May be its not worth it? (but worth a try)
Yukon said:
Has anyone given any thought to the expansion taking place in the lower cowling due to the radiant heat rejection.......?
I don't think its a matter of does it work. It's more about do you need it, at least year round. If you do, it could be a life saver in hot climates. Of course our Kay-nook friends in the hinterlands to the north are "a-boot" cool temps, Eh!
Good point about temp expansion of air. Reading the NACA report the added heat I recall is a small factor. The following quote does not really address temp, it does show what the exit is all about, in their analysis.
"The flow density in the lower plenum
is determined from the temperature rise across the engine.
This information heretofore has not been supplied by engine
manufacturers, and estimates based on experience must be used.
Typical values range from 50?C to 70?C. The exit area (5)
is then sized to accelerate the flow so that its static
pressure is the same as the local external flow (6). The
exit area acts as the system throttle in that the flow rate
and associated pressure drops will adjust so that the exit
pressure matches the external pressure. For a given flight
condition, expanding the exit area increases the flow rate,
and conversely." NASA Report
As inlet shape, position, size and internal smoothness of the plenum play a factor in efficiency of the inlet, the exit is probably more than just AREA. Shape, location and internal flow leading to it are all players.
You could have two lower louvers and reducing the main exit to just accommodate the exhaust? You could also add a movable "slider door" (cowl flap) on the louver. Its easier putting slider flaps on the louver than in stock cowl air exit area, which is busy w/ exhaust pipes & engine mount tubing.
However???? Since vertical induction Lycs already have this fwd scoop and lower bump, the exit just tucks iback n the same frontal area. Why not just use that for the exit, verses adding more exits? However you are right, if you NEED it you need it. I think most think you will not, even in the southwest usa. With the fwd facing induction and scoop-less cowl, may be the louver makes more sense.
The goal of the designer is the accelerate the exit air and make it parallel to the free airstream. It's easy to say and hard to do.
Tom Martin said:
I am perplexed to say the least. The system is helping to raise the engine temps but it is not helping with speed and in fact may be reducing speed.............In level trimmed flight when I pull the cowl flap closed, the nose goes down in a very significant and repeatable way.
A couple points, observations about your cowl flap. It ONLY reduces exit area. As an observation its an exit air restrictor more than a cowl flap.........
Most cowl flaps actually increase exit area, hanging out more in the breeze when open, which promotes more airflow. Your design reduces mass air flow, which will increase engine temp as you say. I like you would think it would reduce cooling drag but in retrospect probably not, at least not with a simple hinged flap in the cowl exit. It is a cowl flap but I am just making a distinction between "classic" cowl flaps which tend to open out into the slipstream.
My observation is exit air velocity or "THRUST" was reduced. I'm sure this could cause a pitching moment change.
Thinking about it, less thrust OUT the lower cowl would make the nose pitch down. Closing the cowl flap, exit area is less so cooling is less, but you lose thrust as well? I suspect air speed changes may be small, hard to measure. It would be interesting if you determine you lose SPEED.
In theory you cowl exit should be sized for MIN area and the cowl flap adds area. However this is hard to do. It is easier to make the cowl opening the max and use a flap to reduce the exit area. Nothing wrong with that set-up. I know I'm learning from your experiment. Please keep up with reports.
I was thinking along the lines of previous post, stalling the inlets as well. If the inlets "stagnation point" moves outside the inlet with the cowl flap closed, it could cause more spillage drag. There is no free lunch? It all has to work together.
Clearly the exit is the throttle of the cooling system (and affects ever thing up stream in the cooling system.
Are cowl flaps on a C-182 to increase cruise speed or more about controlling temp in a climb or Hot/Cold climate operations? You'd assume it improves cruise speed, but how much? Any C-182 flyer's out there know the speed difference cowl flap closed/open in cruise? May be the gain on a RV is so small it's hard to measure?
May be Richard Vangrunsven knows SOMETHING? May be this is was why he did not use cowl flaps. He knew a well balanced and designed cowl would not gain much speed when balanced against the complexity and weight of a cowl flap? In 1993 Van wrote about the value of the cowl flaps, and difficulty executing a design with the existing RV geometry. Van has published cowl flap ideas for the RV-4. He even tried them on the RV-6. Bottom line he "did not find real noticeable benefits......"
Van also made similar observations about turbulence from a cowl flap (or exit air restrictor flap) not helping cooling drag (due to the turbulence). Since the air is not accelerated and smooth as it exit it does some harm (aka thrust or at more drag). Van also felt real cowl flaps, aka C-182, where a pain, high maintenance and required removal every time you un-cowled the engine. I think Van has tried and considered cowl flaps and just found it did not earn its way onto the design or meet the KISS total performance mission of the RV. However in hot temps or fly in wide ranges of temps, it may be a necessity.
It is like retract gear.............if your retract gear doors don't fit tight and seal well when up, its not much better than a well stream-lined fixed gear set-up. I recall a retract on a RV was good for only 6 mph. Of course when you get going real fast, say well over 250 mph, retracts become a necessity as drag increases prohibitively. May be cowl flaps are not quite right for most RV missions.
I think seasonal Cowl changes may be a good trade off in hot climates for those who need it. You can 'louver' the cowl in the summer and remove/block them in the winter?
db1yg said:
Alex, BTW, contrary to some beliefs, when I added louvers, albeit cut down versions of Alex D.'s product, there was no measurable difference in cruise speed (IAS or TAS) at specific power settings.
Good to know. There's nothing like flight test. My worry was a louver may cause more drag (and less thrust) because they're not facing totally aft. There's no way you can accelerates the air, so the exit air out of the louver is slow, merging with fast outside air. In theory that is plume or mixing drag, but if the air's not shooting out, its more just leaking out, drag is minimal. However there is no thrust gain either. Which may be a fair trade off. Drag is all academic if your engine is red lined temp wise.
AlexPeterson said:
I believe the exit has enough area, but there does seem to be some misbehaving going on. I think there is much to learn from these (pics): (cowl exit tuff test)
May be Alan's mod, extending the cowls exit aft, gives a net gain in light of the reverse flow tuff test pics? May be Alan will do tuff test?
(Alex great pictures BTW)