NMP Velocity stack
 

Hello Troy,

I flew the 160DZ with the VS since flight 1, but I fell the VS is restricting the air so much that is lacking power, I got 7800RPM with APC 17x12 and CP 30% High Perf Heli.

Is there another one in the market?, which one do you use?

Regards and thanks

Alejandro

RE: NMP Velocity stack
 

I am running the NMP Velocity stack that you are using. It depends on the model you are flying. I can run a 160DZ with the stack in my Astral XXc at just a shade under 10lbs and its excellent performance perfect for what I'm doing. I use the stack


However on my Adventure Biplane using a 160DZ and the Velocity stack it needs the extra power on the top end. So when running a 160 in this model I run the engine without the stack. However with a 170 in either model the stack is a really good thing for both sound and smooth precise throttle control response.

My guess is that you 160 could be a little tired and needs a new ring or you model is likely a bit on the heavier side.

The YS engines have a very good throttle and torque curve. However like with most glow engines the last little amount of throttle barrel travel doesn't always yield an increase in rpm but does change in noise level. So what we started trying is putting a restrictor in the carb to eliminate this "dead spot" in the throttle travel, and reduce the noise. I had seen the Japanese pilots doing this and felt it may improve the torque curve. A friend and I ordered some of the stacks from Japan and the cost of about $45 USD each. By the time we paid the tax and got the shipping with some other little goodies sold in the Japanese market.

I did a bunch of testing with different diameters on the Velocity stacks. I tested 8mm which is the size of the original restrictor in the YS 140DZ. I then tested every size up to 10.5mm by 0.5mm changes. (8.0, 8.5, 9.0, 9.5, 10.0, and 10.5mm) I found the best performance-noise and effectiveness to be at the 9mm size. By the way the bore on the carb is 12.25mm

The 8.5mm to 9.5mm all performed about the same. You lose about 200rpm from full throttle wide open rpm. The 10.5mm version only lost about 100rpm and you could not hear any difference in the engine sound wise or feel any performance difference between with or without the 10.5mm. The 10.5mm was exactly the same performance, noise, and everything as if it was not there. As we restricted below the 8.5 and down to 8mm the result was the engine started losing more rpm (300rpm for the 8.0mm) from the top but really didn't improve the low end torque situation much more than the 8.5-9.5mm

So this is how we decided to have NMP produce the version you have. We found in several different configurations that the "sweet spot was between 8.5-9mm. This gave the best noise reduction, and throttle curve with the least reduction in top end rpms. Therefor the NMP velocity stack is at 9mm.

Yes it takes some power off the top end. However the engine still has its low end torque. If your model and situation doesn't work with the stack then I suggest you pull it out. Its not going to work in all applications. However a good model with excess power to burn benefits from velocity stack in the improved throttle response and noise reduction.

By the way you engine is down about 200-300rpm from what I am getting on my 160DZs with that same prop. this could be elevation related, air temps, time for a new piston ring? and many other things going into the rpm numbers you are getting. By the way 7900rpm on that prop is right on the noise limit for F3A. That is where I would run the engine for a competition. I actually reduce the throttle travel of the engine to get 7900rpm on the 170's with a similar prop. Wide open with the velocity stack my 170's will pull about 8300-8500 on that prop. So I dial down the ATV to get 7900 and this is where I fly the monoplane. The Bipe gets a different diameter prop and a slightly different setup. It needs an extra few RPM due to its extra drag. But I still run the velocity stack on the 170 in this model and reduce the top end rpm with ATV. For the 160DZ in the bipe I just remove the Velocity stack and run the same curves adjust the ATV to get same RPM as the 170 with the stack.

Both engines perform about the same in this configuration. You really have to play with props, and noise. Also the model is a big variable to see the performance changes when using a device like this to tune the setup. I have found the Velocity does the same thing regardless of 170 vs 160. The 160 just has a little less to give up at the top than the 170. And the prop selection will be a big determination on the performance of your model. The best full power rpm doesn't always work the best for throttle response and the noise limits.

The good thing about the NMP stack is it can be removed in about 30 secs. Its easy to do a bunch of testing back to back.

Troy Newman
Team YS

RE: NMP Velocity stack
 

Troy,

That was a very good explanation, thanks for being so specific.

My engine has only 8 flights plus one hour on the bench and I'm still running it rich, I will lean it on four more flights, the airplane is big, the CAmodel/CPLR Osmose, the weight is fine, just the fuselage are and increased drag is the thing, and at that altitude with the engine still running rich it;s the problem, my engine leaned out on the last flight and quitted, I guess it is getting better, the idle is much more stable, I think I'm going to rework one of the velocity stacks to 10mm and try it again.

