So just what is a "Stunt Run"

This "Stunt Run" thing has me a little confused..

Is it something we should all be striving for and if so just what does it mean. I have heard 4 strokes give a constant speed but that isn't always desirable, a 4-2-4 setup gives you a boost at some of the right places but again I have heard a constant speed pipe set up is better. Never ran a pipe so don't know if they give you any kind of break or do they run more like a 4 stroke. Dan's OS 20 is suppose to be the ultimate but I don't remember seeing anything explaining it's run characteristics.

Just what is the ultimate "Stunt Run" ?

LAST EDITED ON Oct-01-04 AT 12:14 PM (CDT)
 
>This "Stunt Run" thing has me a little confused..
>
>Is it something we should all be striving for and if so just
>what does it mean. I have heard 4 strokes give a constant
>speed but that isn't always desirable, a 4-2-4 setup gives
>you a boost at some of the right places but again I have
>heard a constant speed pipe set up is better. Never ran a
>pipe so don't know if they give you any kind of break or do
>they run more like a 4 stroke. Dan's OS 20 is suppose to be
>the ultimate but I don't remember seeing anything explaining
>it's run characteristics.
>
>Just what is the ultimate "Stunt Run" ?

Bob,

Thanks for asking this one. I've been meaning to ask exactly what the philosophy/physics of the pipe is/are. Pure constant speed set by the resonance, with more or less torque at the crankshaft in response to load, or a "break" into/out of resonance as dictated by load. I'm sure Brett can set me straight.

Andy

Hi Andy:

I've been learning alot about the big pipe set-up during the second half of the summer. Don't know if you heard my plane run last sunday but it is a good example of a pipe setup breaking in and out of 4 cycle. Right after takeoff, it drops into a throbbing 4. Pull it up and it just breaks into two near the top of the hemishpere. On the way down, it bumps back down and gives the braking effect of a diesel truck in low gear.

THis is a little different than the some of the other pipe runs you might have heard on Sunday--as they tend to run them a little faster with less pronounced break.

The time of the break (load condition really) and how hard a change it is can be adjusted with prop, fuel, compression, plug, and pipe length.

But man when it is right, its like flying the beast in slow motion! Even when the lap times are 5.1 or 5.2.

FWIW

Curt,
The Katana looks real nice! I am glad to hear that you have the pipe set up working the way you like it. It is really something when it is on (which is most of the time) and it doesn't take as much fiddling around with as some think. (or maybe we have just been lucky!)
I can't speak to the RoJetts, I don't have one, but the needle is pretty broad on the big PAs when everything is set.
I had notions of trying the 4 stroke engines, but now that is not part of the plan, but I know they work for many.
C'ya at Huntersville.
Bill <><

>This "Stunt Run" thing has me a little confused..
>
>Is it something we should all be striving for and if so just
>what does it mean. I have heard 4 strokes give a constant
>speed but that isn't always desirable, a 4-2-4 setup gives
>you a boost at some of the right places but again I have
>heard a constant speed pipe set up is better. Never ran a
>pipe so don't know if they give you any kind of break or do
>they run more like a 4 stroke. Dan's OS 20 is suppose to be
>the ultimate but I don't remember seeing anything explaining
>it's run characteristics.
>
>Just what is the ultimate "Stunt Run" ?

"Stunt Run" is a very poorly defined term that *usually* means a mild 4-2 break at relatively low revs. This phrase is a pet peeve of mine. The two most successful stunt engines of the last 15 years (40VF and PA61) don't meet the criteria for a "stunt run" by this definition.

By they way - there is no such thing as a "constant speed" pipe run. Piped engines change revs to just about the same degree as any others that work well. They sometimes don't change *state* from 4 to 2, and might run in a constant 2 or a constant 4. But they *are not* "constant speed" in any sense. In fact, it's quite easy to get far too much speed change even if its running in a constant 2-stroke. That's also the trick with the 20FP - the same effect allows some regulation, but properly running, they are not topped out lean in level flight and have a good response to maneuvering.

LAST EDITED ON Oct-01-04 AT 12:26 PM (CDT)
 
EDIT: I was typing while Brett was, so it isn't a direct response to his post. I hope it is in agreement with what he is pointing out, though!

In laymen's terms, it all depends on which method you prefer.

It is a "run" where a constant "manuevering" speed is achieved. The plane doesn't slow down when going up, and doesn't speed up when going down

This can occur with a "wet 2 cycle" (OS 20FP), a 4-2 break (ST 46, Fox 35), a solid 4 cycle (PA 61-65), and can be achieved with a pipe or muffler.

So many factors CAN come into play, so I just depend on set ups of those who are running the style I wish to run.

