M

MTBehnke

Well Known Member
I've been experimenting a little with drag reduction ideas and thought I'd share my results.

I've been checking my airspeeds at 8000' DA using the 4-way GPS spreadsheet to determine TAS. I've got a number of data points charted from different flights and think my measurements are relatively accurate.

My -9A is still unpainted and I had shot high-build primer on all my fiberglass parts. This left a slightly rough texture to the surfaces. I never sanded them until recently. Once I sanded the cowl and wingtips to 240/320 grit testing showed I picked up around 1.5 kts in the 160 kt TAS range.

Later I also sanded the rest of the fiberglass surfaces (fairings, wheel pants, etc) but couldn't measure any additional improvements.

I decided to experiment a little with cooling drag. My CHT's and oil temps are very good, so I assume I have some margin to play with. Over the last few days I made up some inlet reducers. These reduced the height of my inlets from about 3.5" down to 3.0", reducing total inlet area from around 44 sq. in. down to about 38 sq. in. My exit area hasn't been modified from the standard RV-6 cowl exit (for my IO-360). They transition under the cowl to about mid-way along the length of the ramps.

100_5099.jpg


I did a short test flight this morning. My CHTs were higher, between 340-360 degrees (vs. 310-330 normally). OAT was still pretty low - ground temps were only around 62 degrees. My oil temp was very slowly climbing at 2700 rpm, showing 202 degrees by the time I reduced power to repeat the 4-way GPS runs at slower speeds. My oil temps normally don't exceed 200 even in climb.

The results of the inlet area reduction (drum roll please) - no measurable improvement in airspeed.

I was thinking of playing with the exit area as well, but given no change with the inlet reduction I don't think I'd see any difference.

I do have a little rework to do with my wheel pants and fairings which I assume will provide a little more improvement. And of course, I fully expect the standard 10kt speed increase when I finally get it painted this winter.
 
Some interesting work has been done re. airflow thru and out of the cowl. You might want to contact Bob Axsom before you give up in this area. Some speed here is available. It's all a question of effort vs. results.
Terry, CFI
RV-9A N323TP
 
MTBehnke said:
I was thinking of playing with the exit area as well, but given no change with the inlet reduction I don't think I'd see any difference.
Click to expand...

I'll place my bet that you will see an appreciable difference, especially if you can reduce the area without creating any flow seperation on the bottom side of the cowl. (i.e. just blanking it off won't do)
 
The trick is to reintroduce the air exiting in line with the airstream. Great improvements can be made if you can do this effectively. Nice looking Cardinal in the background, is it yours?
 
Easier ways to reduce drag !

I see you have not painted her yet. Than there are much easier ways to reduce drag!: Close off the sides of the hor. stab. and elevator and the vert. stab top. According to Bob Axsom, and some other people who have done this, you will gain 2-3 kts. I used PUR foam and fiber glas. The beauty about the elevator is, that you will ad no additional weight (because you will be removing the additional weight from the lead counterbalance when you balance the elevator)

Also you can install cover plates, a la Bob Axsom, on the rudder and elevator hinge holes. This should give you another 2-3 kts speed increase. (for about 42 g. weight increase)

I am planning on doing some of the same on the wings, ailerons and flaps (where possible) and hope to get 165 kts cruise, or better.

Kind regards, Tonny.
 
MTBehnke said:
The results of the inlet area reduction (drum roll please) - no measurable improvement in airspeed.
QUOTE]

Your the first honest builder on this site! The gains from small aerodynamic improvements are generally less than the margin of error for the flight testing. It will probably be difficult to detect anything smaller than +/-1.5 knots and impossible to accurately quantify it.

Keep experimenting and you will eventually see some improvement from a collection of mods. Some other easy fixes might come from sealing controls and a wing root fairing. Good luck! -David
Click to expand...
 
Different approach same result

In my testing I reduced the cooling air inlet opening by reducing it with incremental 1/4" slices of shaped balsa plugs building out from the inboard side of the opening on each side of the spinner. It was very measurable in increasing CHT but provided no measurable increase in speed for me either. Tom Martin reported similar results on his EVO Rocket.

Bob Axsom
 
Inlets "fix" themselves somewhat

The lack of gain from reducing inlets is probably because "excess" air not taken into the inlets simply splits to go around the cowl. I.e. imagine streamlines going towards the opening(s). At some point just forward of the cowl, a split occurs, sending the streamline either into the cowl or around it. The size of this perimeter is determined by how much the flow rate is into the inlets.

As others have said, the exit area is the key...

If you haven't already, search for cooling exit modification and you will find several fun threads.
 
Exit Area

A few months ago I was trying to find cooling drag solutions and other than in all cases raising temps, there was no measurable speed differences.

