Critical Mass Design

[photo not recovered: 39794.jpg]

It took weeks and the best efforts of David Russum to figure out how to get some video added to my Al’s Models disks 2 and 3. After finally achieving a happy result in the sale disks, I looked at my collection of Digital8 video tapes for another project, more to occupy time than making another sale disk.

I like designing stunt ships and was attracted to a set of six one hour Digital8 tapes on the design of the Critical Mass. I played one to refresh my memory of its content and found myself watching the entire set. I thought the content good enough to justify a better presentation.

Years ago, I had edited this six hours onto three Sony Hi8, two hour tapes copied onto VHS. These VHS tapes could be played on VHS machines but the situation now is rapidly becoming one of what is VHS? If I had any intent of doing anything other than playing these tapes for anyone other than myself, they needed to be in a newer format. DVD would be a possibility but the subject matter wasn’t suitable for HD or Wide Screen. I copied the three Hi8 home movies to standard DVD using the Pinnacle Essentials" program which subsequently crashed every time I tried to use since.

[photo not recovered: 39795.jpg]

The three DVDs made from the Hi8 tapes had a disappointing lack of resolution. I posted a request for information here on Stuka Stunt looking for some way to copy my higher resolution Digital8 originals onto DVD. John Hill suggested a program called DVD Shrink which would slow frame rate instead of suffering a major loss of resolution. About this time, I replaced the guts of my computer and bought a program called Pinnacle 15. I was somewhat familiar with this program because I had also owned and used Pinnacle 7 and 10.5. Pinnacle 15 had variable frame rate adjustments and also a setting which would fill the DVD automatically with the best quality possible. Using this program gave me much better looking videos than the VHS or Hi8 copies. It took me a couple of days to understand the program well enough to actually make DVDs. The first day David helped. The program was anything but intuitive. I wound up copying about 40 pages of selected information from the online Pinnacle 15 manual.

Al

Attachment #1, (jpg file)
Attachment #2, (jpg file)
Attachment #3, (jpg file)
Attachment #4, (jpg file)
Attachment #5, (jpg file)

[photo not recovered: 39836.jpg]

Al

Attachment #1, (jpg file)
Attachment #2, (jpg file)
Attachment #3, (jpg file)
Attachment #4, (jpg file)

Out of curiosity, how did you renforce the end joints of the spacer? Besides just puting fiberglass and epoxy on the outside? I would think that end joints along with the epoxy/fiberglass on the outside only would not be enough to keep the nose on.

Matt,
[photo not recovered: 39893.jpg]

There were 7 longerons cut when the nose was removed. 5 Of the cut longerons were reinforced with 10 double splices and the other 2 cut longerons with single splices. Each splice covered the 1 1/2" extension, were beveled, and extended an inch into front fuselage element and an additional inch into the rear element. Each of the 5 cut longerons were doubled with splices on each side. The two cut longerons not doubled were single spliced. These were located at the top of the dummy exhaust bypass. Additionally, between the doubling splices there were narrow 1/16" balsa splices glued inside the front and rear fuselage segments between the doubling splices. These were mostly gluing surfaces to form shelves for attaching the new 1 1/2" sheeting and to insure the alignment of the new and old sheeting. These narrow balsa shelves are visible in a photo above showing the fuselage almost ready for the new sheeting. This photo also shows shows doubling splices on each side of upper cut longerons and a single splice above the dummy bypass longeron. There are no butt joints in the repair. All are lap joints. Finally, the entire area was given two layers of half ounce glass after sanding old finish back into the original glass covering of the fuselage. Much of the new glass has also been sanded through as the spliced fuselage alignment was checked frequently with a long straight edge ruler.

7 Longerons sounds like a lot of heavy balsa structure, but, if you will check the bellcrank mounting photo, the shell of the airplane is molded 3/32" contest balsa and a number of 1/8" x 1/4' contest longerons and some very thin bulkheads. There were some structures, mostly plywood 1/32" bulkhead laminations, inside the fuselage cut away in the repair to control, as much as possible, the weight of the modifications. Originally, the airplane actually built light until I went crazy with paint.

I think it will stay together.

Needless to say, the cowling also had to be extended and mated again with the new fuselage profiles. Do you think I have enough cooling air outlet?

