Came across a detailed article on the technicalities of tuned pipes. I was impressed and found it very well written. Hope someone else finds it useful.

Also notice the link at the bottom for the tuning software wizard.

Cheers,
DAVE

[link]http://en.wikipedia.org/wiki/Expansion_chamber[/link]

looks interesting and there is very little talk on the subject.................can you go ahead and post the link?

This lead me to ask a question that's been bugging me. When speaking of how to tune pipes, suggestions are often given in the form of specific dimensions, "to the apex of the pipe". Not too confusing, apex is biggest part of the pipe, technically point where the pressure wave is reflected. On a muffled tune pipe however, the apex is not a single point of the pipe, the pipe's largest diameter may extend for 6-10 inches. In that case, the "pressure point" would be at the rear of the apex, where the pipe begins to narrow toward the tip. This point is generally located at approx. 90% of the pipe length for muffled, and approx. 70% for un-muffled.

SO, if a dimension of 12" true inches is suggested with a un-muffled pipe in mind, how does that apply to a muffled pipe? Is it correct to assume that a muffled pipe will be approx. 20% shorter than an un-muffled to achieve optimum tuning? Besides the fact that muffling sacrifices rpm...

FWIW, I'm setting up a YS 45 with a macs #1250.

Cheers,
DAVE

Haha, very funny indeed. My bad.

[link]http://en.wikipedia.org/wiki/Expansion_chamber[/link]

Dave, I am quite interested as well. Could you please post the link for us.

We must have been typing at the same time. Thanks!

The reference point as I've always been told is the point where it first reaches maximum diameter. Get a flashlight and peer down the inlet of the pipe, you'll see things are different inside than out.

MJD

I've have looked before, but never with this specific subject in mind. I see what you're saying. 'Preciate it. I'll use a pushrod to determine the measurements.

DAVE

The first max diameter point is generally pretty obvious externally, the double wall usually starts there.

MJD

this is a tool which can help on the design and usage of tuned pipes as well http://www.mh-aerotools.de/airfoils/javapipe_en.htm

The forum could really do with a sticky, so people can understand with clear simple instructions the black art of pipe tuning.

more info here.

http://www.rcaerobats.net/tuned_pipe_info.htm


If you tune for max rpm on the deck, then surly as the engine unloads in the air, the pipe will detune itself. have you got to set below peak rpm on the deck to allow for this.


One thing that isn't clear is what to do with main needle when tuning pipe? Is adjusment needed each time you cut the header? I assume you go richer the shorter the header.
Or do you leave it alone to measure the results.
If you shorten and dont tune hs needle then are you seeing the optimum rpm increase.?

If you do a mod to an engine...ANY MOD, I don't care what it is....and it forces you to open up the needle from where it was before the mod......congratulations, you have just done something to increase horsepower.

If you are tuning the pipe length in small increments and gaining rpm with each cut, the engine will tell you it needs more fuel. I anticipate the need for more fuel by opening up the needle before the next start up. This is a must for true speed engines with a very high fuel demand.

What causes the blown plugs by a short pipe? heat?
Can the pipe detune itself as it unloads in the air and drop rpm because the peak rpm on ground was set at optimum manifold length?

it causes a very lean condition which in turn fires the plug. unless your engine is a pumped one you have to allow for enough extra fuel for the higher rpm, this is compounded by the further resonance jump when the engine unloads demanding even higher fuel delivery. some set their pipes to peak on the ground, they then fly and the engine goes lean..another plug goes bye bye[&o]

The more radical the set up, the more the mixture needs to be set extra rich to compensate for the explosive increase in rpm. A typical C/L speed run is a pretty dramatic example of that.

Think about it - the rpm will not drop in the air.. but yes, it can go past optimum rpm. This can act as a form of governor, I believe this is how piped CL stunt models are generally setup.

Yes, you need to open up the needle to anticipate the flow conditions when the pipe is on boost - rpm will jump, air and fuel flow requirements increase, and a more open setting is required to achieve the right air:fuel mix at higher rpm. You might think that it comes out in the wash - more rpm, higher air flow rate through the venturi = higher fuel "suck" = same A:F mix. But it is not a linear relationship. And the pipe is cramming mixture back into the cylinder as well, messing things up. If you remember the axiom, more rpm needs more open needle, you're in the right direction.

Here is how I deal with the process, now anyways:

Make a decision ahead of time. What flight prop are you going to use? This is much the same as deciding what rpm you are going to run it in the air, but choosing the prop based on some knowledge of the engine and the aircraft's needs is easier. For example if you are bopping around on a muffled .46 and it spins a 10-7, and the performance is great but you want to bump horsepower to get a bit more.. well then, stick with the 10-7 and tune the pipe system to work with it.