First thing I did was to cover the louvers in the lower cowling. Then developed a set of 3 different sized plates that fit over the exit area to reduce the area. I don't have the numbers with me but the inlet to outlet ration prior to starting was about 1:2.5. Again the plates did nothing but raise the temps.

I was about to try reducing the inlet areas with inserts, it had the Sam James inlet rings to the plenum, but had to put the plane up for sale :( and now she's gone.
 
I believe Alex has it right.

An example from a theoretical RV-8 model @8000 ft and 200 mph:

0.3125 sqft inlet and 0.40 sqft exit. Mass flow is 4.66 lbs, exit velocity (for an optimum exit shape) is 147 mph, and drag is 10.8 lbs.

0.25 inlet and 0.40 exit. Mass flow is 3.89 lbs, exit velocity is 128 mph, and drag is 12.7 lbs.

Observe the relationships. Reducing inlet area while maintaining the same exit area reduces exit velocity and increases drag. It's all about loss of momentum.

OK, stick with the stock 0.3125 inlet and reduce exit area to 0.20. Mass flow becomes 2.87, exit velocity becomes 190 and drag becomes 1.3 lbs.

This is the power of a variable exit, or a small fixed exit with its required flat, fast climb.

Notes:

First, don't treat these numbers as actual. Changes to various model assumptions change the actual numbers, but not general relationships.

Second, the designer's task is to turn optimized theory in hard parts. It all goes to **** with lousy pressure recovery, a messy exit, or a dozen other mistakes. Even the best designs can't match optimum values because of practical considerations.

Third, I'm very much just a student at these things, so don't expect deep enlightment from this quarter.
 
Sounds good, however..

AlexPeterson said:
The lack of gain from reducing inlets is probably because "excess" air not taken into the inlets simply splits to go around the cowl. I.e. imagine streamlines going towards the opening(s). At some point just forward of the cowl, a split occurs, sending the streamline either into the cowl or around it. The size of this perimeter is determined by how much the flow rate is into the inlets.

As others have said, the exit area is the key...

If you haven't already, search for cooling exit modification and you will find several fun threads.
Click to expand...
Dick Martin's very fast RV-8 with Sam James cowl did, he said, pick up speed from reducing the openings for cooling. Maybe there is more drag from backpressure at the openings rather than from a well streamlined inner ring. Didn't A.J. (last name?) experiment with this on a -6, too?
 
smithhb said:
Mike,

I'm waiting to see if Larry Vetterman's idea proves valid.

http://www.vansairforce.net/vetterman/subcowl_2.pdf
Click to expand...

Bret and Mike,

I ran into an RV-7 at a fly-in this spring where the guy made a Larry Vetterman's fairing for the bottom of his fuselage. He said the speed gain was right at five knots, just as Larry said it would be.

I plan on making one once I have my new cowl fitted. But since I'm changing my engine, prop, and everything, you won't be able to compare my gains. However, I will have the option of removing the fairing and see what kind of speed difference I get.

Stand by, I should be flying in about 12 months or so.
 
My experience seems to support Dan's calculations.

I was curious about inlet area/drag/speed implications so I "borrowed" several sizes of inlet rings for my James cowl equiped RV9a. The stock rings for my 0360 engine were 4 5/8" diameter so, in addition to the stock rings, I tested 4", 3 3/4", and 3 1/2" rings. Each aluminum ring was the same O.D. but the I.D. was progressively less due to thicker wall area. To deal with the greater thickness w/o additional flat plate area, the thicker rings had a nice smooth radius machined at the forward lip. This common o.d. allowed easy switch out as the "keyway" on the James fg cowl fit all of the rings. Speeds/cht/ot data was collected at the same d. alt and power settings. Four way gps speeds were captured and then run through Kevin Hortons equations to capture tas. The engine data was logged and recovered from my AFS 2500 engine monitor.

I have a full page of data on this but the bottom line was that as I went to the smaller inlets w/o mods to the outlet, my hottest cylinder cht's went up by around 12 degrees and the tas actually went down slightly (maybe 2-3 kts). I went back to my stock rings.

There is much more to this fluid dynamics stuff than meets the eye!!!

Cheers,

db
 
Thanks, Dave B.

You just saved me some work and some money! Theory is nice but data usually counts for more.
 
Flight data is good.

Dave, theory does match your observation, but to illustrate the real story I need to expand beyond the two samples offered in the previous post.



I plotted a series of inlet-outlet ratios from one extreme to the other (the scale at the bottom). The plot assumes an exit area of 0.4 sq ft and plots inlet areas from 0.05 sq ft to 0.7 sq ft. If your RV-9 does in fact have an exit area around 0.4, your change in ring sizes shouldn't make much difference in drag. All it would do is change CHT due to a reduction in mass flow. The drag change you made would be between the vertical black lines, large inlet being a 1-1.71 ratio, small inlet being 1-2.98 ratio.

