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The Shrinking Exit

Dan,

I appreciate the effort you have contributed to this project. I'm no engineer and I find it difficult to follow your logic so I looked up a video which explains it in terms that I can understand. Maybe this will help others as well.

https://www.youtube.com/watch?v=RXJKdh1KZ0w

Aww, that guy can't hold a candle to Bud Haggert.

Seriously, cooling performance starts with capturing as much of the available dynamic pressure as possible. The previous post merely illustrates the dynamic pressure available in prop outflow, and how a design might be tweaked to collect more of it. Too many RVs are seriously crippled by cooling issues, and it doesn't have to be that way.
 
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Too many RVs are seriously crippled by cooling issues, and it doesn't have to be that way.

I follow your work like a hawk. I didn't miss Physics Class, but I might as well have.
I have done all I can, short of cowl flaps, and you describe exactly the conditions that inhibit my cooling in climb. I never considered the props influence in this stage of flight and the correlation to prop "blast", for lack of a better word. I assumed, incorrectly, that the angle of the inlet may be the culprit. Perhaps it has some influence too...
My CHT's are very manageable, seeing very short durations of 400-410 deg.F when heat soaked and climbing aggressively with standard cowling at full power. It is easy to work with and has never been problematic in how I operate my particular machine.
I think most of us bought a kit from Vans trusting that the design would not require further experimentation or modification. For the most part, and for most RV's, that promise is realized.
However, if "crippled" by cooling issues, your work provides folks a path to resolve, or at least a better understanding of what is going on.
Great stuff as always Dan.
 
I wonder if Propeller outflow or Cp improves with a three blade prop.
I seem to remember that the three blade is more aggressive close to the hub.
 
Rockwell Retro Encabulator

Dan,

I appreciate the effort you have contributed to this project. I'm no engineer and I find it difficult to follow your logic so I looked up a video which explains it in terms that I can understand. Maybe this will help others as well.

https://www.youtube.com/watch?v=RXJKdh1KZ0w

I watched this youtube with the hopes of understanding things a bit better only to realize that it is not mostly in English but gibberish. Nevertheless appreciate everyone's contribution, specially Dan's. Learning a lot here.
 
Some of you may be familiar with Dan Raymer's "Aircraft Design; A Conceptual Approach"

https://www.amazon.com/Aircraft-Des...preST=_SY291_BO1,204,203,200_QL40_&dpSrc=srch

More recently, Raymer released "Simplified Aircraft Design for Homebuilders", which is well worth a few bucks:

https://www.amazon.com/Simplified-A...preST=_SY291_BO1,204,203,200_QL40_&dpSrc=srch

Anyway, I was amused to find this passage in Simplified Design:

Raymer%20-%20Exit%20Ratio.jpg


Like all rules of thumb, reality depends on details, so don't get too locked on the given ratios. Given a variable exit, a wider ratio range just means a wider range of available mass flows for cooling, from a little to a lot. As noted in the last sentence, the exit is the throttle.
 
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Returning to the propeller's contribution to climb cooling, here's a paper some readers will find interesting:

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19980214913.pdf

In the past I've soapboxed a bit about low velocity ratio inlets. The ever-popular NASA CR-3405 illustrates better pressure recovery given low Vi/Vo, "Vi" being velocity through the inlet opening and "Vo" being freestream velocity.

AIAA 80-1872R expresses essentially the same thing as a ratio of stream tube area to inlet area. The stream tube area is the cross section of an imaginary tube of air (viewed at a point well out in front of the cowl) whose mass is equal to that which will actually flow through the cooling system. Flow restriction determines the cross section area of the stream tube; usually the cowl exit is the throttle. I say usually only because it is possible to build cylinder baffles so tight that they set the flow rather than the cowl exit (hello Bill), and in fact the researchers here used a variable orifice plate to simulate baffle restriction and vary total mass flow for their measurements. The stream tube expands as the air slows immediately in front of the large inlet, and as it slows, the static pressure rises. All other air in front of the cowl flows outboard and around the cowl.

