Author Topic: Bad Vibes... (Read 3649 times)

wwwarbird · « **on:** August 24, 2006, 07:59:01 PM »

I've got a heavily modified Twister that is now a Stuka replica that I finished right before the Sig contest in June. Those who attended may have seen it. Anyway, this thing is built just like I've built every good profile, nice long motor mounts to the L/E, solid 1/8" ply doublers to just past the spar with a 1/2" balsa tripler, you know the routine. The original powerplant was a Woolard Fox .35 that ran beautifully on a stock Twister all last summer.
Here's the problem, since day one, when you fire the plane up it VIBRATES LIKE H---! I have tried two other Fox .35's, one known good runner and another one brand new out of the box, with exactly the same results. I have tried 7-8 different wedge tanks and a 4oz clunk, same results. I've tried practically every prop made (all balanced) in the 10-4 to 10-6 and 11-4 to 11-6 range, even the 180 degree rotation trick, same results. With and without spinner, same results. I have tried every combination of the above, same results. The ONLY time I got the model to run smoothly was when I used my syringe as a remote fuel tank (not attached to the model) and it ran fine.
Now, I know the Fox .35 is no swiss watch, but there has got to be an answer. All the engines run fine except on this model. This has been hugely frustrating because I have gotten in about ten flights in, enough to tell it's gonna be a great flier, but the plane is going to tear itself apart if I continue as is. I would prefer to not butcher up the nose for another powerplant. Any suggestions???

Marvin Denny · « **Reply #1 on:** August 24, 2006, 08:41:50 PM »

Mr wwwarbird. That is definitely a frustrating problem. I have had the same problem on twisters, Skyray 35s, and Shoe Strings.
Two things you might try. 10 X 5 APC prop (balanced), and go up to 10% nitro and 25 to 28% castor oil. Use a cooler plug.
Second is install a 3" X 3" X1/8" plywood gusset on the inboard side between the nose and wing. A sorta triangle brace or stiffener if you please. REALLY GLUE IT ON SECURELY and in line with the thrust line so you do not get up or down aero forces.

Let us know if either of these things work.

Bigiron

Jim Kraft · « **Reply #2 on:** August 24, 2006, 08:56:15 PM »

One thing I would try if you haven't already, is to make a couple of aluminum plates to put between the motor and the mounts. Then snug the mounting bolts down really tight. I use socket head bolts and an angle allen wrench so they are very tight. Also helps to put aircraft nuts on the other side of the tee nuts. This has worked for me on several that I have had vibration issues with. Also a couple drops of Armorall in a gallon of fuel to keep it from foaming will help on the ground as the engine is running rough from bubbles in the fuel line. Sometimes, nothing works, but getting the engine really tight can do wonders.

Marvin Denny · « **Reply #3 on:** August 24, 2006, 09:05:52 PM »

You are right Jim!! I forgot about that. Thanks for jogging this old brain.

Bigiron

Tom Perry · « **Reply #4 on:** August 24, 2006, 09:19:10 PM »

I like to drill two or three 1/8th or 3/16th holes down through the hard wood mounts and epoxy in small hardwood dowels. Sorta like detuning a tuning fork. try this if the other suggestions don't help. It really helps strengthen the front end also.

By the way for trivia nuts. I'm fairly certain that strength is the only eight letter English word with only one vowel.

I must be getting tired.

Bob Reeves · « **Reply #5 on:** August 25, 2006, 04:36:16 AM »

Just to add to what has already been said, vibration problems in stiff noses are almost always a resonance problem. The structure just happens to have a natural resonance (like above, think tuning fork) right where the engine likes to run. You either have to change the resonant frequency of the nose structure or change engines.

Any of the three suggestions already mentioned might work, aluminum pads, triangle gusset, dowels. They will all change the resonant frequency of the nose and possibly cure the problem. Once fixed a problem with a Flight Streak by drilling straight back behind the engine between the motor mounts just into the leading edge of the wing and epoxied in a dowel. This fix was hidden and didn't require any major refinishing.

I would first try aluminum mounts then a dowel as above, if that doesn't solve the problem you can add a gusset and/or dowels down through the motor mounts.

wwwarbird · « **Reply #6 on:** August 25, 2006, 04:18:42 PM »

Thanks guys. I did also try the aluminum pads, but that didn't help either. I've heard of the Armor All trick but never tried it. I hate to resort to that because it's just not a proper fix in my mind. A fuse/wing gusset isn't an option in this case. The dowel trick might be a possibility. I might end up doing some butchering here, but I hate that thought also.
Maybe I've just built a nice wall hanging...I hope not.

