Airbox

Has anyone ever developed an intake silencer for a model engine?

LAST EDITED ON Nov-18-04 AT 10:14 PM (CST)
 
>Has anyone ever developed an intake silencer for a model
>engine?
>

Not for muffling purpose on a stunt plane that I know of. However, the airbox idea is *very interesting* to me for several applications.

The reason is that it seems to be clear that the intake configuration, shape, and orientation sometimes makes a very large difference in the way the engine runs. I was stunned by the improvement in run symmetry with various intakes - like spigot venturis (*HUGE improvement*) and even a lowly diffuser made from a stocking. But it goes back to the good old days of the schneurle wars, where it was noted by several that long, skinny intakes helped the otherwise wild differences in the run symmetry from inside to outside.

The theory at the time, I think probably wrong, was that the intake was "tuned" like a tuned mini-pipe. That's a real effect that can happen, but I think much more likely, the effect was one of removing a lot of the off-axis airflow at various angles of attack. Same think with a diffuser. The spigot is a different theory, but related - air blowing cross-axis to the venturi results in varying local pressure at the "dribble hole" in the regular venturi. Sticking the outlet in the middle of the venturi avoids this problem.

The airbox/plenum idea is the ultimate along these lines. A sufficiently large "accumulator" on the intake *should* result in nearly dead-constant airflow into the intake. The Belco engine with the intake on the "top", is the only example I can think of.

Might also have to wonder what's going on around a 4-stroke intake, buried in the fuse, behind the engine, and in the heat and odd pressure environment. Not to mention that a backfire can set the whole plane on fire (which has happened - Uncle Jimby's Saito flambed the inside of his engine compartment, but fortunately "blew out" before it burned the entire airplane up. My 4-stroke design had an airbox that sucked from the top of the fuse (with a screen over it to make the gigantic hole less noticable). Never built it, of course.

Bear in mind this is only slightly better than educated guessing, but there's a whole lot more to the idea than simply how much suction you are getting.

Brett,

One thing I can tell you is that ram air situations are not optimum for 4 Strokes. Tried setting up a couple of "scoops" to ram air into a sort of plenum box that the venturi drew from. Too much pressure and it became impossible to needle. Be fine on the ground but once in the air it would go overlean and quit. Every time. Perhaps smaller intakes would have been the ticket, but I gave up on the idea while still in test phase.

Randy,
Your ram air would probably work great if you had an OS programable fuel injection. A crash would be very painfull though$$$.

Steve,

The idea was to stablize the amount and direction of the intake air. I had planed to work out a sort of vent system that closed somewhat as the the airplane accelerated and opened as it slowed. In the end, it was too complicated and I thought the system would probably pick my first official flight to fail.

I decided to just put a series of baffles that brought air into the intake area without raming it into the venturi. Seems to work OK.

LAST EDITED ON Nov-22-04 AT 11:04 PM (CST)
 
>Steve,
>
>The idea was to stablize the amount and direction of the
>intake air. I had planed to work out a sort of vent system
>that closed somewhat as the the airplane accelerated and
>opened as it slowed. In the end, it was too complicated and
>I thought the system would probably pick my first official
>flight to fail.
>
>I decided to just put a series of baffles that brought air
>into the intake area without raming it into the venturi.
>Seems to work OK.

I would have put the intake on the side, so it wasn't effected by airspeed or pitch angle of attack. I think this is probably the effect Randy noted - but that's what I want it to do!

>Brett,
>
>One thing I can tell you is that ram air situations are not
>optimum for 4 Strokes. Tried setting up a couple of "scoops"
>to ram air into a sort of plenum box that the venturi drew
>from. Too much pressure and it became impossible to needle.
>Be fine on the ground but once in the air it would go
>overlean and quit. Every time. Perhaps smaller intakes would
>have been the ticket, but I gave up on the idea while still
>in test phase.

I wasn't considering ram air. Just something to isolate the intake from the variations (and in the case of a 4-stroke, from the hot air likely to be waftng around in the dead spot behind the engine).

