cowl flaps

[RV8R999]

I discontinued the test at 6K upon completion of the climb and executed a low power decent back to the runway rather than run up to Vmax as I wanted to check the condition of the baffle material prior to running up to full RPM.

I believe I have it setup better now and will try a speed run tomorrow - this working thing gets in the way of fun!

 

[RV8R999]

Vy Climb to 6K PA, Flaps Full Open, Louvers Open, Cowl/Prop Sealed

During this run, cowl flaps were held full open during the Vy climb to 6K.

I provided OATs to give an ideal of the variance from run-to-run. Today was the warmest and yet the CHTs remained the coolest reaching a max delta of -26deg from the no cowl flap run (baseline configuration). Clearly the combo of flaps and complete cowl sealing has a consistent and significant impact on #2 cht cooling during climb.

Interestingly CHT 1,3,4 do not exhibit the same sharp bend near peak as CHT 2 does, rather they tend to peak from 1:30 to 2:30 after CHT 2 and level off. We cannot make any quantitative conclusions based upon configuration since I didn't attempt to consistently start at the same temp for any of these cylinders. Although from some other data during the same profile peak CHT doesn't appear to be sensitive to the CHT at which T/O power is applied. Several of my early tests I started with CHT 1, 3, 4 below 250 and they still peaked within 5 degrees of the values depicted in these charts with steeper gradients during the first 2 minutes. Sorta negates my Heat Soak theory.

Note: I removed #2 Cyl baffle dammm early during phase 1 and opened the inlet ramp area to the lower cyl baffle in that area with a generous radius to allow cooling air to reach the lower cyl fins. #1 cyl baffle dammm is in place. Wonder if this helps create the interesting shape of #2? I still have no earthly idea why #2 CHT rises so much quicker on the ground than any other cyl yet cools so much sooner once airborne? I'm starting to lean towards inlet geometry/placement and choice of prop (thick wood blade root).

Oil Temps are not affected by any of these configuration changes:

From a purely cooling perspective the cowl flap with sealed cowl appears quite effective during extended Vy climbs - hence the popularity in so many certified models - duh.

Jury is still out on drag reduction as
I haven't had a chance to run any level flight performance tests in depth. At first glance with cowl flaps closed at Vmax of 191MTAS 65 deg OAT which is about 63% power at 6KPA my CHTs range from 290-310 and OT's rock steady at 185. on Avg I get 55 FPM better ROC with Flaps closed than open but this isn't the drag comparison we are looking for. We want cowl closed vs stock exit since this is the data point with a choked exit area.

Note for Skybolt users: Sealing the firewall perimeter reduced the bowing between fasteners but not entirely although I do not believe air is leaking any longer. This is simply a result of flexing due to pressure exerted on the cowl

I really need to conduct a dedicated level flight profile and need time to to determine the W/sigma values for test day lapse rates and I haven't had the time. Tomorrow I'll do a mid-speed drag comparison by setting power and altitude and vary the cowl flap setting from full open, half open, full closed and note the changes. I may wait for Saturday so I have still air.

 

[RV8R999]

Exit Ramp Mod

After several level flight perf test runs under test day (not referred) conditions I cannot discern a notable performance increase or decrease with the cowl flaps closed, cowl sealed, and louvers open which corresponds to an exit/inlet ratio of .79. During a WOT climb to 8000ft DA at 130 mph, with cowl flaps open, the highest CHT was 356. At Vmax with flaps closed CHTs are 290-310 steady state, 8K DA. I'm leaving the louvers as the last parameter to change if I decide to do so. However, in an effort to possibly capture augmentation from the exhaust and reduce the exit size further I formed a new exit ramp shape with the apex located just prior to the flap exit:

This further reduces the exit ratio to .5 with flaps closed and louvers open.

I also added baffle material to the edge of the cowl flaps to ensure a tight seal in the closed position.

Wx looks great for tomorrow AM so I'll try it out.

 

[rvmills]

Ken,

Couple thoughts from a knucklehead in the peanut gallery (not knocks, I'm really watching this with great interest).

