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General control line discussion => Open Forum => Topic started by: Dane Martin on October 17, 2018, 05:11:52 PM
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So. Maybe more involved than needed. But! How does one go about "setting the lap time". Yes I've been reading Paul's trim article. But my query is more of, how do you know if you need more RPM or more prop pitch. And then, let's say you determine you need more RPM. Where do you say, "I need more venturi, vs needling it in"?
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Not sure, but my first response if I felt I needed more RPM would be reduced prop pitch rather than anything with changing engine hardware.
But don't take my word, I'm still figuring this stuff out at the junior level....lol
Gary
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Well my cl background was speed and combat. So I know how to get more or less rpm. But how would a stunt flyer know that's what he or she wants?
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First you need to know what you want to do, speed up or slow down. Changing the ground RPM is the quickest and easiest. I ALWAYS take my prop box to the flying field with a good selection of know good working props. . The short answer is, try it then fly it! If you are in the ball park already and looking to make small changes, line length can figure into the mix also.
Type at you later,
Dan McEntee
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If it's showing a tendency to run away on you, it needs less prop pitch!!!
As far as whether to needle it in or give it more venturi, here's what I know from OS LA and similar engines:
- Schnuerle engines like to run fast, old-style baffled engines like to run slow, except (Veco 19) when they like to run fast, too. So a Fox 35 wants a 10-6 prop, but a Magnum 36 wants an 11-4 or 11-4.5
- You can coerce a Schnuerle-ported engine to do a 2-4 break and all that, with a big high-pitch prop -- until it finds its happy place and pulls your arm off with 4-second laps.
- If you get it needled to the point where your level lap times are good and you either can't hold a needle setting or it tends to run away, then your venturi is too big.
- If you get it needled to the point where your level lap times are good and it leans out at 45 degrees or overhead, then the venturi is too small.
- If you can't find a happy place between the above two venturi sizes, then either the engine is too small, it's a dog, or you've got the wrong prop on it.
- For Schnuerle engines, in general (and someone will disagree with me!) if you want more power, use a smaller prop, and then dink with mixture and venturi.
- Just changing a prop and getting immediate and obvious improvement without changing anything else could well mean that the engine wasn't optimized for the prop you had on it, not that the new prop is better.
- As an example, I have 45-ounce planes that like running a 12-3/4 x 3-1/4 prop in front of a 46LA. That same engine turns an 11x4 in my 64-ounce Atlantis and works very nicely (now that I have the venturi size sorted out).
Note that the above implies a lot of messing around with venturi sizes. You can either make a ton of venturis and swap them around, or you can do like I do and get some fine nylon mesh, cut it into squares, and hold it onto your venturi with an O-ring. Get it at a fabric store, or the next time you drop $$$ on airplane parts stop by a florist and get some flowers with it on (your wife will appreciate it -- or if she knows you well, she'll search your shop for new shiny things).
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Thanks for the replies gents.
What started this was; I was flying my TEOSAWKI. I actually flew the pattern the first flight. Not a nice one, but comfortably. After a couple flights that day, I thought, this thing feels slow. And it was. It was running rich, so I leaned it out to a good engine run. It was better, but a little slow. I didn't want to change the engine run so I went from an 11x4 to a 11x4.5. That perked it up to what I thought was a good speed for this plane. 5.0 second laps. So I've been thinking about:
1) If the engine is running "correctly", is changing the prop the first thing to try? Kinda sounds like that's what Dan does?
2) if the engine runs correctly for a given venturi size / air fuel ratio.... well I guess, is there only one instance per engine where this is true? For example, can I tune an os 46 la on a 6mm venturi and make it run properly, but also tune it on a 7.5mm venturi and make it run properly? And if so, how would you know you should switch? Tim, is this what you're explaining?
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Thanks for the replies gents.
What started this was; I was flying my TEOSAWKI. I actually flew the pattern the first flight. Not a nice one, but comfortably. After a couple flights that day, I thought, this thing feels slow. And it was. It was running rich, so I leaned it out to a good engine run. It was better, but a little slow. I didn't want to change the engine run so I went from an 11x4 to a 11x4.5. That perked it up to what I thought was a good speed for this plane. 5.0 second laps. So I've been thinking about:
1) If the engine is running "correctly", is changing the prop the first thing to try? Kinda sounds like that's what Dan does?
2) if the engine runs correctly for a given venturi size / air fuel ratio.... well I guess, is there only one instance per engine where this is true? For example, can I tune an os 46 la on a 6mm venturi and make it run properly, but also tune it on a 7.5mm venturi and make it run properly? And if so, how would you know you should switch? Tim, is this what you're explaining?