To tune the pipe in boost condition on the bench, assuming the pipe/header system is starting on the long side (i.e. the "metal safe" side) the easiest method I know of is to unload the engine by reducing the diameter of the prop. You can reduce pitch instead, but you are usually stuck with integer changes, and you have less control over the magnitude of the change as a result.

For engines in a somewhat "regular" state of pipe tune, trimming the prop to 90% of it's diameter should get you close. i.e. I would trim the example 10-7 to 9.1". For higher strung, peakier setups 85%. You'll find a mirror of this recommendation in ILS's dissertation on breaking in high performance engines as well. Same reason - getting to flight rpm on the bench.

Bench run the engine on the pipe system with the flight prop trimmed to 90% diameter. Warm it up, floor it, and tune momentarily for peak rpm. Record the tach reading.

Trim 1/8". Redo the above. Did the rpm climb? Good, you're going the right direction. No, or did it drop? Then you were short in your initial estimate of length. Better drop the flight prop size or get another header. Oops.

Repeat until the rpm changes are approaching nothing. If you hit no change stop. If your last change looks like almost no change, stop. And, with it running full bore on the bench prop on the pipe, set the needle on the rich side of peak as well, not right on it except for brief tach readings of peak. A clean 2 cycle, but back off from peak to the richer side of 2c.

Now without moving the needle, if you fit your flight prop the engine should now struggle a bit to jump onto the pipe, and should appear somewhat rich. A quick line pinch should see it jump up in rpm but drop off again. Don't move the needle. You are looking at a reasonable facsmile of the ground setting for flight with that prop and pipe setup.

Then as the aircraft gains airspeed and the prop unloads, this allows the engine to get on pipe and the rpm will climb up to, hopefully, pretty much where you were running it on the bench. And if the needle was set to the correct ground-rich setting, the mixture should be decent in the air.

Setting the needle lean on the ground is common for those not totally accustomed to pipe tuning - it may appear comfortably rich running at zero airspeed (I don't like the word static) but as the rpm climbs, as mentioned, the A/F ratio will shift lean as fuel draw fails to keep up with the intake flow. Or more axiomatically: it needs more fuel, open the needle.

The amount of raggedness on the ground compared to in flight depends on the state of tune of the engine and system. The extreme example is of course, F2A speed.. 19-20k or something on the ground, then mad whipping to help the engine unload and stage onto the pipe. very, very critical, peaky setup - very narrow operating range. Sport stuff is far more forgiving in relation.

MJD

Good stuff mike.

Currently my mag is set with peak rpm on the ground and on the pipe, flight is on the pipe too and more importanly the launch is on the pipe.

There must be some rpm tolerance on the pipe as it unloads in the air where the increase in rpm still keeps it sweet on the pipe.

What causes the unloading in the air. Its a secondary boost for the engine?

Yes there is some tolerance, dictated by the design of the system. The less radically timed systems and muffled pipes tend to have a broader powerband - in this case referring to the boosted rpm range. It's a curve like any hp curve, with an upslope as you approach boosted conditions and peaking at highest power level, then dropping off again. The sweet area is wider and lower on some systems, narrower and taller on others (hence the term "peakier"). On a sport/pattern engine with a typical muffled tuned pipe system setup you will not see the hyperactive jumpiness you would except with a more radical setup like in F2A and other small engine CL speed stuff, the extreme examples, or open piped DF engines. These higher timed engines coupled with big fat narrow range open pipes have greater boost levels and higher potential peak horsepower, but in a more narrow operating range. So ultimately the boost level and manners of the system that you use are dictated by needs/wants. The old adage "you don't get something for nothing" always seems to surface. For RC speed flying where you care little about the 3/4 throttle settings, but need "FAST, some sort of cruise while I calm down, idle, stop" you can put up with more peakiness. For sport flying where you want some boost but want some tractability to be retained at mid throttle settings, a muffled tuned pipe system with the pipe not set on the short side is probably ideal. And you can see why as the system's state of tune gets higher and higher, carbs get more pointless/useless.

Unloading - think of how a wing works as you increase the angle of attack. Now think of the rotating wing called a prop - as the airspeed increases, the angle of attack of the prop blades is reduced (relative wind), less energy is being wasted as drag/turbulence and the efficiency of the propellor system increases. Fine pitch props reach pitch speed - the speed at which further increase results in a reduction of thrust - at lower airspeed. They also work more efficiently at low to zero airspeed due to the lower aqngle of attack, hence the much higher "static" thrust. Higher pitch props, conversely, bog the engine down on the ground and at low airspeed with the high angle of attack of the blades and resultant blade drag, so the engine is pulled off tune more and more as pitch goes up. As you can imagine there is a sweet spot in terms of airspeed for any prop, as well as for any engine/tuned exhaust system - your job is to home in on both those spots simultaneously.

