What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

The Shrinking Exit

... if it's truly over center, that means it would be jammed, right? I'm confused on why you describe it this way.

Perhaps a poor choice of words. Let's try a picture.

The highest load on the door linkage would be when the door is closed, because that's when internal cowl pressure is the highest.

If we arrange things so that the arm linked to the door becomes parallel to the link as the door reaches the closed position, there is no resting load on the actuator. That's good for the plastic gears, which are subject to a bit more heat than the designer intended. Even better, as the door nears closure and resistance rises, this sort of arrangement provides a progressive increase in linkage ratio. Thus a little bitty actuator can pull the door shut without strain.

Top is door open, bottom is door closed.

Linkage.jpg
 
Last edited:
Aha! Had to go back to your original photos and blow them up and compare with your diagrams, bit I get it now and also see how the over center arrangement with the door closed relieves stress on the actuator.
Thanks
Erich
 
Great mod Dan. 4 kt increase is a pretty significant drag reduction. I think I know what Bob Axsom will be doing this winter :)
 
Dan since you had your cowl manometer-ized I would be interested in knowing the cruise pressure differences between iterations of the cowl exit.
 
Aha! Had to go back to your original photos and blow them up and compare with your diagrams, bit I get it now and also see how the over center arrangement with the door closed relieves stress on the actuator.

Here's one with the door more than half open. As the actuator pulls on its center arm to close, the arms on the ends will eventually parallel the linkage rods. Linkage ratio goes to infinity.

The black heat shrink thing on a wire is a temperature probe. The actuator was subject to about 160F in this location. As you can see the location shields it from radiant heating.

Variable%20Exit%20Actuator%20Ver1.jpg


Great mod Dan. 4 kt increase is a pretty significant drag reduction. I think I know what Bob Axsom will be doing this winter :)

Thanks, but, well, let's remember this was a shakedown cruise. To use Bob's excellent example, accurate measuring is needed. In addition to airspeed, it would be nice to cycle the door and measure a lower cowl pressure rise and an exit velocity increase.
 
Last edited:
Point of clarification...

I believe the 4kt increase is from a door open to door closed position not an adidtional 4kt increase over the previous exit of same area without a door installed, correct Dan? Unless this exit has smaller area than previous??

Dan's idea, as was mine, was to shrink the exit as small as possible for cruise drag reduction with acceptable penalty in CHT while adjusting the door open for the WOT climb condition to provide CHT margin.
 
Last edited:
I believe the 4kt increase is from a door open to door closed position not an adidtional 4kt increase over the previous exit of same area without a door installed, correct Dan?

I added the red for emphasis.

Pushed over and established the usual settings for fast cruise, WOT, set RPM, lean to 100-125 ROP for power, trim, altitude hold on. Indicated TAS settled in at the usual 181 knots, which is what I had with the previous exit.

Now the good part.....thumbed the switch to close the door, and picked up four knots....

As I read this, he gained the 4k above his normal speed/power settings.

I wonder if there is going to be a max speed increase, and if so, how much.

Pretty sure that is what most folks are interested in :rolleyes:
 
Last edited:
Point of clarification...
I believe the 4kt increase is from a door open to door closed position not an adidtional 4kt increase over the previous exit of same area without a door installed, correct Dan?

Again, the numbers are preliminary, but with the door closed it appears to be faster than previous. A few GPS triangles will tell the tale.

Remember, the fixed exit area (what remains with the door closed) has been reduced as compared to the previous fixed exit. Even the tailpipe clearance bump has been reduced. I also added an internal flange to capture the trailing edge of the horizontal intake fairing; you can see it below the actuator in the above photo. Sealing that joint means one less loss of internal cowl pressure,as the previous exit panel was bulging outward along that line. Increased internal pressure + smaller exit = increased exit velocity = less cooling drag.
 
yep, with smaller exit would expect speed increase.

So here is the test - how small will you go? I'm going to reduce mine an additional 15%.
 
