cowl flaps

[DanH]

Originally Posted by RV8R999

If I'm not mistaken you are using the VANS hinge method on your cowl halves correct?

Formed-in-place glass flange with nutplates and screws:

http://www.vansairforce.com/communit...nge#post276028

Hmmm....just had a thought not previously considered. The clamp force of a Skybolt fastener in tension is less than or equal to the stiffness of their little internal spring.....so a Skybolt can't stop a panel from blowing outward just a little bit when subjected to internal pressure. Good reason to add those rubber flap seals.

[rvmills]

Ken and Dan,

You're right, don't use duct tape, and definitely don't use gorilla tape! It'll pull paint, and heat seems to make each leave a lot of gunky residue.

In most areas I use 3M 471 in the 2" width. Wide, flexible, does curves well with a little patience, and comes off fairly clean, especially if you don't leave it on too long (and goo-gone gets the residue). Doesn't pull paint, unless you have a chip and pull it off too fast. Kinda pricey.

However, on the cowl seams, I've been using a 1" wide clear vinyl tape that Mark Frederick brought to Reno this year. I believe it is this tape, as found on amazon.com. I'll contact Mark and double check, and ask him to look at this thread too. This stuff is very reasonably priced ($5 for 36 yds).

For the 3M 471, here's a link to today's search that shows pages of options for 471, in 1/2', 3/4", 1.5" and 2" widths. Not cheap stuff, especially the 2", but different colors have widely varying prices ($22 to $37 for a 36 yard roll)red and blue seem cheapest), but if its for testing, color is not an issue! Amazon has the best prices I've found (if ya beat it, please lemme know!)

Watching this thread with great interest and learning, as always. I think I've worked up the courage to work cowl issues this winter (those flames in $1,000/gallon paint have intimidated me till now!). Exit area work via a Dan-like changeable shrinking exit, closing that gap at the prop flange, firewall cleanup and internal ducting, and a new induction air intake scoop like a Rocket scoop are items on the drawing board.

Great discussion and great work Ken. Thanks for blazing trails to all of you!

Cheers,
Bob

[RV8R999]

Originally Posted by DanH

Formed-in-place glass flange with nutplates and screws:

http://www.vansairforce.com/communit...nge#post276028

Hmmm....just had a thought not previously considered. The clamp force of a Skybolt fastener in tension is less than or equal to the stiffness of their little internal spring.....so a Skybolt can't stop a panel from blowing outward just a little bit when subjected to internal pressure. Good reason to add those rubber flap seals.

wow..very nice work and undoubtedly well sealed.

agree about skybolt clamp force. Also wondering how much leakage through the skybolt hardware itself...I'll go check (don't ask).

What are your thoughts on attaching rubbers to the firewall perimeter? I have not found any adhesives which provide any longevity although I have not tried proseal (next to adel clamps the second most evil inanimate object).

[RV8R999]

Originally Posted by rvmills

Ken and Dan,

You're right, don't use duct tape, and definitely don't use gorilla tape! It'll pull paint, and heat seems to make each leave a lot of gunky residue.

In most areas I use 3M 471 in the 2" width. Wide, flexible, does curves well with a little patience, and comes off fairly clean, especially if you don't leave it on too long (and goo-gone gets the residue). Doesn't pull paint, unless you have a chip and pull it off too fast. Kinda pricey.

However, on the cowl seams, I've been using a 1" wide clear vinyl tape that Mark Frederick brought to Reno this year. I believe it is this tape, as found on amazon.com. I'll contact Mark and double check, and ask him to look at this thread too. This stuff is very reasonably priced ($5 for 36 yds).

For the 3M 471, here's a link to today's search that shows pages of options for 471, in 1/2', 3/4", 1.5" and 2" widths. Not cheap stuff, especially the 2", but different colors have widely varying prices ($22 to $37 for a 36 yard roll)red and blue seem cheapest), but if its for testing, color is not an issue! Amazon has the best prices I've found (if ya beat it, please lemme know!)

Watching this thread with great interest and learning, as always. I think I've worked up the courage to work cowl issues this winter (those flames in $1,000/gallon paint have intimidated me till now!). Exit area work via a Dan-like changeable shrinking exit, closing that gap at the prop flange, firewall cleanup and internal ducting, and a new induction air intake scoop like a Rocket scoop are items on the drawing board.

Great discussion and great work Ken. Thanks for blazing trails to all of you!

