downunder

I know you don't agree Dar because you've been saying the same thing for years. But how many engines in trainers overheat simply because they're using a 4" pitch prop? You mentioned 3D models but I've never heard of anyone complaining about overheating with their less than 4" pitch. How fast does a model fly with a 4" pitch? Considerably slower than the airspeed coming back from the prop. Not only that but we're talking about running in where the richer mixture keeps the temp down anyway.

Of course the "big names" say to drop an inch off diameter because that's an obvious way to decrease the load. The ultimate goal is to decrease load, it's got nothing to do with cooling.

Rudeboy

G'day mate,

I don't need a book. I just know that nearly all two stroke glow engines in the .40 to .90 range on the market today make their power in the 14000 to 16000 rpm range (check your book). So that's what I prop for most of the time... there are exceptions (depends on the plane), but breaking in a new engine on the bench is not one of them.

I'm with Downunder on this one... it's pretty hard to cook an engine by underpropping it. 4" of pitch at nearly 15000 rpm will give that engine plenty of air.

Then again, a 10x6 is a very good bench mark prop for the .40 to .55 range of engines.

XJet

That's a great theory, but the efficiency of a prop drops really quickly as the RPMs go up. It's all to do with the drag increasing at the square of the speed increase. Double the RPMs and four times as much power goes into just overcoming the prop's own drag -- not into creating thrust. Things get much worse as the tips get close to the speed of sound.

For this reason, it's often better to prop the engine so that it turns somewhat slower than its max-HP RPMs, what little you lose by slipping down the power curve you gain through reduced prop-losses.

The smallest prop I run on a ball-raced 40-50 sized engine is 10.5"x6. A 10x6 usually spins so fast that it's not as efficient as the slightly large diameter version.

Even on trainers, I'm running a 12x4 which (my TT46Pro) spins at around 13,200 and provides unlimited vertical as well as plenty of stright-line speed (unloading to about 14,000 in the air -- giving a speed of about 50mph.

I prop my TT61GP with a 12x6 and it pulls around 12,300 on the ground. My ST90 gets a 13x6 and it manages around 12,000 on FAI fuel.

DarZeelon

Bruce,

True!


This RPM on a 12x4 APC is 1.183 HP... Beyond what an OS.46AX has been proven to put out with a 10x6...
What are you feeding your TT?

Could these .46ish engines be putting out more, or even much more HP, lower in the RPM band, instead of at 16,000 RPM?

proptop

My Feedback: (8)

The APC 12 X 4 is an awesome prop...I use it on all of my .45-.50 size 2 strokes. (except the ones I want speed out of that is )
W/ a Tower muffler and on 15% fuel, I have seen 14,000- 14,200 R.P.M. here at 500' above sea level on a cool fall day ( temp in the 50's ) with both a TT .46 Pro and an O.S. .46FX.


Question: Will the engine produce more torque if it has something to work against?
I mean, by propping up, and making the engine work a bit harder, won't the BMEP be increased, all else being equal? That would change the shape of the torque curve, wouldn't it?

By propping down (smaller ) the engine will be singing up near the H.P. peak, but WAY beyond the torque peak, right?

Not sure where I'm going with this...it's late... (just thinking out loud here...)

DarZeelon

Brian,


I insist it does have everything to do with cooling.

Of course, break-in objectives for ABx engines dictate that a low load prop is used.
But then, there are alternative to how this is obtained; by a smaller diameter, a smaller pitch, or a little of both.

It is much more difficult to start a new, 'pinchy' engine with a small diameter prop.
The decreased inertia of that small prop may lessen the chance that the engine will go over TDC after firing.

Yet the 'big names' all state to use a 1" smaller diameter prop; even a chopped down, larger prop.
And never has any of them suggested to use a high inertia 12x3... or any other low pitch prop.


Have you ever asked why? I have and it is all in the cooling effect.


And as to those 3D planes that suffer from over-heating in a hover; it is only because of the stagnant air-flow created by the low pitch props that they all use.

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Proptop,


Are you suggesting those .46 TT and OS engines can spin an 11x6 APC at 13,800-14,300 RPM?

This would take the same effort from the engine as spinning a 12x4 at 14-14.5K.


Try checking the numbers again, since the [link=http://www.rcuniverse.com/forum/m_4432418/tm.htm]OS.46AX manages[/link] about 12,300 with the 11x6. I know for sure the TT Pro is more powerful on this size.

XJet

It's fuel mixed using Cooper Fuels Plus C oil using just 5% nitro (Nitro is around $80 a gallon here so we don't use much). I gained over 600 RPMs changing from Morgans Cool Power castor/synth blended oil to Cooper Fuel's "Plus C". At first I thought I'd made a mistake but others in our club tried it in their engines and experienced exactly the same power increase.

My TTs (and other engines) now also run cooler and idle better. After quite a bit of running, then stripping down an engine, there's *no* perceptable wear and the top of the piston is actually *cleaner* than before I switched oils. I would not have believed the difference if I hadn't seen it for myself.

