Dave Presta

My Feedback: (3)

I don't remember seeing a post on NACA ducts, so I thought I would take some pictures and show how I did mine. This is the same method that I use on my race planes, and we have tested that it is much more effective then just a hole in the fuse.

Now this takes a little more time and patience but the good news is that you don't have to fire up the dremel and make any dust.


The flash has a ventral fin on the bottom so it takes another step or so to get the centerlines marked on the fuse.
I use the centerline of the fuse to take measurements in the stab area, then transfer them to the bottom of the fuse.
I carry out the lines past the template so it can easily be re-aligned if the marks get erased.

I know what your saying, this is very standard. The next step is what some of you may or may not be familiar with.

Dave Presta

My Feedback: (3)

Taking a standard x-acto knife and a flame, I heat up the blade until it the tip just starts turning red. Cutting along the lines, the blade needs to be heated up very frequently. I basically scribe the whole thing first, then come back and start cutting all the way through the glass. This step takes a lot of patience, and has to be perfect because there is no sanding or shaping with this method.

Dave Presta

My Feedback: (3)

The NACA duct is just that, not a hole, so in order to make the ducting the outline shape needs to be cut and hinged down to form a duct.
I cut everything but the very forward edge, this will remain in place to hinge the duct.
To complete the duct you need to make the sides (I am using masking tape to check depth). Using either 64th ply, or preferably thin fiberglass strips, glue to the sides of the ducting inside the fuse and trim the excess flush with the duct.

Mart (Ham)

Nice job! Strawboss but to late for me, this time.

highhorse

My Feedback: (2)

Ok I'm the dummy. But I don't understand what you're using the ducts for.

They're not engine intakes, are they?

seanreit

My Feedback: (60)

They are typically used for Cooling!

Dave, interesting solution, were you expecting fuse temps to be high? Are others having to install thermal blankets?

Airforce7

My Feedback: (31)

http://en.wikipedia.org/wiki/NACA_duct

Dave, where's the air going to go? I know your trying to cool the pipe, but will you do an exit duct too? Looks like the pipe is a near match with the tail cone when I was looking at Darcy's Flash at CA Jets.

JoelP

My Feedback: (20)

If you're using a Tam's pipe on the Flash, you will have plenty of space around the pipe where it exits the tail cone for the air to exit. I used eight 1/8" ply spacers to center the pipe in the tail cone.

Airforce7

My Feedback: (31)

Cool

Pitts S1S

My Feedback: (9)

I just saw the other day on BVM website, that he has premolded NACA Ducts for sale now. Just cut the opening out, and glue to the back side.
They were designed for the Electra Jet.

Dave Presta

My Feedback: (3)

They are to help cool the rear of the fuselage. The instructions call for it, not my idea, so I decided to do it this way instead of just cutting out holes. The included exaust consists of two pipes, with some spacing in between for cooling, but the instructions ask for some help so air to get into the rear of the fuselage to help flow in-between the inside and outside pipe. I was thinking about ordering a Tam pipe, but the guy's who are currently flying the flash with the same powerplant dont seem to be having any problems, so I will stick to the stock setup for now.

SDCrashmaster

Dave, shoot a pic from inside the fuse so we can see your ply sidewalls.
Tks,
P

Billy

My Feedback: (15)

Yeahbaby, looks like you got it. Can't wait to turn and burn with ya. Should be exiting having seven of them in the pattern.
Have you figured out the scheme yet?
Billy

AFTERBURNER1

My Feedback: (80)

Razor Blades, Blow torch, rulers and cutting.... Looks like a lot of work!!

Dave Presta

My Feedback: (3)

Not for you it isn't. The great part was when I called you this evening.

(Duk) so what the &$#@ is a NACA Duct? And why are you putting them on your plane? How much do they cost? Do I need to order some? Does McBride have any? Call him and find out, because if he does then I want some too.
(me) it shows you on page 12 of the instruction manual.
(Duk) What the &%#@ is an instruction manual? You mean that book with all the pictures? Oh I threw that away when I opened the box, right before I shipped the kit to Hal........that thing would have cost me an extra $.80 cents to ship............and what the hell am I gonna do with it!!!

