Mustunt IV, Part 11, Basic Building Techniques

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Well, we finally got a break in the winter weather and David Russman was available to take some inflight photos. So far the handling characteristics evaluated are a bit sketchy as I got in only one flight.

First, the engine run wasn't all that great. I launched in a barley breaking two stroke. The engine ran that way for about three minutes enabling me to make an initial evaluation of the Mustunt's handling. Line tension was good. The Mustunt IV was stable and grooved well in level flight. The corners were checked with a number of inside square maneuvers. They were OK but nothing to brag about. I was surprised that there was no observed "hinging" in spite of the airplane carrying, what I thought would be, an excess of tipweight. There was no tendency to stall buffet in corners. This indicates that the wing loading (weight) is OK.

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Lazy eights were tried next with the engine showing a definite slowing on outside maneuvers. The initial tank height adjustment was off and the tank needs to be raised. I was using an ill considered 1/16" spacer under the tank which spaced the tank pickup 1/16" below the spraybar. It should have been 1/8", for the initial flight, to align the pickup with the spraybar. The tank will probably wind up just a bit higher than that. Ultimately, I will make height adjustments for a very slightly faster run inverted. Its a matter of taste. I like a little extra power in the top of the vertical eight and hourglass. This can be done without making a noticeable difference in the halves of the square horizontal eight, but it is a delicate adjustment. The bottom of laser cut, LA .46 powered Mustunt tank compartments are an 1/8" below the bottom of the engine mounting lugs. This is 5/8" below the spraybar centerline. The top of the compartment is 3/4" above. This is enough room to adjust a typical 1" deep tank down 1/8" or up 1/4". A minor issue with the Mustunt is that the engine has to be removed to get the tank out for adjustments. This isn't easy to do in the field.

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After the first three minutes, the engine sagged into a four stroke run with a considerable loss of power. This LA .46 has very little time on it and should be OK with a bit more running. This limited further maneuvers to inverted laps and some outside loops.

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Preparation for the next flights includes making up a new handle with wider line spacing to tighten the corners. I was using a standard EZ-Just Hot Rock with 4" line spacing. I will modify a handle to increase the line spacing to 5". The corners could also be tightened by increasing elevator sensitivity. On airplanes with small wings or relatively high wing loading, this is usually a poor trim choice. A flap/elevator ratio change to increase the sensitivity of the elevators would reduce the deflections of the flaps. The Mustunt IV is a very small airplane with only 500 square inches of wing area. It depends a lot on flaps for lift, particularly in tight corners. For now, an increase in line spacing at the handle will give both tighter corners and more lift from the flaps. I typically use 5" line spacing on my smaller airplanes and 6" on the big Bearcats, Cavalier Mustang and Critical Mass. Flap deflections are trimmed for lift, then elevator sensitivity and nose weight are adjusted for pitch rate (corner). There will be plenty of time to try more sensitive elevators.

I'm also reworking a couple of props which might pull better as the engine settles down. This too, is a necessary trimming function.

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This seems a lot of explanation for just one flight, but this is, after all, a tutorial which has, so far, exceeded 36,000 words and 340 photos. When the weather improves again, there will be opportunities to further explore the capabilities of the Mustunt IV. So far, I see no problems, just trimming experiments for competitive performance.

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Shock gears make such pretty landings, it would be a shame to omit them.

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And here are a couple of photos of the finished airplane taken under better lighting than those taken on my loft at night.

Again, thanks to David Russman for the inflight photos. They are great.

Al

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Geez AL, Thats Awsome!! You the man!!
David G. Miller
Pasco, WA

Al,
You build some very nice looking planes.

I wish I had half your talent.


Paul Taylor

~I wish I knew everything you have forgotten about C/L.

Check out my Blog@
http://rootbeard.rcuniverse.com
A link to a slide show of a fun day: http://www.youtube.com/watch?v=V-_sR0ZydSQ

Al,

Now THAT is impressive....both on the ground and in the air. Thanks for taking the time to do the tutorial....it's invaluable instruction.

The 36,000 words and 340 photos would, of course, make a great publication. Have you given any thought to making that information available in one file for us to download? That file would also be a great contribution to a PAMPA library if one was ever established.