Dan, that is a very elegant solution. I appreciate the time and effort you have put into this ?project?.
 
+1 for Axel's sentiments

Dan,

Big thanks for your efforts in posting all of this, and for taking the time to educate the folks "in the back of the classroom."
 
sweet!

that's a sweet door. looks like a butterfly valve turned sideways. I'm impressed by both the thought and workmanship. could you get by with only one actuator arm... like a butterfly valve?
 
Ya'll are quite welcome.

Steve, I doubt a single pushrod would work for long, at least not in this general configuration. I'd expect the hot fiberglass door to warp torsionally.
 
Another great invention

Dan I am amazed of your craftsmanship and inventiveness. You should have lived on the other side of the world :)
 
Force On Door

could you get by with only one actuator arm... like a butterfly valve?

My max measured lower cowl pressure was 6.75 " H20 (relative to ambient static at altitude) = .24 PSI at Vmax. My door is 6" x 4" = 24 sq in. Force on the door at Vmax is about 5.76 lbs. Mine is hinged at the forward edge so the hinge and door feel the full load . Because Dan's is hinged and actuated at the center, when the door is closed, neglecting the slight pressure gradient across the span of the door, the force on his forward half should nearly oppose the force on the aft half resulting in nearly zero moment about his neutrally located hinge line - elegant design for sure! (the exit area begins to accelerate as it moves closer to the exit so static pressure in the cowl is not going to be uniform but for purpose of illustration and in consideration of the short span of the door I'll call it uniform...I know, I know)

Dan, your door may actually stay mostly closed without actuators since as the the door begins to open the dynamic pressure of free stream will act mostly upon the aft portion of the door creating a closing moment. I cannot tell from photos if the fwd and aft portion of the door are of equal area?


As Dan pointed out the heat on the glass plus the load will allow the door to warp. I saw this on the first iteration of my setup when I only had one actuator. After a flight with the door fully closed on shutdown the side without the actuator was still open about 1/2" my actuator arms are located near the aft edge of the door not at the hinge line. My first actuator was simply a push-pull cable (standard Van's carb heat type). After a Vmax run with the door fully open I returned to find the cable bent from dynamic pressure loads on the door when fully open.

This is super cool (no pun intended) - Dan really needs to start an engineering firm specializing in composite aircraft modifications - maybe competition to LoPresti!
 
Last edited:
Rigged my old lipstick camera on a pedestal mount under the belly and taped yarn everywhere.

Good news and bad news.

Bad: At this time the door does not fully close in flight. It appears be a combination of factors. Heat and internal pressure is bulging the panel at the hinge line, and some of the deformation has become permanent despite using an epoxy with a better Tg. I also think the aluminum pushrods are lengthening quite a bit in flight due to their location very close to the exhaust pipe.

Good. The flows look excellent. With the door open you see some reversal just aft of the door chute. Nothing else seems to be doing anything objectionable. There is a little disturbance in the primary exit (the big one with the exhaust pipe), but watch how it smooths out when the door closes. The last 10 seconds or so is taken at 180 knots.....nice clean streamlines. Me like.

So, for now, don't copy the physical structure, but you might want to think about the aero stuff. It's all about exit velocity.

http://youtu.be/nA5PY7PYBsU
 
Last edited:
An examination of the cowl exit panel quickly located the problem. The pivot points lacked support; the edges of the door opening were bowing outward due to internal pressure.

So, I added some ribs along the sides of the door opening to support the pivots. They are simple hardwood cores covered with 2 plies of 9oz and aluminum reflector material:

VE%20with%20Ribs.jpg


Compare with the photo in post 147.

The weather cleared today and I had a chance to fly it. The door now stays shut as designed. Here's a short (boring) clip from the Belly Cam:

http://youtu.be/aIBXAE2Ezn4

New speed numbers in the near future. I did have the opportunity to fly an NTPS triangle about 2 weeks ago, just to confirm the TAS indication which was last calibrated during Phase 1. It's still accurate. 185 knots indicated calculated at 184.9...cruise at 9500.
 
