Choosing the Plane-Engine-Props Combination?

[Kafin Noe’man]

Hi all,

For the past few days, I've been reading and learning a lot about plane-engine-props combination.
So, here's I'm trying to map what I've learned so far, and hoping this will be my guidelines in the future.

*I'm using Brodak's chart as the starting point.

This process of re-writing what I've learned help me to understand and remember better, hopefully it's already in the right direction, if it's not, please let me know where I got it wrong and what needs to corrected.

Looking forward to hearing and learning more from you guys!


Best,
Kafin

[Dave Hull]

Kafin,

Pretty good so far. Depends on how much detail you want to incorporate in your diagnostic tool. You could consider:

--When you say that you do not have "...adequate power/pull to the plane" do you mean it won't climb over the top in a wingover, or that the line tension is inadequate? If the level flight speed and line tension are good, but it seems to struggle to get over the top and the engine is sounding labored, then I would try more nitro in the fuel (assuming you are running nitro). This assumes you have tried a reasonable range of props. Next would be a slightly larger venturi.

--The line lengths are also a trim and tuning variable. Shorter lines take much less power. You don't want uncomfortable lap times (fast) or conversely extra-long lines to try to handle excess engine/prop. The long lines can make the aerodynamic trim problem harder.

--Maybe another way to relate things is that the engine likely will have a certain "happy speed" where it wants to run. So you match the pitch of the prop and your line lengths to get a lap speed that you like. As long as you are not starting off with one variable too far off you can usually converge on the best combination.

--The engine/venturi/prop combination represents a certain amount of available thrust at a given speed. So if your plane has a certain amount of drag you need to overcome, you need an appropriate amount of power. What is on the kit box should get you close, but may not exactly be optimal. For example, not all .40-sized engines make the same power, or make it the same way. You're not going to get a more docile engine than a Fox .35 and it will run old-school slow and steady gallon after gallon but it won't be happy hauling a big/heavy/draggy airplane. Compare that to an OS .35AX and you have a ton more available power but a very, very narrow operating range. A little bit off with the prop size and nothing seems to work.

--Running a larger prop that requires more power might be handled by a slightly larger venturi, not necessarily a larger engine. But the power needs to be stable, meaning if you upset the load (such as turning hard causes high induced drag) it has to come back to the original setpoint and not "run away."

--The timing of the engine advances with overheating conditions. If you run the engine too close to peak power you can see this effect.

Since nearly all of these design and setup choices are evolutionary, looking at a combination that has been very successful for others and matches the kind of flying you want to do is the best place to start, and then tweak from there.

Therefore, another way to help you perhaps is to list your actual setup with full details and find someone with a successful directly comparable plane and engine combination and see how the rest of the two setups compare.

Good luck!

[Steve Helmick]

To complicate matters further, but gain some insight, read Randy Smith's pinned articles at the top of the Engine Forum.

He says that if you need more power, you can increase the venturi size, or increase the size of the muffler outlet. Both allow for more airflow through the engine (i.e., pump). I've found this to be spot-on. I'd rather open up the muffler outlet than increase the venturi bore.

Finding the right propeller is vital. For your OS .46LA, the 11.5 x 4 APC is really hard to beat, although some swear by the APC 12.25 x 3.75. IMO, the latter is too much load unless you increase the venturi/muffler outlet and/or nitro content. Tim Wescott likes the 11x4.5 Thunder Tiger Cyclone prop on his .46LA, but they're really hard to get. Apparently TT stopped producing all propellers.    That prop works very well on other .36 > .46 engines. Loved it on my Magnum XLS .36 (aka ASP)...also sadly discontinued. 

ZZProp makes a CF copy of the TT 11x4.5, FYI, but from my experience, when you commit to CF props, you need a pitch gauge and must learn how to correct the pitch settings. It isn't all that difficult, but it is rather intimidating. IMO, if you give two guys several of the same CF props and one pitch gauge, the props of one fellow will end up being rather different from the props of the other. I think ZZProp may have a forum down in the Vendor's Section. Don't ignore all the other forums on Stunt Hangar! 

The APC propellers can also be re-pitched, and actually carved/scraped/sanded into something that will work quite well. I've completely re-carved APC 12.25x3.75's into an 11.5" diameter that works pretty well on my piped OS .46VF AAC. Not my favorite prop for it, but it works fairly well, and relatively inexpensive. Steve   

[Dave Hull]

Steve,

At one point, Randy Smith had a bunch of the TT 11x4.5 props made. If he hasn't sold out, then they should be easy to get....

Dave

[Mike Hazel]

Hey Steve,

Thanks for the mention  (ZZprop).  By the way, I do not have a vendors forum here.