Bob Reeves · « **Reply #7 on:** August 25, 2006, 05:39:11 PM »

If you don't want to mod the airplane, get yourself a Brodak 40, drill out the mounting holes so it will fit in place of the fox and your problem will be solved. Alternatly you could get a Double Star 40 and it will bolt right in. The Double Star is a bit more costly than the B 40 and I personaly think the B 40 is a better engine.

If you are worried about drilling the mount holes don't be, this stuff is supose to be used and it's not like it's a collector that's no longer made.

W.D. Roland · « **Reply #8 on:** August 26, 2006, 09:50:24 AM »

try this if all else fails
sounds like possibly the airframe may have its natural frequency at the same as the engine.this will lead to airframe failure.this can be cured by changing the mass in the engine mount area as this will change its natrual frequency.
usualy only takes a smal amount.
this is comenly done on formula cars and aircraft.in the case of lets say a tubular suspension link just a small weld bead somewhere along it lenght will stop the vibration that results in failures.

David

wwwarbird · « **Reply #9 on:** August 26, 2006, 11:53:22 AM »

With the frequency thing in mind, I'm thinking of drilling back horizontally behind the engine and between the maple mounts, and then adding a small bit of lead. The plane seems just a tad tailheavy with the few flights I have gotten in anyway. Think that might do it? $:-\$

Bob Reeves · « **Reply #10 on:** August 27, 2006, 04:29:05 AM »

Quote from: wwwarbird on August 26, 2006, 11:53:22 AM

With the frequency thing in mind, I'm thinking of drilling back horizontally behind the engine and between the maple mounts, and then adding a small bit of lead. The plane seems just a tad tailheavy with the few flights I have gotten in anyway. Think that might do it? $:-\$

Just might, problem with this situation is you don't know till you try it. BTW: Don't try to melt the lead and pour it into the hole, the odds are pretty good you will end up with hot lead running down the outside of your nice paint job. Don't ask

W.D. Roland · « **Reply #11 on:** August 27, 2006, 01:57:23 PM »

what Bob said

David

wwwarbird · « **Reply #12 on:** August 27, 2006, 02:50:00 PM »

I think it's the next move. I'll be getting out the drill as soon as I get a chance. Fingers crossed...

Wynn Robins · « **Reply #13 on:** August 27, 2006, 10:39:32 PM »

your problem is that you are using foxes.........pretty simple solution....get an OS or 3

wwwarbird · « **Reply #14 on:** August 29, 2006, 05:43:55 PM »

I hear ya Wynn, and I have a lot more O.S.'s than Fox's in my personal inventory. It just happens that in this case, I built and set up the plane for a Woolard Fox that was an excellent runner on a previous airplane, and I'm trying to get it to work without redoing the nose of the model. I am definitely more of an O.S. fan than a Fox fan, but I have seen plenty of good running Fox's too. In this case, it just appears to be an unlucky or coincidental frequency problem between model and powerplant. Stay tuned...

Bill Sawyer · « **Reply #15 on:** August 29, 2006, 05:52:33 PM »

I did a figure nine with a modified Ringmaster and snapped the nose off. To repair it I glued the nose back on and then used a long drill bit ( 12 incher ) to drill two holes in the fuselage - one going up over the wing and the other going under it. I started the holes in the area behind where the motor backplate is. Then I inserted two carbon fiber rods in the holes. It is as stable as it was before - probably more so.

wwwarbird · « **Reply #16 on:** August 29, 2006, 05:53:02 PM »

Yesterday I took and drilled two horizontal 3/8" holes in the fuse, between the maple mounts and behind the engine. One 1" deep and one 1/2" deep above the first. No scientific reason for the varying lengths, that's just what seemed right at the time.

Then I mixed up some 1hr epoxy with some #8 shot and filled the holes full. $:-\$ I then hit the field with great anticipation last evening and found that this did make a significant difference.

I think now I am in the "NORMAL" range of Fox .35 vibration.

Hopefully with some fiddling with prop selection and things I can get this worked out completely.

Bob Reeves · « **Reply #17 on:** August 30, 2006, 12:19:34 PM »

Cool, good to hear you are making progress..

Years ago I built several Twisters always box stock and always with OS 35S'.. Never had a bit of problem with vibration and the engines all ran great..

Came back into the hobby several years ago and second airplane I built was the TwistMaster (Modified Twister) longer motor mounts, aircraft ply doublers all the goodies everyone today is recommending.. I could not get a 35S to run worth a hoot on that airplane and after a month of fiddling I finally gave up and put an FP on it. Wasn't happy with the FP so it now has a Saito 40 but no vibration problems.

