Oh No! Disaster!

It has been apparent since I built the flaps for Sanggletooth that there was something wrong with the appearance of the airplane. It looked much better with the flaps off. For awhile, I thought I might have used incorrect ribs and wound up with too large a wing chord. Then I thought, maybe it was just that the airplane was missing the large spinner. It was tethered to the corner of the work table with a mounting bracket screwed to the engine mounts inside the engine compartment. I dismounted the airplane and installed the engine, cowling prop and spinner. It still looked seriously out of proportion, somehow.

I got the original Snaggletooth down from the wall and sat both Mustangs side by side on the floor. It was even more apparent something was wrong. I save pieces of interesting airplane crash parts for information and comparison. I dug out the broken fuselages of the first Snaggletooth series, the blue Mustang V, E2-S, and the green Mustang VII, G4-G. Using these two fuselages and the original Mustan VI, Snaggletooth, I determined the fuselage of Snagletooth 2 was too short! It was supposed to be ½” longer than the other three airplanes. Snaggletooth 2 was going to have approximately double the power of the original Snaggletooth. I was concerned with the possible destabilizing effect of the increased power on the longitudinal stability of Snaggletooth 2. The extra length is intended to correct this.

Instead of being ½” longer than its sister ships, it was ½” shorter, a 1” mistake. How could I have possibly made a 1” mistake in laying out the moments of Snaggletooth 2? It took a while to figure out. First, all three of the other airplanes of the Snaggletooth series had 16 ½” tail moments (a term not technically correct, but in this case, Means the distance from the flap hinge to the elevator hinge). Snaggletooth 2 was only 16”. I got out the plans for the first Sanggletooth Mustang, E2-S. That drawing showed a 16” tail moment and I based the drawing of Snaggletooth 2 on that original drawing. Adding ½” to that drawing gave me a moment of 16 ½” after the extra 1/2" was added. Surprise, none of the three sister ships used the original design moment of 16”. They were all 16 ½”. I don’t remember why or when I built those airplanes all with a tail moment ½” longer than the plans. It was thirty years ago and I suspect I saw then what I see now. The airplane looked out of proportion. The tail was too short.

OK that is just half of the story. Snaggletooth 2 with its extra ½” should have been exactly the same length as its sister ships, but instead, it was ½” shorter instead of being the same length or ½” longer. I finally figured out what happened to the other ½”. When I draw my plans, I typically copy the first pencil drawing. Then as significant changes are made I update the copy and use one of the new copies to make more changes as the need arises. By now I’m using 4th generation copies. I never thought much about distortions in the copy process. I knew they were under 1%, so I never worried much about them. When I measured my latest drawing, the fuselage on my latest plans was about an inch and half shorter than the original drawing. I had been accumulating 1% errors with each new changed copy. This shortened the tail moment almost exactly ½”. A 1% error is 1/4" in 25" and the fulselage is about 40" long. I was losing almost 3/8" every time I copied the plans. It could only happen to me.

Well, I fixed the drawing and ran new copies at 103% to restore the proper dimensions to the latest drawing. This not only restores the ½” to the tail moment, but now, the extra ½" finally brings the tail moment of Snagggletooth 2 to 17". The 17” tail moment does increase the actual Snaggletooth's tail moment by 1/2" as intended.

But what about Snaggletooth 2 with its 16” tail moment. It is about to get its short aft fuselage cut off, go back into the jig and have a new aft fuselage built, complete with new stabilizer and vertical fin, which I can’t save. It will be an interesting project. I’ll keep updating Stuka Stunt for those interested in the progress of the rebuild. Snaggletooth 2 looks like hell. It is the prototype for the new updated, technologically improved Snaggletooth 2. It has to conform. It is time to bite the bullet and reach for the saw. I have formed a plan for the rebuild which saves most of the existing structure. With luck, I’ll lose only about a week. Stand by.

It flew great.

I keep hearing that too, and it always seems to come from the same direction... huh, weird...

I saw Al fly at our last contest. He flew the BBQB. It's hard to compliment Al, as he is a very complex personality, but I am going to try...

... there were more than a few good nationally ranked fliers at this contest. I was standing next to Frank McMillan. Al put in flight, I dare say it was the best of weekend...

At the end of the clover I said, "pretty dang salty".
Frank said, "did you say soft?"
I said, "no, I said salty, I did not see any soft".