Al

Attachment #1, (jpg file)

[photo not recovered: 39926.jpg]

Some readers of Stuka Stunt may not be aware that my Critical Masss is a semi-scale version of a real aircraft. Here's the real Critical Mass.

[photo not recovered: 39927.jpg]

And my semi-scale stunt version.

[photo not recovered: 39928.jpg]

The Critical Mass racer was a T-20 Sea Fury modified for racing, mainly by removing the front cockpit, clipping the wings and substituting a P&W 3350 for the much smaller Centaurus engine.

[photo not recovered: 39929.jpg]

I could build a reasonably semi-scale Critical Mass except that there was just no way to get enough area into a "clipped" wing as used on the real racer. Increasing the chord for more area with a shorter wing threatened to make a really ugly airplane. I decided to just go with the same wing and moments as used on my Sea Fury. The wing would be totally out of proportion to the real Critical Mass, but the longer wing had the necessary area and wing loading to make a "pretty" Critical Mass competition stunt ship.

[photo not recovered: 39930.jpg]

The design was elegant and numbers good for a competition airplane. Leave it to me to screw up a potentially winning project. Still, it is pretty, and after modification should be a very pleasant flying decently capable stunt ship.

Al

Attachment #1, (jpg file)
Attachment #2, (jpg file)
Attachment #3, (jpg file)
Attachment #4, (jpg file)
Attachment #5, (jpg file)

<[ Forums include posts, replies, comments, debates and opinions. I would welcome replies.>]

Ha!
I gave up on that theory Al! A waste of time at the very least.

Working on an email reply to yours and including a few ideas used on the FW and improved ideas thought up during construction.

Hopefully can get to it this weekend.

David

[photo not recovered: 39840.jpg]

Attachment #1, (jpg file)

Beware of the Hun in the Sun .

I think Al's going electric , as he needs a round engine

Hey Al,

Why don't you make it an electric? You won't have to build those fancy tanks, although they are a work of art! And you'll get a constant motor run every time with NO filter in the fuel line

Beautiful ship. Keep us informed, GA

Always liked the looks of that one. Several years ago I did a painting of the front end - an "in flight" view - which appeared on the cover of an issue of Stunt News.

Mike Keville

[photo not recovered: 39939.jpg]

Attachment #1, (jpg file)

[photo not recovered: 40036.jpg]

Cutting into a finished airplane isn't exactly a new undertaking to me. I've done it before for reasons other than the repair of structural damage. In this case, my reasons were much the same as lengthening the nose of the Critical Mass. The original Millennium, which I named "Millie" on the cowling, was a bit more tail heavy than I had hoped for. Nose weight worked but I wanted to see the effect of lengthening the nose. The result was pretty much as expected with the CG being moved a bit forward which permitted removing the nose weight. This experiment wasn't worth the effort as far as trimming exercises go, but it satisfied my opinion that the new Millennium II would profit from a slightly longer nose. In fact, this proved to be the case as the Millennium II (Millennium Cavalier) turned out to be the only airplane that I can remember building which needed no nose weight for optimum trim.

[photo not recovered: 40037.jpg]

Looks familiar, doesn't it? The technique for the modification was the same but much more complicated on the Cavalier with its larger fuselage and more complicated internal structure.

[photo not recovered: 40038.jpg]

Here is "Millie" after the rebuild . The Nose also sports a new name signifying its rebirth. It was then called "Millie 2" which shows in this photo. It wa a good airplane but had a succession of repairs to its paint, caused, probably, by exhaust contamination getting past the seals of the removable wing. It ultimately discouraged me from trying to make heavy use of it, and I had a new one under construction, so it now hangs on Windy's ceiling in his Spitfire bedroom. The new Millennium Cavalier is a delightful airplane. It profited in many ways from experiences in building and flying Millie.

[photo not recovered: 40082.jpg]

This is the Millennium 2, or in this case, I call it the Millennium Cavalier.

[photo not recovered: 40035.jpg]

Meanwhile, the repair/modification is still coming along.

Al

Attachment #1, (jpg file)
Attachment #2, (jpg file)
Attachment #3, (jpg file)
Attachment #4, (jpg file)
Attachment #5, (jpg file)

Nice looking model!

