Lightweight Ducting

[Thruster88]

2 hours ago, skippydiesel said:

 

 

My last aircraft ATEC Zephyr, used the two conical  Rotax filters - worked well. (Maxed out at about 125 Kt - Normal /Eco Cruise 100-110 Kt Indicated)

 

The Sonex is a significant step up in performance - outside/cool air supply to the engine, is part of this. (Max Cruise 152 Kt True - Normal/Eco Cruise 130 Kt True)😈

I don't see the down side with ditching the cold air intake and going pod filters. Very slight loss of power at full throttle but how often is that used. For any part throttle operation the throttle just has to be a little more open to compensate for the less dense warmer air. I see coolant carburetor heaters so carb ice should not be a problem. 

[skippydiesel]

1 hour ago, Thruster88 said:

I don't see the down side with ditching the cold air intake and going pod filters. Very slight loss of power at full throttle but how often is that used. For any part throttle operation the throttle just has to be a little more open to compensate for the less dense warmer air. I see coolant carburetor heaters so carb ice should not be a problem. 

If anything the airbox arrangement makes carby ice more likly, than conical filters sucking warmed air.  As you noted, the manifold heaters (claimed to prevent icing) are part of the existing system. My last 912 with just conical filters, never even hinted carby icing. (This is not a claim that Rotax 912's do not experince carby ice)

 

When I was trying to isolate the cause of my leaking carby bowl breathers, I ran the Rotax conical (pod?) filters - no leak.

 

Hot inlet air, will have some (?) performance loss implications. I agree, likly small, however this is not the only consideration. A lot of work & cost, went into the existing air box. One of my flying objectives - economical (fuel used for NM covered) touring, so no unnecessary fuel consumed (throttle setting).  Together ,this means that it would take a genuine safety or performance issue, for me to ditch the airbox.

 

The photo I posted was of the unfinished item - now looks a lot more professional,  sporting a black satin look, heat resistant, ceramic, finish.

 

I have no complaints about the airbox performance -  now that it's not using the carbys for part of its support system. The clearances, due to limited space, is a minor problem that only impacts on my choice of airbox to carby duct, the topic of this thread /discussion😈

[Thruster88]

Thin wall, 2mm, silicon 50mm tube could be an option. Possibly it may only need to be clamped on the carburetor allowing more movement. 

 

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Also it would be wise to plumb the carburetor breather tubes, bearing in mind they need to sense air pressure near the carburetor, so that any discharge of fuel exits the bottom of the cowling to reduce fire risk.  

[skippydiesel]

Thanks Thruster,

 

Will look into the Silicon tubing, looks promising. As you suggest, IF it is a tight fit, security clamping at just one end may be acceptable.

 

Float bowl breather tubes are plumbed into the airbox (same air pressure at carby inlets). The airbox has drains that exit into a temporary catch can,  so that any fuel leak can be monitored.😈

Edited March 6 by skippydiesel

[Jack Jones]

Stainless or aluminum formed duct sections + short flex joins – More rigid approach, but avoids all the internal wire issues while still handling heat well.

Realistically, there isn’t a perfect “SCAT but no wire” equivalent that keeps all the same properties. Most people either accept SCEET/SCAT wire, or switch to silicone and redesign how it’s supported
 

[skippydiesel]

Hi Jack Jones,

 

Thanks for your thoughts. 

 

I have a quantity of aluminium concertina duct  (a Gates product to use as cold air "blast" tubing) that is pretty much what you describe. I feel that it may not stand up to constant movement/vibration all that well, so haven't gone down this track with the carby/airbox ducting.

 

I have purchased a thin walled silicon tube/duct on lsBay (made in China) that claims several times the heat tolerance required and likly has the characteristics I have articulated.

 

As the de wired SCAT is doing  a good job at the moment, I will not be trying the silicon duct for some time - will report back when I do.😈

[Moneybox]

16 minutes ago, skippydiesel said:

Hi Jack Jones,

 

Thanks for your thoughts. 

 

I have a quantity of aluminium concertina duct  (a Gates product to use as cold air "blast" tubing) that is pretty much what you describe. I feel that it may not stand up to constant movement/vibration all that well, so haven't gone down this track with the carby/airbox ducting.

 

I have purchased a thin walled silicon tube/duct on lsBay (made in China) that claims several times the heat tolerance required and likly has the characteristics I have articulated.