Jimmbbo

Nice lookin' duck... er, duct

From what I know and have observed about NACA ducts (did wind tunnel model design in a former life, and have seen them installed in many FS aircraft), they work best when installed on a surface sloping down into the relative wind, which provides the highest pressure and minimizes separation entering and within the duct... The idea is to give high pressure air on a ramped surface somewhere to go. and the NACA duct inlet provides good pressure utilization with less drag and fewer icing problems than a scoop...

Installing them in a decreasing taper location may or may not allow them to work properly...

Unable to determine from your installation where the duct is located, but had to add my $.02...

Cheers!

Jim

highhorse

My Feedback: (2)

I'm not an aero engineer, but I stayed at a holiday inn express last night. So here goes.

Didn't I read that the accelerated engine exhaust itself creates a vacuum? If so, will the gap between the pipe and fuse allow air to be drawn aft from the inside of the fuse to fill that void? If it does, then these NACA vents will not need to provide air under pressure. They just need, in this case, to allow air to be DRAWN into the low pressure area inside the fuse.


Even if the location isn't conducive for the intended function of the NACA scoops then they'll still make really good looking vents !!

baron-noir

My Feedback: (1)

Thanks for the tip, I was wondering how to make proper NACA ducts for some of my electric sailplanes and I will try your method which seems simple and reliable. But where did you get the NACA shape, you did "guess" on your own or you scaled up or down based on some example ?

Dave Presta

My Feedback: (3)

Highhorse,
I agree with you. I do some engineering stuff only when I need to, but this was not one of those times. The instruction manual just says to install the ducts to assist air being drawn into the rear of the fuse and help flow in-between the pipes to keep the rear fuselage cool. I guess when fully bipassed the rear gets warm because there is really no other place for air to enter the fuse. Guy's have been flying them without the bypass cover, and this has given them plenty of air to keep the plane cool. You hit the nail on the head, I really was not concerned about the aerodynamics of the ducts, rather just making really cool looking vents. I mean, if the instructions call for vents in the shape of NACA ducts, then why not make them.


Baron-noir,
I know how to draw my own NACA ducts to whatever scale I need, but this was much more simple..................the template was included in the instructions.

HighPlains

My Feedback: (1)

There is a bit more to the design of a NACA duct.

The short version, one guy's implimentation:
http://www.wisil.recumbents.com/wisi.../naca-duct.htm

The real 1945 NACA document, all 57 pages:
http://ntrs.nasa.gov/archive/nasa/ca...2005060952.pdf

GR7Racer

If you need a pattern for the outline of a NACA duct, you can get one at the NACA report server site. Do a search for "submerged inlet" and a report will come up that talks about the design and testing of this inlet. On page 50 of the 57 page report is a nice clean outline of the duct that you can scale and print out to whatever size you need.

Here's the link...

http://ntrs.nasa.gov/archive/nasa/ca...2005060952.pdf

Dave,

I've cutout square holes in fiberglass before by scoring with an x-acto blade, and then following the score with the small no.11 x-acto saw...you've probably done the same. It leave a very clean edge. Do you think that tool would work on a curved cutout like you are doing?

Nice detail...I'm envious of your Flash. Tony and I are looking at a Lightning, and I'm hoping you'll let the big one play with the little one.

KMac

Dave Presta

My Feedback: (3)

Kenny Mac,
I use the hot #11 blade because when the blade is cold it is very easy for the knife to slip when trying to follow a curve. You need to have a deep enough scoreline in order for the exacto blade to follow your line without wandering out into the open. Depending on how thick the surface is of course. If you have a fairly thin glass fuse, then it would be much easier. With the hot blade the knife sinks into the glass as your following the line making it much easier to control. Just my opinion.
I cant wait, tell Big T to order that Lighting and come play with us.
Miss you bro.

Reverend Russ

Dave,

From what I have been told, substantial nose weight is needed to balance a flash...the Tam pipe is 6 ounces lighter! Why not save the weight and allow for lightweight batteries in the nose to lower up to a pound or more? Not to mention the Tam pipe fits like a glove and doesn't need extra cooling. I will post some pics soon.

Russ

baron-noir

My Feedback: (1)

Very cool, now we have the ultimate source for NACA ducts drawing easy to scale up/down. Even the scale at which the NACA did the testing (1/4) isn't that much away from our models (1/3 for gliders 1/5-1/6 for jets are common)
Thanks

wikman

Hi all
Just wondering...
Is there a heating problem in the Flash?

Best regards
Anders Wikman