It does make it easier to become more precise in manuevers!
Bill <><

OK, so I would now define a pipe in this context as a device used to modify the power output characteristics of the engine (torque and speed as a function of load) to produce a performance envelope which better matches the desired power output characteristics demanded by the aircraft during maneuvers. It can be set up to produce a wide variety of power output characteristics which can include a 4-2 break, but need not do so. The pipe changes the engines performance because of its resonant properties which produce pressure waves at the exhaust port which are in phase with the engine's intake/exhaust cycle. A wide variety of possible power delivery characteristics are possible by varying the tuning of the pipe.

In a nutshell, the pipe literally let's you *tune* the power output characteristics of the engine.

How am I doing?

Andy

Sounds pretty right to me--but I'm a novice.

Despite being a self-proclaimed wordsmith, I will not even attempt to answer Bob's question, or to elaborate on Brett's response.

Instead, many of you have (rightfully) been scratching your collective heads over not just two BBS columns in recent issues of Stunt News dedicated to the same topic, my full-on approach here on SSW as it relates to a very direct answer to Bob's question via a very specific assemblage of hardware.

About a year ago, I was asking the same question as Bob.

Thanks to an entry-level, small-bore engine, a couple ARF Flite Streaks, some really good advice from a friend, my questions have been answered. In full. Too bad words fail me. C'mon by this weekend, Bob! We'll go flying!

Well Dan I have to say I truly wished I could come out and spend about a week playing with airplanes. Too bad my situation is such that this can only be a pipe dream right now. I am sure I would leave knowing alot more and wishing I had more time.

I thought it was the quick trip to the Porta-Pot after inhaling castor fumes all day!

At 80 plus feet per second, there is no human being in this world that says to himeself " I think I am going to start my pullout right about....now!" You would have only 1/16 of a second before your prize possesion was toast. In world class track, if you leave the blocks too soon after the gun goes off, you are considered to have false started. So please do not puff your chest out and tell me that you think your way to the turn point and then execute it. It can only be done when the model is travelling at a highly predictable rate of speed. And then only by other learned behaviour.

So the first rule of the perfect "Stunt Run" is that the speed must always be highly predictable at any point in the pattern.

You have just started your round loops and the first one is fine, the second a little faster and the third (how long did you say your arm was after the third loop?) faster still. But hey, this is how your plane normally behaves in a little wind.

The second rule of the perfect "Stunt Run" is that it not only must be predictable, it must produce the same speed for all segments of any particular manouevre.

As you get to the square eights, your engine begins to pick up a bit and keeps picking up till it quits at the end of the flight. This is fine by you because you can adjust to the increased power output and it keeps the plane from slowing too much since you have to hit those corners hard to impress the judges.

The third rule of the perfect "Stunt Run" is that the speed must remain predictable and constant from manoeuvre to manoeuvre. We do not want to be "adjusting" during the pattern. Just "flying". (oh and don't "hit" the corners so hard and your plane will fly a lot better)

You watch a top flyer doing the pattern and you put your watch on the plane; 5.5 seconds. Another top flyer is on the circle and you time him; 5.2 seconds. Another gets airborne and the time is 4.9 seconds. Same day, same air, same high level of polish on the pattern. You wish you could fly like that. So you say to yourself that GodLike flyer "A" told you that he "REALLY LIKED" flying at 5.5 seconds per lap. And who are you to argue?

The fourth rule of the perfect "Stunt Run" is that the speed you fly your plane will be different than the speed someone else flys theirs. It is dependent entirley on the model (line length, weight, balance, flap area, elevator area, power delivery system etc. It is not dependent on some preconceived notion of what the perfect speed should be. It is the plane after all that is doing the flying, you are only doing the steering. The plane MUST fly at the correct speed to be manoeuverable enough to execute the pattern as described in the rule book and not fly faster than that speed to preserve as much margin for error as is possible. Remember, the ground appears bigger sooner the faster you go.

You are feeling pretty good about your flying. Your engine purrs just fine and your speed is predictable, the same in all segments of a manoeuvre, the same from one manoeuvre to another and just right to make the model highly manoeuverable but not too fast to make you have a heart attack every time you point the nose at the ground. How could it get any better?

The fifth and last rule for the perfect "Stunt Run" is that the speed you have chosen to fly at must be capable of dealing with ALL types of weather. Most predominantly the wind. Even though you may be able to lay in a great pattern at 5.5 seconds per lap, you will not be able to do so in a strong wind. You could fly slower on less windy days and faster on windier days, but that is a loosing proposition. You need to train yourself to fly at one speed only. The one that gives you the highest percentage chance of scoring well in all conditions. Like Jack says, there is only one swing.