Back to old tired car analogies, in terms of prop pitch, think of your engine as peaking at 5,000 on the tach, and imagine launching from the line in 1st, 2nd, 3rd gear respectively and how you would have to manage the clutch (prop slippage - turbulence, heat) to accomplish that, and the resultant performance as you pick up speed and approach 5k. Not a perfect analogy, but it's in the right direction.

In terms of rpm tolerance, the tuned muffler systems - Jett, Nelson et al - are very good examples of less ultimate boost (but pretty decent nonetheless), but broader powerband.

On your Magnum, sounds like you need to trim the header a bit (you've beem muttering about that haven't you?) or increase prop pitch a bit to reach top speed potential. In which case you'll have to launch a little more rich and toss harder. Or enjoy it where it is with lots of boost at launch and a little less top end. Perfect example of the decision making process with these things.

MJD

as the airspeed increases, the angle of attack of the prop blades is reduced.

As in blade flexes?

As in the whole aircraft is flying with the nose lined up with the airstream when you're going fast [ideally]. When you are flying slow, the nose is up more and the prop blades are presented to the air stream at more of an angle.

The prop blades de-pitch right off the bat if the prop isn't stiff enough, then once you reach the point where thrust equals speed you would figure that the prop blades relax to their original pitch.

No - it is due to the direction of the relative wind. The higher the forward speed, the lesser the angle of the blade in relation to the airstream it is intersecting. The prop works most efficiently at an airspeed that causes the prop blade to function at the prop airfoil's sweet spot.

MJD

p.s. in this diagram, you are situated to the right of the engine with the aircraft facing to the right, i.e. the normal starting position for most people. Left = "static", middle lower airspeed, right, higher airspeed.

Quote

A curious thing happens though when the model plane starts to fly. The inflow begins to pick up speed as the model picks up speed. The prop gets more efficient, that is, the drag on the prop is reduced as the plane picks up speed. We know this as the unloading of the prop. At some point as the inflow air speed increases, the low pressure in front of the prop turns the inflow to the direction of the prop rotation. The result of this is to decrease the effective angle of attack (pitch) of the prop. If the initial pitch of the prop is too low, the increased air speed can reduce the pitch to zero or to a negative value where the prop becomes a brake. To compensate, speed planes run very high pitches.


some good reading from quote source here.

http://www.clspeed.com/tech.htm

Same thing though the wording is a little odd here and there. The diagram says it all - it is simply a solution of two vectors. As the airspeed increases the AoA decreases - if you extrapolate that line it eventually crosses into zero and then to negative pitch. That is outrunning the prop, and all of us have likely seen that on sport jobs with 4, 5 or 6" pitch props WOT straight down. It is used to good effect on some types of aerobatic aircraft, you can create somewhat of a "wall" with big blades and fine pitch.

Remember that airfoils have an ideal angle of attack, the angle or the small range of angles where they work most efficiently (best l/d), and it is easy to see why there is a process involved in homing in on the best prop for any particular engine and aircraft setup. It is the prop that pulls the aircraft to an airspeed where the working AoA is in the sweet spot of the blade design to get best efficiency from the prop, while absorbing all the power the engine has to offer. Intuitively, the prop blades have to be smaller to absorb the same power as a lower pitch setup with larger blades. There is no runaway situation, the aircraft's drag is constantly increasing as the square of the airspeed increase. The system reaches a limit - but the better the prop choie, the higher that limit within the bounds determined by available horsepower, drag, and the reality of never-perfect efficiency.

MJD

Would a compressed air tank attatched to the carb with tx mixed inflight mixture control give you any more power as on board air ram boost.?

Before you get too fond of that idea, figure out the air demands - displacement in cubic inches times rpm = cubic inches per minute. It's rather a lot. At 6:1 every gram of fuel you carry requires 6 grams of air.. that's why air breathers fly for so much longer than rockets.

Are you pulling our leg here mkI?

A .40 running at 30,000 rpm can gulp alot of air on it's own in just a few seconds. You couldn't get a self contained air system that supplies all the air to that engine airborne, the tank would have to be pretty big to supply not just the volume but consistant pressure. Nitrous systems, from what I hear are used in RC cars, but would seem to be too bulky for small planes.