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  #41  
Old 03-08-2018, 11:58 AM
Bicyclops Bicyclops is offline
 
Join Date: Oct 2012
Location: LA, California
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Default 18 kts

I closed my very large cowl flaps at cruise and gained 18 knots. I also got a substantial pitch down. I don't usually open them at cruise, just sometimes forget to close them right away. That and forget to turn off the boost pump. ;-(

Ed Holyoke

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Originally Posted by simatos View Post
has anyone opened the cowl flap at cruise speeds? Not just in the climb??
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  #42  
Old 03-08-2018, 08:23 PM
simatos simatos is offline
 
Join Date: Sep 2014
Location: vancouver canada
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I just wondered if its safe to open in cruise if you choose to make a hard climb and you put the cowl flap out in that situation. Thanks
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  #43  
Old 03-08-2018, 08:46 PM
Bicyclops Bicyclops is offline
 
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Yeah, it's safe as far as not causing drastic aerodynamic effects. Depending on the cowl flap - how it's built, how it's actuated - you might be able to bend or break the cowl flap or mechanism, but it's not going to bring the airplane down. I get a significant pitch-up and a bunch of drag at cruise speeds, but it doesn't come anywhere near affecting the controlability of the airplane. If you are having trouble controlling CHTs in climb, crack the cowl flap open if you've got one. If you only have to open it a little to get the cylinders cooled off, there is no reason to fling it wide open. If you really do need it wide open, inspect it closely afterwards and you'll have your answer. My homemade cowl flaps don't have any problem holding up to the highest speeds I've thrown at them. Another thing is that, if you really are making a hard climb, you won't be at cruise speed for long. Lower airspeeds will reduce the aerodynamic loads on the flaps.

Ed Holyoke

Quote:
Originally Posted by simatos View Post
I just wondered if its safe to open in cruise if you choose to make a hard climb and you put the cowl flap out in that situation. Thanks

Last edited by Bicyclops : 03-08-2018 at 08:52 PM.
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  #44  
Old 03-09-2018, 06:13 AM
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DanH DanH is offline
 
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Location: 08A
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A "cowl flap" need not be a flap, or extend very far into the airstream, or require much mechanical power to open or close.

Original implementation was powered with the smallest Firgelli linear actuator(L12). It was powerful enough given the linkage design, but developed internal freeplay over time, assumed due to temperature. It's now powered with the familiar trim servo.

The lower cowl itself required the addition of reinforcement ribs, as shrinking the exit raises lower cowl pressure, and the entire cowl tries to blow itself into a round balloon shape, and this system requires cutting a pretty good size hole. The deformation didn't allow full closure, as seen here in the first video. Note the gap increase at 180 knots as compared to 120. It's a direct response to increased cowl pressure.

https://youtu.be/nA5PY7PYBsU

BTW, while there, note the disturbed flow when open, just aft of the rear slot, and just in front of the tailpipe. That's the disadvantage of anything extending into the airstream. As a flap gets larger and/or the open angle gets more acute, the disturbance gets larger. Pure drag. Everything goes smooth, including the tufts in the primary exit, as the auxiliary exits close and primary exit velocity rises. A slot exit like this one would produce less drag when open, thus there is less airspeed delta, open vs closed. The pitch trim change is very small.

A center hinged door means one of the two auxiliary exits is a recessed slot.

This video taken after internal cowl reinforcement, to check for full closure. Didn't bother with tufts, as the first video told me all I needed to know on that score:

https://youtu.be/aIBXAE2Ezn4

Folks who are adding Nimmo's flaps while retaining a full size stock exit don't need to worry much about cowl deformation due to internal pressure. On the other hand, I've never videoed one at 180 knots.
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Last edited by DanH : 03-09-2018 at 06:17 AM.
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  #45  
Old 03-09-2018, 08:08 AM
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AX-O AX-O is offline
 
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Quote:
Originally Posted by DanH View Post
On the other hand, I've never videoed one at 180 knots.
this one is close at 179 KTAS.
https://www.youtube.com/watch?v=CdGLPGgpyQA

