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  #61  
Old 08-22-2007, 05:04 AM
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gmcjetpilot gmcjetpilot is offline
 
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Default Cowl flap down up speed difference? No jet thrust here?

Quote:
Originally Posted by Nuisance
Here are a couple of pictures of cowl flaps on my 185.

You can see that the shape is better with the flaps closed. Ole' Clyde Cessna figured it wasn't so important at climb speed. What is really interesting is that the area is only 41 sq. in. when closed, and even then part of it is blocked by the boost pump. hmmmmm.
John have you done any speed test with the cowl flaps, ie in level fast cruise flight. I suspect it will not make much speed change. It should a little just from the cowl flaps not hanging out in the breeze. Just curious, its not a homework assignment. I have plenty of GA plane time with cowl flaps, singles and twins and all I remember is I used it for temp control.

Quote:
Originally Posted by Yukon
Several posts, including the last have eluded to "thrust" a proper cowling produces. From everything I have read over the years, a good cowling actually propells the aircraft forward, rather than producing drag. Think about it.......only 29% of the heat energy is converted into mechanical motion, the rest is discarded out the exhaust and cooling system.

The cowling is like a jet engine. Cool air is forced into a restricted space, large amounts of heat is added, and is allowed to escape out a restricted
exit, at high speed. I think our cowlings have been optimized by Van to take advantage of this principle, but at the expense of climb cooling. I'm starting to think a good set of cowl flaps is a good idea for those of us in the desert.
That way, the "jet thrust" could be optimized for cruise, but the cooling still be available for climb.
I think I said or alluded to "thrust" My bad, I miss spoke. I was referring to exhaust adding (in theory) a small amount of thrust. To have thrust you need a change of momentum, action / reaction. The fact is air is leaving the cowl is slow, slower than the free air stream and also lower pressure (as the tuff pictures show).

Even though heat energy is added by the time air is in the lower cowl, the air has expanded in volumn and lost pressure going thru the cowl, engine fins, baffles and all the junk under there. Cooling DRAG is a way of life. It's a large part of the total airframe drag, so small improvements are very worth while.

After air goes in it actually it's expanding, losing pressure and velocity (energy) while doing the work of cooling. The only time you have a chance to acclerate the air is right at the exit, which is the firewall. Aft the firewall you are outside airframe in the slip stream. Typically we run our exhaust aft the firewall. We can hope to get some thrust out of the exhaust but to optimize you would need to make a nozzle shape. Again trade offs, more thrust by may be less HP due to more exhaust back pressure? It's always something.

We can acclerate the exit air as much as possible. One way is to reduce the outlet as Dave Anders suggested (less area w/ same flow = faster velocity). Total pressure has to be great enough to re-accelerate the air, so making it more efficient upstream (minimizing pressure loss) helps. This is where Dave Anders says you should have a smaller outlet area than larger relative to the inlet.

The second way is the exhaust augmenter (gas injector) tube. Here is an example.



It's pretty ingenious, using the fast moving exhaust to create suction and accelerate the cowl air (using the energy in the exhaust to add back to the cowl air). It does improve cooling and lowers drag. However in a twin you have room to hide this big tube, from the firewall to the wing trailing edge. With our configuration you have to cut the pipe off right at the firewall and run the augmenter tube fwd into the cowl. Not easy.

Alan and others found mixed results with their RV exhaust augmenter experiments. Not surprising, with the augmenter tube totally out in the breeze, which adds parasitic drag. Also being under foot was negative. There's little room in the RV. I think the T-34 Mentor has an augmenter. Also as Alan said the augmenter is louder, which is also a factor.

From Wikpedia:
The 310 first flew on January 3, 1953 with deliveries starting in late 1954. The sleek modern lines of the new twin were backed up by innovative features such as engine exhaust thrust augmenter tubes and the storage of all fuel in tip tanks in early models. In 1964, the engine exhaust was changed to flow under the wing instead of the augmenter tubes, which were considered to be noisy.

In the above the word thrust is used. A better word really should be injector. There have been studies (check NACA server under exhaust gas injector). The tube diameters and length need to be designed properly. They are not small dia or length.


Bottom line airframes are made with the concept of an augmenters from the drawing board. Some famous multi-engine WWII planes and Airliners have augmenter tubes. I am not going to try it based on the reports I have heard here. There is always the practical aspect, adding more weight and so on. Also cutting the pipes short affects the efficiency engine. Trade offs as usual.