What do you think about the 18.1 prop on the 160?, and I will removed the velocity stack, I'm used to the YS throttle curve anyway, I will give up on the benefits for the extra punch, I will measure it with my dB meter to see how it is doing anway.

Best regards and thanks for the advices

Alejandro

RE: NMP Velocity stack
 

The 18.1-10 not the 10W version I think is the best for the 160. I don't like the 10W on the 160.

I would sya the first thing your should do is get the engine runnign properly and performaning properly then add things like the velocity stack. This will tell you what it does and you can feel the difference.

I think it makes the YS throttle curve even better...but it does sacrifice a couple hundred RPM from the top.


Troy Newman
Team YS

RE: NMP Velocity stack
 

Hello Troy,

I flew my Osmose without the VS, and what a difference, but, I also flew this time at sea level, so I got unlimited vertical at about just a little above half power, what a beast!

The 18.1 was slowing the engine a lot and was overheating alittle so I landed after 5min flight, but temperature was 90F, so I guess that affected the engine a little, I could see the difference in flatenning the THR curve, but I preffer to have the extra power for know.

Thanks for all your help again!

Alejandro

RE: NMP Velocity stack
 

Hi Troy,

Could you possibly post a picture of the throttle curve you use for the DZ160/170. I realize it wont tell too much but it would be interesting to see what shape you have settled on.

Regards,

Peter

RE: NMP Velocity stack
 

If you set up your linkage so that the servo travel is matched top and bottom. My Travel adjust is 115% HI and 118% Low. I'm using the outside hole on the YS throttle arm. And I'm using a small servo wheel that gives full travel at the basic 115% ATV each side. I set my Idle trim since the JR has a mechanical trim on throttle to the mid notched position, and then I adjust the ATV up or down to the idle to the desired rpm for flying.

If the idle adjustment is off more than about 5-10% from the Hi throttle stick ATV I adjust the linkage to get symmetry there.

Now for the big secret of my throttle curve. Its a super secret and you can't tell anybody. But I just shot a photo of it.

It is attached below. I really don't think it was the magical inspiration you were looking for.

It comes up a few extra percent above idle to help with stall turns and small throttle corrections at idle. Say a 45 degree line into the wind is a little slow and loosing its angle a couple clicks of throttle stick helps maintain the line.

Then the line goes pretty much linear form there.

Each engine will be a little different. This is the curve on my 170DZ in the Astral XXc. Prop is a highly modified 18.1-10W that has been pitched, clipped and re-shaped. Basically the prop started as a 18.1-10W and now it is a own design using he 18.1-10W as a blank.

Velocity stack installed. WOT on CP 30% Heli is 8200-8300rpm.
Once I get the engine running properly and the ATV adjusted to give me WOT throttle and idle with nearly equal ATV % values...

I then dial the ATV back at Hi stick to get rpm of 7900rpm. Which is 90% on the High throttle side for this engine on this model.

The Engine now makes great HP and also passes F3A sound test with the prop I'm using. The full wide open system is 95-96db(A) so it will make sound on the AMA classes without any reduction in the RPM. But F3A rules of 94db(A) the 7900rpm is the magic number on my version of the prop.

I show up at a new flying field and I will use my tach to set the RPM right at the 7900 that I need. I fly all my sequences at this number. The model tends to perform about the same even with going up in elevation or down to sea level using this trick. Basically the curve stays pretty close to the same with changing conditions as well.

The curve below is magic for sure.....?

If I run the engine without the velocity stack the RPM number needs to be about 7700-7800 on the same prop to make the 94db(A) limit and the curve gets raised at the 3 points below in the photo. I would say without the stack these points each get increased by 6-10%. I fly it an tweak the curve to my needs.

RE: NMP Velocity stack
 

Thanks Troy.

I will keep your secret safe. We dont want the WWW to find out;)

Seriously, thanks for the effort in responding. I guess I was looking for a better throttle curve than I currently use. I believe there is a lot to be gained in this area of model setup. Throttle reponse and setting to suit the particular model will gain you more points in competition then say changing aileron differential by 1 to 2%.

Some days when I fly and the engine feels great in mid-range you can draw really nice radius's in your cuban's and immulman turns. When the engine is not running right and it effectively feels like an on-off switch it becomes very difficult to draw smooth lines.

I am currently playing with my own modified 18.1x10W cut down to 17.5x10W with the tips thinned down. I love the prop but I feel I need a little more pitch on those windy days. The front of the model is a little draggy so it might be fine with a cleaner airframe.