The non-issue (to me) of engine RPM's

RknRusty wrote:

PV Pilot wrote:Because he didn't eat his Wheaties.

I spewed coffee out of my nose. Didn't see that one coming.

Rusty: Actually I expected it; consider the source. (j/k)

BTW, deep fried turkey is pretty good if you do it right. I have heard war stories about folks dropping frozen turkeys in hot grease...a definite no no.

For our engines, it is important to run a prop that both loads the engine to take advantage of the available torque at near peak RPM. We do this by varying diameter and pitch. We need to consider the airframe to pick the prop.

George

Congrats to Geo! You nailed as well a I could have.

I only hope that everyone caught and understood your comment.

SuperDave wrote:...I only hope that everyone caught and understood your comment.

OK Here's some additional thoughts...

When you run on the lower end of the torque curve, perhaps by using a large diameter prop, you are at a lower RPM, which lowers the venturi velocity, with the end result being decreased fuel draw. This becomes important if you are using an external tank. That is why older diesels, which were intended to swing a club, have smaller venturis.

When you run at the higher end of the torque curve, eg. smaller prop, unless you go to extremes and exceed the design parameters of the particular engine, the engine will run fine. Bearings, such as the conrod lower end and the crankshaft have a characteristic that the higher the RPM, the more pressure excerted by the lube to keep the rotating part centered in the bearing (less wear). That is one of the reasons why insufficient lube wears out an engine quickly.

The prop - should be chosen for the airframe in question, then an engine that will handle it. Of course in competition, you need to follow strict guidelines. For something like a scale Piper Cub, however, a small prop doing a gazillion RPM just doesn't seem appropriate, even if it supplies the needed power.

I realize that many already know this stuff, but it might save a new guy's engine.

Your thoughts?

George

George:

When I initiated this thread I hoped that this very same information would be drawn out. There are no strict guidelines due to the many veriables: engine, airframe, weight, prop diameter and pitch.......

In essence experimentation is required to achieve maximum performance which often means compromising one factor against the others and so it is with life itself. It's illusionary to think one can have the best of everything.

Nice job.

SuperDave wrote:

RknRusty wrote:

PV Pilot wrote:Because he didn't eat his Wheaties.

I spewed coffee out of my nose. Didn't see that one coming.

Rusty: Actually I expected it; consider the source. (j/k)

Thank you!,, I get caught the the Superdave question tornado with no one throwing me a lifeline and this is how I get repayed??. Big thanks to all!!

PV Pilot wrote:

I get caught the the Superdave question tornado with no one throwing me a lifeline and this is how I get repayed??. Big thanks to all!!

Oh come on PV do I not cause exercise of the ol' gray matter (brain) with my questions? Consider me a catalyist that promotes a reaction.

SuperDave wrote:

PV Pilot wrote:

I get caught the the Superdave question tornado with no one throwing me a lifeline and this is how I get repayed??. Big thanks to all!!

Oh come on PV do I not cause exercise of the ol' gray matter (brain) with my questions? Consider me a catalyist that promotes a reaction.

Admin wrote:I think the closest thing you will find to a transmission and clutch is the RC Jerobee McLaren MK 8B with the Cox engine and the Sea Bee boat. http://www.mh-aerotools.de/airfoils/cox_models.htm

The 1989ish Cox GTP nissan car that I ran and still have has a centri clutch. Has the dragonfly style throttle ring and would idle nice without movement once adjusted properly. I ran several gallons thru that car and raced it a few times with the 1/12 scale electrics. Single gear ratio and open differential. I seen one sell on the bay for around $250 a few years back.

PV Pilot wrote:

Admin wrote:I think the closest thing you will find to a transmission and clutch is the RC Jerobee McLaren MK 8B with the Cox engine and the Sea Bee boat. http://www.mh-aerotools.de/airfoils/cox_models.htm

The 1989ish Cox GTP nissan car that I ran and still have has a centri clutch. Has the dragonfly style throttle ring and would idle nice without movement once adjusted properly. I ran several gallons thru that car and raced it a few times with the 1/12 scale electrics. Single gear ratio and open differential. I seen one sell on the bay for around $250 a few years back.

That came with a Sanya radio right?
What kind of speed do you get? I always thought a reedie .049 was too small for a 1/12 scale rc car. I have a .050 frr engine in my 1/16 buggy.
I feel it would do better with a .074 or something I was thinking maybe a Norvel .074 heli motor.

nitroairplane wrote:

PV Pilot wrote:

Admin wrote:I think the closest thing you will find to a transmission and clutch is the RC Jerobee McLaren MK 8B with the Cox engine and the Sea Bee boat. http://www.mh-aerotools.de/airfoils/cox_models.htm

The 1989ish Cox GTP nissan car that I ran and still have has a centri clutch. Has the dragonfly style throttle ring and would idle nice without movement once adjusted properly. I ran several gallons thru that car and raced it a few times with the 1/12 scale electrics. Single gear ratio and open differential. I seen one sell on the bay for around $250 a few years back.

That came with a Sanya radio right?
What kind of speed do you get? I always thought a reedie .049 was too small for a 1/12 scale rc car. I have a .050 frr engine in my 1/16 buggy.
I feel it would do better with a .074 or something I was thinking maybe a Norvel .074 heli motor.

Mine did not come with the sanya radio. I bought the ARR version, and then futaba micro reciever and micro servos for my radio and cut my own control wheels for the servos. I never speed trapped it and the AMB lamp counter systems were not readily availible at that time, it was all manual lap clicker counters and go until the timer stopped when racing. It was pretty fast, as a vauge description. Not very fast out of the hole, but it would sing pretty good down the long straights. The electrics still had me by far, even back then,but it was fun to hear it sing by and see the smoke trail.

I never ran cars so I have a question that may seem stupid to you guys. I understand that some race cars run only about 9% lube while our airplane engines need ~20%. How can they do that? My first thoughts are because they are running ball bearings, or because of the high RPM. That could explain needs for the lower end, but not the cylinder.

George

gcb wrote:I never ran cars so I have a question that may seem stupid to you guys. I understand that some race cars run only about 9% lube while our airplane engines need ~20%. How can they do that?
George

An interesting question and one I've also wondered about. The engines run faster and I would think hotter since many are enclosed in car bodies, yet car fuels are often 20/10 and 30/10 blends.

A significant amount of fuel running thru a offroad/onroad car engine,,big heat sink head and finned engine blocks to pull that heat off, other little engine tips and tricks maufacturers use to make them last.

Fuel lube packages are usually more aggresive with anti-foamers and stabilizers, ect.

Novarossi makes alot of engine parts for other engine manufactureres, so they dyno these parts to make sure they can handle the heat and stress.

I always ran a 30% nitro 12% oil blend fuel when I raced 1/8 scale offroad, and it worked the best for me. Some guy's would get down to 7% oil, but that makes your tuning window very narrow, but allows for the engine to gargle less oil as it runs,,again freeing up rpms. As with all my engines, I would replace the steel main bearings with ceramics, to free up the rpms and take some of that load stress out of the motor, helped with the fuel economy as a by-product as well.

The cox GTP nissan head was unfinned and then had a big clamp on heat sink to pull that heat off.