J-4 project-3.14159265359

skywagon8a · Mar 13, 2019

CharlieN said:
With the nice sun out it gives me a chance to get a few more shots of the flap templates.
View attachment 41876 View attachment 41877

Charlie, I've been thinking about your flaps. I know that could be dangerous. ;-) The purpose of the slot between the flap segments is to convert the higher pressure air below the flap into an accelerated flow over the top of the next surface in order to force the air to stick to the boundary layer following the surface to the trailing edge. In order for this to be efficient the trailing edge of the forward segment needs to hold down and redirect the upward flow. In looking at your pictures it appears as though the higher pressure air flow will just blow straight through the slot without being redirected aft and down. I'm thinking that the gap should be narrower in order for this to function as intended.
This is why I asked you about a wind tunnel on your computer.

This function also true on the stock Cub flaps.

Southern Aero · Mar 13, 2019

Charlie

In my research on gaps in flaps it appears to be about 1 to 1 1/2% chord is standard. (slats and slots too) If you have a double slotted flap the slots would need to be 1/2 as wide if using this "standard". And slots would need to be equal in dimension to keep a larger slot/gap from bleeding more air than the other..... but then it may be desirable to have one flow more than the other. Since they source from the same high pressure air, if one was larger in area than the other it would deprive the smaller one from some flow. I'm sure speed and wing loading could effect these percentages. Also the position of the leading edge radius of the flap has a certain position it needs to be in relative to the end of the structure above. I'm sure there are some engineer types on here that could elaborate on this.

CharlieN · Mar 13, 2019

Both of you are spot on something I do not feel is fully optimized yet. At this point from the old books I have worked with such as Abbott and Doenhoff's and a few others, these are close. But what does close mean when there has been so much more technology since the books written in the 30's to the 60's.
I truly do want to get more development on these before they get built. This was one reason I took the chance to cut all these "dolls" out on the CNC router. It is nice to swing the parts and see and measure the slots through the full motion.
I feel the basic shapes of the airfoils is close considering there are many trains of thought between the shape and size of the foreflap in relation to the main flap.
A bit over a year ago I changed the airfoil from 12% thick to 13% as well as moved the CP back about 10%. The thickness of the wing at the rear spar was NOW ¾" thicker. Going into that change I was real happy with the flaps, with that change I do not think I have had the "warm & fuzzy" feeling that it is right.
I am working with one of Mr Ribletts optimized airfoils and I sure do wish I committed to this project while he was still alive. It would be nice to have had his guidance on technical tweaks like this.
I am hoping once I get out to Oshkosh to work out the bugs in my new computer and get Solidworks to do some magic.

CharlieN · Mar 13, 2019

Sure, now I am looking for the actual drawings where I was working on the motion geometry. If these slots need to be closed up it can be as simple as different actuator arm geometry. It could well require changes to the pivot locations should the trailing edge of the fore flap needs to be raised or lowered a bit. Something I have done a few dozen times and can do again. :roll:

skywagon8a · Mar 13, 2019

For what it's worth the flaps on my E-AB Cub are 110" long using the original airfoil. With the flaps up the wing trailing edge extends to the maximum thickness of the flap over it's spar. When the flaps are full down 56 degrees the gap is a maximum of 1/2". I did a test with yarn taped to the bottom of the wing at the point where it curves up into the flap cove. Then rows of yarn on the bottom of the flap to the trailing edge. When the flaps are full down the wing yarn flows up into the slot. The other yarns all flow straight aft in a straight line. Except in the section within the prop wash where the trailing yarns indicate a lot of turbulence.

I made the wing trailing edge from balsa wood using the flaps as a sanding block shaped it to match the flap leaving just enough room for a couple of layers of fabric with the wing trailing edge coming to a point. The result was a nice smooth flowing transition from the bottom of the wing up through the cove to the top of the flap.

tempdoug · Mar 13, 2019

YOUr picture shows the flap all the way down, could we see a picture with the flap back up about 4-6 inches? use what you have there just not going down that far??

