|
-
POSTING RULES

-
Donate yearly (please).
-
Advertise in here!
-
Today's Posts
|
Insert Pics
|

09-07-2009, 08:25 PM
|
 |
|
|
Join Date: Oct 2005
Location: 08A
Posts: 9,500
|
|
<<what happens to the chart if the inlet size is held fixed, and the exit size varied?>>
The above is plotted for intake/exit ratio, so you can derive what you want with some calculator banging. However, here's a direct answer.
0.3125 sq ft intake area (stock RV-8), 8000 ft and 200 mph. X axis is exit area in sq ft, and in the middle you'll find 0.4, roughly the stock exit size.
Again mass flow and cooling capacity are quite proportional. However, look at the drag rise with increasing exit size; obviously you want the smallest exit which delivers the mass flow necessary to cool the anticipated HP. Note the stock exit will theoretically cool over 300 HP at this altitude and velocity; it was sized for climb speed so it is way too big for high speed.....the downside of a fixed exit size.
Note as exit size decreases, drag decreases much faster than mass flow. Kinda makes one wish for a variable exit nozzle.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
|

09-07-2009, 10:54 PM
|
|
|
|
Join Date: Nov 2006
Location: Metropolis, Illinois
Posts: 8
|
|
Getting thrust from intake air
Quote:
Originally Posted by DanH
Flight data is good.
BTW, the yellow line ("exitV') is poorly labeled. It should read "exit velocity ratio". In this example it never quite reaches 1. If it rose above 1 the drag and exitV lines would cross and the result would be thrust. Dream on.....
|
If I remember correctly, I saw a Discovery Channel show on a nuclear-powered aircraft engine prototype...no moving parts. The engine heated incoming air in the honeycomb-shaped core and they got thrust out of the other end.
We have all this expanding heated air coming off the cylinders and exhaust pipes, not to mention the exhaust itself. Maybe we could tap into the potential thrust of the air exiting the lower cowl instead of trying to suck it out of a square hole.
I wonder what would happen if the inside of our lower cowlings were shaped more like the expansion chamber of a rocket nozzle?
__________________
Dan Babenco
Metropolis, IL / Paducah, KY
RV-7A QB N383DB "Last 10%"
|

09-08-2009, 02:03 AM
|
 |
|
|
Join Date: Nov 2007
Location: Keller, TX
Posts: 151
|
|
Heated air
For another example, the radiator on the P-51 gives a net increase in thrust and not drag as you would think. This plus the super critical airfoil made it about 40kts faster than the Spitfire with the same engine.
|

09-08-2009, 02:05 AM
|
|
|
|
Join Date: Mar 2005
Posts: 5,685
|
|
Some thoughts
Larry Vetterman's experiment with the fairing on the stock outlet and small exhaust pipe and cooling air outlets outboard of the current stock location is good evidence that the cooling air outlet configuration can be improved. My work with baffling in the lower cowl to create three chambers in the cowl and channel the cooling air flow showed that even with the stock outlet the drag can be reduced sufficiently to increase the speed of my RV-6A by 4 kts at 6,000 ft density altitude. This took a lot of experimenting to work out and many things tried did not increase the speed or even decreased the speed.
At the moment I am very handicapped with what I can do because of responsibilities here at home but the brain still ponders the possibilities.
1 - I think Larry's fairing idea is a must do for me but I don't like the shape or the louvers.
2 - I think that if I can smoothly curve the sides of the fairing inboard then back to form a concave sidewall fairing aft of the FAB and nosegear and maintain most of the vertical dimension of the stock outlet it might be a better shape than the teardrop and it would allow for two needed cooling features.
3 - I need an outlet for the oil cooler and other zone 3 (the chamber between the upper and lower cowl baffling and the firewall) air that I currently vent through a narrow slot at the top of the stock cooling air outlet. This would be provided by a fixed vertical slot at the rear of the new fairing.
4 - A flap on one side of the fairing (or both if necessary) operated by a manual vernier control could be opened for ground and low speed (climb) operations.
Bob Axsom
|

09-08-2009, 07:20 AM
|
 |
|
|
Join Date: Oct 2005
Location: 08A
Posts: 9,500
|
|
Quote:
Originally Posted by Dan Babenco
We have all this expanding heated air coming off the cylinders and exhaust pipes, not to mention the exhaust itself. Maybe we could tap into the potential thrust of the air exiting the lower cowl instead of trying to suck it out of a square hole. I wonder what would happen if the inside of our lower cowlings were shaped more like the expansion chamber of a rocket nozzle?
|
We don't really suck it out of a hole; pressure in the lower cowl is higher than ambient pressure. Heating would contribute to raising pressure further, a good thing. Upper plenum pressure must be even higher or air doesn't flow down through the cylinder fins, thus very good pressure recovery at the inlet is required; it's a system. And although the best practical exit wouldn't look like an expansion chamber, "rocket nozzle" is a pretty good thought picture. It sure as heck wouldn't look like a standard RV exit. Think velocity.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
|