Last edited:
Dan I am amazed of your craftsmanship and inventiveness. You should have lived on the other side of the world :)

Why, so they could send him to the gulag, like all of the brilliant Russian aircraft designers? We are Americans, we do NOT live to suffer! :D Awesome work Dan.
Charlie
 
Follow up...this weekend I replaced the tiny Firgelli servo with a standard Ray Allen T2-7A.

RAC%20Servo%20600P.jpg


Early in test I had noticed some freeplay developing within the Firgelli servo and expected it to strip a gear due to heat, which measured at about 160F in the servo location. However, it has been hanging in there since Christmas, so bravo to the designers.

That said, OSH is coming and I didn't want to chance a failed air door inbound from Ripon. The RAC servo is larger and more powerful, meaning the load applied in this app should only be a small fraction of normal capacity, i.e. low load on the gears when hot.

Figelli servo travel was 10mm (0.3937") and the T2-7A is 0.7", so the servo arm in the linkage was lengthened to 0.75". That gave me about 1/8" less door opening, good because the front edge of the door was contacting a pipe when full open. The RAC servo takes a full 10 seconds to cycle end to end and has internal limit switches, so it's easy to select partial openings. The Firgelli cycled so fast I pretty much just ran it full open or full closed.

More reports at OSH, over beer ;)
 
Last edited:
Playing around last weekend on a run to Oklahoma. Nothing much to do in cruise so I got some data.

Upper plenum - aircraft static delta vs exit door position in seconds of servo motor run. (picolo tubes in upper plenum)

Exit velocity vs door position (exit pitot-static probe)

All pressures are inches H2O

position......pressure.....% of Potential Q....Exit delta
Open.........11.36........ 0.7126.................0.91
2..............11.63.........0.7296.................1.15
4..............12.03.........0.7547.................1.27
6..............12.3...........0.7716.................1.35
8..............12.3...........0.7716.................1.45
closed.......12.35..........0.7747.................1.46


12.35 / 11.36 = 8.7% plenum pressure increase...pressure recovery rises when the exit is throttled.

1.46 / 0.91 = 60% exit velocity increase. Note that exit throttling is quite different as compared to the usual "cowl flaps" concept.
 
Last edited:
Dan,
Where in Oklahoma were you? Would have loved to talk with you when you were here.

Did you measure any speed changes from open to closed? Curious to know if there were any noticeable differences?

I am still very interested in the AirChia design for my exhaust exit but have not done anything besides think about it.
 
Dan
Do you have the corresponding IAS and CHT numbers?
I am especially interested in the difference as you squeeze the opening the last few times.
On my plane there is a spot where IAS reaches a point where it actually will decrease slightly if I close it down too much.
I have found that I never have to open or close the unit now that I have found that sweet spot, and I am considering glassing the unit shut. (Now maybe if I lived in Alabama I might want to open it up occasionally in the summer!)
 
Last edited:
Dan, Where in Oklahoma were you? Would have loved to talk with you when you were here.

Just across the border from Ft Smith.

Dan, Do you have the corresponding IAS and CHT numbers?

No...cross-country cruising, so I wasn't going to fly three leg circles. CHT goes up and down with cooling mass flow and/or mixture adjustment, as you would expect.

I am especially interested in the difference as you squeeze the opening the last few times. On my plane there is a spot where IAS reaches a point where it actually will decrease slightly if I close it down too much.
I have found that I never have to open or close the unit now that I have found that sweet spot, and I am considering glassing the unit shut. (Now maybe if I lived in Alabama I might want to open it up occasionally in the summer!)

Door operation is non-linear; door motion is less and less per unit of servo run time as the door nears closed. Go back a few posts to the door linkage discussion and you'll understand why. Speed increase appears to be proportional to exit delta values.

Don't remember your exit configuration very well. Got a picture?