It was certainly plenty "hard enough"...

There Al, I am through bragging on you. Maybe you could respond to one of my post sometime, huh?

But seriously, sorry to hear about the mishap.

I was just musing the other night about an airframe I've hauled around from location to location for, um, twenty years at least. It was more or less intact, although uncovered, for all that time. During the last move, a book fell on the wing and shattered the spars, a number of ribs and broke off the remnant.

Now I had already observed that there were design errors built into the airframe during the first round of construction in the early '80's that would dictate taking the plane partially apart.

When I was a kid, the only focus was on assembly. You built to put things together, not take them apart. If you shattered the airframe against the ground ( unless it was a clean break one could simply epoxy back together), that was it. the engine, tank and wheels came off and the rest it went in the trash barrel. There was another $6.95 Flight Streak kit down at the hobby shop.

So I came away with the vague feeling that 'disassembling' a partially built airframe was somehow wrong, going in reverse gear. Hence I was getting close to throwing this long harbored partially built airframe away, since (a) it was broke and (b) I'll have to take it farther apart to get to where I can put it farther together.

Now Al comes along and tells us all he's going to get out the saw and cut the tail off an airframe that looks close to going to finish just to rebuild the whole thing so he can move the tail back a half an inch.

That says a lot.

Getting it as right as you can is the goal.

Accept no substitutes.

Given how much better his finishes are than anything I ever produced, I can see where he doesn't want to spend the time to put one on an airframe that is not what he designed it to be.

But until I read his post I would not have thought of so drastic an action to un-do a mistake.

Tip of the hat, Al.

Cy.

This sort of error reminds me of a theory about the All American's 2" longer inboard wing panel - that someone really meant to make it 1" longer, so they slid the wing 1" to the left..

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"I watched the Indy 500, and I was thinking that if they left earlier they wouldn't have to go so fast." - Steven Wright

Just kidding Al, you do nice work.

You are still a little young in the gills, too, I see. I remember when they first came out and the Nobler was a whopping (out of my league) $ 5.95 and the Flite Streak (and Ringmaster and Mustangs--got a couple of them) were still a bit high at $2.95. I was delivering newspapers for a nickel a week per customer at the time. That's all week. Every day. A nickel. Them airplanes were expensive!

Fly high. Don't crash. I still got 'em. Both of 'em.

I look forward to seeing the finished product. If it's anything close to the BBQB, it'll be a humdinger.

At first I didn't understand the thrust of your comments. Yes I could have "justified" the shorter tail moment based on "new" theories of stability. Sorry for being so dense.

I didn't exactly fit the theory to the practice, and I'm not an engineer, but I do observe. Increasing power on propeller driven airplanes usually entailed an increase in the length of the tail (P-40N) or larger tail surfaces. See below, The 27 litre Merlin Spitfire and the 36.7 litre Griffon Spitfire. Looks to me like a little more tail moment or area is appropriate, so I added 1/2" to the tail moment.

When I design I use tail moments more or less correctly, that is from the Quarter chord of the wing, including flaps, to the quarter chord of the empanage(horizontal tail group). This discussion uses "tail moment" in the typical stunt meaning or from the wing ROOT hinge line to the elevator hinge line. My airplanes nearly all have wing/flap hinge lines swept forward 2", so my airplanes have about an inch longer tail moment if the average wing hinge is used.

Time to add an extension to the engine and a false nose.

Sorry to hear about the accidental fuselage shortening. It takes one HE MAN to admit that kind error! Hopefully, your fuselage design will allow for adding a 1" section just forward of the stabalizer? That should be doable, right?? Kind of reminds me when I droped my Intrepid off a wire while painting clear and broke the fuselage in half just ahead of the stabalizer. Took me a while to get that put back together, but fly it does - 5 years old now!

Man it hurts just to see this, best of luck in your re-build.

The three photos below show the stabilizer frame with elevators attached, the vertical fin with the rudder attached and the new laser cut bulkheads and jig pockets to finish the repair.

Actually, I goofed. My correction to the drawing was 103% and should have been less than 102%. Not only are the drawings slightly out of size, they are slightly out of proportion because full size modifications to the drawing were also enlarged. These drawings are not really of use to me. I have about 12 of them. I also have about 10 of the last revision where the fuselage is an inch too short. I don't think these drawings are useful in the building sense but they are too pretty to just throw away. I'll give one to anyone willing to return postage.