Only thing missing is the "Taco Bell" graphic.

I wonder where you could get that?

http://cfcgraphics.com/Taco_Bell.html

Charles

Al, as usual, excellent craftsmanship. How did you restore and check alignment of the motor bearers when re-attaching the nose?

Dick,

I pretty much just keep the construction gap precisely the same all around and trust that nothing significently changes in the overall alignment in the process. Alignment has never seemed to be a problem. Flight characteristics of Mille 2 remained same.

Al Rabe

[photo not recovered: 40049.jpg]

Charles,

I'd bet that that Taco Bell graphic is available on the web. If not, I'm fairly sure that they are available in some form at my local Taco Bell restaurant. I think you know that graphics are easily copied.

I Could have used the graphic on my critical Mass but thought that would be carrying semi-scale too far. It doesn't do anything for the airplane. I doubt that Tom Dwelle put it on his Critical Mass because he thought it was pretty.

Don't you think my Critical Mass looks better without the graphic?

I just noticed the link to the Taco Bell graphics site. They gotta be kidding. $139 for graphics? They would probably charged me to put them on my model.

Al

Attachment #1, (jpg file)

About the only thing that could possibly make that plane look better, would be if it was hanging from MY wall! hahaha

Again another great how-to article Al. Always a pleasure to read your stuff

That link is to a page in my website. That price isn't bad when you consider what you get. First off, the model the set fits is 80" in span. Not tiny.

The graphics include both sides of the fuselage, both sides of the vert stab, and both sides of the wings, tops and bottoms, plus odd and end graphics that also go with the set.

Any element can be ordered by itself.

Yes, your model looks much better without the Taco Bell.

Gotta ask this, after all those repairs and extra paint, what does that model weigh?

Also, I'll ask this question here. I guess there's other tanks for CL besides the "wedge" type that I have plenty of?

What did you use for a tank?

Your model looks great!

Charles

Charles,

The graphics are, for the most part, not the correct graphic used on Critical Mass. Also, If I wanted that graphic, I'm perfectly capable of finding one to copy even if I had to photograph the front window of the nearest Taco Bell and run it through PhotoShop.

By relocating the engine and removing two sets of nose weight, the model will be approximately 6 ounces lighter than it was before cutting.

I build my own tanks of various configurations. Most of my tanks are variations of rectangular.

Al

[photo not recovered: 40258.jpg]

Well, the Critical Mass is almost finished with its modifications. The construction and finishing are complete. Tomorrow, I will mount the engine tank and muffler on a test stand and put a bit of running on the engine before reinstalling it in the airplane.

[photo not recovered: 40259.jpg]

I'll get it reassembled in the next couple days and post a few glamor shots of its reincarnation by mid-week. Its amazing how much different in appeasrance airplanes are with and without spinners.

Al

Attachment #1, (jpg file)
Attachment #2, (jpg file)

[photo not recovered: 40476.jpg]

Putting the Critical Mass back together again has turned out to be a bigger problem than I thought, or maybe, I should have said problems.

The first problem was that, in the past two years and head trauma, I had forgotten how to build. I knew what had to be done but had to discover all over again just how to do many of the building tasks. I hadn't spent much time in the shop and forgot where everything was and even that I had some seldom used power tools that simplified some tasks. Actually, this has been a good experience which seemed to clear any number of mental cobwebs.

The tank that was in the airplane was a BBFB size and contained about 6 1/2 ounces. That sound's a bit small but seemed enough for the Saito and the prop I was running. Since the old tank had been sitting for more than two years and there was room for a larger tank, I decided to build a new, larger, tank.

In the photo, can you see why one of the backplates was rejected as being unsuitable for my tank?

The first design for my new larger tank was made to the specifications of the new, larger tank compartment. It didn't fit. Somehow, the compartment had lost some of the new width I had intended for the rear of the compartment. The next design contained a error in the new dimensions that left uneven space at the front and back of the compartment with the tank installed. The third try works fine and bumps my installed fuel by a little more than an ounce. Quantities are calculated using the volume in cubic inches divided by 1.8045 to find ounces. The tank designs are drawn on paper and copied with my printer onto manila folder material to make tank models and templates for layout onto tank metal.