If your exit is closer to 0.3 sq ft, the black lines would shift right; large inlet 1-1.28, small 1- 2.23. That would result in a CHT rise and a drag rise, which is what you report.

Student observation: note cooling capacity (the blue HP line) and mass flow (the magenta line) are proportional. Given a fixed exit area, increasing or decreasing inlet area always results in decreased or decreased CHT...which is exactly what everyone consistently reports here in the forum. However, note cooling drag (momentum loss) is not proportional.

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.....
 
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<<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.
 
Getting thrust from intake air

DanH said:
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.....
Click to expand...

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?
 
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.
 
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
 
Dan Babenco said:
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?
Click to expand...

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 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.
 
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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?
 
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
 
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! :D)

A
 
Peter Garrison and Melmoth

Andy_RR said:
...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! :(
...
Click to expand...

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.
 
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I believe the factory prototype RV-6 used an updraft cooling system. I did have an annular inlet around the prop (there were no openings in front of the cylinders. Old-timers will remember "old Blue" with this cowling configuration.

Not sure how it worked but it did win the "ugly ducking" award from most observers at the time.

Not sure how it worked and perhaps some of the factory guys can shed some light on this.
 
outlets

Hey Fellas:

Lancair did a bit of work figuring out where to locate the outlets -- they found the outer edges of the belly had the lowest pressure, so they located the outlets there.

In addition, they used bluff body outlets -- these create low pressure at the outlet lip by using a specific airfoil cut at its lowest pressure point.

Since my side mounted adjustable outlets on the 550 F1 didn't work (very ignorant designer, I think. I see him in the mirror when I shave...), I decided to try the above mentioned outlets in place of the cowl flap doo-hickeys.

By golly, someone out there knows their respective stuff -- it worked!

http://img443.imageshack.us/i/dcp6382.jpg/
http://img171.imageshack.us/i/dcp6385.jpg/

Engine temps are more even, and oil temp is acceptable, tho it could be better. Climb cooling is fine; cruise cooling appears to be OK too. I need to fabricate the interior ramps to smooth the flow (I think) to get better oil cooling.

I notice no speed degradation from the increased flow -- it could be the flow is directed better, both inside and out, but not increased?

The induction air for this engine comes from the upper plenum, and I see exactly ambient pressure -- I expected a bit of ram, but it's not there. The inlets are very small -- about 36 sq in. This could be the cause of the lack of upper plenum pressure? Outlet area is about 42 sq in.

Upper cowl outlets should be shunned -- any fluids or gasses escaping from the infernal combustion device will find their way onto your plexiglas, and you won't be happy. Trust me on this one -- any oil escaping from my outlets attaches itself to the sides of the fuselage, and the H stab..and V fin...in other words, it is not hidden as a bottom outlet would help with, assuming the same leak rate.

Another design fault of such an outlet location (side or upper cowl) is HEAT. It appears that the cooling air (now very hot, thanks to that infernal combustion device)) flows in a nearly laminar manner, and happily transfers its BTUs into the side skins... which are also very happy to transfer their BTUs into the cockpit. Now, this may be a bonus at 15000'MSL, or in winter, but it's no picnic in any temps over about 70F.

So, I added an additional cockpit/pilot cooling air inlet onto the boot cowl:

http://img406.imageshack.us/i/dcp6377.jpg/

Geez that's a lot of air moving thru the airframe... but she'll still give me 200KIAS at 6000' DA/2300RPM easily. My E6b sez that's about 251MPH, but only 14MPG...."speed costs money; how fast ya want to go?" Normal cruise is 175KIAS/11GPH/20MPG which is more reasonable to the wallet.

Next mods will be the ramps on the inside of the outlets, and then an induction air scoop w/filter to get the MP where it should be. The scoop will have to go on the top of the cowl as was done on the Allison powered p40s and P51s, so I'd better make that look right...

Summary? There ain't no free lunch. Cooling drag on these ships is already very low, tho some improvement can be seen with careful modifications. The augmenter idea is probably the best path, but the cockpit noise associated with such a design is a major downside. NACA was able to get a Martin B26 with multi tiered augmenter system to run full power on the ground indefinately, and it actually produced about 120HP of thrust in flight. Of course this system took up the entire aft nacelle, but it was more or less empty to start with. The system was never used on production ships.

NASA tested louvers on a C-421 (I think) -- very draggy. Don't go there.

As for the under-spinner smiley inlet, you can see it in action on a very popular aerobatic stars' biplane - I think Sean is his name -- the cooling capacity is very good: he can hover his plane for what looks like an amazingly long period of time. It's not an updraft system -- nor was Van's -- it's simply another style of inlet.

