It seems like a cogent discription Ed. I have been studying the problem for some time. I like your no core example, much cheaper but it will only fly once. (And for a short time!) The question I have been pondering is, does it really help us to consider a exit ducting to direct our exit flows. The data you presented seems to indicate that it does. The dynamics of the pressure drop across the core contain compromises related to the efficiency of the heat exchanger, flow of the water in it and air through it. Many of the designs I see lately pay very little attention to the exit and re-merging the flow. In core-in-the-standard-inlet systems such as yours the exit ducting may not be practical. This is a problem I have see with the Eggenfellner Sabaru installations as well. At least the rotary can have some exit area without the cylinders right there in the way! The exit question tends to favor the chin scoop. The problem is that this has always proven to be a high drag choice. Currently I'm favoring a vertical side radiator (or radiators) ducted to the outside (cowl) blowing into the engine area with a diversion duct to turn the air towards the normal rear bottom exit. Possibly with a cowl flap for climb. These have never been easy choices. Often we intend an elegant solution, only to be rebuffed by the need for hoses, wires, and exhaust pipes and other unimportant stuff like that. ;-)
Thanks for all your research,
Bill Jepson
-----Original Message-----
From: Ed Anderson <eanderson@carolina.rr.com>
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Sent: Thu, 8 Nov 2007 5:05 am
Subject: [FlyRotary] Re: Total,duct, Ambient or Velocity????
Hi Bill,
It is my opinion, based on my limited knowledge of the topic, that dynamic pressure in the duct is the most significant factor. If you don't have it - you have no flow. If you do have it you will have flow but you could have significant Major losses - that's why you may need other types of pressure measurements to figure out the problem. In fluid flow talk, they appear to refer to loss of energy through wall friction as a major loss as it is not recoverable (but this is minor at our speeds) , while trades between dynamic and static in the duct result in "minor" losses which may or may not really be minor.
Here is my understanding, you would like to convert dynamic energy to static pressure increase in front of the core as that slows down the velocity reducing drag and tends to give you more even velocity distribution across the core (assuming little or no separation of flow from the duct walls). You would like the greatest pressure drop across the core which results in the highest velocity through the core tubes generating turbulence for better heat transfer.
However, there is a balancing point, more pressure drop generally means better heat transfer from metal to air, however, it also generally means less mass flow because of the resistance. Too much pressure drop = too little mass flow and overheating, too little pressure drop = great mass flow but higher duct drag and less heat transfer per unit time which can also lead to overheating.
I like to use this example to emphasize the point. You would get maximum pressure drop by placing a solid board across the duct - however, the air flow would be nil and cooling likewise. On the other hand, if you remove all obstructions in the duct (including the core) , the pressure drop would be nil, the airflow would be maximum but cooling would still be nil. The only significant difference is the no core approach is cheaper and causes less drag {:>)
In any case, all the literature I have read seems to indicate that the difference in pressure between the inlet and out let of the duct is a (if not THE) key factor. That dynamic pressure is the only thing (assuming no fans/blowers) that will move significant air through the duct. Since this dynamic pressure is referenced to the dynamic pressure available in the freestream flow as that is what it starts out as, I personally think referencing dynamic pressure measurements to ambient air is what we are mainly interested. This is rather than referencing it to the duct static pressure as shown in the diagram. But, you have to remember this is all from the guy who has not done any duct instrumentation.
But, my reason for focusing on dynamic pressure is that you can infer a lot from your duct dynamic pressure readings about what is going on in the duct. If your dynamic pressure is down, then your static pressure is up and vice versa. If you have dynamic pressure then you have flow while static pressure does not necessarily tell you that.
However, it all really depends on what you are trying to figure out on what measurements you take.
It would appear if you know how to interpret what you are measuring then all provide some useful information.
That's about the extent of my limited knowledge.
Ed
----- Original Message -----
Sent: Thursday, November 08, 2007 12:28 AM
Subject: [FlyRotary] Re: Total,duct, Ambient or Velocity????
Ed, The slide is a good way to explain the various references. I am still confused as to what will give you the "best" data. The static in duct pressure compared to the total, or to the velocity? It probably doesn't matter if you use the same method all the time.
Bill Jepson