The exit ramp re-shaper is a neat idea. In looking at the photo, I'm wondering if the forward seam where it attaches to the ramp and the airfoil apex in that location will each contribute (in their own way) to tripping the exit flow. The seam by perhaps adding interference drag, and the airfoil at that position by causing separation. Either might have the opposite of the the desired effect (smoothing and accelerating that exit air to the speed of the surrounding air (which I know you know).

Then again, that apex location may be good from the standpoint of imparting some acceleration to the exhaust air, and sucking out the surrounding exit flow. The fore and aft position of the airfoil apex is probably something that could be played with.

Just looking at it, I was wondering if that airfoil could somehow be formed as an extension of that nice radiused lip that you lucky RV-8 guys have. In other words, build the airfoil as an extension of the radiused lip, to keep the entire thing smooth. Not sure if that would mess up the coanda effect of the radiused lip. Could also place the apex too far forward.

In the current photo, it just seems like the air comes off that lip and out of the exit, then hits the seam, then accelerates up (actually down) your added ramp airfoil. Seems like a lot happening to the air there.

Still I think its a cool experiment! Any thoughts of putting an airfoil shape on the top (inner) face of the cowl flap, opposite your ramp airfoil, to make it a Paeser-like venturi?

Again, just some thoughts that I hope add value...very interesting stuff!

Cheers,
Bob

 

[DanH]

...I cannot discern a notable performance increase or decrease with the cowl flaps closed, cowl sealed, and louvers open which corresponds to an exit/inlet ratio of .79.

I formed a new exit ramp shape with the apex located just prior to the flap exit...This further reduces the exit ratio to .5 with flaps closed and louvers open.

Ken, do the 0.79 and 0.50 area ratios include the open area of the louvers? You'll need to close them off if you really want to really see what the variable exit work is doing.

Cool weather has arrived (only 70F in south Alabama tomorrow), so I'll return to the slot exit in the AM. With it installed, inlet/exit ratio is about 0.32, not that I think ratios are in themselves significant. The figure of merit is probably total exit area, about 18.5 sq in.

 

[Sig600]

So with that exit ramp are you trying to create a Venturi effect to draw the cooling air out quicker? Cool tinkering ideas, I'm surprised the cowl flap idea didn't yield more dramatic results.

 

[Tom Martin]

I agree with Dan, close the louvers. Your engine temperatures are very good which means you do not need that extra exit area, especially in the cooler fall weather. The cowl flap will make no difference until those louvers are closed. I could not get any significant data on my installation until I sealed the prop/spinner gap. Each time I closed the flap the air just spilled from that area; enough to actually change the trim of the aircraft.
At the end of the day I only use my cowl flap as an easy way to inspect the bottom/aft part of the engine area. It will lower the engine temps during climb but with it closed I never see temps over 400 in climb so it is not really necessary.

 

[RV8R999]

Ken, do the 0.79 and 0.50 area ratios include the open area of the louvers? You'll need to close them off if you really want to really see what the variable exit work is doing.

Cool weather has arrived (only 70F in south Alabama tomorrow), so I'll return to the slot exit in the AM. With it installed, inlet/exit ratio is about 0.32, not that I think ratios are in themselves significant. The figure of merit is probably total exit area, about 18.5 sq in.

Yes both .79 and .5 include louver area. Without louvers ratios are .66 and .4 (exit area of 21 sq in). I agree closing them off is a good idea and will probably do so soon, but in an effort to maintain some configuration control I'm changing one thing at a time.

 

[RV8R999]

I agree with Dan, close the louvers. Your engine temperatures are very good which means you do not need that extra exit area, especially in the cooler fall weather. The cowl flap will make no difference until those louvers are closed. I could not get any significant data on my installation until I sealed the prop/spinner gap. Each time I closed the flap the air just spilled from that area; enough to actually change the trim of the aircraft.
At the end of the day I only use my cowl flap as an easy way to inspect the bottom/aft part of the engine area. It will lower the engine temps during climb but with it closed I never see temps over 400 in climb so it is not really necessary.

agree. I've completely sealed all those areas as well. Louvers are not required for cooling any longer, especially during the cooler season. I'll fly with them open, gather some data, then close them off. Thanks for offering your experience here!

 

[RV8R999]

Ken,

Couple thoughts from a knucklehead in the peanut gallery (not knocks, I'm really watching this with great interest).