Brian (Cooper) says this is an "advanced oil" and I have to say it sure performs like no other oil I've tried. I'm a an old cynic from way-back and it takes a lot to impress me these days -- but I am impressed.

DarZeelon

Bruce,


I thought this may have been the case...
I am still trying to find someone to market Brian's oil here... Everyone is bent on Morgan's...

Harry Lagman

Not in comparisons I've done. The AX spins the same prop about 300-400 rpm more than a TT .46 Pro in the 12,000-13,000 band. The typical AX number is 12,300 thru 12,600 rpm with 5-10% nitro on the APC (not MA) 11x6.

The only place the TT might compete with the AX is at a higher rev band (say 15,000-16,000), but I have not compared them yet at this rpm level so I don't know. I know the TTs unload nicely in the air.

The AXs at our field outspin the TTs, the GMS .47, the ASP .46, the FX .46 and even the ST G51 engines when all are running APC 11x6 props. What does the MVVS .49 do - really do, rather than hearsay - with a non tuned quiet muffler?

DarZeelon

Grant,


Please read what people say in [link=http://www.rcuniverse.com/forum/m_2082094/tm.htm]this thread[/link], about the TT.46Pro. The results others are seeing may surprise you.

----------------------

As to the MVVS .49, It does not come with any muffler, but you may opt to buy it with a standard one (for trainers...), or even with the extra quiet A silencer.

It is designed, port-timing-wise, to be at its best with the tuned silencer and most buy it with it.

Because of this it is slightly more affected by the use of a normal muffler type.

Its performance with the normal muffler is comparable to the AX; better when props 11x6 and larger are selected.

Harry Lagman

It would be interesting to see what the .49 would do with its different pipe options with the APC 11x6 as a reference. I have been pretty impressed with the performance of a couple of piped MVVS .77s at our field.

DarZeelon

Grant,


We have gone way off subject (OS.55AX, remember?).

Let's see what we can do on another thread with the MVVS.

NM2K

My Feedback: (14)

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What was OS thinking when they brought out the .55AX? If their .46AX is clearly head and shoulders above all of the competition, I would have waited until someone else topped the .46AX and then introduced the .55AX.

The Webra .55 is probably the only engine that is challenging the .46AX's lead, if what you say is true, not that I doubt your word. However, not enough folks buy the Webra .55 to make it a serious economic contender for first place position in mid-sized sport engines. I would have waited.

The GMS .47, like the current Magnum .52, is tuned to spin up. Loading it down with a 3D prop makes it look bad, all QC issues notwithstanding.


Ed Cregger

proptop

My Feedback: (8)

I was thinking the same thing Ed...why a .55...maybe it will replace the over-weight .61FX instead?

Dar...I think the prop constants or whatever formula you're using is wrong in this instance? You can't compare the 12 X 4 with the much thicker bladed 11 X 6...they are 2 different designs.
The airfoil is much thicker out in the "meaty" part of the blade and especially at the tips putting a lot more load on the engine than the 12 X 4. The 12 X 4 reminds me of a beefier "E" prop or a larger version of their pylon props.

The tach #'s I got were recorded with 2 different tachs too FWIW, so...(?)

I have noticed that all my engines really crank out the power in the crisp (dense ) fall air

Anyway...going 'round Robin Hood's barn to get back to the topic...I am wondering if the amount of load placed on the engine and thus the resistance causes the BMEP to increase and if that could cause an increase in the temp / press. inside the chamber / cyl. and effect the rate of wear (pinch ) among other things?

The biggest prop I think I would feel comfortable running on the .55AX would be an 11 X 7 or 12 X 6 or maybe a 13 X 4W?
If the .55AX has the power delivery characteristics of the .46AX then it's one strong running engine. I would prop it to turn somewhere in-between the torque and H.P. peaks.

Fuelman

My Feedback: (6)

The original thread starter was worried about his piston/sleeve pinch.
One thing I noticed over the years in the FX engines is that the cold pinch seems less if all the head bolts are not torqued sufficiently or if a couple are lose. Maybe it is the same with the new engines as well. Worth a try.

fc911c

Yes you are correct when I pulled head off to check things, I installed it back and the pinch magically reappeared. I was wondering why that happened.

thanks
Frank

proptop

My Feedback: (8)

Distortion around the top of the cyl. case and sleeve Frank...
As you torque the head bolts it causes distortion around the top flange of the sleeve and the top of the case.
Some engines get more pinch as the bolts are tightened, and others loose some.

Just to mention MVVS one more time...my .49 has virtually no pinch after the head bolts are snugged down?
I have tried to just barely bring 'em down but when I tighten them just enough to keep the head from leaking, the pinch disappears?!?

DarZeelon

PropTop,


Your MVVS has a head that has an insert part.

This insert portion may be slightly tapered, opening the sleeve top as it is tightened, thus undoing the "pinch".


The OS.55AX head lies flat on the sleeve top.

fc911c

Makes sense, thanks for the tip.

Frank

proptop

My Feedback: (8)

Thanks Dar...I'll have to check that out!