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RV-14 Speed Modification

mountainride

Well Known Member
I have an RV-14 with the standard IO-390 A3B6 that has been running fantastic. I am really impressed by the Van's team getting the cooling just right for an aircraft without a cowling flap. Just enough cooling for hard climbs and pattern but not excessive drag in cruise. My CHTs will sit in the low 300s in cruise during the summer at the altitudes I normally fly at in Colorado (7-15K). In a shallow 175 kias descent with 16 inches of MAP in I have trouble keeping them above 300. In fact with the 5 inch oil cooler modification my oil generally sits on the vernatherm at 185 unless its a summer climb. However if I do a ROP VY climb my chts would rise to the high 300s and oil lower 200s maybe even higher if I didn't add some airspeed like I normally do.

I can't help but think if I could remove some of the cooling drag in cruise to bring my CHT's up to ~350, I would gain some performance. My idea was to remove the bump out on the original RV-14 lower cowl, glass it flush and install the cowl door design for the new EXP engine. The cowl flap would go in the cowling instead of the tunnel (where my single exhaust stack exits) I already have the push pull cable installed for cabin control. From what I can tell there are not any clearance issues. I would basically just be reducing the exit area which to my understanding has the biggest effect on cooling drag.

I don't think I would add the operational complication of a cowl flap or wear from running 40 degree hotter for a knot or two but the gain of 5-10 knots would be worth it. What do folks think of the idea or value of a modification like this?

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You might check out Mr. Vetterman’s mod he did to his RV7 . Also have a good look at the exhaust exits of a Ted Aerostar .

I would be inclined to leave the 14 cowl exit as is, Just add a tear drop on the floor skin behind the exit that re joins the air flow like the aft half of a wheel pant.

For the exhaust&cooling air have a look at the AeroStarr exit. It would be easy to put this scallop shape into the aft tear drop cowl.

There is No doubt that there are a few knots of cruise speed to be gained through the reduction of cooling drag and reducing the turbulence of the existing exit.
 
Shrinking your exit can yield unexpected results. Vans has a proven geometry, and if you did that your odds would be good. 5-10 ktas - is . . . uuuuhh optimistic.

The open vs closed difference was 3 MPH (2.6 knots) when Marc Cook flew the -119 version RV-14 with Greg.

I did something similar on my 7 and, after substantial work, got zero speed increase but did get temp increases.

Bill's configuration remains an oddity; it has no significant lower cowl pressure. Can't generate much exit velocity without pressure. It appears the system's entire pressure drop is across the engine baffling, and indeed, it has very restrictive cylinder wrapping, a leakproof plenum lid, etc.

Along those lines, do note a major difference as compared to the angle valve 390; the parallel valve has significantly less cylinder fin flow area, i.e., given equally tight cylinder wraps, the parallel valve will be more restrictive to cooling flow.

Might be a skunkworks project in progress for the 14...
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Just as Dan has said, the most I see in closing the cowl flap is about 2-3 knots but I can hardly ever close it unless I am high up (about 13-14K) when the OAT is my cooler and not generating as much power.

My experience has shown that the EXP119 is good for those who live in a cooler climate. My CHTs are always cool (lower 300 range) but oil temp is on the upper side of the range which I am not happy about.

Consider this, that if you install the cowl flap and change the lower cowl as you are planning, you will have even less exit area than the one I have (EX119 cowl)
 
Interesting question. I have a non-EXP with dual Vetterman exhaust. I installed the cooling flap during the initial build so I don't have any before versus after data. I didn't reduce the outflow area any and now feel I could have. My CHT's are cool and my oil runs 187-191F with the cooling flap closed. I find myself only opening the flap on hot days for landing to reduce under cowl heat and reduce fuel vapour idle loping.
My gut tells me you could reduce the lower cowl exit area substantially and not create temperature issues that couldn't be addressed using the cooling flap.

Cannot speak to any speed benefit.

Attached is a good shot of the EXP cowl and exit area compared to my set up.
 

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I suspect that a good portion of performance gains from exp119 upgrade come from the change in the cooling drag. Anecdotally I am gathering the a3b6 runs a bit cooler, possibly due to sump size but also the cowl exit. We have two sources between Glenn and Merhdad that have some real world data to compare the operating temps of both engines. Vans came out of the 5 inch oil cooler line specifically for the 119. It makes sense they could reduce total cooling while shifting the split to allocate more to the oil cooler.