Brett,

Yea, I know. My original idea was exactly that. I was concerned about hot air swirling around behind the engine and constant variations in pressure. I was trying to move outside air through the area by using intakes that vented air behind the engine. The original idea caused a big increase in air pressure behind the engine and the result was the engine quitting right after take off. Subsequent alterations kept fresh air coming in without a huge increase in air pressure. Still not sure about variations in pressure, but the general idea seemed to work OK.

>Brett,
>
>One thing I can tell you is that ram air situations are not
>optimum for 4 Strokes. Tried setting up a couple of "scoops"
>to ram air into a sort of plenum box that the venturi drew
>from. Too much pressure and it became impossible to needle.
>Be fine on the ground but once in the air it would go
>overlean and quit. Every time. Perhaps smaller intakes would
>have been the ticket, but I gave up on the idea while still
>in test phase.

Ron B.
F4Fguy

Brett:

Your right,back in the fifties,race car designers were installing "ram air" pipes for more power at speed. The result? Some improvement in power over a narrow range at the top end. Then some bright young engineer actually thought about it and removed the "back of the box".The result? Even more improvement and much more consistent running throughout the rangeThe reason? Lower air temp is much more important than the (very) small gain in ram air pressure,and it's constant regardless of speed.

As to tuning,one of the reasons FOX 35s are easy to handle is the short inlet path,putting it's frequency far outside the tuning range.Tuned intakes are certainly possible ,and may even be reasonable, but making one work for stunt will be at least as difficult as the first tuned exhaust pipe,probably more so due to the difficulty of measuring the parameters. For example,the system has to be treated as a resonant flask,with a varying volume/frequency. Given the irregular shape of the interior volume and the fact that the resonance is affected by ALL the timings and piston positions,it's a daunting task.

ETA did it years ago on the ETA 15!

I think I read somewhere that 10% of the engine noise in our 2-stroke engines comes out the intake.

Tony, my education is lacking. Can you describe ETA please?

>Tony, my education is lacking. Can you describe ETA please?

Ian,

I've heard about the ETA .15 tuned intake, but never seen one!
I have however seen a tuned intake work quite spectactularly on a Cox Pee Wee. About 10 years ago there was a bit of interest in indoor team-racing on 20 foot lines using these engines in our region. One of our club members experimented with K&S Aluminum tubing extending from the 0.020 intake back to just past the wing T.E. I guess the optimum length would have been about 50 to 75mm or so. Gave a noticeable increase in revs and a 10-15% increase in range from the standard tank. It would work best with rear intake motors. I'm surprised that someone hasn't tried it with modern teamracers.

I too have only 'heard' of the ETA 15 tuned intake - in the Irish club at Milton Keynes sandwiched between Irvin Ella (my flying buddy and engine collecter) and Dick Place in conversation.

Tony

There's a very good article in Model Airplane News December 1995 by Tore Paulsen. The subject was sound. A considerable portion of the noise on an engine comes from the intake. He had dimensions for an intake pipe resonator. Part I of the article was in the November issue.

LAST EDITED ON Nov-19-04 AT 10:34 PM (CST)
 
I have used these twiced before. I didn't get the results I wanted. on my models they delayed the cycling of the engine, and made it run like it did not know where it was in the pattern . Sort of a "dumb motor run". I like them to come on and off quickly.
I am also pretty sure that Windy had one on his Nats plane around early to mid 90s. may have been the yellow side winder or the Toxic Avenger. It would be interesting to ask him what he found?

As far as noise reduction goes, I am sure there is noise from the venturi but I could not measure any difference. I think the problem on my plane was the prop makes far more noise than anything coming from the venturi

Engineers found in engine sound studdies, that anything that moves makes noise.
I have been in engine test rooms with fifty or so engines all running at the same time and the exhaust being run through the roof .
The noise was unbeleivable.No putt putt, but what sounded like
tons of gear noise.It was impossible to tell which engine was running and which wasn,t unless you looked at the crankshaft.The operator had to use a tach to tell if the engine was running properly.
In four cycle engines,engine clearances (part tolerances}and intakes account for a large portion of noise.
In Nextel cup the cars use an air box with the intake located at the base of the windshield because it is a low air pressure point on the car as well as a place where the air is cooler.
Cooler air means denser air which means more oxygen molecules.
To me venturie shape and length is another example of black magic.
Ive seen them make a big difference, but like life ,they seem to be a compromise.You give ,you get.