Alex

 

Remembering back, It seems as if the manifold pressure, at WOT seemed to be always less than ambient. Ram would increase that. Of course, a charger (turbo or super) would increase the "ram" pressure even more. In Normally aspirated, there are two types of alternate air. Manual controlled-- ie levers and cables, etc, and automatic, which is exactly as you described.(and what I will be using)

 

The usual and critical part of the induction system for blockage seems to be between the air inlet and the filter. The "flapper: door generally has a light spring holding it closed. Remember that the throttle plate is downstream of  the filter. When the throttle plate is open, the flowing air between the plate and the trochoids (of course that part of the induction system downstream of the plate) has the least resistance  and thus the MP is the highest-- going near but not to ambient. As the plate is closed, the little trochoids are sucking against a partially closed orifice and the MP goes down. (of course, the MP is measured between the plate and the trochoid intake port.

 

The intake manifold pressure upstream of the throttle plate does not suffer the same fate, as a matter of fact, logic (who says that is right) says that the pressure in that part of the manifold might even be inversely affected.

 

Now if you have a blockage before the alternate air flapper (assuming automatic, the trochoids are sucking against that blockage making the pressure in the plenum between the blockage and the throttle plate decrease below ambient, the door swings inward (high ambient vs low internal) and the engine can still breathe.

 

In the planes that I have had with the automatic alternate air, if memory serves me correctly(no guarantees) the flapper plate was part of the air cleaner assembly.

 

Rich

 

 

Ya don't want to pay the fuel or maintenance on anything burning kerosene..I take your point on less airflow. Can you tell me which pressure is greater in the intake manifold, the influence of ram air pressure or intake vacuum pressure. Or does it vary from one to the other according to the cycle. 

For an "automatic" homegrown system on a 2 rotor would it work to have a spring loaded door opening inward into the intake manifold, fed by interior cowling air when the main system is blocked. It makes sense to me to have a door open for a negative pressure in the intake manifold which i imagine would be the case of a blocked regular air intake and ongoing intake vacuum pressure. Would a door like this be kept shut under normal conditions of positive ram air, to preserve ram air which is what we want?

Alex M

On Sun, Jul 7, 2013 at 12:08 AM, <ARGOLDMAN@aol.com> wrote:

Interesting concept, I can see how that would eliminate a lot of FOD, however that is not how alternate air systems work for piston/rotor type engines.

 

That is almost the reverse. My guess is that the airflow in the PT6 is hughly greater than in a 2 or 3 rotor engine and the kinetic energy of the FOD is significantly less. Additionally, the alternate air is to allow air to flow if the induction system gets blocked. this valve works in the reverse. The greater the pressure upstream the more it will open and in fact will vent the presure (FOD/rain etc) overboard

 

If the intake becomes blocked as in a filter or induction icing, the pressure is reduced tending to seal the manifold.at this valve.

 

Would that we could all fly Turboprops

 

Rich

 

In a message dated 7/6/2013 1:12:11 P.M. Central Daylight Time, alex.molteno@gmail.com writes:

Hello Lynn

Are you referring to a similar system you see on the Pratt and Whitney PT6 series engine, basically reversing the flow of air 180 degrees before entering the compressor (throttle body for us i guess). The inertia of the FOD or ice particles doesn't let them "make the turn" into the throttle body and they exit the duct via a bypass flap. In this case the opening of the flap creates the 180 degree turn and spillover air exit?

Alex M