Stream%20Tube.JPG


Regular readers know I operate without any practical climb CHT restriction, and do so with less exit area than the stock RV-8 cowl. One key factor is good pressure recovery, and 80-1872R shows why.

Prop%20Contribution.jpg


Fig 5 plots considerable propeller contribution to upper cowl pressure in climb, as compared to cruise, just like the mirthful measurements in post 198. At the same time, note the difference is pressure recovery as the ratio of stream tube area to inlet area gets larger. Low ratio inlets (expressed in area or velocity) recovery a little more dynamic pressure in cruise, and a lot more in climb.

Fig 6 illustrates an important detail; varying the mass flow varies the pressure recovery. Increasing the exit area makes the stream tube larger. Mass flow rises, but pressure recovery falls. However, it falls less for the large inlet. What it means is opening a variable exit (a cowl flap, if you insist) in cruise results in a little less drag penalty when the system has a large inlet, because overall system pressure remains higher, thus exit velocity remains higher.

Fig 6 is for the cruise condition. Now look at the climb condition in Fig 7. All the inlets perform better when flow is throttled (A), but open the throttle so mass flow is equal to cooling demand (1.0 W/Wc), and the small inlet falls on its face (C). Meanwhile, the low area ratio (i.e. low Vi/Vo) inlet hangs in there at 0.8 Cp or better (B).

On a slightly different front, sometimes builders express reservations regarding drag, and the blunt nose typical of a large inlet, small exit cowl. Check out this video, 26:18 to 27:45: https://youtu.be/rxvoDbZpoY8
 
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It?s alive! Alive, I tell you!!

So this thread is still alive - good news!

About two weeks before OSH 18 I decided to change to the “Official Dan Horton Round Inlets” in an effort to get the engine temps under control. Heck, things were so bad I had nothing to lose.

Example: I had an ADI system in the plane to control temps in the climb (and when racing at Reno), else I would have flown everywhere at 55% power and 3500MSL...that system weighed 75lbs when full. Now not necessary - even at continuous race power.

Two weeks of frantic fiberglass work followed...what fun!:rolleyes:

My original baffling system was a close copy of what Cirrus uses, as my engine is the CM IO-550N (310HP). I thought that should have worked fine!

Before temps were ~400ish if I tried to run power above ~70%, with oil consistently 205-225F, and easily 240F in a normal climb. Bad juju.

After temps are CHTs around 340F in climb, about 300F in cruise, and oil temps on the vernatherm, or around 180F - after taping off an area of the cooler!

No mod to the outlet, so it’s probably a bit larger than required in cruise. That part is about 32sq in, as WAS the total inlet area.

I am not an engineer, nor will I ever actually get into those studies. But I can tell you that his system is the best I have seen. The baffling changes were easy - many of you know that the Baffling War is not easily won, but the changes required to fit this system are not difficult. Search Cessna 400 baffling - piece o’ cake to copy, and no need to fabricate any fancy ductwork.

In fact, inlet ductwork might hurt this design. I can say it is not necessary - my ship has none. There is a large plenum just behind the inlets.

In case you are thinking that aircraft speeds will be impacted, in my case nothing changed except CHTs and oil temps - speeds were the same. I suspect your results would be the same - but remember I am overcooling by a fairly large margin. Speed gains should be available, so I guess I’d better figure out some outlet changes that include a cowl flap of some sort.

So, in my opinion, the system design is solid, and easy to get right - overcooling might be your only problem!:eek:

Thanks, Dan!
 
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I am not an engineer, nor will I ever actually get into those studies. But I can tell you that his system is the best I have seen.

Thank you sir, but clearly I didn't invent this stuff. Being the Forrest Gump of aviation, I was just lucky enough to be in the right time/place for a very good CFD guy to tell me "GA does it wrong"...which was before more recent designers (see the Acclaim, and the TTx, and the current Carbon Cub cowl) started doing it right.

Still, it can be hard to change minds...

“Official Dan Horton Round Inlets”...