Trimming, for me, is harder than it was learning the pattern. I hope, as you continue to trim out the "Mustunt IV", that you will continue to make the trimming process part of your tutorial. It would be of immense help.

Great stuff !!

Cheers.

Warren Wagner

Warren,

Maybe you have no idea of just how many pieces of paper would be required for a complete download of all 11 parts. It makes a stack mote than 1/2" thick. Meanwhile anyone can download them all from right here. Print them individually from here or copy and post them all into Word and print it all out at once from that big Word file.

Warren,

Check out "The Mustunt IV Building Tutorial" which may answer your questions about downloads. I just pulled it up to page 1.

Al

Al,
On reading your post I was interested to note your cure for rich outsides is lowering the fuel tank relative to the spray bar.

I have likely confused myself but I would have thought to make it run richer in outsides, the tank should be raised.

My personal simple rule of thumb is to correct a rich mixture problem is move the tank in the direction of the force acting on it for leaner mixture.

So, if too rich inverted, move the tank up. (I assumed a simplified static situation not allowing for centripetal forces, but seems to work in practice)

I then surmise in outside loops, assuming in a simplified situation centripetal forces act outward from the centre of the loop,
the tank would still need to move up to cure the problem.

I the back of my mind I suspect I have missed something. If I have I'd be grateful for your insight.

Note I have assumed up is always up, not dependant on the attitude of the plane.

Thanks,
Peter

Peter,

You are right of course. I don't know how I could have been so careless in my diagnosis. Probably has something to do with being 72, I guess. I'm very glad you pointed out my mistake. There was very little room in the .46 Mustunts for lowering the tank but lots of room for raising it. I've already respaced the tank in the airplane. It will save the unnecessary trip to the field ro realize I moved the tank in the wrong direction.

I have edited the Part 11 post above to correct my error.

Thanks again,

Al

Hey Al, I think I asked this before, but, is there a way to put all this on a CD? I have thought of trying it myself. If I do and get it done will let you know.
DOC Holliday

>Hey Al, I think I asked this before, but, is there a way to
>put all this on a CD? I have thought of trying it myself. If
>I do and get it done will let you know.
>DOC Holliday

DOC,

I tried several times today without success. I am hopeless with a complete lack of computer skills. When I copied part 1 (as anyone can) and pasted it in Word, both pictures and text were too big and ran off the right side of the page. I copied it anyway from Word to a CD. On the CD it appeared the same. I'm sure there is a way to do it and the size of the files don't appear to be a problem because the forum photos are so small. My attempt at part 1 ran only 260 kb so I'm sure it will all go on an a CD. I do think the CD should be copies of the Stuka Stunt pages as they appeared here. Anybody can highlight these pages and copy them. I'm at a loss for what to do next. Maybe someone with computer skills can help.

Al Rabe

>Hey Al, I think I asked this before, but, is there a way to
>put all this on a CD? I have thought of trying it myself. If
>I do and get it done will let you know.
>DOC Holliday


Doc,

I expect the CD will come later. I definitely intend to make one which includes all of the 345 photos and 37,000 words of the tutorial parts, as posted here. Meanwhile, I just finished the 8 VHS video set which covers the same step by step building of the Mustunt IV. In some ways, videos better describe some of the techniques used. The set is available for $125 including U.S. postage.

Al

hi al
it looks so good in the air
hope my bearcat is half as good ..still waiting on kyle for bits
paul winter

Paul,

Check your e-mail

Al

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I just thought it might be interesting to compare the Mustunt III and Musttunt IV.

Al

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We had another nice flying day yesterday. I was able to get in six more flights. I seem to be of the opinion that if a little bit is good then, it stands to reason, that a whole lot has got to be better.

In my first example, on the first flight the engine ran slower on outsides. The tank was moved up 3/32" before flying again. Now the engine runs faster on outsides, slower on insides. It looks like the final adjustment will be with the pickup somewhere between 1/32" and 1/16" above the spraybar. Today, I set the new height at 1/16" above the spraybar. While the tank was out I checked the filter. When using a new tank, I think it should be checked after about 10 flights, then again after about 25 flights. If all is well, after that I only check filters when the engine is out or trouble suspected. Also, I pressure check tanks, particularly new ones, any time they happen to be out of the airplane. They live in a high vibration environment and are subject to developing leaks.

