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Model Forum / Radio Controlled / Air Models / November 2006Prop alignment relative to cylinder
I orient my props so that they are horizontal just before the cylinder reaches TDC. Since I usually flip start my engines by hand, I felt that this gave me the most arc to easily flip the prop through to get it started. I recently read a post where it was recommended to line the prop up vertially when the cylinder is at TDC to reduce vibration. It seems to me that, if the prop is properly balanced, it wouldn't induce any vibration no matter how it was oriented. However, I know that on a glow engine the power is not smoothly applied and has some "bumps" in it so I'm open to discussion on the topic. Any idea? Preferences? Explanations? Thanks, Steve > I orient my props so that they are horizontal just before the > cylinder reaches TDC. Since I usually flip start my engines by hand, I > felt that this gave me the most arc to easily flip the prop through to > get it started. I used to do that but haven't for some time. Did I get better at flipping or just get too lazy to set the prop every time I put it on? (easy guess) I recently read a post where it was recommended to > line the prop up vertially when the cylinder is at TDC to reduce > vibration. I don't balance my props as much as I used to either and I bet that's a bigger vibe than prop orientation. > It seems to me that, if the prop is properly balanced, it wouldn't > induce any vibration no matter how it was oriented. However, I know [quoted text clipped - 5 lines] > Thanks, > Steve Wonder where you read this? mk I'm with you on that one. I am totally anal when it comes to prop balancing, and the once in a blue moon when I've dropped an off-the-shelf prop on a model when I didn't have a balanced one at hand, hoo boy was I shocked at the difference. Someone is really going to have to convince me that aligning a balanced prop vertically makes a difference. I set mine as mentioned - horizontal just as it starts to feel some compression, in order that the prop stops horizontally if/when the engine quits. MJD > > I orient my props so that they are horizontal just before the > > cylinder reaches TDC. Since I usually flip start my engines by hand, I [quoted text clipped - 23 lines] > Wonder where you read this? > mk > I'm with you on that one. I am totally anal when it comes to prop > balancing, [quoted text clipped - 8 lines] > it starts to feel some compression, in order that the prop stops > horizontally if/when the engine quits. Prove it to yourself by placing the heavy blade opposite the piston on the crankshaft. It will smooth out quite a bit, most of the time. There will be exceptions, but not many. Ed Cregger > I orient my props so that they are horizontal just before the > cylinder reaches TDC. Since I usually flip start my engines by hand, I [quoted text clipped - 12 lines] > Thanks, > Steve I began flying models via control line. When flying control line, the good thing to do is to align the prop horizontally when the engine is up against compression. That keeps the prop up and out of the way when landing. Old habits die hard. Many years ago, I did used to know a trick about where to put the heavy blade in order to help balance out the heavy piston and connecting rod. But now I forget what it is. Back then, no one balanced props that I knew. Ed Cregger | > I orient my props so that they are horizontal just before the | > cylinder reaches TDC. Since I usually flip start my engines by hand, I [quoted text clipped - 23 lines] | | Ed Cregger Hope I don't have this backward. A lot of engines are made where the counterbalance on the crankshaft does not completely balance out the piston and connecting rod. So, the prop should be installed with the heavy end down when the piston is top dead center. Does this jog your memory? In practice, I also balance the prop and install it so that it is horizontal when bumping against compression. Control line flying and some years of experience of running Fox engines on R/C planes has saved a lot of props.  SignatureJarhead | | I began flying models via control line. When flying control line, | | the good thing to do is to align the prop horizontally when the | | engine is up against compression. That keeps the prop up and out | | of the way when landing. Old habits die hard. Well, that's a good idea even today, even for R/C. In a dead stick, it makes it less likely that a dorked landing will break the prop. | Hope I don't have this backward. A lot of engines are made where the | counterbalance on the crankshaft does not completely balance out the | piston and connecting rod. So, the prop should be installed with the | heavy end down when the piston is top dead center. Your prop should not have a noticibly `heavy end' -- and if it does, that means it's very much out of balance, and is going to shake the hell out of your plane. Perhaps your engine isn't completely balanced internally (by design), but trying to correct for this with the prop is likely to hurt a lot more than it helps. SignatureDoug McLaren, dougmc@frenzied.us `Look! There! Evil!.. pure and simple, total evil from the Eighth Dimension!' --Buckaroo Banzai Steve Wrote: > I orient my props so that they are horizontal just before the > cylinder reaches TDC. I use a different approach altogether. I want the