The CAFE foundation did a lot of cooling work -- best you look at their results for a direction to proceed. Come to think of it, they use a smiley inlet on their Mooney...so I guess it does work! Might also peek at some of the Formula 1 racers at Reno -- extreme air flow control is their holy grail.

Carry on!
Mark
 
F1Boss;356904 The induction air for this engine comes from the upper plenum said:
Mark, got a few photos of those inlets, in particular from the engine side?
Click to expand...
 
inlets

Hey Dan:

Here ya go -- the upper cowl has ramps molded in as would be done in any installation. I'll get you a shot of those ASAP. There is no room for a plenum -- the intake system fills the upper cowl completely.



Carry on!
Mark
 
Mark,
Given the mass flow required to cool the 550 and the small inlet size, they would have to be high velocity inlets. High velocity inlets without optimum internal difusers probably don't net much pressure recovery.
 
AGREED!

DanH said:
Mark,
Given the mass flow required to cool the 550 and the small inlet size, they would have to be high velocity inlets. High velocity inlets without optimum internal difusers probably don't net much pressure recovery.
Click to expand...

Which is exactly what I have seen on the MP gage. The inner ramps that direct the flow along the inside of the inlets are at the wrong angle (too steep) but the induction air filter is there (dictating the ramp angle), so that's what had to be done. Once I get the induction scoop in place, I can re-set the inner ramps to a shallower angle, and maybe get a bit more pressure in the upper plenum. This will allow smaller, higher velocity outlets...I hope...

It sez experimental, and I'm experimentin'! The only problem is that if I get 'er going another +6KIAS in cruise, Ol' Mel is gonna be on me to put them 12" N numbers on 'er. Ugh.

Carry on!
Mark
 
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F1Boss said:
Once I get the induction scoop in place, I can re-set the inner ramps to a shallower angle, and maybe get a bit more pressure in the upper plenum. This will allow smaller, higher velocity outlets...I hope...
Click to expand...

Ya know, there are reasons Mooney used these low velocity, external recovery inlets on the Acclaim. ;)

 
Yeah but

I beat the factory Acclaim in the Texoma race last year with my stock inlets on our RV-6A.

Bob Axsom
 
That's gonna leave a bruise!

Bob Axsom said:
I beat the factory Acclaim in the Texoma race last year with my stock inlets on our RV-6A.

Bob Axsom
Click to expand...

OUCH!;)

And with about 50% of the HP too...

What the heck is an 'external recovery' inlet? I may be showing a large amount of ignorance here....beware! Could be that 'negative knowledge' thing happening (you'll know less after we discuss it than you did when we started).:confused:

Those are some really large inlets, BTW -- likely required due to the lower IAS with the higher cooling requirements (the Acclaim has a twin-turbo'ed/twin intercooled 550 -- I'll bet that engine puts out a LOT of heat!). If I recall correctly, the normally aspirated version (Ovation?) has inlets that are virtually identical to mine -- at least that was the look I was trying to copy...

2ec12td.jpg


Doesn't look like much of a ramp behind 'em either? Lancair/Corvallis/Cessna & Cirrus really surprise me with their lack of such on their normal installations -- kind of like the inlets are only round for styling?



Carry on!
Mark
 
Bob Axsom said:
I beat the factory Acclaim in the Texoma race last year with my stock inlets on our RV-6A.
Click to expand...

I'm proud for ya', but a 1.125 mph faster race average ain't exactly running away ;)

Missed the point. My previous comment relates to the practical aspects of obtaining pressure recovery.

Mark may (or may not, we're both guessing to a certain extent) have a problem with the shape and angles of his inlet ramps (diffusers), which are critical because he is using very small high velocity inlets. He says his current induction setup compromises the ramps....one of those practical problems which can compromise design and steal performance. To bypass the problem he is thinking about adding an external scoop for combustion air and hopefully reshaping the ramps for better results.

Now consider a practical aspect of the large, low velocity, external diffusion inlets on the Acclaim. There are no ramps at all, just a baffle plate across the front of the engine below the propshaft to separate the upper and lower plenums. The hard parts necessary for internal pressure recovery with a high velocity inlet are not required.
 
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<<And with about 50% of the HP too>>

65% assuming Bob has 180. Bob is also pulling 35% less total airframe drag...which is why the speeds are close. Nothing about the result (similar speeds) tells us anything about cooling drag, just the sum of all drags.
 
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Yep, 180 hp

The engine is a 180 hp O-360-A1A ordered throught Van's, drop shipped from Lycoming. All of my speed gains to date are from drag reduction. I hope to work or other performance enhancement areas in the future. Cost is the problem.

Bob Axsom
 
FYI - For those of you who are thinking about making a Vetterman's belly fairing, Avery's has the louvers on sale. I don't know how much longer the sale will last so you had better act quickly.

(They also have the brake rivet tool on sale for $31.)
 
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