CharlieN · Mar 14, 2019

I will get the fuse back outside in a few days and shoot more shots with the flaps in many positions. I can shoot some video as well.
One thing I noticed yesterday before heading out to host our VMC program was the final gap between the foreflap and main opens up in the last few degrees of travel, basically from 75 to 80°. I need to get the pictures to explain that better.

skywagon8a · Mar 14, 2019

That makes sense. Under those conditions it would be appropriate to limit your flap travel to 75*. It seems likely when moving from 75 to 80°, because of the gap widening there could be a sudden stall of the final flap. This would be caused by the sudden removal of the high velocity flow over the final surface.

CharlieN · Mar 14, 2019

I am not sure if it would be a stall or just what. As I have worked with this flap system it has always left questions, A year ago I "walked away" from the geometry once I felt I could lock down any portion to do with the fuselage allowing me to get going on the build. Even when I get going on building the wing I can get pretty far since the flap system has little affect on any structure forward of the aft spar.
I expect I will have allot to do in the 40 hrs once this is flying. Not sure when the designing will end on this plane. This would not be the first of my planes that did not need a few tweaks after the first flights.
In this build the linkages between the flaps and ailerons will probably need to be "touched".

tempdoug · Mar 14, 2019

it just looked like with the last flap down that far some of the air might go up infront of it instead of all of it going down, but what do i know. but what your doing is cool.

CharlieN · Mar 14, 2019

The foreflap and the geometry of the slots play a very important roll in invigorating, directing and controlling attachment of airflow over the surfaces, both upper and lower.
When designed right these offer very powerful increase in lift especially when going to the extreme as I choose to do. Even with errors in the engineering they will be impressive, but why leave performance on the table.

CharlieN · Mar 14, 2019

Another goal reached, the conical engine mount is complete and tacked together.

I need to get this fairly completely welded before I hang any weight on it but in a few days I will be at a point and weather will allow I will hang a motor and move it outside for pictures. I will also weigh the fuselage with and without the engine to get a baseline.

CharlieN · Mar 15, 2019

I took the frame outside to get pictures of the flaps, and once back inside I hung it with scales on it. It is not light but light is not the total goal when gross weight will be close to 1K higher than an original J4.
This frame with engine mount weighs #109.

Pictures soon.

CharlieN · Mar 15, 2019

Here is a series of images of the flaps rolling back. These came out well but the Vid I took seems to default to 16:9 aspect that cropped the top and bottom off, took away allot of information.
If my math is right the maximum gap at full extension seems to be 2% chord. The gap opens progressively.
The ratio of extension does not actually open any true gap between the main wing and foreflap until what would be used for landing.
See if these offer some decent information.

skywagon8a · Mar 15, 2019

This looks very interesting. Will you be able to adjust the ratio of travel between the two sections easily after you start flying? In pictures 4&5 it appears that the forward flap is lagging behind while opening the second gap prematurely.

What is your planned flap length? They are going to be extremely powerful.

CharlieN · Mar 15, 2019

The actuator arm can be tweaked a little but it really needs both ends of the link moved to truly get the ratio happy, but yes I can build these so they can be changed once the flying begins. And I think I should.

Cord of the foreflap is 10.77"
Cord of the main flap is 20.75"
Span is 120"
I have built and flown a plane with less wing area than these flaps have.

The ailerons also work as a fowler flap and are essentially the same cord as the main flap.

CharlieN · Mar 15, 2019

Oh, in the pictures above, the tape rule is positioned so the gap can be reference, not for trying to show the distance from the pivot.

skywagon8a · Mar 16, 2019

How did you say that you were going to operate the flaps, mechanically with an overhead lever? Those flaps are going to be very powerful with high air loads. You may not be able to conveniently move them without some sort of mechanical advantage linkage or a hydraulic system. You might want to consider a hydraulic hand pump with a restricting check valve in the line. The actuators could have return springs incorporated within thereby only needing one hydraulic line.

CharlieN · Mar 16, 2019

Correct, overhead lever. The mechanism starts to go over-center at full extension such that there is little to no load on the latch. Geometrically the last 10° of flaps takes very little effort compared to say about 20° of deployment. With the air loads being very high in the last amount of travel this should be fine.
It also makes the system so there may well be less chance to be broken by someone who thinks they need all the flaps at 50 or more.