prop running 2:00 o'clock to 8:00 o'clock when the engin comes into compression. I found it helped make flipping the prop easier with a motion acros the fuse, whereas anything more horizontal tended to result in liftin the model a bit when the engine came into the compression area. Personal preference, nothing more. Having said all that, I haven't flipped a prop by hand in decade because my fingers cost way too much to repair. My hands don't get an closer to the prop than the handles on the electric starter irrespective of the engine being started. I earned this grey hair and I'm keeping it ! -- the-plumbe > Steve Wrote: >> I orient my props so that they are horizontal just before the [quoted text clipped - 17 lines] > > I earned this grey hair and I'm keeping it !! Over the years, I did drift to using the two o-clock position, for just the same reason as you described. I'm with you on using a starter these days. Ed Cregger > > I want the prop running 2:00 o'clock to 8:00 o'clock when the engine > > comes into compression. > > > > I found it helped make flipping the prop easier with a motion across > > the fuse, whereas anything more horizontal tended to result in lifting > > the model a bit when the engine came into the compression area. I guerss that's the way mine really are too, not really horizontal but just a little bit past as they come through TDC. Thanks for all the replies. This seems like something to experiment with a little bit but I don't forsee spending a whole lot of time on it. Since I try to get the props as balanced as possible I don't know that they'd do that well counteracting any out of balance forces within the engine. Now, I do have a prop that balances at an angle (about 30). Would this mean that the hole is slightly off center? Thanks for all the info... Steve Yup. If the hole is dead nuts center, that is located on the chordwise (hope that term says what I hope it does) center of gravity, then blade imbalance will cause the offending heavy blade to drop to the bottom of the arc. But if the hole is shifted towards the LE of one blade, then you'll get what you describe. Are you talking about an APC prop perchance? Those should never be indexed by the through-hole, but rather the through-hole should be opened up to larger than shaft diameter, and the appropriate bushing (close fit to the crankshaft) fitted to the molded in recess in the rear of the hub to. It is accurate, whereas the post-molding drilled holes are often not, and in fact APC admitted that to me on the telephone when I inquired. My phone call was the result of ordering about two dozen pylon props, and staring in horror at the clearly visually off-center through-holes. I had noticed some misalignment before but this number of props at one time in my viewfinder really caught my attention. The same goes for balancing - make sure the balancer cones are centering the prop hub via the molded recesses, not the drilled hole. If you do both, balancing becomes a much more useful procedure. But if you only do one, you're often not gaining much. > > > I want the prop running 2:00 o'clock to 8:00 o'clock when the engine > > > comes into compression. [quoted text clipped - 18 lines] > > Steve > Yup. > [quoted text clipped - 27 lines] > balancing becomes a much more useful procedure. But if you only do one, > you're often not gaining much. It is time we all rebelled and refused to buy props that needed balancing. This has gone on long enough. If the hole isn't true, the hub isn't true, or the balance is out, it is a DEFECTIVE product and should not be sold to the public. Ed Cregger | > Yup. | > [quoted text clipped - 35 lines] | | Ed Cregger So when are you going to start making your own props? ;-} SignatureJarhead > | > Yup. > | > [quoted text clipped - 54 lines] > > So when are you going to start making your own props? ;-} And what makes you think that I haven't? <G> Ed Cregger >> | > Yup. >> | > [quoted text clipped - 58 lines] > > Ed Cregger I reckon the whole fun of flying planes is to find out what makes it work better and then applying it. I think props are made as accurate as possible, but if the quality of a brand is suspect, just dont buy any of that brand. Consumer pressure works quite well. If their quality inspections dont pick up problems, you are quite in your rights in most places to return and get one that is OK or a refund if not, an exchange on another item. Moaning wont fix it, fixing it will. Just make sure you are aware of how to balance a prop. A lot of badly fitted bits can do damage to a prop. I had problems just last week with a prop that didnt slide to the rear of the thread causing the prop to spin off on startup. Easy fixed, hard to see the problem, enlarge the hole somewhat with a correctly sized drill bit, and get it centre. Just landing the wrong way ie nose first will do a major rebalancing act on your prop. If your really concerned throw on a new prop, but check its all OK first. You are supposed to use your brains not sit on 'em. As far as making your own props, its easy to do, but hard to get them right (I think I'm now going to get a whole lot of prop builders say the opposite) Proper pitch to develop thrust where you need it at the torquey side of revs to start with. Its up to you why you want a size or pitch, just get it right. Jonno |
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