Question for the Nelson Guru's
 

I posted this in the pylon forum as well but it doesn't seem to be very highly watched these days so I'll post here in hopes that somebody here can offer some insight.


So recently I picked up a couple of Q40's and a quickie, all with nelson short stroken engines.
I'm not new to Nelson's, but in all fairness, I have never raced with them so never worried about a perfectly tuned engine. As such, in the past I never used a Tach on them.
Well, I burnt a lot of plugs in the past, wrecked a couple engines, and/or just plain had crappy runs so.....
I bought a Tach for use with these engines.
Now the Q500 engine is pretty consistent once needled, it generally peaks around 19,000 - 19,400. I back it off about 400 before launch and go.

The QM40 engine I'm running right now on the other hand, depending on the day will peak anywhere from about 21,900 - 23,500. I'll back it off around 500 and then go.

My question is, does air temp and humidity make that big of a difference on those engines to see a 1600rpm difference between runs? I'm using the APC carbon QM40 props (7.5x7.4 I believe), using 15% nitro with a castor synthetic blend, I add a little oil to give me around 19% oil. tanks are tetra bladder tanks etc.

I don't need every last RPM as I'm just flying them for fun, but I like to know why the peak would be so different. I want to make sure I don't lean the engine out to far and damage anything.


Another quick question. The old Nelson HD plugs with the 3/8" hex, I seem to get between 10 and 20 runs before the plug performance is degraded. I recently bought the Merlin Nelson plugs from Jett and peaking the engine the same as I did with the HD plugs, I'm only getting 1 or 2 runs before the plug is burnt out completely so I'm running the engine richer just to try to keep the plug life better. What am I doing wrong? I've never used these plugs before.

Thanks in advance
Jeremy

Weather can have a huge impact on these engines, as does altitude. Since you're not racing, I suppose you always fly at the same site? That simplifies things. Get a 1cc syringe to measure the head volume. This way you can play with liner shims, while still holding the same head volume.

Stick with the Nelson HD's, they are very high quality. I'm still using a plug in one engine that I started using at the last race of last season. If you back it off rich enough, you shouldn't be blowing plugs, they last very long if you stay on the rich side of peak. Don't peak the engine before launch, work you way up to a good setting and remember where the needle was set for the next flight. The engine should have the nice rich cracle to it in flight. It won't cost you any speed, but it will save plugs. Never mind if it seems to drop a stage down on the pipe when on the ground, it will pipe up as soon as you launch. You can also briefly close the needle a bit to get the engine staged on the pipe, then open it back up and launch.

Jeremy, When you speak of the Nelson heavy duty plug are you referring to the flat coil Glo Bee style, or the traditional spring coil "peep eye" plug that has a scribe line cut in the wrench hex? There are several different plugs sold by Henry Nelson, Mike Langlois, and Dub Jett. There was the original Glo Bee that used a separate collet: later there was the single piece Glo Bee: there was a single piece with a standard element: and one with the heavy duty element. The Merlin Glo Bee style plugs are nice, but in my experience don't last as long as the original Glo Bee's. On the occasions that I sport fly my old short strokes, I'll run the heavy duty "peep eye" plugs. As Rudeboy said, try not to peak the engine on the ground before launch, especially if it's an AAC setup. Something happens when they get hot on the ground, that keeps them from making full horse power in the air. Some of the Q-40 racers hold a finger over the venturi to keep the engine rich on the ground before the flag drops. That way they don't have to needle the engine up just before launch.

The short answer is to get in touch with Performance Specialties to place an order for props, plugs, a spare head, spare needle, etc....and ask Dave to steer you in the right direction.
In general, use the least amount of nitro, compression and propeller load that gives you what you want.
The HD Nelson head is wound with heavier wire than the standard.
If you aren't starting out your testing under compressed, under propped and with minimal nitro...then there's no point in talking about what it takes to find the correct combo on your own [with out having Dave around].
Guys with model diesel engine experience must learn this basic set of rules and high performance glow engines follow the same rules if you want any sort of longevity and repeatable success.

I don't think Dave still sells anything for the Nelsons, it's all handled by Mike Langlois at aero precision machine now.
Also, I don't think you can still get stuff for the SS engines (I'm not 100% on this though, I just don't have SS engines).
Standard Duty plugs are no longer available, only HD's.

Make a test prop by cutting down and balancing a 7.4" Q40 prop to about 6.25" diameter. This will simulate the prop load in flight. Use this prop as a benchmark prop for all testing.

If you really blow too many plugs (and not because you are just running the engine lean), raise the head some (if you have shims that is). Don't overdo it, since you will also be blowing plugs if you raise it too much (because the engine becomes difficult to needle, and you will likely end up too lean.

And again, a lean run will blow your plug regardless of where the head is set at. So first try a couple of flights with a nice rich mixture, and see how a new HD plug looks after that.

Also, 500 rpm rich of peak sounds like too little. I don't use a tach for setting the needle, but I'm sure I'm setting them way richer than 500 rpm rich of peak.

I think Gary Freeman still has some short stroke stuff available if you're interested.

that prop is to small of a load in flight for that engine. I run the 7.4x7.8 to 7.4x8.0 on my short strokes. The long strokes like the 7.4x7.5 and 7.4x7.6.

since you are running that light load it will unload more in the air. get some 7.8 props, run 15% nitro. I dont set by a tach but get them close to a good sounding peak and then back off slowly till hear the rpm drop ever so slightly. it seems to me with the 7.8 to 8.0 props that it like to be very close to peak on a Q40 model. the LS need to have a larger rpm drop. The LS with stock timing runs good on a 7.6 prop but can goto .005 on sleeve and .008 more on the head and rune the 7.5 prop for more rpm. this should bring timing to 196 or so.

Thanks for the input guys. The airplanes came from a well known pylon racer here and I'm basically just using the same set up as he was. I spoke with Darrol Cady and he is sending me some of the green Merlin plugs (similar to the Nelson hd plugs). The last time I ran nelsons was when I lived in alberta and was at 3400'asl. Out here at sea level I find is a much different game. I'm pretty sure the biggest issue I have had is just not backing off the needle far enough for the amount they unload in the heavier air.
Thank you
Jeremy

now one runs the 7.5x7.4 props. You really should try the 7.4x7.8 - 7.4x8.0 on the engine.

you should check the deck clearance as well as it shrinks as the engine ages...and yes 500 is too little more like 800 rpm to be safe...adding shims is trivial and quick..thenwatch the engine unload better and plugs last longer....

I just measured a new LS Q40 engine: timing was 197.3° from last light to last light...

what shims are in it rude?

0.008 and 0.003 under the sleeve

yeah thats stock. I will ask Richard again tomorrow but from what I heard stock is 193 or 193. doing the light method does add a little bit as I have done them that way when in low light and can see light pretty early lol

Airraptors post was dead on.. Nelson HD and Merlin Green (a cold plug) should both run fine, but you are underpropping the SS a little bit..I would go to at least a 7.6 pitch, if not a 7.8.. Good luck.

Absolutely. More than air, temp and "relative humidity", water grains in the air do way more damage to performance.
If anyone is still looking at this thread.

When playing with competition engines of this caliber - QM40 & Q500
One needs to understand how the volumetric efficiency is affected by altitude , temperature , humidity & fuel ratio

I'm no rc expert, but I race drag boats, with blown alcohol engines.... air (quality, meaning temp, baro pressure, water grains, etc.)makes a ton of difference... unbelievable...