Yak w/ side exhaust augmenters. Exhaust pipes end just aft of the last 'S' in the "MISS". Down side? Exhaust soot/stains.
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Last edited by gmcjetpilot : 08-22-2007 at 07:50 AM.
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  #62  
Old 08-22-2007, 05:14 AM
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Default Taxiing on hot days

This is a great thread with a lot of good advice. Van probably knows a lot of this and has a big industry connection base but in reality, the cooling system is a compromise between desert summers, long slow taxi times behind a bunch of airplanes (think of Sun 'n Fun) with 30 airplanes ahead of you.

Alan has very small inlets as we've seen but how does the CHT look during a midsummer lengthy taxi? Or a long hold for traffic? Remember that springtime and winter temps will cause dramatic CHT/oil variances. We have airplanes knocking around 200 MPH or better, the -6's close to redline TAS. I personally like 145-160 MPH climbs but not everybody will do this.

Nevertheless, I'm enjoying this thread and seeing careful step by step testing as Alan is doing is neat.

Regards,
Pierre
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  #63  
Old 08-22-2007, 06:51 AM
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Default Big oil cooler

Quote:
Originally Posted by pierre smith
Alan has very small inlets as we've seen but how does the CHT look during a midsummer lengthy taxi? Or a long hold for traffic? Remember that springtime and winter temps will cause dramatic CHT/oil variances. We have airplanes knocking around 200 MPH or better, the -6's close to redline TAS. I personally like 145-160 MPH climbs but not everybody will do this.
Pierre remember he has a secret weapon (that he mentioned), a huge oil cooler ducted to its own external scoop he can manually open. He has the air/oil cooled IO360.
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Last edited by gmcjetpilot : 08-22-2007 at 06:59 AM.
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  #64  
Old 08-22-2007, 07:13 AM
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RV8RIVETER RV8RIVETER is offline
 
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Default

Quote:
Originally Posted by pierre smith
Alan has very small inlets as we've seen but how does the CHT look during a midsummer lengthy taxi? Or a long hold for traffic? Remember that springtime and winter temps will cause dramatic CHT/oil variances. We have airplanes knocking around 200 MPH or better, the -6's close to redline TAS. I personally like 145-160 MPH climbs but not everybody will do this.
Pierre

While exhaust augmentors/ejectors provide a big benefit for reducing drag by energizing the exit flow, I think they also can play an important role here. By increasing the pressure drop across the cylinders at high AOA or taxi.

As for the thumping.... NACA report 818, which details the ejector study states that there is no performance difference with regard to straight or curved ejectors, so it may be possible to minimize the negative effects by slightly turning down the outlet. Also, while a the optimum L/Da ratio was 8, good perfromance gains were had at L/Da ratios of 6. Also the experiment was conducted with a single cylinder 85hp engine. One would think that with 4 cyl engine decreasing the time between power pulses may have a sightly different effect. I guess what I am saying is for real world practicality we may not have to use the "optimum" configuration and can arrive at a design that is both helpfull as well as easy to live with.

There is that engineering compromise again.
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  #65  
Old 08-22-2007, 07:50 AM
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rv969wf rv969wf is offline
 
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Default Heres the answer

Quote:
Quote:
Originally Posted by pierre smith
This is a great thread

Alan has very small inlets as we've seen but how does the CHT look during a midsummer lengthy taxi? Or a long hold for traffic? Remember that springtime and winter temps will cause dramatic CHT/oil variances. We have airplanes knocking around 200 MPH or better, the -6's close to redline TAS. Regards,
Pierre
This is a worst-case scenario. I live in the panhandle of Oklahoma; we had one week this month where it never got under 100F during the day. I flew my ?6 one afternoon in this yucky hot air and I peeked my CHTs around the 340F-350F range at 100F ROP. In the pattern with power pulled back and slowed up the CHT?s dropped to around 325F. On the ground taxing the CHT?s drop to 280F. It doesn?t matter if I?m facing into the wind or not. Idling at 700-800 rpm and the mixture leaned for maximum idle speed, the CHTs will settle in the 270-280F range as long as you want to sit there on the ground.

The oil temp is not an issue either. I?m using a huge 11 row SW dual bypass cooler just for this reason that has the external air supply.

Coating the exhaust with a barrier will also help with under cowl temps while taxing in hot weather and I do agree that most airplanes need huge inlets or cowl flaps to keep them cool on the ground for lengthy times.