The exit ramp re-shaper is a neat idea. In looking at the photo, I'm wondering if the forward seam where it attaches to the ramp and the airfoil apex in that location will each contribute (in their own way) to tripping the exit flow. The seam by perhaps adding interference drag, and the airfoil at that position by causing separation. Either might have the opposite of the the desired effect (smoothing and accelerating that exit air to the speed of the surrounding air (which I know you know).

Then again, that apex location may be good from the standpoint of imparting some acceleration to the exhaust air, and sucking out the surrounding exit flow. The fore and aft position of the airfoil apex is probably something that could be played with.

Just looking at it, I was wondering if that airfoil could somehow be formed as an extension of that nice radiused lip that you lucky RV-8 guys have. In other words, build the airfoil as an extension of the radiused lip, to keep the entire thing smooth. Not sure if that would mess up the coanda effect of the radiused lip. Could also place the apex too far forward.

In the current photo, it just seems like the air comes off that lip and out of the exit, then hits the seam, then accelerates up (actually down) your added ramp airfoil. Seems like a lot happening to the air there.

Still I think its a cool experiment! Any thoughts of putting an airfoil shape on the top (inner) face of the cowl flap, opposite your ramp airfoil, to make it a Paeser-like venturi?

Again, just some thoughts that I hope add value...very interesting stuff!

Cheers,
Bob

Hi Bob,

The seam is very low profile so I'm not too concerned about a drag contribution there. The location of the apex was certainly something I thought about. In the end I decided to place it where it had the most impact in reducing the exit area as well as providing additional velocity just prior to entering freestream. If I slide the apex farther forward then any additional velocity gained through this "nozzle" is reduced as the air is diffused when the area opens up again. The choice to locate on the ramp rather than the flap was to help keep the exhaust off the belly of the plane and lesson some minor vibe issues - not sure this will help but given the choice I thought I'd try this first. Placing another one on the flap would be an interesting experiment too. I'll see how this works out first. Thanks for the comments!

Ken

 

[RV8R999]

So with that exit ramp are you trying to create a Venturi effect to draw the cooling air out quicker? Cool tinkering ideas, I'm surprised the cowl flap idea didn't yield more dramatic results.

yes to some degree trying to utilize the pumping action of the exhaust to entrain and accelerate the slower cooling air but the nozzle itself will increase velocity to all the exiting area with or without exhaust augmentation.

What I'd really like to do is add a converging nozzle to my 4-1 exhaust and blast it right into that nozzle - next week

 

[RV8R999]

level flight data

Flew 8.5 hours this weekend most of which was very smooth, straight and level cross country providing ample opportunity to test the cowl flaps in level flight.

Fabricated a fixture which allows me to position the flaps such that the exit area is exactly are as if it were stock.

BLUF: 3 MPH TAS increase over the stock exit area with FLAP full closed at Vmax and about 2 MPH TAS increase in the 60% power range.

6 MPH TAS delta from full open to full closed (louvers still open) at Vmax and 4MPH TAS at 60%.

It is time to close off those louvers as they are biasing the exit open which minimizes the impact of the cowl flap - as has been pointed out by Dan and Tom.

Any thoughts on adding a converging nozzle to the exhaust pipe? I have the same pipe you have Dan with a 4-1. My thought was to converge this exit to the diameter of the individual pipes.

 

[DanH]

Any thoughts on adding a converging nozzle to the exhaust pipe? I have the same pipe you have Dan with a 4-1. My thought was to converge this exit to the diameter of the individual pipes.

Can't do it.

Exhaust duration is something like 250 degrees (50 BBDC to 20 ATDC? Too lazy to look it up). Engine fires every 180 degrees. So, you would have two cylinders exhausting through the area of one pipe for two periods in each crankshaft rotation.

Here's a graphic, any cylinder and next cylinder in the firing order. Hatch is valve open period. Put them together and the solid areas are where they would interfere with each other.

 

[xblueh2o]

A most interesting thread.
Question for the group,
Would tufting to check local airflow be a benefit? It would be interesting to see the exit area airflow in the various configurations.

