jet engines for use on UAVs and that requires an in-depth understanding of fluid dynamics, metalurgy, thermodynamics, engineering, manufacturing processes, electrochemistry and most of the disclplines that are also intrinsic to the field of piston-engines.

Unlike many here, I actually *make* engines and I also test various engines (sometimes to destruction) for use in unmanned aircraft systems that are often worth six-figure sums.

They don't let me do this without being *very* sure of my abilities, knowledge and objectivity :-)

Well since I *do* do this stuff every day (including evaluating glow engines for mini-UAV uses) I guess you're wrong in this instance.

I'm not about to get into a pee'ing contest but please don't underestimate my qualifications or experience in these fields.

OUTCAST

For those of you who are not familiar with the term hypereutectic allow me to explain.
This point of saturation in aluminum is known as the eutectic and occurs when the silicon level reaches 12%. Aluminum with silicon levels below 12% are known as hypo-eutectic (the silicon is dissolved into the aluminum matrix). Aluminum with silicon levels above 12% are known as hypereutectic (aluminum with 16% silicon has 12% dissolved silicon and 4% shows up as primary silicon crystals). Silicon levels over 20% cause pocketing of the silicon crystals, significant reducing strength, there for most hyper-eutectic alum in our field would be around 14-15%.


It is the primary silicon that gives the hyper-eutectic it’s thermal and wear characteristics. The primary silicon acts as small insulators keeping more of the heat in the combustion chamber and reduces heat transfer, thus allowing the rest of the piston to run cooler. The technically pure aluminum has coefficient of thermal expansion 22-24x10-6 mm/mmK, but 20% Si alloying decrease it into 16-18x10-6 mm/mmK. In other words it expands less per BTU absorbed.

To make this simpler to understand hypereutectic aluminum has an expansion rate of approx 12.5 {micro/inch [F-deg] }
Brass has an expansion rate of approx 11.2 {micro/inch [F-deg] }.

In internal combustion engines the piston and cylinder do not operate at the same temp. While the cylinder is cooled by air or water, the piston is only cooled by the flow of what ever substance flushes the bottom of the head. It is not uncommon for automotive pistons to rum at temps approaching 500-deg F, we however use meth for fuel and our pistons run probably 100 or so degrees cooler {that’s a guess but probably fairly accurate}. Our cyl temps rarely exceed 250 deg F.

So as XJet professed the expansion rate between Brass and Alum per BTU absorbed is fairly close, but do to the difference in temps the total expansion difference is significant.

Fastsky

Charlie P. , from your description the weird little Perry carbs do sound like they work well but no better than any of my OS, ASP, or TH carbs. Once I decided to take the time to set the low end properly to where it should be for each engine which usually takes around a tank, maybe a bit more then the engine will idle very smooth for 2 minutes or 5 if I want. Then when I hit the throttle, the reponse is instantaneous with no hesitation going to full rpm. Therefore I am happy with my original carbs and won't switch to Perry. I will keep them in mind however based on your review in case I try a different brand of engine that is finicky to run with the stock carb.

Charlie P.

Can't argue that. If my SK came with one of those carbs it might be OK, too. ;-) (What's a "TH"? "Tower Hobbies"?) The reason I tried the Perry is that it cost half what a replacement carb for the Thunder Tiger would have (Something like $65 for the carb on a $90 engine?!?). A replacement piston and sleeve for the TT-Pro 46 would cost MORE than a whole new engine from Tower; I'm serious. I costed on out at the LHS that has TT parts. (I sent the TT back when it was two years old and they replaced the sleeve and piston under warranty even though I told them it was crash damaged). Just after I got it back I managed to shear off a chunk of the casting that holds the needle-valve, and that part (the barrel/housing) isn't available seperately.

The carb on my SuperTigre was the futziest one I've ever had. Ran beautifully once adjusted but required adjustment as the weather changed and had a narrow range where it "grooved". That, and the air-bleed models OS made/makes could get the 'ol blood-pressure up; though they would usually run . . . just not at their best.

Fastsky

Yep, I am calling the Tower Hobbies TH for short. I have one 60 and three 75 sizes. and am pleased with all of them. On the Super Tiger carbs, hate em, and whats with the poky thing hanging out the side?? IS that supposed to be an extension that you clear a cowl with and then bend?? [8D]

Charlie P.

LOL. Darned if I know. The butt-end of the needle, I suppose. I was afraid to bend it for fear I'd snap it or otherwise screw up the needle. I stuck some tape on mine to make a little flag so I could see the adjustments easier.

And speaking of Tower Hobbies . . . I've worked for a couple manufacturing firms in the past and at both we made product that a reseller would distribute under their name. I wonder how many engine brands are all produced under the same roofs? I don't picture a big "Tower Hobbies" factory in some foreign land. I'd bet $1 that Kangke (a NY Company associated with Phil Kraft of the Das Ugly Stik and Kraft radios fame), Magnum, GMS, Tower, SuperTigre, and some of the really off brands like Mark are farmed out to various manufacturers under contract. Probably all made in a Norinko plant. That would explain why some examples within a single size are gems or lemons in the hands of different fliers. "Tool up for another run of Kangke 50's when those Tower 61's are finished."

XJet

And this is why 2-stroke engines are different.

The underside of the piston (and the areas exposed to the transfer ports) are bathed in air/fuel which absorbs a *huge* amount of the piston's heat by way of the latent heat of vaporization.

This means that (unlike a conventional auto-engine) the pistons actually run rather cool and the cylinders run somewhat hotter. This is exacerbated by the fact that the brass liner isn't a particularly good conductor of heat and there are thermal losses at the interface with the crankcase casting from which the cylinder cooling fins are formed.

This is why ABC engines lose their pinch when really hot -- the cylinder *does* expand faster than the piston.

So, if you have a piston that's already an "easy" fit in the cylinder -- running it really rich will do it absolutely no harm at all and it won't cause any additional wear or stress on the conrod/crankpin as Kange suggest with the SK90.

Not so -- for the reasons I've quoted. If your assertion was correct, the pinch in an ABC engine would get tighter as it got hot, not less.

Think about it.

As you've said -- this isn't your area of expertise -- but it is mine.

OUTCAST

Are you kidding me?
Are you trying to tell us that the cylinder, covered with cooling fins, with 50 times the cooling surface area, pummeled by 80-100 mile per hour wind from the prop blast, runs hotter than the slug that goes up and down inside it, that’s bathed in fire, because of the minuscule amount of air that manages to get between the piston head and the wrist pin?
And this is your field of expertise?

asmund

I`m not exactly sure why and how, but when running an ABC engine too hot it looses compression and pinch because the sleeve expands more than the piston, and the engine looses alot of power. Once it cools down everything is back to normal again. This happends sometimes when we race our nitro cars hard (too hard that is) I have also managed to do this with some of my aero engines. The engine turns over very easy when this occurs, so it is not the piston that expands the most and seizes to the sleeve because then it would be real hard to turn it over. People denying this obviously have never ran an engine too hot and therefore can`t understand/believe this