A few hours ago I started redrawing the motion ratios allowing for 70° of full deployment rather than 80°, this is tedious work.
I think I might go downstairs a weld for awhile since the frame is laying on it's side now.

CharlieN · Mar 16, 2019

I decided to stick to it,
With the location of a few holes in the linkages changed and the length of the links themselves changed I can easily change-reduce the gaps in the flaps.
I will later on change the pivot location of the foreflap so it brings down the trailing edge as it travels and see what that looks like since that may allow the full 80° travel.
Time for breakfast.

skywagon8a · Mar 16, 2019

With these big barn doors full down they could create air flow disturbances which render the horizontal tail inadequate. Watch for that, we wouldn't want a sudden nose down pitching moment.

It could indicate that the horizontals should be higher on the tail.

CharlieN · Mar 16, 2019

skywagon8a said:
With these big barn doors full down they could create air flow disturbances which render the horizontal tail inadequate. Watch for that, we wouldn't want a sudden nose down pitching moment.

It could indicate that the horizontals should be higher on the tail.

I agree, I have designed the horizontal and elevator to be quite generous but the thing I do not have a true feel for is if they get downwash or blanketed.
I truly expect the first few hours in this plane will need to be flow when truly on one's A game.

CharlieN · Mar 16, 2019

I have just spent some time working with flap pivot location. A little change makes a big difference. I obviously knew that since I went thought this many times till a year ago when I felt it was right.
But there are quite a few options to change the gaps both before and after the plane is ready to fly.

CharlieN · Mar 18, 2019

I have been welding on the fuse but took half the day to do some plinking in the yard followed by a nice dinner with friends.
This morning I have been reviewing drawings of the rudder and elevators. I thought I had these designed to be, say 3+3 from Supercub sizes. Looks like my tooling is made for just +3 chord, ele and rudder.
I am now second guessing if this is just what I want.

I know I had drawn many variations all of them being larger than the original J4 surfaces.

I would like to hear a few opinions especially about planes that can slow way down?

skywagon8a · Mar 18, 2019

I've been able to fly my Cub with the double length flaps and stock sized tail surfaces as slow as a measured 28 mph in ground effect with no loss of elevator control. The wing just would not fly the plane any slower.

The Twin Bee with the original sized horizontals moved 3 feet further aft than the original Seabee could also fly at off scale low speeds without loss of elevator control.

At altitude both airplanes will buck and roll the wings with indicated speeds near zero, but neither one ever losses elevator control prior to the wing giving up it's lift. This with power.

With your flaps, I would look at increasing the aspect ratio prior to increasing the chord. Look at the Helio stabilator.

CharlieN · Mar 18, 2019

skywagon8a said:
With your flaps, I would look at increasing the aspect ratio prior to increasing the chord. Look at the Helio stabilator.

I intermittently give thought to building with a Stabilator and have always passed it by, but as I started writing just now I realize there is no reason to not investigate it. I doubt I would change but I always look at options.

skywagon8a · Mar 18, 2019

I wasn't suggesting using a stabilator only to look at the aspect ratio that the Helio is using. The high aspect ratio is good for low speed operations. A balanced PA-18 style horizontal with a high aspect ratio ought to work well for you.

skywagon8a · Mar 18, 2019

I've even seen the indicated speed in the Twin Bee go to a negative 5 or 10 mph without ever loosing elevator control. Of course this was with full power, full flaps, lightly loaded and it felt as though I was laying on my back the nose was so high. I've no idea if it was actually going backwards but certainly the airflow would have been strange around the pitot tube.

CharlieN · Mar 18, 2019

skywagon8a said:
I wasn't suggesting using a stabilator only to look at the aspect ratio that the Helio is using. The high aspect ratio is good for low speed operations. A balanced PA-18 style horizontal with a high aspect ratio ought to work well for you.

I recognize you were not suggesting so but my mind is always going toward the "what if's".

Sounds like that Twin Bee was an interesting bird.

skywagon8a · Mar 18, 2019

The Twin Bee is an interesting and amazing flying machine. Now if we can just get these people who don't understand how to fly them to stop wrecking them. There aren't that many left that I know of out of the 23 that were built.

J-4 project-3.14159265359

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