Gary,
The inlet pitot tube was set up to measure dynamic pressure. It recorded the difference between total and static - just like the aircraft pitot tube. At first I was surprised by the readings, but the numbers were confirmed to within a few percent by computing mass flow via all the pressure measurements.
The 360 inlets are so small that it drives up the inlet velocity and prevents the more common external pressure recovery. Most inlets, including those on subsequent Lancair designs, are much larger relative to the airflow required by the engine. This does pose a problem for the 360 in terms of pressure recovery since it relies so heavily on internal diffusion. The upside is that there is virtually no disturbance to the external flow.
For the diffusers, constant angles were ruled out simply due to space and geometric constraints. Even-though we have extended prop hubs, more length is really needed to conform to any conventional rules of thumb on diffuser angles. I too couldn't find much published on bell mouth diffusers. The bell mouth, with appropriate surface roughness, is an attempt to keep flow attached as long as possible. There was no expectation of keeping it completely attached except along the upper surface. If the flow is slowed sufficiently before separation, losses can be brought to acceptable levels. Note that the non-standard inlets ahead of the diffusers already start the diffusion process with much more controlled geometry. If your sharp exit has sufficiently slowed the flow, I imagine it would have the same effect. Starting with slightly larger inlets would help immensely in this regard.
The exit of the 360 is in need of serious help. The firewall to exit transition is a hard 90 degree corner. Subsequent Lancair designs incorporated a nice exit tunnel to help get the air out in a more controlled fashion. In the 360, the lower engine mount attachment sits right where an exit ramp would be located. As such, the exit area is over-sized to account for its inefficiency. On my plane, the area could be cut down as there is more than enough air flow, but there is one minor problem. I am carburetted and therefore have a cowl scoop that starts pointing back up towards the fuselage once clearing the carb area. I can't increase this angle any further since flow already separates from the scoop at very low angles of attack (at speeds above ~ 190 KIAS). Additional difficulties in cleaning up the exit are created by the retracting nose gear
penetrating this area. I do have some ideas for the exit, but they have been placed on the back burner pending completion of other improvement projects. If and when I get a new exit up and running, I will instrument the engine compartment again for a good comparison.
Chris Zavatson
N91CZ
360std
From: Gary Casey <casey.gary@yahoo.com>
To: lml@lancaironline.net
Sent: Wed, February 17, 2010 11:38:55 AM
Subject: [LML] Re: Cowl pressure
Chris,
Thanks for posting the link to the most excellent report. But I have a couple of questions: You said you used a "pitot" probe in the inlet and reported the velocity at about 100% of aircraft velocity. But aren't you really measuring total pressure, which is the sum of the velocity and static pressures? So some of the pressure recovery could have been ahead of the inlet, which I think is a good place to have it. That reduces the amount of recovery that has to be done in the diffuser. I've seen two(at least) recommendations as to the shape of the diffuser section. Conventional wisdom seems to specify a constant angle to prevent separation at the downstream portion. Another recommendation was to use a bell-mouth exit as you have done. I couldn't follow the rationale for the bell-shaped exit so I used a nearly-constant angle with a sharp exit. Comment? Finally, I see that the pressure
below the engine is very low and I would expect to see some losses as the low-velocity air exits the cowl. Wouldn't it be better to construct a small-area converging nozzle to provide for the exit? The lower cowl pressure would go up, but that would provide the energy to accelerate the air to something closer to the free-stream velocity. True?
Thanks again for the excellent information.
Gary
Paul,
That one doesn't have an easy singular answer.
Figure 15 in the link below shows required mass flow and pressure drop and how this relates to CHT. Unfortunately it doesn't cover the most desirable CHT range.
You can see what I measured on my plane in Figure 13.
http://www.n91cz.com/Pressure/PlenumPressure.pdf
Chris Zavatson
N91CZ
360std