Then there was a problem with the Saito .91. It had no compression. I took off the rocker box covers and found both valves stuck open against their valve springs. The loose rocker arms had allowed the pushrods to fall out of alignment. I teased the valves back into reluctant motion with a bit of wood and freed things up with WD 40. At various times during the resuscitation of the engine I spun the Saito a couple of seconds at a time with my starter. Finally, it all loosened up and I was able to align the pushrods and adjust the valve clearances.

On the test stand the Saito wouldn't start. I tried a new plug, fresh fuel, recharged the batteries on my power panel and starter, and adjusted the valve clearance again. After several hours of frustration and physical discomfort from kneeling on concrete, I finally tried opening the needle a bit more and it ran. Then the test stand tank leaked. I installed the new flight tank, prop and spinner and put about another hour on the engine to pick the prop to be used, exercise the flight tank and set the needle. I still have no idea about the condition of the ball bearings. The Saito seems to run well and there is no noticeable black in the exhaust.

[photo not recovered: 40478.jpg]

Before the running was finished the test stand required some maintenance. One of the plastic straps was broken and two were worn enough that the test stand was getting loose when the engine was running. This test stand was made to allow the engine to be turned upside down while running. The mount rotates and can be locked with the engine either upright or inverted while running. It is easy enough to replace the clamps but is still another trip to Home Depot, interrupting the afternoon of test running.


[photo not recovered: 40479.jpg]

With the engine ready to go back into the airplane, there is still one unresolved issue. The installed muffler was a stock Saito unit like the bottom muffler in this photo which screwed to the bent steel manifold. Saito mufflers have a life of about 400 flights. The mounting threads on these mufflers typically wear out from vibration. I had custom mufflers made for the BBQB Bearcat by Scott Dinger ([email protected]) which were highly satisfactory. I sent Scott a full size model which he copied to make beautifully welded duplicates. These are quieter than the Saito mufflers and attach to the manifold with flexible silicone tubing. This minimizes the effect of engine vibration on the muffler. These mufflers weigh about the same as Saitos and can be made to fit inside the cowlings of both the Saito powered Bearcats and Critical Mass. It will be a significant improvement. Since the new design worked better on the BBQB than the stock Saito muffler, I don't remember why I didn't use one on the Critical Mass when I built it.

I don't plan to fly the Critical Mass any time soon, but I have decided to get out with a couple of my less important airplanes and do some flying. The rotator cuff still doesn't work but I have decided that normal wrist and elbow motions should suffice.

Stand by for the next chapter of the rehabilitation of the Critical Mass and my somewhat beat-up body.

Al

Attachment #1, (jpg file)
Attachment #2, (jpg file)
Attachment #3, (jpg file)

Wow, Al! That sounds like quite a challenge you are meeting. I'm amazed as always at the quality of your presentations and the systemmatic and creative approaches behind them. 'hope your flights prove satisfactory and bring you a bit of fun! Thanks again for sharing your usual thought provoking and informative efforts.

SK

Al,

When I was 9 years old, I clamped my OK .049 in a blscksmith's vise to test run it. It's amazing that I didn't destroy the engine totally in the process (I did have a hard time starting it).

My cousin, a free flight flyer, used a hand-held test stand to break in all his engines (even the McCoy 60). It was made of wood and at that time was comercially available (fewer lawyers per capita then).

As a result, I made an aluminum model of his wooden one that works for everything except radial mounts.

[photo not recovered: 40485.jpg]

Amazingly enough, nothing ever shakes loose, the pull is less than the tension in the lines with a big stunter, you can reach everyplace and check the engine for profile or inverted running.

The best part is that you can put it on a table or sawhorse to start it and not wear out your knees on the concrete.

My far future project is to build a Hughes H1 for the 90R.
I'm thinking it should have about 950 sq in and weigh in at 80 Oz.
Any advice ?

Rex

Attachment #1, (jpg file)

[photo not recovered: 40492.jpg]

Sometimes we are too casual about handling these model engines. We do the same things over and over and sometimes forget just how dangerous these engines and props can be, particularly a big engine like this Saito .91 swinging a 16" prop at 10,000 RPM. That is a prop blade passing in front of you about 330 times each second.