Probably should point out that they don't need to be round. It's just sort of a handy shape. Any outline with sufficient area should work if the inlet lip radius is reasonable and the exterior shape of the cowl does not promote separation. See posts #15, 16, and 18.

In fact, inlet ductwork might hurt this design. I can say it is not necessary - my ship has none. There is a large plenum just behind the inlets.

Correct. It is another happy little detail about low ratio inlets.

The rubber inlet ducts on mine are purely for sealing. I'm thinking about 400 style baffling with molded perimeter seals if those ducts wear out.
 
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Cooling

I have recently reread and studied the excellent series of articles in Kitplanes by Dave Anders, IIRC he believes that the traditional "mail slot" inlets are just a bit more efficient then the round. But the round are MUCH easier to seal to the plenum.
The earliest data on the round inlets I have found is a paper by Mississippi State University. Their test subject was a one of a kind Piper Aztec that Piper donated to the Univ. I don't remember the date of the paper but it is quite old.
The late Dr. August Raspet was the director of the Aeronautics program at the then Mississippi State College in the mid to late 1950's. He was killed testing a modified production aircraft. The EAA Raspet Memorial Trophy is given in his honor. The first homebuilt he tested was the very first Wittman Tailwind. Among other tests, ten flights were conducted with the prop removed from the Tailwind and the cowl sealed. The Tailwind was towed to altitude behind a 450 hp Stearman.
 
I have recently reread and studied the excellent series of articles in Kitplanes by Dave Anders, IIRC he believes that the traditional "mail slot" inlets are just a bit more efficient then the round.

Efficient in what way Jim ? External drag? Pressure recovery? Something else?

BTW, the inlets tested in AIAA 80-1872R are neither round or mail slot.

The earliest data on the round inlets I have found is a paper by Mississippi State University.

That would be NASA CR3405. The data collected is not specific to round inlets, but rather, it is data collected using round inlets because it was convenient. From the text:

The three axisymmetric inlets were investigated because of
the existence of an experimental data base and analytical
design procedures.


https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19810013485.pdf
 
Thanks for review of AIAA 80-1872R, Dan. I am going to have to spend some time to understand the definition of "large" inlets. Mine are returning around 90% pressure recovery and it stands to reason, that it would drop if the restrictions between the upper chamber and the exit were reduced resulting in higher velocity and mass flow through the inlet rings. The prop appears to be a huge help, always wondered how much.

The velocity in my top end is so low - - - that I sucked in my two uncovered foam cowl plugs a few days ago and the CHT's still stayed under 350F and even as they ever are!! Bruces covers plugs (with flags) were ordered when I got home.

I was thinking of adding a 2 or 3" baffle bypass ducted to the cowl exit with a throttle valve might provide some insight to what happens to engine cooling, airspeed, and inlet pressure recovery with change in mass flow. My curiosity has not been matched with the energy and time to do the testing. If it gave me 10 kts, maybe:rolleyes:
 
Thanks for review of AIAA 80-1872R, Dan. I am going to have to spend some time to understand the definition of "large" inlets. Mine are returning around 90% pressure recovery and it stands to reason, that it would drop if the restrictions between the upper chamber and the exit were reduced resulting in higher velocity and mass flow through the inlet rings. The prop appears to be a huge help, always wondered how much.

The velocity in my top end is so low - - - that I sucked in my two uncovered foam cowl plugs a few days ago and the CHT's still stayed under 350F and even as they ever are!! Bruces covers plugs (with flags) were ordered when I got home.

I was thinking of adding a 2 or 3" baffle bypass ducted to the cowl exit with a throttle valve might provide some insight to what happens to engine cooling, airspeed, and inlet pressure recovery with change in mass flow. My curiosity has not been matched with the energy and time to do the testing. If it gave me 10 kts, maybe:rolleyes:

90% PR is very good on a working system. Did you instrument your aircraft to get this data?
 
I am going to have to spend some time to understand the definition of "large" inlets.

