Question on Showplanes Cowling (Alternate Air)

[lr172]

Originally Posted by rocketman1988

Ok, we can play the what-if game, just for fun.

In the showplanes scenario you would need to block TWO 5" diameter inlets, separated by two feet, at the same time.

Admittedly, flying through a flock of birds could provide this scenario but it is highly unlikely, they would need to be large birds, and it would take TWO. With the standard cowl, it would only take one bird in the intake and alternate air may be available. Point is, if you figure a one bird direct hit on either cowl, air is likely still available in either situation.

Icing; encountering enough ice to block both inlets certainly is possible but I would submit that if you were in those conditions, you shouldn't have been anywhere near there to start with. Once again, possible but highly unlikely. In this case, we assume both cowlings encounter enough ice to obstruct the inlets. The win here goes to the stock cowl, as you said. Again, if you are in an area with that much ice, I would question the pilot more than the cowling design.

Debris. I am curious as to where one might find the amount of debris, in flight, that would obstruct the intakes...it would literally take a shovel full on each side. or in the case of the stock cowl, one shovel full. As far as plugging up the filters, the showplanes system uses standard K&N units. These are comparable to those used on many off road vehicles that operate in environments far "dirtier" than where our aircraft operate...and they still operate. Assuming you were able to plug the showplanes intakes, the stock cowl would also be plugged but alternate air may still be available.

I suppose you could make the point that volcanic ash could plug them up but again, why are you flying through an ash cloud? In this case, I see no advantage to the stock system either, in that opening the alternate air bypasses the filter and allows volcanic ash into the engine.

One final thought on the alternate air system on the stock cowl just to note that there have been some failures with the system over the years. The win here would go to the showplanes cowl which has no mechanical moving parts.

In summary, once again, there is no perfect system. There will always be an unacceptable failure mode. The good news in this discussion is that the risk of that mode occurring is very, very small, in either case.

Won't challenge much said here and agree the risk is low. HOWEVER, my fear is not typical icing that would cause me to exit an area quickly. It is rain followed by a temp drop and that is a very real scenario in an IFR climb. If the filter gets very wet from flying through rain, a drop in temp to near freezing could cause real issues. Also remember that we are dealing with a lot of air movement in this area, so the temp drop of the filter will be quick to follow the ambient, not to mention latent heat of evaporation that can drop the filter well below ambient. I was once flying in clear skies below a cloud deck (very low dew point spread) at altitude (2-3* above freezing) without a trace of ice or frost on the airframe. All of sudden airspeed drops and pitot heat fixes it. Not a scientist, but smart enough to know that I don't really understand how things freeze over or how likely that could occur on an air filter.

[hgerhardt]

Years ago, there was an airline pilot who flew his carb'd RV-6 into a snowstorm which apparently clogged the air filter with snow. The stock heated alternate air system did nothing as it ducted in hot air UPstream of the filter, so with a clogged filter, no clearing of the snow was possible. After this crash is when Van's released their retrofit bypass air valve kits which duct in air downstream of the filter. They have these kits for carb and FI installations.

This Show Planes intake system could also easily clog the filters with snow. With two symmetric intakes, why would one clog and the other not?

Looks to be easy enough to modify the Tee section at the throttlebody intake by making a flat at the bottom and adding a Van's bypass air door.

Here's the Van's doc on the bypass systems (this covers vertical and horizontal induction): http://www.lightaircraftassociation..../Fab-sb_05.pdf

And below is the Show Planes intake system. The filters are clamped to the Tee, and if those ducts fill with snow, both filters will both clog.

[rocketman1988]

Originally Posted by hgerhardt

Years ago, there was an airline pilot who flew his carb'd RV-6 into a snowstorm which apparently clogged the air filter with snow. The stock heated alternate air system did nothing as it ducted in hot air UPstream of the filter, so with a clogged filter, no clearing of the snow was possible. After this crash is when Van's released their retrofit bypass air valve kits which duct in air downstream of the filter. They have these kits for carb and FI installations.

This Show Planes intake system could also easily clog the filters with snow. With two symmetric intakes, why would one clog and the other not?

Looks to be easy enough to modify the Tee section at the throttlebody intake by making a flat at the bottom and adding a Van's bypass air door.

Here's the Van's doc on the bypass systems (this covers vertical and horizontal induction): http://www.lightaircraftassociation..../Fab-sb_05.pdf

And below is the Show Planes intake system. The filters are clamped to the Tee, and if those ducts fill with snow, both filters will both clog.

Once agin, what is the likelihood? Are you going to intentionally fly into a snowstorm that could produce the amount of snow necessary to plug those intakes?

If the answer is yes, then I question your judgement as a pilot...and with all of the flight and weather tools available today, accidentally flying into that area should be non-existent.

As always, build what you want.

[rocketman1988]

Originally Posted by lr172

Won't challenge much said here and agree the risk is low. HOWEVER, my fear is not typical icing that would cause me to exit an area quickly. It is rain followed by a temp drop and that is a very real scenario in an IFR climb. If the filter gets very wet from flying through rain, a drop in temp to near freezing could cause real issues. Also remember that we are dealing with a lot of air movement in this area, so the temp drop of the filter will be quick to follow the ambient, not to mention latent heat of evaporation that can drop the filter well below ambient. I was once flying in clear skies below a cloud deck (very low dew point spread) at altitude (2-3* above freezing) without a trace of ice or frost on the airframe. All of sudden airspeed drops and pitot heat fixes it. Not a scientist, but smart enough to know that I don't really understand how things freeze over or how likely that could occur on an air filter.

Yes, there is always a possibility when you are in those conditions. It is difficult to get a decent comparison between a pitot tube which has a nominal diameter of about 1/4" with an induction intake that has a diameter of around 5".

You also need to take into account that the filter element is oiled and is hydrophobic, ie repels water.

Definitely things to consider; it all comes down to what you are comfortable with.