09-08-2009, 09:29 AM
|
 |
|
|
Join Date: Sep 2005
Location: 1T7, Kestrel Airpark , Texas
Posts: 773
|
|
Dan hit the nail on the head with "it's a system". Nothing magic about individual components; inlet size, outlet size, ratio. They all must work in unison to produce the optimal effect.
Just think about how a standard config cowl flows in very basic terms. Airflow enters the inlets and a certain amount is converted to pressure to move across the cylinders. I say certain, because while you can do the math for a given airspeed and inlet size, as the Miss. St. data shows some is going back out the inlet and some is being leaked away through various gaps, holes, ect. This air emerges from the cylinders into a very large space with highly turbulent flow and the exit which is on the other side of a large volume space has air flow back into at as well.
The system design goal should be to control the cooling air flow from the time it enters the inlets all the way to the exit, with air velocity gradually increasing as much as possible. Then merge the outlet air with the free airstream with minimum upset.
Each builder must decide what the engineering goal is for their system, because maybe good CHT's and simple design are good enough for some. While others may want the best efficiency they can possibly get.
__________________
Wade Lively
-8, Flying!
N100WL
IO-360A3B6D, WW 200RV
Last edited by RV8RIVETER : 09-08-2009 at 09:38 AM.
|

09-08-2009, 11:24 AM
|
|
|
|
Join Date: Dec 2005
Location: Sedalia, Colorado (KAPA)
Posts: 320
|
|
Then use the exhaust
To Dan's point about it beign a system, it would seem that if we:
A. Were careful not to send any more air through the cowl than necessary to cool the engine,
B. Sculpt the discharge points for minimum internal and external drag (keep the junk out of the way),
C. Focus the exhaust energy to augment the flow, and finally,
D. Correctly size the discharge to achieve the desired discharge velocity, we should be able to end up with net thrust.
Before doing all this work, it would be interesting to know how much zero cooling drag would increase what is now a 160 kt, 8000 foot cruise speed on a 160 hp 6?
__________________
____________
Duane Zavadil
RV-6a, IO-320
|

09-08-2009, 12:39 PM
|
|
|
|
Join Date: Sep 2006
Location: Bozeman, Montana
Posts: 858
|
|
Top of Cowl Exit
I have considered putting an exit on the rearmost portion of the top of the cowl. It appears to be a low pressure area, I can see some exhaust leak staining indicating exiting air in one spot where I left a little gap in the fiberglass.
One could put in a flap, or I thought of just cutting some slots in the cowling. A shutter type mechanism would allow you to stop the airflow and remove the drag.
I see the two advantages of the top of the cowling as being a lower pressure area than the bottom and better cooling of the engine compartment after shutdown.
Hopefully someone will chime in with the CFD Air Pressure image of an RV. I have seen it in the forums, but can't find it.
Hans
|

09-08-2009, 01:35 PM
|
|
|
|
Join Date: Oct 2006
Location: Melbourne, Australia
Posts: 427
|
|
Hans,
The low pressure area on top of the cowl is actually near the forward edge, rather than towards the windscreen.
I remember reading about an aircraft some guy had built that used a reverse-flow cooling system, where the cooling air entered below the engine, flowed vertically upwards through the cylinders and then exited on the top side of the cowling via vents mounted in the foward section of the cowling. Apparently placing the vents here gives a good increase in cooling flow at high AoA, due to the negative pressure area increasing in magnitude. Sadly, whatever I type into Google doesn't help me find it again!
The downside of having cooling vents in the top is when it rains and soaks your engine - OK if you hangar it when not in use, I guess.
I must say, I like Larry Vetterman's ideas a lot. I've been thinking about having this fairing moveable like a cowl flap, hingeing it at the rear edge of the existing cowl. It would cover the majority of the exisiting exit area, allowing enough leakage past the exhaust pipes to give the minimum flow area By making it hollow, when it opens up (hinging downwards), you can regain most of the flow area again. If you shape it properly, the extra flow can be accelerated before it exits between the cowl flap and bottom skin of the fuse.
(hard to describe without pretty pictures I know!  )
A
|

09-08-2009, 01:45 PM
|
 |
Forum Peruser
|
|
Join Date: Jan 2005
Location: Austinville, Alabama
Posts: 2,458
|
|
Peter Garrison and Melmoth
Quote:
Originally Posted by Andy_RR
...I remember reading about an aircraft some guy had built that used a reverse-flow cooling system, where the cooling air entered below the engine, flowed vertically upwards through the cylinders and then exited on the top side of the cowling via vents mounted in the foward section of the cowling. Apparently placing the vents here gives a good increase in cooling flow at high AoA, due to the negative pressure area increasing in magnitude. Sadly, whatever I type into Google doesn't help me find it again! 
...
|
I believe you are thinking of Peter Garrison and his Melmoth I/II. I'm not sure if both of his designs used updraft cooling, but I believe one of them did. Also, there may have been others besides Mr. Garrison as I don't think the idea originated with him.
Two of my RV friends poo-poo'd me recently for reading FLYING magazine. Mr. Garrison's monthly writings alone are IMO worth the cost of the subscription. Of course, that is just MHO.
P.S. After I posted above, I found web page describing Melmoth 2. Here is a link to Peter Garrison's page of Cooling Air as used on Melmoth 2.
__________________
Don Hull
RV-7 Wings
KDCU Pryor Field
Pilots'n Paws Pilot
N79599/ADS-B In and Out...and I like it!
?Certainly, travel is more than the seeing of sights;
it is a change that goes on, deep and permanent, in the ideas of living." Miriam Beard
Last edited by rv7boy : 09-08-2009 at 01:51 PM.
Reason: Added Melmoth 2 link.
|
| Thread Tools |
Search this Thread |
|
|
|
| Display Modes |
Linear Mode
|
Posting Rules
|
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts
HTML code is Off
|
|
|
All times are GMT -6. The time now is 08:14 AM.
|