This is far too complicated to fully discuss in the few minutes I have right now, however changing prop/venturi/nitro was common in the days we ran muffer-type engines, as was adding or subtracting exhaust outlet area. The fact that you have to change it with major parts swaps just for normal variations is a severe defect that was solved by tuned pipes. That's why very few people are able to be competitive using muffler engines any more.
When you are changing venturis over such a huge range .236 to .295 (6 mm to 7.5 mm) would require fundamentally different approaches. With the .236 venturi, you would have relatively low power, and would need to run a lot of pitch (6-7") at relatively low RPM, because the efficiency of the prop would have to be high. With a .295 venturi, you would have tremendous power that you would have to dump somehow, probably with a combination of small diameter, low pitch, and correspondingly high revs. The usual adjustments of venturi size are more on the order of .005" and that can be a pretty big effect.
The key to understanding it is that the venturi sets the overall power and way the power falls with RPM. The power needed to fly the airplane falls in to a very narrow range. You have to either have a very efficient prop (which is generally bad for maneuvering, like a 4-stroke and 7") to transfer more of it to the airplane, or you have to have a very inefficient prop (which is generally good for maneuvering, like a 40VF and 3.5") so that the net power applied to the airplane will result in the right speed.
Once you are in the ballpark, you can make very small adjustments for controlling the way it works in the maneuvers and slightly adjusting the "operating point" (how close to a 4-stroke or two-stroke, or how it varies) with other very small adjustments.
The right starting point in many cases, for 2-stroke engines the way we normally run them, is to get a choke area of about .017-.022 square inches. That's about .273 with a ST spraybar, and about a .257 with an OS FP spraybar. This covers a huge range of engines and applications, it's more or less what is run on the 20FP, the ST46, and David's PA75. The starting point for the OS40VF is about .270 with an ST spraybar (0.0174 square inches).
.236 with an OS spraybar is WAY off the lower end, and .295 is over the top at the high end (although not way out of bounds).
Tuned pipes avoid the need to change them wildly from moment to moment by providing a way to run extremely powerful engines and still control the output with a frequency/RPM dependent way. You can fund muffler-based setups that at least don't require a lot of changes from morning to afternoon and are fairly predictable but you are compromising on something at some point, and they generally are far down on performance from other systems, which is why they aren't used very much competition since about 1988.
The 20FP is a special case that happens to work out for our purposes, the muffler is just right to make it run correctly and powerfully without getting "wild", and that's why "improving" it or "saving weight" by removing the muffler usually screws it all up, and why the solution is to put it back the way it came from the factory.
Brett
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So in a way, I've been extremely fortunate to have built an airplane and bolt on a known good set up engine and just fly. But in the case of my twister at VSC, I didn't have the right power. So, at the coaching of Chris McMillan and Joe Gilbert, we increased venturi size and decreased pitch. I don't think it changed the airspeed (not noticeably anyway) but it felt like it could pull a tree stump, it had so much power. I understood how it worked after the fact, but I wouldn't have come to that conclusion without them.
Is this type of airplane tuning more experience, then?
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So in a way, I've been extremely fortunate to have built an airplane and bolt on a known good set up engine and just fly. But in the case of my twister at VSC, I didn't have the right power. So, at the coaching of Chris McMillan and Joe Gilbert, we increased venturi size and decreased pitch. I don't think it changed the airspeed (not noticeably anyway) but it felt like it could pull a tree stump, it had so much power. I understood how it worked after the fact, but I wouldn't have come to that conclusion without them.
Is this type of airplane tuning more experience, then?
That's essentially what was described above, if you need more "power", add venturi, and reduce the efficiency (by reducing the pitch) to compensate. Same or similar lap time/level flight speed, much more "power" (by the stunt flier definition).
Brett
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That's essentially what was described above, if you need more "power", add venturi, and reduce the efficiency (by reducing the pitch) to compensate. Same or similar lap time/level flight speed, much more "power" (by the stunt flier definition).
Brett
Now it makes complete sense. And until it's actually felt, I think it would be very difficult to explain to someone. It felt like my plane lost a pound from one flight to the next. Total eye opener. Much like flying the Skyray. Feeling what a plane feels like with line tension equal throughout the flight!
Tim, I reread your post a few times. I'm thinking I understand what you mean, but I haven't tried to use the same engine in that big of a difference in planes yet. I've only built in my narrow comfort zone.....
