|02-26-2013, 12:45 PM||#11|
I am: Mike D
Join Date: Jun 2009
Location: Mono Centre
Feedback: 1 / 100%
Total Props: 1
Re: Rossi DF Engines With a Prop
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Sources for CF AMA "D" control line speed props - which are in the right range:
Mike Hazel - ZZprop - you can find him through Delphi forums, control line speed forum
Steve Wilk - Eliminator prop - google Eliminator prop.
RocketRob - on RCU extreme speed forum - has a CF APC 9X10 made for him by Eliminator and sells it himself
Do not run stock APC 9" props on this engine, even clipped down 10". Severe risk of shedding blades. For bench running a common recommendation is to trim a wooden Zinger 11x10 down to about 8 - 8-1/4" for 10cc engines, go a bit larger for 15cc - 8-1/2"-9".
For setting up the pipe on the bench, you need to reduce the load on the engine by trimming the chosen flight prop down to 85-90% diameter. By tuning the pipe for max rpm on the bench with the lighter load, you can do a good job simulating the rpm in the air with unloading. As an example, setting up the pipe on my OPS Speed .65, I trimmed a CF 8.25x10 down to 7". Seems insane on an engine this big at first, but it is not - these props spin up in DF rpm territory which is what you want. See
A better example of a piped engine idling on a small prop is here:
Regards carb size - you must have the larger bore size to permit the engine to breath properly at high rpm. The powerband is defined by the timing - intake, exhaust, etc. The ability to ingest a full measure of intake on each cycle depends on the carb throat diameter - for higher rpm you need more cross sectional area because the amount of suction does not change with rpm nor does the displacement, so to flow the same volume in less time you need more area - simple. Bigger carbs don't "make" more power, but they allow the engine to make as much as it can for the given setup. Likewise, too small a carb will restrict high end power. Too big a carb and the engine will be a bey-atch to tune and throttlability will go south in a hurry. Big enough = good, too big - bad. 13.2
BTW, you might be surprised at how much air is flowing into the engine per second.. at 22K: 13.2cc x 22,000rpm / 60 seconds/minute x volumetric efficiency (.85 is close enough) = 4.1litres/second! Run it half that rpm and you can drop the throat in the carb to half the area (0.707 times the diameter) and not choke the engine.
If the carb is any good, the engine will idle okay. Ducted fan engines have to idle and throttle or they are not much use to the owner. You don't need a tick over idle with a small narrow high pitch prop anyhow, the thrust at low speed is weak.
Last edited by MJD; 02-26-2013 at 08:25 PM.
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