 

As the de wired SCAT is doing  a good job at the moment, I will not be trying the silicon duct for some time - will report back when I do.😈

I just spotted this thread because we’re out prospecting for gold, came for three days a week ago. Skippy it seems to me that that air box, similar to mine, is too chunky for the space. You don’t need all that air volume so a slimline box would have fitted much more easily, perhaps made from rectangular aluminium tube rather than round. I realise it may be a bit late but I think it would simplify your ducting/vibration issue. 

[skippydiesel]

I hope your prospecting went well Moneybox.

 

In my limited experince, Rotax 9 carburetor engines have two air inlet system, that can be used;

• Simple, no ducting, air supplied from within engine bay, through individual carburettor conical shaped filters.
• Rotax or other airbox, supplying cool air from outside engine bay to both carby's. Air filtered at some point in the ducting. The airbox will usually be mounted separate from the carby's so there will be  flexible connections to duct air from airbox to carby's

Note: The carburettors are connected to the inlet manifold via "rubber" vibration isolators. If you should somehow restrict the action of the isolators, the carburettors will spill fuel, through the float bowl vent and possibly cause accelerated wear of internal components.

• The simple, no duct air filters, allow for the carburettors to move ie no impact on the rubber vibration isolaters. 
• In the Rotax airbox, the box is mounted on the Ring Mount and flexible ducting connects it to the carburettors. The flexible ducting minimises the chance of interfering with carburettor movement.

The initial problem with my custom airbox was that it was mounted on the carburettors themselves, almost completely negating the action of the rubber vibration isolators - fuel was being ejected from the float chamber vents.

 

My first solution was to dispensed with the air box & fit the simple conical filters. The fuel spillage stopped BUT my engine was now breathing hot air (+10C above ambient) from the engine bay.

 

I really wanted to use the airbox but how to mount it so as to allow the carby vibration isolators to perform as designed?

 

I don't have a Rotax ring mounting system to mount/connect to.

 

Make/install a firewall mount but I really didnt want to take my aircraft out of the air/back to the workshop if I could avoid it. 

 

The solution seems to be to mount the airbox on the engine itself.  This would remove the weight/movement restriction on the carby's AND at the same time have the airbox move with the engine, a definite plus. However a second issue arises, the space between carby's & firewall is such that the airbox outlets and carby inlets are only about 15 mm apart (very close). Flexible ducting depends, to some degree, on the length/distance of the unsupported section to allow movement ie the shorter the distance the less movement can occur. This means that whatever flexible ducting between the two, that is used, will need to be very flexibly, so as not to restrict the movement of the carby's. 

• The origional thick walled "rubber" hose/duct was way to stiff.
• Metal concertina duct may work for a while but I suspect that fatigue cracks would rapidly form, allowing unfiltered air to enter the system.
• Variose other suggestions ie using modifies rubber concertina suspension / steering rod covers, were considered
• SCAT tube seemed to be the solution however the coiled wire stiffener impeded the flexibility of the duct over such a short distance (15mm) - solution, remove the wire.

Removing the wire needs to be done with considerable care, as there is a tendency to damage/rip the flexible silicon wall. The result is a soft, lightweight flexible tube that works very well.

BUT I wanted a solution that did not involve damaging the SCAT and possibly reducing its service life in the process. 

 

I have found/purchased some soft, thin walled (2mm) silicon ducting (no wire). It looks like it will do the job, although heavier and possibly slightly less flexible than the modified SCAT.

 

I am in no hurry to try the new stuff as the  SCAT is doing the job for the moment😈

 

 

[rodgerc]

Buy one….Cut away most of the centre portion of the bellows so you have two ends to connect to your Bings with hose clamps….Make up two split ring flanges from some 0.040” 6061 T6 sheet that will secure the “cut ends” to your airbox with pulled rivets….Go flying. 

[skippydiesel]

Looks good Rodgerc.

 

I would be concerned about ;

• The ability of the concertina not to fatigue in the "valleys"
• My ability to detect fatigue cracking in those "valley" should it occur.

 

The stuff I have purchased was suggested by Thruster88 (above) .

I am hoping that it will do the job very well.

The claimed dimensions(ID) and heat tolerance are a perfect fit.

The smooth, flexible wall should perform well and any deterioration show up quite readily.

If it does as I hope, the minimum amount I purchased, should provide plenty of replacement material (if required) & last for several 100hr service intervals.

😈