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Now it makes complete sense. And until it's actually felt, I think it would be very difficult to explain to someone. It felt like my plane lost a pound from one flight to the next. Total eye opener. Much like flying the Skyray. Feeling what a plane feels like with line tension equal throughout the flight!
Tim, I reread your post a few times. I'm thinking I understand what you mean, but I haven't tried to use the same engine in that big of a difference in planes yet. I've only built in my narrow comfort zone.....
I wish I could find one of Brett's rants explanations of what "power" really is to an engineer vs. how most stunt people use the word -- it's very informative. (Short story -- real power is the energy per second that the engine is putting out at the moment. Stunt "power" is how well the engine regulates the speed of the plane).
In the "how to" part of my post I'm just trying to go into detail about how I achieve what Brett was talking about. He's right (of course) about getting consistency with muffled planes. I'm still flying with a muffled 46LA because I haven't wanted to drop the cash onto a tuned pipe setup. I suspect I'm well into a level of piloting ability where I'm being held back by not having a tuned pipe.
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Rule of thumb is to adjust airspeed with prop pitch, to find a speed that the airplane flies "happy". Adjust prop diameter to load the engine to where it's happy. From there, adjust line length to get a lap time that makes the pilot happy.
It seems easy enough to recognize when the engine has too much (prop) load. But how to tell that it's not loaded enough has me puzzled.
Brett has posted that more prop diameter will increase the airspeed, but I don't see that. Less diameter will allow the engine to rev higher, so with the pitch staying constant, I'd expect a bit smaller diameter would increase airspeed. Brett is expecting increased prop efficiency to do the job, while I'm expecting increased horsepower to do it. Brett's probably right, but if we're talking of changing 1/2" plus/minus in diameter, it goes against my experience. n~ Steve
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Brett has posted that more prop diameter will increase the airspeed, but I don't see that. Less diameter will allow the engine to rev higher, so with the pitch staying constant, I'd expect a bit smaller diameter would increase airspeed. Brett is expecting increased prop efficiency to do the job, while I'm expecting increased horsepower to do it. Brett's probably right, but if we're talking of changing 1/2" plus/minus in diameter, it goes against my experience. n~ Steve
Larger props are more efficient (everything else being equal), meaning more HP is transferred from the engine to the airplane, meaning faster. The 20FP uses the opposite effect, you use a smaller prop to reduce the speed and still permit the engine to run the way it wants.
Your theory about more RPM+same pitch = faster is true, also same RPM and more pitch = faster, but only if you leave the prop diameter and other characteristics the same. The prop is not a wood screw wedging itself into a piece of pine, and the airplane definitely *does not* go the speed you might expect from the RPM*pitch, in fact, it's more like 60% for "large" props and lower and lower as the diameter goes down. We have used this effect, whether anyone realized it or not, for ever, even in the ancient ST46 days or earlier, to adjust where the engine ran compared to how fast the airplane went.
By the way, what we measure as pitch is not a direct measurement of the effective pitch of the propellor. It would be if the airfoil was "flat" with the top of the airfoil being the same as the bottom. What actually matters is at what forward speed the thrust goes to zero, divided by the RPM. This is always more pitch than you measure using the back of the blade. That's why when you find the inflight RPM and multiply it by the pitch, the airplane seems to be going "faster" than you would expect, usually, A LOT faster. For example, I know that my engine it going about 10800 rpm in the air in level flight at about 5.35 seconds a lap (~78.7 feet/second), and it has a measured pitch of 3.75". 180 revs/second and .3125 feet of pitch = ~56 feet/second, or MUCH slower than it is actually going.
This is an effect of the prop airfoil elevating the angle where the prop blade lift goes to zero well above the back of the blade angle. You can try to figure out the effects of this analytically, and there are ways to do it by considering the camber of the prop airfoil at a bunch of places. It deviates further and further as you go toward the tips, by a tremendous amount near the tips, typically. Doing that, a 12.5-3.75 Eather UC measures 3.75 at the tip, and through analysis you get something like 8-8.5" of effective pitch. Same thing with a flat-back might be 6", because the camber is lower. I figured the net effective pitch of my particular prop at the time I did the RPM measurement to be about 6.5-7", say it's 7", pitch*rpm = 105 fps, airplane is really going 78.7, only 75% of the pitch*rpm. The efficiency might be around 55-60%.
Effective pitch*RPM = 0 thrust. You actually need about 2.2-2.5lbs of thrust to make the airplane go the speed you need (.32 -.35 HP). If you figure about 60% efficiency, you would expect the shaft HP of the engine to be about (.53-.6 hp) at the in-flight revs.