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My second excess was to jump from 4" line spacing to 5" line spacing at the handle for more sensitive controls. Boy, did I get more sensitive controls! Now it turns like a combat ship. With a little practice though, it was possible to smooth the corners enough to evaluate the appearance of the corners. I really like the Mustunt's appearance in corners. Many airplanes seem to "fly through" even very tight corners. Mustunt IV corners look a bit different. Inside corners seem to rotate about the Mustunt's wing with the tail slapping down smartly to align the fuselage with the corner exit. Outsides are the same. It not only turns tight, it looks like it is turning tight.

OK, so how am I going to calm it down? I'm not sure that the 5" handle will too big when the airplane is fully trimmed. For now, I've added a 1/2 ounce of noseweight to slow things down a bit. I have a second reason for trying noseweight. The Mustunt has a slow, "floaty" glide after the engine quits, and resists towing or whipping to lengthen the glide. This characteristic is typical of tail heavy airplanes. It is a common mistake to simply adhere to someone else's opinion of proper CG location. I think that optimum CG location can only be determined by observing which balance changes help the airplane's characteristics. In any case, noseweight will improve both the turn and glide. Then We'll see about another line spacing adjustment.

In the photo, there are three modified EZ-Just Hot Rock handles. The one on the left is a straight 4" spacing but reinforced for some of my airplanes with very heavy line tension which will make standard EZ-Just handles creak audibly. The middle handle is the new 5" handle modified for the Mustunt IV. The right 5" handle happens to be the current handle used on the new BBFB Bearcat. The BBFB handle also has a hole in the lower skid for a thong. Weekend aviators around here don't seem to feel much need for thongs. With eight airplanes there yesterday, none of the handles were equipped with thongs. I just threw the BBFB handle in to show an example of one of my many similarly modified EZ-Just handles having spacings from 4" to 6 1/2". The two handles on the right have plywood "skids" to protect the handles when they are being dragged across hard surfaces when the lines are being rolled up. I pretty much modify handles to suit particular airplanes. When trimming is complete, that handle is dedicated to that airplane. I guess it goes without saying that I like the feel of Hot Rock handles.

The last new issue was engine runs. First, the run was way too short. I don't know the venturi size, but it is obviously too big. I solved that by shoving a bit of balsa into the venturi. This works well enough as a restrictor. I flew the World Championships with balsa in my engine's venturi. Later, I may have a smaller venturi made, but for now, this works fine. The second issue was the character of the run. It tended to launch running very slow, then fly fifteen or twenty laps before suddenly "coming on" to a normal run where the pattern maneuvers can begin. This is probably glow plug or the engine running too cold. I'll try a new Thunderbolt RC Long plug, and take a scrap of aluminum sheet and brass wire with me to the field so that I can block some airflow to the cooling fins until we get warmer weather. Bill Wilson thinks some LA. 46s run too cold in this weather. He is able to compensate by plugging some of the holes in his tongue muffler with screws. He claims that on cool days there is no noticeable power lost. One of these methods will probably work.

There is a bit of "wobble" in the top of very tight outside squares and some loss of line tension in the second corner of the hourglass. These will require an adjustment of the movable rudder pushrod. I cranked out a bit of rudder offset and increased and moved the pushrod to an inner hole on the rudder horn to increase sensitivity.

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One very important preparation for any flight, especially first flights, is checking the prop for balance and accuracy, and possibly reworking it. Most of the better flyers rework their props. Here is the equipment I used to rework two Zinger props for the Mustunt IV. I used the Prather pitch gage to check pitches, then ground and sanded on the back of the prop until the pitch of both blades were reasonably accurate. The front of the prop was shaped using dial calipers to thin the blades to the same thickness at about 4 stations. Much of the fine shaping was done using a single edge razor blade in a holder with its edge turned for scraping. Final shaping and sanding was done using a very precise balancer to determine which blade needed more work. Finally, the wood is sealed with CA dripped on, spread around, then wiped off using a Kleenex. Props which work well on the airplane are eventually painted black with yellow tips. My reworked 12-6 worked well for these initial flights. I also prepared a 12-5 1/2.

