AndresAM

Mr Hanson,
This proves what you have been saying for a long time. I have a question, I just bought a ZDZ Super 80 (On sale now on Troy Built models for $675 ) and was wondering if a 25x10 was a better choice than the 26x10 (or maybe the new mejzlik 26x8) to get the best power out of the engine?. The reason for asking is that if a 26x10 is the right choice for a DA100 maybe a little less prop is better on the 80. Has anybody else tried a 25x10 on a ZDZ 80 (I don´t know if Mejzlik has a 25x10 size).
Thanks,
Andres

rmh

The non piped ZDZ80J using -say a in cowl muffler - will give best performance if you avoid overloading it - buy a 26x12 wood whtever -- cut it to 25" and smooth and sweep the tips - a good quiet setup
or
buy Mejzlic 26x10/8 -any prop that lets it easily run 6500 on the ground a tad rich
NEVER prop it for screamin in level flight - that is too lean -on any prop
fwiw - the 26x10 mejzlic is only about 25.5" long

I use the ZM 26x12 -and a header and JMB can - quiet great running setup - for super snortin --the Skorepa 26x10 or Mejzlic 26x10
On full pipe the reves go up to 7300 on the ground -set right for full power on up lines - this gives me 74--7500 in level flight -- with a well broken in engine

don't force it!
let it rev at least for a few gallons and by all means baffle it and keep it cool

amjflyer

In fact i was thinking about baffling the DA. At the moment the cowl is just open in the normal places at the front (inlets either side of the prop driver). And most of the bottom of the cowl is scooped open (about 5:1 ratio outlet to inlet for air through). However im not sure where i should put the baffles. Just behind the cylinder heads, and underneath? I would welcome and ideas. Ive been told extra cooling could get another 300rpm or so out of it.

rmh

you are on the right track-
but rather than look for 300 rpm improvement --think of it as NOT loosing 300 rpm
anyone who has run a bunch of engines can tell you taht a cool engine vs a hot engine - can loose up to darn near 1000rpm (failure mode) and 300 loss is common .
so
A :forget the big props that is all BULL S HIT if you want more power
B: rework the back edge of the cowl with a lip or lips to create spoilers
C: try to direcet every last bit of inlet air thru th fins --including wrapping the flow around the backside of the fins
Having done this - it is worth it - that is the hot spot on any twin
Some will state that "their DA don't need none of that there stuff"
BULL-again
all engines benifit from it and the work needed to induce suction thru the cowl is well worth it
remember - you can not blow air thru the cowl -it has to be a flow of high to low pressure .

amjflyer

If anyone has tried baffling a twin i would be very interested in install photos. Im going to give this one a try! Thx

sillyness

My Feedback: (25)

http://www.cactusaviation.com/Tech/baffling.html

amjflyer

brilliant thanks!

rmh

That setup is good in many cases but does not maximize baffling.
Ideally - the air is held close to the cylinder and is pulled thru due to low pressure outlet(s) in aft section of cowl
The wood is 1/64 ply - the lip on the bottm of cowl creates a turbulance to make low pressure area in cowl
the large top sheet blocks air from flowingover top inside cowl - ALL air has to go thru fins
the lower baffle is attached to cowl and sets just at lower cyl barrel level- and surrounds the end of the head
The low pressure area (the opening with the lip at the bottom) now must pul thru the fins - no other source of air
This is how full scale really works
On the latest YAK 55 by CF- there are large working louvers all around rear of cowl - using these plus a good shield up front and a couple of plates -- you should be able to work the crap out of your engine with no significant temp rise
temp rise = serious power LOSS

RTK

My Feedback: (1)

Now that is some serious baffling Dick, I might just have to copy that

mmattockx

One thing missing here is to move the air inlets out away from the spinner and the center of the prop. The air flow there is stagnant at best and often reversed and flowing OUT of the cowl opening. The way to do it is like the Lancair cowls, they have an inlet well spaced out from the center and they cool very well (200+hp from two very small openings). This tip came from John Roncz at the EAA Oskosh fly-in in '95, not my own observation. Air also seems to like sharp edged protruding nozzle like openings for recovering pressure from velocity, as done on many F1 cars and MotoGP race bikes. The Lancair's also do this part well.


Mark

rmh

Yes good idea -I did this on my YAK and alsso on my 33%edge the blue ring is an extension of the cowl
Many modelers do the YAK with the spinner right up against the cowl because they think it looks better
- bad idea-

amjflyer

Having looked at my cap inside the cowl the trick here is to manage to get the baffles in the very tight space and still be able to remove and replace the cowl without difficulty or breaking anything. I can see this is getting closer and closer to full size practice and actually it would be better if the cowl was split in half and the top was removable seperately rather than removable with the screws around the rear into the fuz. The commercial kit and arf manufacturers should take note and either build in some baffling or at least the provision to easily install baffling. After all, my jet turbine aircraft arf's come with bypasses and fittings for tail pipes ready installed which is almost the same thing.

mmattockx

Yes, it is getting to be like full scale cooling practices, because they are more efficient and a decent model to follow in general. I believe Dick gave you the best guide for how to implement the baffles, with the top and rear portions mounted to the airframe and the front/sides/bottom portions on the cowl. Is it a PITA to implement? Yes. Is it worth it? Absolutely. In a perfect world, the cylinders are tightly jacketed with sleeves tight over the fins and all air coming in the cooling openings is forced to travel through the jackets, past the fins. Dick's idea of a spoiler lip to create a decent low pressure zone for extraction is excellent, as well. Not a good idea for a full scale aircraft, as it adds drag, but on a seriously overpowered R/C aircraft the drag is way less important than the cooling.