Being a super-nerd, I tested this. We got out the engine dyno, put my test engine on it, with the flying prop, and set it for the usual launch RPM. Then, we stopped it, and then, without changing any of the settings or the needle, put smaller and smaller props on it, and measured the HP VS the rpm. When we got to the known level flight RPM, the engine was generating about .52-.53 HP at the shaft. The prop was much smaller, of course, but that was just a matter of getting the right load to get the right RPM. Experimental results don't get any better than that. The only effect we didn't model was the slight change in the fuel pressure head (lower in flight).
This was all a long time ago, and a very similar experiment with very similar correlation was the thread where I finally gave up attempting to get people to understand what the word power actually means, when no one "believed" the measurements. The same thread discussed static thrust, where I predicted several weeks ahead of time what the difference between a 12-6 and 12-4 Rev-Up would be, when both are set for launch (difference was a factor of 2.5, which anyone who has launched a ST46 and 40VF airplane could have easily predicted) and also matched the shaft HP required to about 3 significant figures - and no one "believed" that, either.
It's more or less impossible to even have sensible discussion about it because there are so many self-proclaimed "engine experts" who typically descend on these threads to provide their wisdom about how "torque flies airplanes" and other magical thinking bullshit theories that are so ingrained that no one can even listen or "believe" actual measurements, or complain about the "advanced mathematics" like multiplying two numbers together. Howard still likes tilting at these windmills in other areas, I more-or-less gave up about 15 years ago, unless someone asks a specific and well-stated question or situation.
Brett
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Brett, if you didn't adjust the NV for the smaller propeller, then the engine ran richer and richer, as the prop load decreased. That means that you were not getting to peak rpm (or peak HP). So I guess I don't see the point of the dyno testing. This was done with open exhaust, right?
I'm still wondering about what to look for in determining that the engine is not loaded with enough propeller. In a previous life, I determined that with a stopwatch, on a variety of go-fast CL models. D>K Steve
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unless someone asks a specific and well-stated question or situation.
Brett
Since I'll take this as I have your ear, when flying my "new" impact, we had it tached at 10,100 to launch. It had a 5.6 second lap, but felt like it had enough power to do manuevers (I did, just milked them) but it felt slow. We leaned it in a little to 10,300 on the ground. We didn't assume it would make a change, and it didn't. The prop was a bolly 3 blade 13x 3.25 (I believe) but it had been worked. Could have even been flatter. During flights, we noticed the lines were too long. Like from the center of the circle, the wing hung over the outer circle. Could the lines alone make up for slower lap times?
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I wish I could find one of Brett's rants explanations of what "power" really is to an engineer vs. how most stunt people use the word -- it's very informative. (Short story -- real power is the energy per second that the engine is putting out at the moment. Stunt "power" is how well the engine regulates the speed of the plane).
In the "how to" part of my post I'm just trying to go into detail about how I achieve what Brett was talking about. He's right (of course) about getting consistency with muffled planes. I'm still flying with a muffled 46LA because I haven't wanted to drop the cash onto a tuned pipe setup. I suspect I'm well into a level of piloting ability where I'm being held back by not having a tuned pipe.
Your descriptions are very clear and concise as an SOP for inflight testing. I will be paying attention to those things while I fly.
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During flights, we noticed the lines were too long. Like from the center of the circle, the wing hung over the outer circle. Could the lines alone make up for slower lap times?
Of course. It's about .07 seconds/ft in the range we are talking about, take off 3 feet, and you are down to about 5.4, or slightly faster.
One thing that you might note- the maximum allowable length by the AMA rules is 70 feet from the center of the airplane to the grip of the handle. Figure that you need about 67' lines, measuring the normal stunt way of eyelet to eyelet. But a lot of people, in fact, I guess the vast majority, run less than the full allowable length on almost all normal airplanes, even those with large tuned pipe engines. I think that David and I are both at 64' eyelet to eyelet right now (rather than the 67 it would require for max length).
Brett
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Of course. It's about .07 seconds/ft in the range we are talking about, take off 3 feet, and you are down to about 5.4, or slightly faster.
One thing that you might note- the maximum allowable length by the AMA rules is 70 feet from the center of the airplane to the grip of the handle. Figure that you need about 67' lines, measuring the normal stunt way of eyelet to eyelet. But a lot of people, in fact, I guess the vast majority, run less than the full allowable length on almost all normal airplanes, even those with large tuned pipe engines. I think that David and I are both at 64' eyelet to eyelet right now (rather than the 67 it would require for max length).
Brett
I'll fix that first, before changing anything. Thanks