The only real way is to test. If the majority of gains come from a change in exit size as opposed to redirection of that airflow, simply blocking the exit would be a good first step. It shouldn’t be hard to match a cowling flap with a variable exit area encompassing what already exists. Another option is to put a shutter internally inside the existing cowling. It certainly would look “aerodynamic” and make use of the tunnel

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I remember when the EXP119 was announced by Van's Greg and I think an engineer explained the changes in a video. I'm sure it is still around. At that time Van's estimated 1/2 the speed increased was due to the engine changes, the lighter engine and the bigger fuel servo, and the other 1/2 of the gain was due to reduce drag by modifying the lower cowl and closing up the tunnel. My finish kit and engine was already ordered so I only gave the cowl flap change a try. I had already ordered Vetterman's exhaust. I may have gained a few knots with the tunnel closed up but feel I'm loosing some speed by exit area of the lower cowl being larger. As I mentioned in my previous post I believe my engine temperatures could handle the exit area reduction.

There is another 14 that happens to be an EXP119 on my field. Anecdotally the EXP seems to run slightly warmer oil temps. Also faster. We have not done side by side runs but I did not keep up to it on a long cross-country.
 
I remember when the EXP119 was announced by Van's Greg and I think an engineer explained the changes in a video. I'm sure it is still around. At that time Van's estimated 1/2 the speed increased was due to the engine changes, the lighter engine and the bigger fuel servo, and the other 1/2 of the gain was due to reduce drag by modifying the lower cowl and closing up the tunnel.

It's what the math says. See "Speaking Truth To Power" at the end of this article:

https://www.kitplanes.com/vans-rv-14a/
 
I remember when the EXP119 was announced by Van's Greg and I think an engineer explained the changes in a video. I'm sure it is still around. At that time Van's estimated 1/2 the speed increased was due to the engine changes, the lighter engine and the bigger fuel servo, and the other 1/2 of the gain was due to reduce drag by modifying the lower cowl and closing up the tunnel. My finish kit and engine was already ordered so I only gave the cowl flap change a try. I had already ordered Vetterman's exhaust. I may have gained a few knots with the tunnel closed up but feel I'm loosing some speed by exit area of the lower cowl being larger. As I mentioned in my previous post I believe my engine temperatures could handle the exit area reduction.

There is another 14 that happens to be an EXP119 on my field. Anecdotally the EXP seems to run slightly warmer oil temps. Also faster. We have not done side by side runs but I did not keep up to it on a long cross-country.

For clarification I do have the 13-row oil cooler with a 5-inch-diameter duct on my setup. I believe it was standard on the FWF kits by then even with the standard 390A.
 
I installed a plug in the scoop to test a basic change in airflow. I did back to back flights with the scoop installed on the first flight and removed on the second. Performance specs are difficult to measure as conditions always change slightly. RPM was 2400 both runs. My results were to opposite of what I expected. My airspeed was slower with the airdam installed. Temps did not have any significant change. I am assuming the low pressure burble behind the dam was stronger than without exit airflow to help relieve some of the pressure.
Changing the change in size of exit airflow did not change the cooling or cooling drag from what I can tell.

Tuft testing would confirm my suspicions. I think going after the low pressure burbles on the cowl exit are going to provide better results than chasing the cooling drag. I think a rear fairing to the fixed cowl flap, or removal of the flap completely (flat with the fuse) would offer positive results. If the fixed cowl flap were removed completely, I wouldn't expect my temps to change significantly, and would be removing a 2x14 inch lip and burble.




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I installed a plug in the scoop to test a basic change in airflow. I did back to back flights with the scoop installed on the first flight and removed on the second. Performance specs are difficult to measure as conditions always change slightly. RPM was 2400 both runs. My results were to opposite of what I expected. My airspeed was slower with the airdam installed.

Picture is a bit dark and I'm not quite clear on what you mean by installing a 'plug in the scoop'. Do you mean you blocked the exit area in the lower cowl such that the only exit for cooling air was via the tunnel? If that's the case then I would expect slower airspeed because you just increased drag. If that lower cowl exit area isn't needed for cooling then you'd need to remove it to reduce drag and improve performance (which I believe is what you proposed doing at the beginning of this thread). I was thinking about doing exactly that on my -14A, but before I ordered the finish kit Van's announced the EXP119 package and did it for me.
 
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