I agree with Pierre that some of the Vans airplanes have already pushed there limit and are reaching or exceeding VNE speeds. But there is always going to be someone out there that wants to go faster or is trying to outrun his buddy. I?m not an air racer, just a cruiser liking to get from point A to B in the most effective way efficiently.

Some of the things that I tried and did might not apply to everyone?s airplane, but it has been a learning experience with great return in the end.
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  #66  
Old 08-22-2007, 09:55 AM
Yukon Yukon is offline
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Default

George,

I hear what you are saying, and I am not suggesting that the cowl provides
a majority of our motive power, but I think it is doing more than most people seem to think. These airplanes display an amazing amount of efficiency utiizing stock GA powerplants, and I am facinated about how they do it.

When I get mine flying, I really don't want to be fussing with cooling problems out here in the desert, so I will probably start out with a set of louvers. If they cost too much in speed, I suppose I will experiment with cowl flaps.
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  #67  
Old 08-22-2007, 12:07 PM
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Default

I wouldn't think you'll have cooling issues with the 235 if you're still going that route. You getting any work done on that thing with the lovely Maya around?
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  #68  
Old 08-22-2007, 12:31 PM
Bill Dicus Bill Dicus is offline
 
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Default Augmenters

One benefit of augmenters on T-34A was experienced duriing a night landing with a folded nose gear. Couldn't stop prop before nose fell thru (even with full nose down trim to try to increase elevator effectiveness). It's amazing how far that nose comes down with no wheel up front! The only damage was to prop tips, scrapes on nose gear doors, and flattening and abrasion of lower exit area of augmentors. They probably prevented some additional damage. Bill
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  #69  
Old 08-22-2007, 05:17 PM
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gmcjetpilot gmcjetpilot is offline
 
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Default I hear you Bro!

Quote:
Originally Posted by Yukon
George,

I hear what you are saying, and I am not suggesting that the cowl provides
a majority of our motive power, but I think it is doing more than most people seem to think. These airplanes display an amazing amount of efficiency utiizing stock GA powerplants, and I am facinated about how they do it.

When I get mine flying, I really don't want to be fussing with cooling problems out here in the desert, so I will probably start out with a set of louvers. If they cost too much in speed, I suppose I will experiment with cowl flaps.
No I hear you Bro! Yea I think we're splitting hairs, but that is what it takes to go fast. However going fast and worrying about temps is no fun. I think you will be fine with the stock cowl.

What I'm looking for, which I think most folks do as well, are little improvements, mods that result in more efficiency with out major mod. Of course those "little mods" often take a lot of work and effort. The faster you go, each successive mod gets harder, resulting in less pay off each time. At some point people just say, good enough.

There's no doubt "Van The Man" has thought about almost everything under the sun, coming up with his combo (the total package), efficient, light, easy to build & maintain, in stock per plans config. There are great ideas and theories that work, like augmenter tubes, cowl flaps & louvers, but practical aspects (trade-offs) come into the picture. Each builder is free to config and mod as they see fit, and that's the fun part of experimentals.

We'll never have zero cooling drag or 100% cooling in extreme hot climates, but we can keep trying to shave drag and increase efficiency, while keeping the temps stay in the green. Keep up the good work!
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Last edited by gmcjetpilot : 08-22-2007 at 05:21 PM.
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  #70  
Old 08-23-2007, 09:09 AM
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Default My Cowling Mod



So I wasn't going to post this b/c the it is such an early test picture, I know, it really looks ugly. The 2 "tunnels" are now slightly larger and made of stainless to avoid the burning problem. (Yeah this was STRICTLY a test part) and no it's not held on just by tape, it was riveted. There was some work done interally to to smooth the air flow a bit. My testing seemed to indicate this mod increased speed by 1-2 knots, but don't rely on my test data. I did a gage R&R on my airspeed indicator and found that even corrected true airspeed for density, and humidity was only repeatable to 2-3 knots, so my observations could be totally in the range of error. This version ran a bit hot but after baffling improvements the cruise CHT was down to 325F, still climb had to be limited b/c the exit area was so small. So as I said I opened it up a bit by making the ramp angle less severe and slightly wider,that seemed to do the trick. I don't have any pictures of the most recent test part. I got all consumed on the baffles and front end of the cowling for a while and have not made a final part. The other problem is that I want to actually fly the plane instead of work on it, so progress is slow. The other thing is I am tired of modifying fiberglass molds. On of these days I will get off my butt and complete this one.
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