Yes, I want temperature control over a wide range of OAT and airspeed...an extension of Mr. VanGrunsven's mantra.
 
Just out of curiosity, did you ever measure the cooling air exit temperature at high speed? Door open vs. closed?
 
Just out of curiosity, did you ever measure the cooling air exit temperature at high speed? Door open vs. closed?

Not open vs closed.

I had done some preliminary measurements earlier with a fixed 4" extension, i.e. the same exit as with the door closed.

TAS....... Exit Temp
knots..... F
133.7..... 228
154.5..... 221
170.6..... 221
191.9..... 209

Increasing airspeed means more mass through the system. Efficiency is reduced as mass flow rises; it's not heating the air as much per unit quantity of air. I'd expect an across-the-board reduction if flown at the same airspeeds, door open. It would simply be more mass flow. That's fine. I don't care about efficiency in initial climb, or when grinding inbound on the Ripon approach. I just want low engine temperatures.
 
Last edited:
TAS....... Exit Temp
knots..... F
133.7..... 228
154.5..... 221
170.6..... 221
191.9..... 209

Increasing airspeed means more mass through the system. Efficiency is reduced as mass flow rises; it's not heating the air as much per unit quantity of air. I'd expect an across-the-board reduction if flown at the same airspeeds, door open. It would simply be more mass flow. That's fine. I don't care about efficiency in initial climb, or when grinding inbound on the Ripon approach. I just want low engine temperatures.

Geez Dan, and I WAS feeling great about getting my temps down to 350 degrees. WOW!
 
Not open vs closed.

I had done some preliminary measurements earlier with a fixed 4" extension, i.e. the same exit as with the door closed.

TAS....... Exit Temp
knots..... F
133.7..... 228
154.5..... 221
170.6..... 221
191.9..... 209

Increasing airspeed means more mass through the system. Efficiency is reduced as mass flow rises; it's not heating the air as much per unit quantity of air. I'd expect an across-the-board reduction if flown at the same airspeeds, door open. It would simply be more mass flow. That's fine. I don't care about efficiency in initial climb, or when grinding inbound on the Ripon approach. I just want low engine temperatures.

Fascinating data. I would have thought the temperatures would have been much higher given the heads are usually well above 350F. I suppose the steel barrels have a large percentage of the total airflow running over them, are a lot cooler than the heads and have a much lower K value than the aluminum heads. This would dilute the hotter air from the heads somewhat.
 
Fascinating data. I would have thought the temperatures would have been much higher given the heads are usually well above 350F. I suppose the steel barrels have a large percentage of the total airflow running over them, are a lot cooler than the heads and have a much lower K value than the aluminum heads. This would dilute the hotter air from the heads somewhat.

The exit air is heated by the heads and the barrels, plus the oil cooler, hot exhaust system, the alternator, and even a little heating from compression at the intakes. Due to the mixed nature of the heating you can't pay much attention to the values by themselves. A better yardstick incorporates intake temperature and average CHT...

(Exit - OAT)/(CHT - OAT)

...the result being a decimal value. Work to push that value as high as possible; maximum heat transfer means less mass flow for the same cooling.

Practical note; when measuring exit air temperature, be sure to shield the probe against radiant heat from the exhaust system. You can see the shield to the right of the tailpipe in post 147. It's fiberglass with a reflective aluminum tape surface. The probe is suspended behind it.
 
Dan, Great data, very useful. Now if we just knew the mass flow. One thing, is the heater bypass air also included in your exit flow? I have been thinking about reducing it with an orifice on the heater valve rather than a full dump.

If we know ambient temp, pressure (density), fuel flow, manifold pressure, egt and engine rpm, we can make a good approximation of heat balance for cooling mass flow purposes.

Thanks again.
 
Last edited:
Oh - exit temperature

Please excuse me Dan & shame on me for not reading more closely. I mistook those numbers for cylinder head temps. :(
 
One thing, is the heater bypass air also included in your exit flow? I have been thinking about reducing it with an orifice on the heater valve rather than a full dump.