Large in comparison to the stream tube, i.e. stream tube area/inlet area = <0.4 Another definition could be inlet velocity/freestream velocity = <0.4

Mine are returning around 90% pressure recovery....

I would caution readers that your case is unusual. Based on a near zero lower plenum pressure measurement, your cylinder and head wraps are very restrictive...analogous to a nearly closed orifice plate in the test setup. So, the stream tube is smaller than typical, thus your inlets can be smaller and still maintain a low numerical ratio.

If the orifice plate were closed completely, pressure recovery would rise to 100%. The inlet would be a pitot tube. There would be no stream tube.

The velocity in my top end is so low...

Is cowl exit temperature high?
 
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Is cowl exit temperature high?

High, large, maybe relative. At 5347' pressure altitude, 53F OAT, cowl exit 138F, CHT average 304F but all at low speed, 117 kts and around peak EGT's.

Calculated dynamic pressure, 7.4064 in-H2O, UPPER=6.47, LOWER=.44, calculated recovery = 87.4%

142 KTAS ~90%, 157 KTAS~91%

Pressure recovery % improved with airspeed, but cowl exit temps went down (slightly).
 
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When you can, get a full set of data at 3500 PA, at least three speeds. Lets see how it compares with the other RVs in the data set.
 
When you can, get a full set of data at 3500 PA, at least three speeds. Lets see how it compares with the other RVs in the data set.

It's on the list, tried a couple of times, but 3500 has been too rough for good data around here. Wx is getting good for that. I'll make a pitot tube from your plans to get some exit velocity, and although expect it to be quite low, it is needed for knowing just how much the exit area might be closed. Something has to be done as the exit is a drag.
 
Wouldn't the "test" for whether an inlet was too large be glide/tuft testing different inlet sizes ?

Perhaps you could expand on the question Marc.

Can you tell us how the proposed test might be conducted, why, and what might be determined?
 
90% PR is very good on a working system. Did you instrument your aircraft to get this data?

Ross, I installed piccolo tubes above and below the engine barrels/heads according to DanH drawings. Parallel to the base of the head and 1" above the fins. I used the same electronic manometer (Dan recommended) and flew triangles to correct the airspeeds. The static side was tied into the aircraft static system. Actual TAS was 2kts +.3 lower than the GPS speed.

If the recovery is 90% I assume the velocity is very slow, as is the mass flow. Cooling is good. Remember Allan Judy's RV6? He had an angle valve and a separate cooling duct for his (large) oil cooler, but the inlets were quite small. Maybe ~2" in dia with well rounded edges.

My challenge now, is what to do with the cowl exit. Near zero lower cowl pressure acting like an air dam. Either add an after body or sleekly reduce the exit area w/o increasing the pressure too much and reducing cooling. Unlike DanH, I do not have unlimited range of cooling. Maybe if we use the 435F and 500F limit, but not if desiring to stay below 400F on a hot day climb. Bragging about speed might be nice, but many faster birds out there already, it's really about the poor aerodynamic efficiency that bugs me. BTW - the aft lower cowl already extends over an inch behind the FW.
 
If the recovery is 90% I assume the velocity is very slow...

It says inlet velocity/freestream velocity, or stream tube area/inlet area, are numerically low ratio.

Velocity, on the other hand, varies a lot at different points in the system.

...as is the mass flow. Cooling is good.

There are only two ways to carry away more heat. You must flow more mass or transfer more heat to the mass you have. Recall I asked about exit temperature? Yours is not very high, so if cooling is good, you must be doing it with mass flow. Think about it; you have a lot of deltaP.

High deltaP across the engine baffles also says flow through the fin passages is at high velocity, at least at some points.

My challenge now, is what to do with the cowl exit.

I really want to see a data set before drawing a firm conclusion, but I'm beginning to think your extensive wrapping is too restrictive. We don't want the system restriction to be the engine baffles. We want it at the cowl exit.

