home *** CD-ROM | disk | FTP | other *** search
- Newsgroups: rec.models.rc
- Path: sparky!uunet!ftpbox!mothost!white!rtsg.mot.com!svoboda
- From: svoboda@rtsg.mot.com (David Svoboda)
- Subject: Re: Props
- Message-ID: <1992Jul28.230434.26635@rtsg.mot.com>
- Sender: news@rtsg.mot.com
- Nntp-Posting-Host: guppie44
- Organization: Motorola Inc., Cellular Infrastructure Group
- References: <1992Jul22.222910.27329@iex.uucp> <1992Jul23.153612.20836@rtsg.mot.com> <1992Jul27.225145.1783@iex.uucp>
- Date: Tue, 28 Jul 1992 23:04:34 GMT
- Lines: 73
-
- In article <1992Jul27.225145.1783@iex.uucp> djones@iex.iex.com writes:
- |In article <1992Jul23.153612.20836@rtsg.mot.com> svoboda@rtsg.mot.com (David
- |Svoboda) writes:
- |>
- |>"Double the diameter and add the pitch" is an old rule of thumb that I use to
- |>determine engine loading. So an 11x6 prop produces a "load index" of 28, as
- |>does a 10x8, so those props will give approximately the same engine load.
- |Note
- |>that a lower pitch prop will unload in the air more than a higher pitch
- |>prop (with the same load index), and will make the airplane fly more slowly,
- |>and climb more positively.
- |>
-
- |Does this hold for both 2c and 4c engines?
-
- Prop load is prop load. I would use it in any case.
-
- |And would it be safe to say
- |that if a plane is a little underpowered, that a higher pitch for a given load
- |index would be preferable? For example if the manufacturer suggests a 11x6 as
- |the optimum
- |prop, and the plane is a little heavy, then a 10x8 would be better. Or are
- |really loading then engine down MORE and thus making the plane even more
- |difficult
- |to fly?
-
- A 11x6 prop would be similar in engine loading to a 10x8. That is, you would
- realize approximately the same engine rpm *at a given airspeed*. Now, remember
- that a lower pitch will give you a lower airspeed, but more static thrust. Think
- of it as a "lower gear". Thus, a low pitched large diameter prop is good for
- a naturally slow airplane, and will give you a good climb. A higher pitch prop
- will have less climb or acceleration, but will pull the airplane with a faster
- top speed (and a faster cruise speed, for that matter). If you are underpowered,
- a larger, lower pitch propeller would be a better use of the engine's horsepower,
- but you have to limit that becuase that airplane will have a minimum speed, and
- an extremely low pitched prop would not pull it faster than that (stall on takeoff).
-
- An example of this principle is my Sig Skybolt, with an OS longstroke 61 with
- Perry micro-oscillating pump. I started out using the 11x8 prop, like I did on
- my last airplane (an old Super Kaos (a Bridi kit, if anyone remembers them)). Now,
- the Skybolt only has a 50 inch wingspan, and the 11x8 pulled it around so fast
- that I thought I would shed some wings. So I bought a great big 12x6 for it,
- and it still turns at about 10,000 rpm (remember, it's a longstroke; it's supposed
- to lug), but it flies considerably slower, but now climbs like a rocket (a slow
- rocket, but a rocket). I fly around at half throttle most of the time, but
- between half and full throttle won't speed the airplane up appreciably, it just
- makes it climb better. (Can you say "Consecutive Vertical Snaps"? :-)
-
- |Do you set a lower pitch prop a little richer on the ground or does it really
- |matter.
-
- A higher loading prop will lug the engine, which makes the mixture very hard to
- adjust. You can easily have a lean run, and have it masked.
-
- |I was taught to fill the tank 1/2 way, warm the engine, peek the
- |RPM on a tach and then back off about 500 RPM. Fill the tank and go fly. This
- |technique was taught to me more for 4c than 2c but I expect it would work
- |in both cases.
-
- I'm not any sort of 4-stroke guru (well, with motorcycles, maybe, but not model
- airplanes :-), but that sounds like a good method of protecting your engine. I
- would further suggest that you peak the engine with the nose pointed at the
- sky, rather than level. And listen and look at it flying--some engine just seem
- to change mixture in the air when they unload.
-
- |>Changing prop brands will skew this rule a bit, because different airfoils,
- |>blade widths and planforms will produce different drag.
- |>
- |There appears to be no easy answer!
-
- Sure there is. Try an alternative, and see if it works.
-
- Dave Svoboda, Palatine, IL
-