 

[gereed75]

Ok guys, here is my phase 2 exit fairing. This is what the air see's coming from under the right side cylinders as it approaches the cowl exit. Note the rounded shape of the lower firewall fairing and the rounded shape smoothing flow at the corner of the exit and on the belly of the airplane.

There is a "shark fin" shaped splitter that separates the right side flow from the left side flow. The intent is to not allow the two flows to collide. Instead this center fairing guides and straightens the flow and directs it out the cowl exit (While also reducing exit area).

Heres what it looks like from aft looking forward

Note also the hole in the end of the center fairing. That is the crankcase vent exit.

Finally here is what the whole thing looks like from outside.

The simple fairing dropped CHT's by about 25 degrees and increased speed 4 KTAS. Adding the center body picked up 2 more KTAS with CHT's going up about 10 - 15 degrees.

I did not tuft test this but I did do some oil drop flow testing. Flow aft of the fairing and "inside" the cowl opening is very streamlined with virtually no evidence of turbulence.

A bit more detail work to finish but I like the results so far.

Bob, with this complete, I will send my prototype fairing, I swear on Davy Jone's locker!

 

[RV8R999]

Very interesting. I plan to add some blending in my cowl flap once I'm sure it is worth it.

Love how you routed the breather tube through the fairing..elegant.

 

[RV8R999]

Can't do it.

Exhaust duration is something like 250 degrees (50 BBDC to 20 ATDC? Too lazy to look it up). Engine fires every 180 degrees. So, you would have two cylinders exhausting through the area of one pipe for two periods in each crankshaft rotation.

Here's a graphic, any cylinder and next cylinder in the firing order. Hatch is valve open period. Put them together and the solid areas are where they would interfere with each other.

this wouldn't work very well now would it. Thanks for taking the time to illustrate the problem

 

[RV8R999]

Lower Plenum

Finally closed off my louvers and fabricated a lower plenum to smooth flow from lower cowl to the exit area:

The right side was fairly straight forward but the left side is way more difficult as I have my oil cooler mounted on the engine mount. I only had time to fab the upper portion today. I'll do the sides and lower part connecting to the exit area later on. Hoping weather clears up for flying tomorrow.

here is a shot of my starter ring with 10 hours flying using baffle material to seal this area. The fainter marks were from the first couple of runs before I had it trimmed the way I want. Similar to Dan, my baffle material shows a slight feathering at the contact point.

 

[RV8R999]

Can't do it.

Exhaust duration is something like 250 degrees (50 BBDC to 20 ATDC? Too lazy to look it up). Engine fires every 180 degrees. So, you would have two cylinders exhausting through the area of one pipe for two periods in each crankshaft rotation.

Here's a graphic, any cylinder and next cylinder in the firing order. Hatch is valve open period. Put them together and the solid areas are where they would interfere with each other.

I've been thinking about this a little more and wondering if considering only the overlap period when exhaust valves are open is over simplified. Isn't the exhaust flow rate from each cylinder going to be a function (radius of crankshaft arm * sin(crankshaft angle) * shaft RPM (rad/sec)) of the velocity of the piston (lets assume the velocity of the exhaust is only a function of piston pumping action for now) - which is non-linear (zero at BDC and TDC, max somewhere in the middle). To know what the minimum exhaust nozzle area we'd need to know the cyclic mass flow rate through each pipe and add those during the overlap periods to determine peak mass flow and size the nozzle to this value.

My head hurts now, I'm going to turn a wrench for awhile...

 

[DanH]

(lets assume the velocity of the exhaust is only a function of piston pumping action for now) - which is non-linear (zero at BDC and TDC, max somewhere in the middle).

But it would be pointless to do so. The highest exhaust gas pressure in the pipe is during the blowdown period near BDC. That's as far away from the middle of the stroke as you can get. The piston does only limited pumping of exhaust in any case. If the pipe length and diameter is optimum for the chosen RPM, then pressure approaching TDC is negative.

Here's an illustration from Smith and Morrison, pipe pressure recordings of a single cylinder test engine. Note that pipe pressure is highly variable at mid-stroke and TDC due to pipe effects, but the BDC region is very predictible.....always high.