Consider this tidbit from Hoerner's Fluid-Dynamic Drag. The given values are for radiators, but something similar would be true for engine fin passages. I think the good doctor is saying that if velocity within the cooling passages is reduced, heat transfer won't fall near as much as drag, i.e. energy loss across the engine baffle. So, open the baffling a little, and clamp down on the exit; slow flow in the fins, maintain higher lower cowl pressure, and generate faster flow through the exit.

Duct%20Loss.jpg
 
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Dan is exactly right on the mark here. Especially this statement:
"I really want to see a data set before drawing a firm conclusion, but I'm beginning to think your extensive wrapping is too restrictive. We don't want the system restriction to be the engine baffles. We want it at the cowl exit."

I've been meaning to get back in touch since we talked about aileron rig and didn't get to talk about your cooling system. What you have now is cooling well, but very draggy.

Instead, open up the cooling wraps on your engine (which if you did nothing else would increase cooling massflow and perhaps drag) and then follow up with dramatic reduction in cowl exit area, preferably by reducing frontal area at the same time. If you restrict the cowl exit to the right amount, you will get back to the same mass flow you have now, but with less pressure drop across the engine, higher pressure in the lower cowl and more exit velocity out of the cowl. As you do this, the airplane should speed up for the same cooling flow.



It says inlet velocity/freestream velocity, or stream tube area/inlet area, are numerically a low ratios.

Velocity, on the other hand, varies a lot at different points in the system.



There are only two ways to carry away more heat. You must flow more mass or transfer more heat to the mass you have. Recall I asked about exit temperature? Yours is not very high, so if cooling is good, you must be doing it with mass flow. Think about it; you have a lot of deltaP.

High deltaP across the engine baffles also says flow through the fin passages is at high velocity, at least at some points.



I really want to see a data set before drawing a firm conclusion, but I'm beginning to think your extensive wrapping is too restrictive. We don't want the system restriction to be the engine baffles. We want it at the cowl exit.

Consider this tidbit from Hoerner's Fluid-Dynamic Drag. The given values are for radiators, but something similar would be true for engine fin passages. I think the good doctor is saying that if velocity within the cooling passages is reduced, heat transfer won't fall near as much as drag, i.e. energy loss across the engine baffle. So, open the baffling a little, and clamp down on the exit; slow flow in the fins, maintain higher lower cowl pressure, and generate faster flow through the exit.

Duct%20Loss.jpg
 
I agree with Dan and Steve. With 90% pressure recovery, you've nearly stopped the air and with no variable geometry exit door you must have lost most of the cooling air momentum.

Here's an early video of my RV in flight with an ASI reading the rad exit velocity in flight. Cycled from closed to open to closed again: https://www.youtube.com/watch?v=uvO11OAe440

Later on, with refinements and higher coolant temps, I was able to cool in cruise very nicely with only 1.5 inches Delta P and achieve 104.5% exit velocity at the same time. I could raise the air exit temp to 92% of the coolant temp at rad inlet. You probably won't be able to do that on an air cooled setup due to the higher pressure losses across the cylinders, large volume changes in the system and relatively poor heat transfer present but Dan made his setup a lot better than factory.

BTW, my inlet area is 29.5 in2 and closed exit area is 16.8 in2.
 
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I've been meaning to get back in touch since we talked about aileron rig and didn't get to talk about your cooling system. What you have now is cooling well, but very draggy.
I assume draggy due to the low pressure area behind the cowl exit is presenting excess frontal area and that it is causing turbulence with its (assumed) low velocity?

Instead, open up the cooling wraps on your engine (which if you did nothing else would increase cooling massflow and perhaps drag) and then follow up with dramatic reduction in cowl exit area, preferably by reducing frontal area at the same time. If you restrict the cowl exit to the right amount, you will get back to the same mass flow you have now, but with less pressure drop across the engine, higher pressure in the lower cowl and more exit velocity out of the cowl. As you do this, the airplane should speed up for the same cooling flow.
OK, so, as the exit area is reduced relative to the inlets, it will need some pressure to convert to velocity though the exit. Then to generate the same mass flow, the engine delta-p needs to be the same. Is that about it?