It is question of photo contrast. That photo shows a wooden stab with masking tape on the elading edge.

Would that MY screw ups looked half so good!

The tail was too short.

I have been accused of that, too!

It could only happen to me.

Wanna bet? Measure twice, cut once, measure twice.....

Sooooo, THAT'S the Snaggletooth? NIIIIICE!

Now back to important stuff.

I'm making progress now.

The first photo is of the Snaggletooth radiator assembly. When I stretched the fuselage, it required more than just moving the radiator back. The new position required the shape to be subtly slimmed so I ground off a bit from the bottom to match the new contour.
The second photo shows the rebuilt radiator.
The third photo shows the airplane going back into the jig I used to originally build and align the airplane. The wing jig accurately returns the fuselage to an exact alignment of the fuselage in yaw and roll. The fuselage jig returns the airplane to an exact alignment in pitch and locates the new replacement parts.
The fourth photo shows how the fuselage is being lengthened with gaps behind the wing trailing edge and a gap ahead of the stab leading edge bulkhead.
The fifth photo shows the beginning alignment of the fuselage. It is almost ready to start gluing in new pieces for adjustment and adding new structure to replace old. I'm guessing everything is aligned within the width of a pencil line. Even if it was off, which I doubt, it wouldn't have any affect on the alignment of the tail surfaces. They will be aligned with their own independent jig.
The last photo is of my finished new tail surfaces. I tried very hard to reduce their weight but there wasn't much fat. I saved 2 grams. The new set,Stabilizer, elevators, vertical fin and rudder, hinges and horn, weigh 60 grams instead of 62. More will be saved by eliminating the shoulder longerons. So far I've saved 8 grams. I'm trying for 14 grams or 1/2 ounce saved which will be eaten up with a bit of added balsa and adhesives. I seem to be on track to complete the repair with no weight gain.
Well, I keep adding photos. This one shows most or the modifications completed. Now it is ready for mounting the stab and making a new pushrod.
One last photo, then I'm hung up until a trip to the local hobby shop after church to get another carbon pushrodt tube. This one shows the stab, in it's jig, fitted to the fuselage awaiting the elevator pushrod. Maybe today I can get the control system aligned and operational again.

It proceeds. By Monday, I should need new molded fuselage top pieces.

What a gorgeous structure.

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"It's not whether you get knocked down. It's whether you get up again." -Vince Lombardi

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"Te audire no possum. Musa sapientum fixa est in aure." (I can't hear you. I have a banana in my ear.)

Very well documented. I did have the pleasure to see your P51 Mustang fly back in Woodvale (1978 WCC in England) and being part of the brazilian team at that time, I was just speechless to see a model that kind perform so well.

Attached is a photo I took at that event and which is part of my website.

That is the living proof that wanting too badly is a very important key element in succeeding.

Congratulations even though sometimes there are thorns to pass by.

Here is the latest photo of Snaggletooth 2 repairs. I had decided to eliminate the shoulder longerons and use molded pieces which would extended down to the thrust line and under the stabilizer. I rejected this idea for two reasons. First, after shaping the shoulder longerons to fit the bottom of the stab, they were thin and too weak to risk gluing the stab to. Also it would have been a bitch of a job to fit the larger molded pieces to the thrust line, top longeron and bottom of the stab with accuracy. So I added the side pieces up to the shoulder longeron. This made it easy to fit the stab saddle before gluing the stab in place. With the side pieces in place, I will grind away almost all of the shoulder longerons leaving just a bit to locate the top molded pieces. I'm still trying to get four more grams out of unnecessary structure.

The elevator pushrod was made to very close measurements. It is basically a carbon tube with wire ends. The final adjustment to the stab location was to move the stab 1/64" forward to exactly align the elevators with a measured Gage (a piece of balsa which is glued to the jig base and just touches the inboard end of the left elevator when the flaps are exactly neutral). The flaps also have a Gage. There is a piece of brass tubing over the flap horn wire extending toward the rear of the airplane. My flap gage is a piece of balsa glued to the table which fits exactly the same when placed under the front of the brass tubing or the rear of the brass tubing to insure the absolute "0" alignment o