Cheers,
Bob

Bob, Thanks for all the info good stuff!

[rvmills]

Originally Posted by RV8R999

wow..very nice work and undoubtedly well sealed.

Concur, very nice. My cowl is done with nutplates and screws as well. Seems fairly tight, and I don't get much bowing between fasteners. However, I have seen small areas of cowl seam tape that appeared to have had some air blow-by. Leads me to believe this sealing is a very valuable exercise. I like the idea of the baffle material seal...cogitating on that.

Originally Posted by RV8R999

agree about skybolt clamp force. Also wondering how much leakage through the skybolt hardware itself...I'll go check (don't ask).

Wear safety glasses! I have this mad scientist with compressed air vision in my head! Perhaps you could take it out on a couple adel clamps and launch them with the skybolt innerds when you find their critical pressure!

Originally Posted by RV8R999

What are your thoughts on attaching rubbers to the firewall perimeter?

Now that would look really strange...oh never mind...mind outta the gutter!

Originally Posted by RV8R999

I have not found any adhesives which provide any longevity although I have not tried proseal (next to adel clamps the second most evil inanimate object).

That is a definite concern...how to hold them in place when there is no air pressure pressing them up or out into the seam...more cogitating!

Cheers,
Bob

[gereed75]

Ken, I suspect that the general improved efficiency that you saw even with the cowl flap closed is due to improved exit airflow. Again, flow at the corners of the stock cowl outlet is bad. Giving that turbulent corner flow a chance to straighten and combine with the free stream air improves the exit flow considerably.

See my cowl exit fairing. Simple but good for 4 Knots and 15 - 20 degrees cooler CHT's.



http://www.flickr.com/photos/68108668@N04/

Bob, I would be happy to send you my prototype made of aluminum if you want to play with it. It should fit your six cowl. Us Navy guys must stick together, especially if it means running down Rockets!!

[RV8R999]

Originally Posted by rvmills

Wear safety glasses! I have this mad scientist with compressed air vision in my head! Perhaps you could take it out on a couple adel clamps and launch them with the skybolt innerds when you find their critical pressure!

You really don't want to know...

Originally Posted by rvmills

Now that would look really strange...oh never mind...mind outta the gutter!

very funny!

[RV8R999]

Originally Posted by gereed75

Ken, I suspect that the general improved efficiency that you saw even with the cowl flap closed is due to improved exit airflow. Again, flow at the corners of the stock cowl is bad. Giving that turbulent corner flow a chance to straighten and combine with the free stream air improves the exit flow considerably.

See my cowl exit fairing. Simple but good for 4 Knots and 15 - 20 degrees cooler CHT's.





http://www.flickr.com/photos/68108668@N04/

Bob, I would be happy to send you my prototype made of aluminum if you want to play with it. It should fit your six cowl.

Interesting...

[gereed75]

I have some additional refinements in the works along the lines of Tom Martin's suggestions back on page 2 -

"4. Internal ducting of air in the lower cowling towards the exit area"

When that boy speaks, it comes from many hours of experience with many mods and his results show it.

I'll continue to post as I learn. Continued good luck.

[rvmills]

Gary,

I'd love to experiment with your prototype...thanks for the offer. I'll pay shipping! The aft edge of my exit is actually cut at a raking angle forward (rather that straight down). Not sure if one of the previous owners cut it to allow better cooling (at the expense of drag). The motor and oil cool very well, but I can see reverse airflow evidence aft of the exit in the form of oil drips swirling and moving forward. Much to be gained here, I believe, and Tom Martin's first advice to mr in this area was to build an extension much like you did. Great first step before I get brave and cut out an area to make a removable section as Dan did.

As for the internal ducting, John Huft did this well, and it was documented on his website, which appears to be down now. I may have a copy of the pics on my computer at home, and will post them if I do. They were basically curved vertical walls on the lower cowl, looked like 2-3" high, and curving from wider just behind the aft cylinders, to narrow at the exit (matched the exit width). John also ran a duct from his oil cooler to the exit too.

Interesting discussion that Ken's cowl flap may be more of an extension than a cowl flap. That may match Tom's experience too. Tom, IIRC, you saw no speed gain from your movable cowl flap, but when you closed the cowl around the prop shaft, your temps dropped and you were able to run with the cowl flap closed (exit throttled). Is that correct?

Its such an interesting interrelated system of systems!

Cheers,
Bob