Fellows,
>
>  I have been thinking up this crazy idea of trying to make intake 
> runner tubes out of carbon fiber. My question is regarding length of 
> the velocity stack. Does the length make a difference? Could a 
> velocity stack be more or less the entire length of the tube? 
> Meaning, if I have a 12" intake tube, could the entire length of the 
> tube be a gradual taper to the diameter of the block opening? Would 
> that mess with the speed the air is traveling in the tube? 
> Aerodynamics is not something I have a very good handle on, and am 
> hoping someone out there in Fly Rotary land can shed some light.
>
> Thoughts???
>
> BTW,  Any reasons why carbon fiber should not be used for intake 
> tubes? May make the velocity tubes a mute point.
>
> Thanks for any thoughts.
>
> Ben Schneider

 

It would take the rest of your life to read all there is to read about intake (tuned) lengths. So here is my view on it.

 

The pipe organ idea is in play, but in my opinion is gifted too much interest. In actual operation just about anything works prety close to just fine. I just reviewed some formula Super Vee stuff and note that a number of intakes were tried. Super Vee was a class where a 1600 CC VW (Rabbit) engines were used in race cars. The stock parts had to be used but could be modified by machining, The intake runners could be up to 32MM, and the mechanical fuel injection (probably from the diesel I think) could be used.

 

HP well above 180 was the result, from VW parts. A straight 32MM tube with a bellmouth. Not very exotic.

 

The tuned length of any system has many harmonic peaks. They look like the primary frequency but are removed from it by a factor of 4. So you have harmonic peaks at multipals of 4. So, 4,8,16 and so on. So there are secondarys that show up to either side of the primary, and some of those are reenforced by combinations of the secondaries, so the 16th harmonic can be quite clear and easy to find on a scope.

 

The outcome is that it would be foolish to build an intake with one primary frequency in mind. Say you want to cruise at 6,000 RPM, and built tubes to peak at that frequency. If you cannot keep the engine at that RPM exactly, the effect is lost. Then you find the engine in a null between peaks and performance is now less than that of the thrown together mess on that plane in the next hanger. 

 

On takoff you would want to have say 6,500 or 7,000 available. Where would you tune for that?

 

So I drop back to what works in most situations. In general long columns of air operate at lower frequencies. Short columns at higher frequencies. Large diameters at lower and small diameters at higher, and so on.

 

The ideal shape for intake runners is tapered. Yep, thats it. not often seen due to complexity of construction, but if you want to talk ideal.................

 

The horn shape at the end of the tube is to prevent a vena contracta that reduces the effective diameter of the tube. A short rounded edge that turns back 180 degrees is the very best and works well in confined spaces like a plenum. For carbs, not so good. At harmonics of, and just off peak frequencies, you will get a ball of fuel standing in space just outside of the horn. In those cases, a longer tapered horn works better to cover up a number of frequencies and make the ball less obvious.

 

The tuned length thing works less well in bent systems, and since the nearly best length will involve runners over the top of the engine, there is at least a 180 turn involved, and for side port engines, two turns involved. So, to start off we are in tuning trouble with a bent system that tends to null our best math picture of ideal.

 

Round runners are ideal for efficiency in moving air through the smallest cross section. However, in a turn

the "D" shape takes over from the round, in that the mass of the airflow tends to move along the outer wall, and at some speeds may be seen moving along the inside wall in the wrong direction, and also may

slow and upset the flow along the outer wall. In most cases the "D" shape may have slightly less cross section than the original round runner. Also there may be improvements should a slightly course or rough surface be left along the flat of the "D" so as to keep flow attached and reduce the higher velocity along the outside. Uniform velocity is better than a number of velocities or even reversed flow.

 

The taper............

 

Large tube diameters have lower velocity and small higher for any fixed depression. If you imagine the rotor as a blade and the intake flow as a sausage being pushed through the blade, then you are one sick puppy.............

 

However, it works for me.

 

So would high velocity at the port face, (where the rotor cuts off each chunk) be better than low velocity?

 

The higher velocity for any unit of time means more sausage (or fuel air mixture) entering the chamber.

 

Yes high velocity is better. But drag in the runner develops at the square of velocity, so if the runner is the same diameter for all of its length, drag will be high, the boundary layer will be thick and easy to upset. Suppose then that the taper was only in the last several inches of the runner. Or if you want the ideal, the taper starts at the horn and runs the length of the runner tightening to the port size at the manifold face. Less drag overal. Thinner more stable boundary layer, and the highest possible velocity at the port face.

 

Another thought..........

 

The primary and secondary ports feed just one rotor each set. The primary ports are those in the center iron, and are smaller than the secondary ports in the end irons. The engine operates for most of its life on the primaries, as little HP is called for in normal driving. The primaries are small, so as to maintain high velocity, and crisp throttle response. 

 

I would join the two intake ports and run a single runner for each rotor housing. Less volume consumed.

Lighter intake system. Smaller plenum. Probably no loss of performance. The molded part then could be tapered, have the "D" shape, the rough inside turn, a length of straight tube to mate to a slip tube to "tune" runner lengths. 

 

Lynn E. Hanover