I agree with Dan and Steve. With 90% pressure recovery, you've nearly stopped the air and with no variable geometry exit door you must have lost most of the cooling air momentum.

I am not sure the same areas can be reached with the air cooled but it might be the right proportions. I looks like you create some thrust!
BTW, my inlet area is 29.5 in2 and closed exit area is 16.8 in2.

Dan, the heads are only wrapped under the baffles, the barrels are wrapped but I did some calculations on velocities and truly believe that it is higher by forcing the air through the fins than letting it just bounce around in the plenum volume. Clearly no data or CFD. The barrel wraps were opened by an additional 2 inches which seemed to increase the lower cowl pressure to just below zero. The concern was to ensure that turbulent flow existed at low speeds for cooling, and it was a good Reynolds number at quite low speeds.

If weather holds and air is smooth, I will get another data set in a few days. I am still not sure exactly why I need more mass flow unless it offsets some exit drag (shrink the exit first).
 
Dan, the heads are only wrapped under the baffles, the barrels are wrapped but I did some calculations on velocities and truly believe that it is higher by forcing the air through the fins than letting it just bounce around in the plenum volume.

Sure. Small passage connecting volumes with high deltaP = high velocity. When you were building and we were talking about wraps (wow, where does the time go?), our mutual belief was that promoting high levels of turbulent flow between the fins would be advantageous....and it probably is, because you're cooling very well. The problem appears to be energy cost.

Clearly no data or CFD. The barrel wraps were opened by an additional 2 inches which seemed to increase the lower cowl pressure to just below zero. The concern was to ensure that turbulent flow existed at low speeds for cooling, and it was a good Reynolds number at quite low speeds.

Yep, I recall your analysis.

If weather holds and air is smooth, I will get another data set in a few days.

...which will allow comparison to other installations.

I am still not sure exactly why I need more mass flow unless it offsets some exit drag (shrink the exit first).

You should end up with roughly the same mass flow, or dial-a-flow if you install a variable exit. Ponder the snip from Hoerner. Again, if I understand it correctly, slowing the flow through the heat exchanger reduces loss (drag, or pressure loss, or energy loss, however you care to think of it) to a much greater degree than loss of heat exchange.

What we're proposing is moving your flow restriction from the heat exchanger to the system exit. Conceptual sketch:

System%20Throttles.jpg
 
Dan,

I was speaking from the recollection of a series of discussions (more literally lectures) by John Thorp. The particular point discussed were his cooling drag studies (or FAA flights) related to the STC's he developed on the Navion. If CR-3805 supports larger inlets are ok if shaped properly.

That thought (to me) is overshadowed by the mass vs speed discussions that followed my point. My "takeaway" from the Thorp discussions is to follow the Reynold's effect to insure turbulence through the "hydraulic tube" at "rated" delta P, and thereafter avoid "excess" delta P by exit control.

If my reply is not responsive, please let me know.

Regards
 
Dan asked: "Can you tell us how the proposed test might be conducted, why, and what might be determined?"

Don, I apologize for not responding directly to this earlier.

John Thorp and I also discussed the "zero thrust" accessory developed by the CAFE Foundation. I recall John as being almost stunned by the intellect exemplified by this idea. I believe he wished he had thought of it much earlier. He approached the problem in the Navion by fitting a full-feathering propeller, but the CAFE approach was better to the point of being "elegant" (or something similar).

The test approach would mimic the Navion cooling drag tests by investigating different configurations by recording descent rates during glide tests under "zero" thrust. Tufting the cowl inlet and exits could provide insight into developing the next "configuration."
 
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The test approach would mimic the Navion cooling drag tests by investigating different configurations by recording descent rates during glide tests under "zero" thrust.

Did you happen to note the name of the referenced AIAA paper?

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19980214913.pdf

Tufting the cowl inlet and exits could provide insight into developing the next "configuration."

Yes, tufts are cheap and easy. Poor external shape around a low Vi/Vo inlet may result in separation; tufts may be useful to detect it.
 
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