I have a question. My 12-5 1/2 rework wound up with thinner blades than I intended. Their thickness is only .180" out to about 3" then tapers to .125" near the tips. I found records which show that I used this thickness for the NATs win with Snaggletooth in '77. Now I'm older, more conservative, and wonder if I was teasing the animals with props that thin, particularly since I seem to have another one?

This is just the first chapter in the long process of trimming a new airplane. I don't intend to report every minor change but wrote this to illustrate the beginning of a very systematic process of adjusting. This process isn't particularly hampered by the flying of a full pattern on each flight. But, trimming changes should be made with a fair degree of frequency. It is much too easy to adapt to a poor characteristic with practice. Then it becomes normal to habitually compromise unconscionably to overcome that poor characteristic. The trimming process is one of simply observing carefully, then making progressive changes to improve the airplane's handling characteristics. This process makes our airplanes more capable, competitive, and fun to fly.

Al

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There seems to be more useful information from today's flights to add to the technique of trimming.

The photo is of a small trimming device which aids measurably to the accuracy of trim adjustments. It amounts to a few wraps of thread wrapped and glued around the down line where it exits the handle. This reference mark could have been added to the up line or both the up and down lines. The idea here is that when a small adjustment is needed, it is frequently difficult to tell whether the handle's heavy line moved any at all through the handle adjuster or how much adjustment was actually applied. There are similar indicators on all of my handles. In this case, the marker is 9/32" from the face of the handle.

Today, with the added noseweight, the turn sensitivity is about what I was looking for except the Mustunt IV is turning just a bit tighter on insides. Normally, my airplanes don't have an adjustment for elevator pushrod length. My larger airplanes have 2 degrees of incidence and surface areas determined mostly through the evolution of my designs. The Mustunt IV also has 2 degrees of incidence with some of that possibly lost in the "pinch" of the fuselage at the stab leading edge. I'm a great believer in incidence. It compensates for gyroscopic precession in tethered airplanes, tail surfaces located well above the wing centerline and adds a bit of longitudinal stability. With the Mustunt's exposed elevator horn, I could either droop the elevators slightly with neutral flap, or take in a bit on the handle down line. I drooped the elevators instead of adjusting more down in the handle to take advantage of a possible increase in stability. One turn of adjustment shortened the elevator pushrod 1/40", or added maybe about a half degree of effective incidence. I probably would have been just as happy with the result of shortening the down line 1/8". Meanwhile I haven't actually flown either. Whether this works well or not, there will certainly be adjustments to the handle to match the Mustunt's handling characteristics to my personal taste.

This past week the tank was lowered to better balance the upright and inverted engine runs. It is still running a bit faster inverted but not as much as last week. The only place the difference can be seen now is in the outside rounds where the engine is noticeably faster than in the inside rounds. The difference isn't noticeable in the round or square horizontal eights. Moving the tank down just another 1/32" should give the run I'm looking for.

The LA is running better now as it builds up a bit more break in time. I checked last week's plug and found the coil crushed up against one side of the plug cavity. It was changed to a new Thunderbolt RC long. The first few overly rich laps were still there like last week though not nearly as many laps before the engine picked up to a normal pattern run.

Next, I tried using atmospheric venting of the tank by the simple expediency of leaving the plug off of the overflow vent on the bottom of the fuselage. With atmospheric venting, the engine run was completely normal from take off to landing without the troubling changes of speed in the first laps. When the engine has more time on it, I'll try uniflow again.

This past week at home I tried taking out nearly all of the rudder offset to "square up" the Mustunt on the end of the lines and added rudder sensitivity to give more right rudder on down. This took the "wobble" out of the outside corners but the reduction in rudder offset cost too much line tension overhead. The increased sensitivity was retained while three turns of rudder offset were cranked back in. Line tension is better everywhere now, especially in vertical and overhead maneuvers. The Mustunt seems to fly yawed out a bit now, but so far, I haven't tried any adjustments to leadout location which may correct that. In any case, its mostly visible only from the inside of the circle and doesn't hurt the appearance of the maneuvers.