Given the focus on efficiency, I'm rather anti-bypass. There is no forced air heater or heat muff; I wired the airplane for electric vests. No blast tubes either.

Please excuse me Dan & shame on me for not reading more closely. I mistook those numbers for cylinder head temps. :(

Jugs that cold would be poking little bumps in the baffling ;)
 
The exit air is heated by the heads and the barrels, plus the oil cooler, hot exhaust system, the alternator, and even a little heating from compression at the intakes. Due to the mixed nature of the heating you can't pay much attention to the values by themselves. A better yardstick incorporates intake temperature and average CHT...

(Exit - OAT)/(CHT - OAT)

...the result being a decimal value. Work to push that value as high as possible; maximum heat transfer means less mass flow for the same cooling.

Practical note; when measuring exit air temperature, be sure to shield the probe against radiant heat from the exhaust system. You can see the shield to the right of the tailpipe in post 147. It's fiberglass with a reflective aluminum tape surface. The probe is suspended behind it.

Fascinating data indeed.

At some point you should arrive at an optimal entry-exit ratio that would achieve maximum heat transfer based on air volume and speed through the compartment and with minimal drag factor based on TAS with a particular configuration - or at least that is my interpretation of the effort - right?

Lots of stuff going on here....that's what makes it so interesting.
 
Dan, do you have a figure for the closed door exit area?

13" x 2.625" less about half the tailpipe area, ballpark 22-25 sq in. Right at the moment I don't remember the exact tailpipe diameter.

Postscript; measured the tailpipe..2.5" diameter, so (13 x 2.625) - (1.25^2 x 3.14) = 29.2 sq in. Call it 30.
 
Last edited:
Follow up....

The Ray Allen T2-7A servo has been operating the cowl door for about one year now. Recall that RA doesn't claim their servos will hold up at the sort of temperatures typical for a well sealed cowl, but they don't say they won't either. So far, I have not noticed the slightest reduction in performance, increase in noise, or any other symptom of a heat issue. It is insulated with fiberfrax felt and aluminum tape, and it is shielded from exhaust pipe radiant heat. The insulation just means it takes longer to reach the overall in-cowl temperature, so it's living with the heat.

The drag reduction is nice, although a fully closed door requires an OAT at 65F or so to maintain oil temp below 200 in fast cruise. Here's a current example; yesterday, hauling back to the office after a truck sale in Mississippi. It was warm at 7500, about 66F, and I'm at best power mixture, door closed, and 69% power per the EFIS. True airspeed checks per the NTPS method say the EFIS is accurate.

EFIS%20-%20Temperatures.jpg


With this exit geometry, door position has strong effect on oil temperature, or more precisely, oil cooler mass flow. Given the above conditions, opening the door about 1/3 will lose 1~2 knots and drop oil temp 6~10 degrees. That's rarely necessary if I get high enough, as OATs in the 10K ballpark tend to be low enough for a fully closed door.

Down low in a hot day, full open costs about 4 knots in cruise, and puts oil temperature under control of the vernatherm, about 187F for mine. Obviously the door is full open for climb, or crawling around in slow flight. Yesterday it was over 90F on the ramp at KPIB. With the door full open, taxi time and a WOT/2700 climb to 5500 @125 IAS didn't break 200F.
 
Last edited:
Dan, what fuel flow are you at in th picture of your EFIS in post 190? I am trying to equate your 390 cubes to my 360 for the other parameters. Thanks.
 
Follow up...this weekend I replaced the tiny Firgelli servo with a standard Ray Allen T2-7A.

Early in test I had noticed some freeplay developing within the Firgelli servo and expected it to strip a gear due to heat, which measured at about 160F in the servo location. However, it has been hanging in there since Christmas, so bravo to the designers.

That said, OSH is coming and I didn't want to chance a failed air door inbound from Ripon. The RAC servo is larger and more powerful, meaning the load applied in this app should only be a small fraction of normal capacity, i.e. low load on the gears when hot.