I don't open it all the way any more after seeing the tufts. That being said, I tested at 169 KTAS/full open (and closed it at the end of the video).
https://www.youtube.com/watch?v=Qb7ypnvV9vw

And at 67KIAS
https://www.youtube.com/watch?v=PYoPUOCfQ8g
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  #46  
Old 03-09-2018, 09:13 PM
simatos simatos is offline
 
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Thats very interesting, Its amazing the cameras don't blow off at those speeds too. I put the EZ cowl flap in today hope to try it out this weekend. Thanks G
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  #47  
Old 03-10-2018, 04:06 AM
NYTOM NYTOM is offline
 
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Default Slightly off topic

Dan I took your advice and Im adding a SS heat shield with fiberfax backing in a installation that looks very similar to yours. I think I obtained the right thickness SS but just wanted to ask what yours is. In the video yours looks nice and snug with no visible movement at all. That stuff is so thin I was afraid of some kind of distortion at speed.
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  #48  
Old 03-10-2018, 06:33 AM
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DanH DanH is offline
 
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Quote:
Originally Posted by NYTOM View Post
Dan I took your advice and Im adding a SS heat shield with fiberfax backing in a installation that looks very similar to yours. I think I obtained the right thickness SS but just wanted to ask what yours is. In the video yours looks nice and snug with no visible movement at all. That stuff is so thin I was afraid of some kind of distortion at speed.
The stainless panel in the video is a polished SS replacement for the standard Vans aluminum 867A-1 exit ramp. It is typical firewall material, about 0.018" thick, not a foil overlay as one would do on a 6-7-9 or 10. There is also a lot of aluminum tape stuck to it in the video, for tufts, and measurement probe hoses and wires.

A foil overlay on the firewall is fine at 0.002". Some folks are using as thick as 0.005" on firewalls and belly panels, for appearance.
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  #49  
Old 03-10-2018, 07:04 AM
Kyle Boatright Kyle Boatright is offline
 
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Quote:
Originally Posted by DanH View Post

A foil overlay on the firewall is fine at 0.002". Some folks are using as thick as 0.005" on firewalls and belly panels, for appearance.
Some folks <ahem> use as thick as .008. Still easy to cut with snips and thick enough to roll the edges slightly. Doesn't deform much at all when riveted.
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  #50  
Old 05-26-2018, 11:04 PM
Hongie Hongie is offline
 
Join Date: Sep 2012
Location: Mackay Australia
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Quote:
Originally Posted by Bicyclops View Post
As others have mentioned, the standard RV cowling works pretty well for most people with proper attention to sealing. One thing I have noticed on many RVs that I have inspected is that the little tab on the baffle which is supposed to be bent to be very close to the cylinder base of #3 is often left wide open. Even if the tab is bent properly, that whole area should be sealed with RTV and often isn't.

I modified my cowl during build a la Dan Horton's "Shrinking Exit" by increasing inlet size and decreasing exit area. I was concerned that I would have excessive CHTs and oil temps during taxi and climb so I took drastic measures to increase airflow through an oversize cooler and I cut in very large cowl flaps. As it turns out, I have to choke off most of the air to the oil cooler just to get into the green and don't need the cowl flaps for taxi and climb even on 100 degree days. Now that the weather has cooled to about 90 degrees (much cooler than that today), my hottest cylinder is about 340 down low with high power set. So - I definitely have excess cooling capacity. My intent is to shrink and extend my exit scoop further which will enclose my exhaust pipe and add some augmentation to the mix. My hope is that I'll reach a point where I do need the cowl flaps for ground ops and climb but not for cruise.

As pertains to this thread, I wouldn't do cowl flaps during build on a standard installation. If attention to detail is employed, it'll cool ok. If it runs hot anyway, then the addition of cowl flaps is a viable method of adding mass airflow for cooling.

Ed Holyoke
sorry for the thread revival.

is the cirlced bit the tab you are talking about?

I'm experiencing high CHT #3

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