As to what will produce the above mentioned mystical run it is no secret at all. Any of the modern engines OS 20FP 40 & 46 LA, All PA engines, All Ro-Jett engines, the Discovery-Retro engines and many more will produce the above run with little effort. NEVER modify and engine (i.e. remove metal, adjust timing etc.)you are just wasting your money. You should be able to acheive a world class stunt run with no more than a couple of venturis, a couple of different plugs, a couple of head shims, a couple of different pitch props and a couple of gallons of different % nitro. If you need to do more then you do not need the above advice.

Interesting how the definition of stunt run changes over time.

15 years ago we had a NATs where 19 of the top 20 flew an ST 60.

Then a stunt run included a 2-4 break that was 100% reliable, and a engine that could handle anything from a 12-5 to a 14-6 depending on the wind and the weight of the airplane.

You knew you had a stunt run as soon as you hit the first square and started it up for the wingover.

Today there are almost as many views of stunt run as there are flyers. Anyone think we will ever again see 95% of the top 20 fly the same engine at the NATs...

Kim,

Well, I will agree with you on one point. The run has to be predictable and repeatable. I probably diverge from your comment pretty significantly after that. But that we can agree on.

LAST EDITED ON Oct-02-04 AT 04:20 AM (CDT)
 
I agree with Kim also on another point. The speed must be constant. Variation of speed during the figure prevents good flight. Your model can fly 4.8 or 5.6 and you can easily make good flight, but variation during the figure completely kills your reflexes. But that is another story. My point is that Kim did not say constant to WHAT – to air or to ground?

If you will imagine perfect setup, which keeps constant speed to ground (for example with some gyro, or accelerometer or whatever) and if you fly in wind, you will easily see that such a setup is counterproductive. Model will have big variations of speed to air and it will make different feeling in every maneuver and thus it will not allow perfect controlling, it will make problems entering figures down the wind, and it will make difficult to keep level in level laps.

So it looks constant speed to air is the way – and we do it – piped engine is approach to that. But again, such model slows down against the wind. It is not big problem in level flight, but if you want fly figures in strong wind, you need line tension -> centrifugal force -> lap speed, so the only way is to fly level laps far quicker then optimal in calm air. So dead constant speed to air is also not the best way.

So the answer to the question is probably in proper compromise:

1/ engine should accelerate loaded and nose up – that is to keep proper speed uphill and downhill

2/ model should accelerate unloaded (relatively to air) – that is not to slow down against the wind

3/ model should keep speed relative to the air

… and now be clever from all of that

It will need some sweet mixture of all those 3 points, but I think there is no universal “magic combo”. I think it always depends on model. I already wrote in past that I tend to believe that constant rpm setup (here I mean really constant RPM not constant power or whatever) is good compromise, and now playing with electric model having constant rpm engine I believe it more and more. It adds power uphill as it loads constant rpm prop, model accelerates in speed against the wind thanx its mass inertia and constant rpm prop well suppress speed variations in air.

But assume that we HAVE such a good compromise for example with strong .60 piped engine with 3-bl prop. Now I change only engine to .40. It has exactly 2/3 of original power so I will need to change to 2-bl prop of exactly same geometry. It should be the same, but what happens? It has also 2/3 of original response of thrust to speed variation. It means it will more slow down uphill; it will more accelerate against the wind. So it will just need little different mixture of those 3 points – more acceleration uphill. (I think Brett mentioned it already several times). It is so also opposite way – great setup on one model can be out of sense on another model.

The same can happen just by change of prop. Little different response of shaft load to speed variation (UCT prop or that Beringer’s trick on tips) or just prop thrust to speed variation (prop diameter and pitch) will need different engine properties (that is like Len does it ) – or - for given engine we can find proper prop (that is like Brett does it )

… I just think there is no general answer … it is one big bag of compromises

Just what is the ultimate "Stunt Run" ?

Anything over 500

Stunt run? Easy. The ST .46 in Kaz's Shark, making that beautiful bubbling sound, always in the precise places, where it should. Just lovely.

No, sorry. You can't have it. Kaz already got it. There's only one.

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"It rubs the lotion on its skin,
it does this whenever it's told..
It rubs the lotion on its skin,
or else it gets the hose again.."

Now, it places the lotion in the basket.
It places the lotion in the basket.
Put the ^%#! lotion in the basket!
Ahhhh!
Aaahhhhh Aaaahhh!"
-Buffalo Bill, from Silence of the Lambs

With an ideal stunt run, you do not hear the engine, because you are completely confident that the engine is doing exactly what you want it to do. Therefore your whole attention is focused on watching the beautiful maneuvers your airplane is making.