My test stand allows some adjustment, but, I think it is safe and has worked well for more than a hundred engine runs. Still, that is an awful lot of energy that is being routinely handled there. What do you suppose might happen if, for instance, I forgot to put the concrete blocks on the skids of my test rig?

Al

Actually, I did forget the concrete blocks and started the engine. anybody want to know what happened next?

Al

Attachment #1, (jpg file)

[photo not recovered: 40498.jpg]

Well, I finally got it through and ready to fly but, I'm sure that will probably wait until I'm flying again. The longer nose doesn't hurt the airplane's appearance as much as I thought. In fact, I'd have to admit its hardly noticeable. Fortunately, the modifications aren't noticeable.

The big winner, of course is the balance is moved forward enough, I think, to make a noticeable improvement in the airplanes stability and level flight groove. The more forward CG and stability should also require somewhat larger control deflections to maneuver. More flaps out will make more lift for tighter maneuvers. The feel on the handle will be much the same if the handle line spacing is increased slightly.

[photo not recovered: 40495.jpg]

Getting the custom muffler into the nose was a bit more difficult than I had thought it might be. The nose turned out to be tighter than I expected. I guess I was thinking in terms of the BBFB Bearcat which has a slightly oblong cowling which is a bit deeper. this is what the Critical Mass looked like up front before I started the modifications.

[photo not recovered: 40496.jpg]

This is what the my design muffler looks like installed in the Critical Mass. I did the photography while the wood was bare expecting it to be easier to understand than if all of the wood were black. The rubber "pig tail" still has to be cropped to make it invisible in the profile side view.

[photo not recovered: 40497.jpg]

Goodness, Stuka Stunt readers must have red spots in front of their eyes by now. Anyway, that's it for now.

[photo not recovered: 40501.jpg]

I just happen to like the looks of the Critical Mass in this attitude as it sits on the workshop table amid the detritus of building.

Al

Attachment #1, (jpg file)
Attachment #2, (jpg file)
Attachment #3, (jpg file)
Attachment #4, (jpg file)
Attachment #5, (jpg file)

Al,

Is it my imagination, or is the flap hinge line bent ?

[photo not recovered: 40500.jpg]

How do you articulate the flaps ?

Rex

Attachment #1, (jpg file)

Rex,

Yes, the flaps are in 4 sections. It was necessary when using outboard dihedral breaks. The inboard and outboard flap sections are joined by a "U" shaped music wire embedded in the ends of the flap segments. This is exactly the way my NATs winning Sea Fury were articulated, in fact the Sea Fury and Critical Mass wings would be interchangeable.

Al

[photo not recovered: 40511.jpg]

Here is the completed Critical Mass with the engine and related equipment all installed, awaiting only the cowling to be ready to fly. I decided not to paint the tank/muffler retainer after all . This, and the under tank spacer have been fuel-proofed with thin epoxy.

[photo not recovered: 40512.jpg]

With the cowling on, there is nothing more to be done to the Critical Mass but to take it off the wall and fly it.

Next, The BBFB Bearcat needs a bit of work. It has flown only twice. I didn't like the run so I decided to set it aside for farther work before flying it again. I think the problem was simply one of having a a too small prop which resulted in a lot of noise and not much thrust. I'll get its Saito .91 on the bench again tomorrow and do a bit of trouble shooting. I think I know now about what what RPM to expect and will try the 16-5.5 carbon (Eather, I think) like the one that I put on the Critical Mass. It should turn about 8750. The Bolly 15-4.8 that I tried on the test stand for the Critical Mass, and likely had on the BBFB, turned a little over 10,000 with the other Saito .91. There is no filter installed so the high RPM run wasn't the result of filter blockage. I'll also use the BBFB flight tank which hasn't held fuel in nearly three years. It'll run or not. I can pull the rear of the tank to check the contents and clunk weight if necessary. It may take a bit of troubleshooting but, like the Saito in the Critical Mass, it will eventually return to easy starting and reliable running.

[photo not recovered: 40515.jpg]

It is a pleasure to handle the "new" BBFB Bearcat. This was probably my absolute best job of building and nicest paint. This airplane needs to be flown.

Check the sale on page 2 of this thread.

Al

Attachment #1, (jpg file)
Attachment #2, (jpg file)
Attachment #3, (jpg file)