The only worrisome item is a vibration, felt through the lines, which occurs only in hard corners. It makes me think the prop tips may be fluttering with rapid changes of the airplanes pitch attitude. I checked with Randy Smith this week for a carbon prop with greater stiffness than my wood props and probably more efficiency and thrust as well. Randy recommends a 12 1/2" Bolly carbon prop for .46/.51 size engines run 4-2-4. I forget just what pitch Randy said. In any case, it is a thin prop, taking advantage of carbon prop stiffness. Engines are able to handle a bit more diamenter in thin stiff props. They are said to turn easily. One of these is on its way here now.

Meanwhile, I'm having lots of fun, and that's what its really all about.

Al

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Reply to post #10...

Outstanding planes Al!

After comparing the three planes it comes to my mind why I don't like the blue painted OS engines. The blue in this case (to me anyway) appears out of place. For all the fine details, the level of accuracy in your final finish then the blue thing hanging out the top is why I strip the blue off all my OS engines. From the .15 to the .46 I have not one blue engine... of course this is strickly a personal preference, it in no way reduces the appeal of your plane(s).

Warbirds and Bi-planes have always been my favorites. Thanks for consistently sharing your craft with us!

I always look forward to what You've built. Your work is very outstanding in my opinion. I only wish I was 1/2 as good.

hi al
nothing in my email

send to

[email protected]

ta
paul

I sent it to:

[email protected]

Some communications are better handled with e-mails than posts on this forum.

Al

hi al
ok
paul

AX

Hi Mr Rabe,

I just have one question?

Have you or anyone considered mass producing "Universal Shock Gear" to

fit .40 to .60 size ships.

I'd be very interested in buying those for some of my future stunters.


Thanks Alex.

Aha! So, all of your handles use cables and not those funny "hard point" connections.

I feel better now, because my handles use cables. Now, the next time I hear someone spouting-off about the joys and benefits of "hard point" I will cooly remind them that Mr. Rabe uses cables!

Floyd in OR

Oregon State Motto: "The Sunshine State--for a couple weeks in July."

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>Again, thanks to David Russman for the inflight photos. They
>are great.
>
>Al
>

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While David's photos are really great, the other major contributor to this project was Kyle Tankersley. Just recently, Kyle finished a beautiful set of plans. I think his effort was more in support of the Mustunt IV and the possiblilty of a magazine article than an interest is producing kits. Since Lone Star went up Kyle mostly got out of the kit business and found work as a machinist. Kyle will still produce kits on request and just shipped the first two Mustunt IVs. Even that depends on the availability of good wood. With Lone Star back in business, maybe there will be more kits from Kyle. Meanwhile, full size copies of these plans can be had from Kyle. His address is [email protected]. His price is $15 plus $4 S&H.

There are two differences between these plans and this 11 part construction tutorial. After building and flying my Mustunt IV, I see where a couple of small changes might make a slight improvement in the construction of new Mustunt IVs.

First, the Part 4 shows me embedding the engine lug crush plates into the 1/8" plywood engine compartment floor. It will be better to sit the crush plates on top of the plywood floor. This will raise the thrust line 1/8" which will have no noticeable effect of flight characteristics and deepen by 1/8" the tank compartment to permit a greater range of adjustments to tank height. This also slightly improves the visual shape of the nose.

Also in Part 4, there are two paragraphs explaining how the tail surfaces were built to the same exterior 1/4" thickness as the original Mustunt II and III. This was ridiculously laborious and just plain too much nostalgia and adherence to old plans. The new drawings show that all of the internal structure of all of the tail surfaces should now be simply be built from 1/4" x 1/16" strips with the option of building the stab airfoiled.

Adding airfoil to the stab would probably have little practical effect on what are already very nice handling characteristics. But, again, this is another feature I'd use if I ever built another Mustunt IV. It probably isn't needed but certainly can't hurt with the possibility that it might, in some way, help.

Airfoilling the stab has another justification. this is not a simple stunt sport/trainer. It is a very sophisticated small stunt ship with very advanced features. Any stunt ship with molded parts, a full suite of trimming adjustments, build up tail surfaces and fully sheeted, dihedraled wing with shock gear is way beyond simple in construction or capability. Its also why this 11 part series actually presents a fairly good explanation of some advanced construction techniques.

Meanwhile, I intend to rewrite and replace pictures to the posted Mustunt IV Part 4, Basic Building Techniques tutorial. Leonard will probably help with substituting the reworked material in a long overdue edit.

Here's another big thanks to Kyle.

Al

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