 

[RV8R999]

Cowl Flap Exit Location Change

after reading NASA report 3405 suggested by Dan H in another thread:

After reading the section of the report on exit configurations I'm going to change the location of my cowl flap hingle line moving it foward of the firewall. As I have it configured now I'm using the geometry of the -8 exit ramp to decrease exit area when the flap is closed. When cowl is open exit area is limited to the stock dimension by the fixed size formed by the cowl and lower firewall. I was hoping to receive more benefit in the wide open position through generation of a low pressure area behind the flap. As the report pointed out this was negligible and most affect was due to area changes alone - which corresponds to the minimal changes in cooling/airspeed I noticed from closed to wide open.

Moving the hingle line forward provides a much greater range of exit area changes and hopefully more significant results is cooling and airspeed. It also eliminates the need for louver bias. Should be an easy mod.

I'm thinking of converting to a covered upper plenum but will wait to gather the pressure/temp data with my current system for fun comparisons later.

also:

I've always suspected the stock inlet geometries should be different for left and right sides - specifically the left side lower lip angle should be steeper and more radiused than the right side. Someone else can do this mod though cause even after a year and half I'm still in no mood to do any repainting!

 

[RV8R999]

Cowl Flap Mod

Had the first free weekend in a long time so I took advantage and modified my cowl flap as per the previous post - Don't worry Dan, I have a cover which returns the exit to stock configuration in about 10 minutes.

I basically moved the hinge line of the flap forward 4" by cutting out the lower rear portion of the cowl exit area. By allowing the flap to deflect a max of about 30 deg I've increased the exit area by 22 in^2 for a total of 69.1 in^2 when fully open and reduced to 32.6 in^2 when fully closed. Stock exit (minus exhaust) is 47.1 in^2.

I used engineering graph paper to trace the inlet shape and calculate those areas for a total of 61.75 in^2 resulting in ratios (exit/inlet) of :

Stock: 0.762
Flap Fully Open: 1.11
Fully Closed: 0.527 (most likely a bit higher since the flap is probably leaky around the sides - I'll fix this)

which leaves the stock ratio right in the middle of the adjustment range where I wanted it.

I don't have a plot of my test today as my datalog battery died shortly after T/O.

WIth flap open I started with #2 cyl at 350 deg as in previous tests and a WOT Vy climb to 6K PA. Temp climbed to 360 by 300 feet then started decreasing and stabilized at 345 by 1500 ft and steadily decreased down to 330 at 6K. OAT at Sea level was 77 deg F. Changing from flap fully open to closed resulted in a 150 FPM ROC increase.

In a Vy climb with flaps fully closed CHTs climbed to high 370's, low 380's then decreased to 360's by 6K. An average 30 deg hotter than with flaps fully open during climb

Flap fully open in level flight with a fixed throttle setting gave 2300 RPM @ 152MPH. Flap closed to stock exit area resulted in a stabilized speed of 153 MPH. Fully closing the flap gained another 2 MPH to 155MPH. CHTs at 155MPH flap closed were 309, 313, 324, 319

In level flight with fixed throttle setting closing the flap from fully open increased CHTs by an average of 5-10 deg with corresponding airspeed increase of 3 MPH.

-Overall the flap is far more effective in this configuration than the original which is expected given the wider range of adjustment.

-A climb rate penalty is the price for better cooling (DUH)

-With lower OAT climbing with flap closed is acceptable

-Drag reduction realized with flap fully closed with marginal CHT penalty.

- OT was 175-180 throughout all flight regimes

modified cowl flap:

I will say there is significant pressure trying to open the flap from the fully closed position. As it is I had to maintain positive pressure on the cable knob to keep the flap fully closed. It will be interesting to measure the pressure changes as a function of flap position soon.

 

[DanH]

I used engineering graph paper to trace the inlet shape and calculate those areas for a total of 61.75 in^2......

Ken, that seems large. Back in 2009 I measured the stock RV-8 inlet at 45 sq in (0.3125 sq ft). Pretty sure others reported similar. My 6" dia round inlets are 56.52 sq in (0.3925 sq ft).

 

[RV8R999]

I'll check my math when I get home. I measured from the leading edge of the lip of each inlet.

 

[DanH]

I'll check my math when I get home. I measured from the leading edge of the lip of each inlet.

That's probably it.....pretty sure I measured the throat.