Mark

vatechguy3

My Feedback: (11)

any thoughts on a biela 26x10 on a da-100 with ks cans?

thanks
tony

amjflyer

That should be a hot setup from what i have heard. 7k rpm +. HOwever i have also heard that the KS canisters have a tendency to 'burn out', but that was on a 200cc setup not a DA100. Could also be becuase the guy was running them too lean without sufficent pipe cooling.

amjflyer

Ok, posting a follow up to this issue as i have learned some things and made some progress and it may be useful to anyone else out there.

When I started the thread static rpm was about 4650 on a MZ 28x10, in air performance seemed only slightly better im guessing 4800 or so, very poor. The ignition was foud to be faulty and this also exhibited problems with one cylinder running only on initial start.

New ignition installed (engine also a little more run in by 3 gallons) - new static rpm check 5000rpm, so a 350rpm improvement straight off.

Changed prop to Mejlik 27x10 - new static rpm check 5500rpm, ok so reduce prop dia. you would expect some improvement, but also throttling seemed to improve as well.

Added engine baffling (as per Dick Hanson recommendations etc) - static rpm on the 27x10 still 5500, hmmm, ok so maybe baffling only makes a difference in the air after all it is about oncoming airflow. So fly it and see...Wow!!! in the air it seems like its gained 300-500rpm alot better, really, alot.

Finally, changed fuel to high octane petrol (super unleaded Shell Optimax in the UK) and added high octane additive (Millers Oil lead replacement and octane booster) - wow again, static rpm 5800, in the air better still.


So after all this I now have a model that I can do some really challenging aerobatics with at last! Ive concluded that i can only better this by changing the well baffled custom silencer ive got, but that would mess up a nice model so i willl live with it where it is. I might try to open up the baffles inside the custom muffler but that may not be possible, it certainly wont be easy.

Tired Old Man

My Feedback: (1)

Keep a close eye on the plugs. The lead additive will probably foul them fairly quickly.

AHPowelson

My Feedback: (37)

Have you ever checked or played with the timing? Maybe it's too far advanced? A significant RPM increase just from raising the octane level is normally (in my experience) almost contradictory.

amjflyer

Change timing by just moving the sensor up and down?

If i have the theory right Octane level should increase rpm since it increases energy in the detonation but does not affect the timing as this is electronically controlled. ie more bang more torque + rpm on same prop. Its not the same as increasing nitro in glow engines which on two strokes invariably does nothing or kills rpm as it changes the timing (since there is no timed ignition on a glow). However on glow four strokes it does invariably increase power but not necessarily rpm (more likely torque, so swings bigger prop). But again engines have to be designed for it as it advances timing, hence why YS / Saito run so well with high nitro.

Actually both the fuel and additive manufacturers claim it also keeps the plugs clean, we shall see...

RTK

My Feedback: (1)

Actually higher octane gas contains less BTU's (energy) then lower octane gas. To take advantage and see an increase in power with high test you need to increase compression and/or adjust timing.
Terry at CH ignition ran some tests and found no improvement with it and the engines ran a little hotter with a stock set-up.
I could see using it if you are running a tuned exhaust, but that would be the only time.

amjflyer

Interesting, well im definitely getting a 300rpm boost with it. Must be something about my setup.

sillyness

My Feedback: (25)

Yeah... your timing is likely advanced too far. The higher octane burns slower... less power robbing force generated before your piston is passing top-dead-center. The faster burning low-octane gas was generating more force that was trying to push the piston backwards prior to TDC. Retard your timing and go back to regular gas. I talked to DA the other day regarding contradictions in my 2 DA-100 manuals. As it turns out they no longer recommend hogh ocate gas... low to mid only. The high ocatane was doing strange things to the pistons over extended periods of time.

Regardless... you should be getting mid 6k with that prop.

shakes268

Straight out of the box without touching the needles mine was spinning a PT Models 27x10 at 5993. Haven't tach'd it yet since I leaned it up a little on the low end but I am positive its over 6k now. If you're not getting even 5900 I agree, retard the spark and go back to lower octane gas.

amjflyer

Im trying to do this scientifically one thing at a time. So next thing will be to either a) try a different exhaust sys (if i can loan one from somewhere) or b) send exhaust to specialists to be 'opened up a bit', as DA think the muffler might be over baffled. After doing that if rpm is still too low i will twiddle with timing...thanks for the feedback.

sillyness

My Feedback: (25)

Geez... I think I'd do the cheaper option first (timing).

Actually, I'd send it in to DA for a free check-up.

Anyone else?

(BTW... I'm getting 6,500 RPM on stock exhuast with a BollyWood 27x10, 13 gallons through motor... tached after flight. It's not tuned lean at all... nice plug color, no sagging, pulls like stink!)