Question on Showplanes Cowling (Alternate Air)

FlightTwins · #1 03-12-2023, 02:18 PM

All,

We've run into a snag on the decision making on our RV-10. We'd like to use the Showplanes cowling but have noticed that the appears to be no provision for alternate air.

For those of you that have used this cowl or similar, how did you install your alternate air device? Pictures appreciated.

Thank,

The Twins

Richard Connell · #2 03-12-2023, 02:30 PM

I think most installers take the view that because the intake is split in two there is no need for alternate air.

rocketman1988 · #3 03-12-2023, 02:44 PM

Either duct can provide air to the engine so alternate air is not required or necessary.

Reflex · #4 03-12-2023, 07:50 PM

Bob,

I'm interested in the answer to this as well. Given your credentials, you likely have the correct answer. However, it would be nice to have a bit of an explanation.

If one intake can become obstructed, why can't two?

Thanks,

Fred

rocketman1988 · #5 03-12-2023, 08:11 PM

Quote:

Originally Posted by Reflex

Bob,

I'm interested in the answer to this as well. Given your credentials, you likely have the correct answer. However, it would be nice to have a bit of an explanation.

If one intake can become obstructed, why can't two?

Thanks,

Fred

As I have often stated, there is no such thing as a perfect design; drill down deep enough and you will always find an unacceptable failure mode.

That said, the likelihood of both ducts becoming obstructed at the same time, well, it would be exceptionally low. You could also say the same about the alternate air system on the stock cowl. Could the primary intake and the alternate air become obstructed at the same time? Yes, but it would be extremely unlikely.

There is more we could say about potential obstructions but again, obstructing both ducts at the same time is very unlikely.

supik · #6 03-13-2023, 03:32 AM

In theory, icing could be the common enemy for both intakes at the same time. The 2 drain holes might prevent the engine from dying completely though.

KatanaPilot · #7 03-13-2023, 03:53 AM

I don't disagree with Bob as he has a lot more experience with this cowling on his RV-10 than I do. The likelihood of dual blockage does seem to be very small. If there is enough icing to block both intakes, I think you would be in a lot more trouble from icing on the airframe at that point.

All this being said, I have been building an alternate air solution for my Showplanes cowl. It probably won't work on most others though, since we have a 2" extended hub Hartzell prop and the very short SDS throttle body - which required us to extend the fiberglass Showplanes intake (there was virtually no trimming of our cowling at the firewall). It's a fairly simple concept utilizing in part, the alternate air components from a Van's FAB. It is not completely developed at this point, so I can't say with certainty that it will work. It may well be a solution in search of a problem, though.

lr172 · #8 03-13-2023, 06:38 AM

Quote:

Originally Posted by rocketman1988

As I have often stated, there is no such thing as a perfect design; drill down deep enough and you will always find an unacceptable failure mode.

That said, the likelihood of both ducts becoming obstructed at the same time, well, it would be exceptionally low. You could also say the same about the alternate air system on the stock cowl. Could the primary intake and the alternate air become obstructed at the same time? Yes, but it would be extremely unlikely.

There is more we could say about potential obstructions but again, obstructing both ducts at the same time is very unlikely.

The stock alt air design covers more scenarios, as it bypasses the filter. A possible scenario is complete filter blockage from something ingested, ice or just overloaded with debris. The alt air entry is in the lower cowl area and is well protected from ice and outside debris, so don't see how that can be obstructed when compared to something sitting out in the free stream air. Not saying that the approach you mention is bad, only challenging the fact that the alternative is no better.

I R A EE · #9 03-13-2023, 07:04 AM

I too struggled as to whether to address this or not. I am with Bob as blockage is unlikely. However the icing that Krea brought up is what pushed me to design an alternate air system. I used to fly a FIKI Piper Matrix and early winter morning I got pushed over the class bravo at Detroit by ATC into very light clouds. After hanging out there for was a very short time I started to pick up a light frost on the leading edges. Not enough to cycle the de-ice boots. Something I would be very comfortable flying the RV in. On the Matrix airframe the air filter is on the nose of the cowl below the prop hub. Admittedly very different from the Show Planes snorkels. Long story short, the frost building on the wings was enough to block the air filter and I had to go alternate air to maintain altitude. The icing of the air filter is complete speculation on my part though the alternate door opened very easy which was highly unusual for that plane nearly confirming my icing suspicion. Additionally when I landed there was nothing on the air filter and I never experienced another situation like that with that plane.

What I have done is to cut the top off of the air horn of the snorkel and install a sliding door on top that actuates toward the engine to open. The area of the fully open door is equivalent to the throttle body. It is actuated with a ratchet cable (same as for the heat boxes). All hardware is external to the assembly so no parts to get sucked into the engine. I am still building so I do not know whether it will work. One concern I have is the amount of force that will be on the alternate air door during the situation when it would need to be actuated. Essentially there is a 540 cubic inch vacuum pump sucking on the door if the air filters ice up. Unlikely to get to that extreme so actuation should be achievable with the cable setup I have. I have a Barret cold air intake and the SDS throttle body that it is fitted to. It was tight getting it all in there. Unfortunately I have it all torn down and in the paint booth now so no pictures. I could follow up with some if you are interested.

Feel free to reach out if you want to chat on what I have done.

rocketman1988 · #10 03-13-2023, 09:20 AM

Quote:

Originally Posted by lr172

The stock alt air design covers more scenarios, as it bypasses the filter. A possible scenario is complete filter blockage from something ingested, ice or just overloaded with debris. The alt air entry is in the lower cowl area and is well protected from ice and outside debris, so don't see how that can be obstructed when compared to something sitting out in the free stream air. Not saying that the approach you mention is bad, only challenging the fact that the alternative is no better.

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.