The only conclusion I can come to is the stiffer nose of the "NEW" Twister was responsible for the trouble with the 35S.. Is a Stiff nose always better, I'm not so sure....

Jim Kraft · « **Reply #18 on:** August 30, 2006, 01:24:33 PM »

Hey Bob; I think you are on to something. I have heard this before, that to stiff of a nose will cause a problem sometimes just as bad as to soft. Maybe the old kits with the short motor mounts was right. I know I have built a couple of Ringmasters using the stock short mounts, and never had a problem with vibes even with a Fox. Of course, the Ring is fairley short in the front to begin with, plus having a pretty deep front end. Glad the extra lead helped.

W.D. Roland · « **Reply #19 on:** August 31, 2006, 01:31:35 PM »

I da oldern days we found that on almost any airplane(fast&slow combat mainly)that a stiff engine mount would gain us a few hundred revs.

we could tell when they were not stiff enough by starting and setting a normal needle and then with our hand hold the engine mount as tight as we could and listen for a rpm increase.

if it didnt increase it was ridgid enough.

if rpm did increase it was not stiff enough.

from what i read modern engines are aparently lots smother than older ones.

my newest engine is close to 20years old

might be along time before i have a modern one
its gona take awhile before the 100 or so good old engines i have are worn out
1/3 or so have never been run!

warbird sure glad your problem is solved!(for this airplane atleast)

David

James Lee · « **Reply #20 on:** August 31, 2006, 03:21:51 PM »

Lost a lot of my own hair with the same problem!! Several years ago built a Skyray with the good 1/8 ply doublers, cheek piece on the inboard and really made it look slick..... Shook like the devil!!! Finally got it to run sorta, but it was never right. Finally won an R/C! trashcan contest with it(But that is another story!!) My old junk skyray with the little motor mounts and lite ply doublers runs smooth as you could want..... I think a lot of us have outsmarted ourselves and made the front ends too stiff for the old shaker motors.. On the next profile I do for a Fox I'm going to try an 'old' style nose with out all the 'new' extra stuff.... Can't be any worse!
Of course my other skyray with the OS 25 FP runs smooooth, but it doesn't growl and snort and smell good like fried castor from the old Fox!
Jim

Randy Powell · « **Reply #21 on:** August 31, 2006, 03:50:52 PM »

Probably already thought of, but did you put a pad between the tank and fuselage? I've solved vibration problems with profiles before by sliding a hard foam pad between the tank and fuse.

Just a thought.

wwwarbird · « **Reply #22 on:** September 01, 2006, 11:02:20 AM »

Randy,

Yep, I put a bit of foam behind the tanks on almost all profiles. I'm hoping to fiddle with this thing some more over the weekend.

Thanks though!

Lee Thiel · « **Reply #23 on:** September 01, 2006, 01:09:52 PM »

I use rtv to secure all of my tanks on profiles with Fox 35s with only one viberation proablem, and that was on a Tomahawk.
Lee TGD

Steve Helmick · « **Reply #24 on:** September 02, 2006, 04:05:33 PM »

I'm not too keen on the idea of running the engine bearers back to the LE. What that does is make a "hinge" at the LE.

The problem with the typical profile (short beams) is that the balsa doesn't connect the two layers of plywood very well. My idea is to put strips of plywood along the outside edges of the maple, and extend that back to the wing's high point. If this would work with straight strips of ply, then I'd use 1/16". If they'd need to be curved, then I'd use two layers of 1/32" ply, and in all cases, I'd have the grain of the plywood across the width (1/2", typically). Making the nose an extra 1/8" thicker wouldn't hurt, either.

As a machinist, I frequently have to tune out harmonics. It's what "speeds & feeds" is all about, but also what a stiff setup is about. Sometimes, with the one part deals, the expedient method is the easy setup, with some sort of scheme to tune out the harmonics. A machinist's jack with a piece of leather on top can do it, or just hold a plastic hammer against the part.

Point is, it doesn't take much to tune out the vibes. Raising the rpm of your Fox would be a likely cure. Try 10-5 and 10.5 x 4.5 APC's.

Steve

wwwarbird · « **Reply #25 on:** April 17, 2007, 05:24:07 PM »

StukaTwister update: I re-did the nose a bit last Friday night and mounted an O.S. FP .40, with a black Tornado 10-6 three blade prop, and now have an excellent flying, vibe free airplane added to the stable. I should have known to scrap the Fox right off the bat.

john e. holliday · « **Reply #26 on:** April 18, 2007, 11:44:29 AM »

Yep, some planes are like people. They just don't like to use the tried and true equipment. DOC Holliday