Dan,

Since Allan has brought an OTS solution to market using a similar actuator, did you ever tear that thing apart to see if it was a pending failure or?

As far as the RA T2-7A. How is that servo holding up?

By the way, thanks to Lars, I now have everything I need to make the piccolo tubes and gather some real data to answer your question in my other thread....
 
Last edited:
About 11.2; the EGT values are degrees ROP.
Dan, a question that is a little bit off the current topic but still of interest concerning the shrinking exit -- Have you evaluated the CHT's, TAS, fuel flow, Oil Temp, etc. while running in the same configuration as you posted in #190 while LOP? If so, what numbers have you seen LOP in that similar environment?
 
Have you evaluated the CHT's, TAS, fuel flow, Oil Temp, etc. while running in the same configuration as you posted in #190 while LOP?

Not seriously. No point. The benefits/shortcomings of LOP operation are independent of exit area.
 
Not seriously. No point. The benefits/shortcomings of LOP operation are independent of exit area.
Understand about the exit area issues not really relevant for a conversation on LOP operations. I was just curious if you tested running LOP compared to ROP and what the delta was for the parameters mentioned.

Hope to talk with you again at OSH.
 
Dan, i am designing a cowl flap for summer ops. How is the RA servo holding up?

I am going to finish my flight test phase before starting all the press/temp measurements under the cowl. I have all the instrumentation ready. I wont make any FWF mods until I am instrumented.
 
Follow up report.

Started flying this oil cooler air supply duct in early 2013. The goal was to reduce the temperature of the air reaching the ducted oil cooler; measurements said the air picked up as much as 17F as it made its way from the cowl inlet to the rear baffle wall.

Finished%20Duct.jpg


The duct dropped average oil termperature as expected, but I later decided to install a larger 10611 oil cooler anyway. I removed the above duct from the plenum lid at the most recent annual. As an experiment, it proved the theory, but isn't really needed now.

The 10611 with inlet and outlet ducting remains. Inlet is fed from the rear baffle wall. Cooler hangs on the motor mount. Inlet diffuser has a turning vane. The outlet duct sits flat against the firewall and terminates in the top of the cowl exit bell.

Oil%20Cooler%20Duct%20Inlet.jpg


OC%20Diffuser%20with%20Vane.jpg


10611%20and%20Ducts.jpg
 
Last edited:
I happened to be looking up some old data from past tests, and just out of curiosity, ran the numbers for coefficient of pressure as defined in NASA CR3405, to wit:

(plenum static pressure less freestream static) / freestream dynamic pressure

The three measurements made in level flight were done at partial power, in each case just enough to maintain the desired 120, 140, and 160 IAS. The resulting true airspeed (NTPS method) is shown here. A Cp in the high 0.8 range is very good.

The curiosity point was the effect of propeller outflow at full power and 100 IAS, given a constant speed BA Hartzell, with low Vi/Vo inlets moved outboard as far as possible, and in close proximity to the blade. Cp turned out to be 1.17 when calculated using 100 knots for the dynamic pressure. The difference between Cp in the 0.8's and 1.17 is almost entirely due to high velocity air from the prop, meaning prop outflow is a very large part of cooling power in a steep climb.

There is a probable prop outflow contribution in the level flight values too, but not nearly so much, as the power settings were quite low, and the difference between prop outflow and aircraft TAS gets smaller as velocity goes up. It might be interesting to get some pressure numbers with the prop stopped, for comparison.

Note, these measurements were taken several exit iterations in the past....a small fixed exit ending at the plane of the firewall, the smallest in the very first post.

Prop%20Contribution%20to%20Cliumb%20Cooling.jpg
 
Last edited:
Thanks Ron,

Like you, I had a tough time following Dan's logic...but your post very much helped me get my head around some of the technical issues. The explanation of the Rockwell Retro Encabulator put everything into easy perspective.

Thanks,
Ackselle
 
Back
Top