Conference vmszoo::rc

    i've been thinking of trying the following experiments on the balsa
    usa cub i'm building.
    
    1.  homemade softmount
    
    the "stock" firewall is a piece of 1/4" thick ply, approx 5x7 inches
    in size.  normally some kind of hard engine mount like a hayes mount
    is bolted directly to this via bolts and blind nuts.  but i intend
    to bolt the hayes mount to another piece of 1/4" ply, about 4x4
    inches square.  the mount and ply assembly then would be attached
    to the real firewall with permatex rtv blue gasket maker.  if you've
    never used this stuff, then you'll surely wonder how it can possibly
    hold a st90 to an airplane.  but i've used rtv for all kinds of
    things, it is incredible.  sticks to metal and smooth hard surfaces
    like crazy and always remains flexible.  anyone else tried this?
    
    2.  homemade muffler
    
    the muffler on my baker p47 is made with a hacked up tatone manifold,
    2 brass tubes, a stainless steel tube, and a soup can.  the only
    thing holding this thing together is rtv blue silicon (the same
    stuff).  i built this muffler not to be quiet, but to simply get
    the exhaust out the bottom of the cowl without melting it.  its
    a bit loud but sounds kinda neat.  anyway, this muffler is holding
    up well.  for the cub i want to build multiple baffles by using
    varying sized tin cans (i can see you all shaking your heads!),
    each inside the other, with holes drilled all around except for
    the outermost can.  the whole assembly will be inside the fuse
    just past the firewall.  exhaust will exit from bottom of fuse.
    having the muffler in the fuse may be even quieter with balsa/
    glass fuse serving as part of silencer.  may be a heat problem
    in there tho and may have to vent it.  anyone tried this?
    
    kevin
                                             

    Thanks for the tip on UPPER CASE, I'll stop shouting.
    
    Regarding soft mounting a pattern ship engine one major problem
    crops up that is kinda tough to solve due to the typically tight
    cowling in a pattern ship and that is somthing called gyroscopic
    precession.  this occurs in a high G pull up maneuver where the
    soft mounts allow the motor to pull down which changes the thrust
    line of the motor temporarily, when you then roll the airplane before
    the motor returns to a normal position the altered thrust line will
    cause a pitching moment right when you don't want it to occur. 
    the solution is to build a rubber ring into the cowl of the airplane
    with about 1/6" clearance around the nose of the motor.  This limits
    the movement but still allows vibration damping and noise reduction.
    My wife has a small business called R/C Noise Reduction Specialties 
    and markets two sizes of Lord Mounts to distributors.  I will provide
    them to any of you for cost ($4.00), they normally go for $12.95
    at the hobby shop.
    
    On mufflers, J'TEC sells a great after market add on that bolts
    into the front half of a stock muffler.  Works Great!
    
    If you are building your own muffler, try designing it so that the
    exhaust gas makes three passes through a chamber of some sort. 
    Example; entry into a brass tube with holes in it surrounded by
    a second brass tube with holes in it surrounded by a tin can which
    leads to the exhaust exit.  The idea of mounting the muffler within
    the fuselage is a good one but be sure to insulate the outer muffler
    shell with somthing fireproof!
    
    When we were first tinkering with soft mounts the rule of thumb
    we kept in mind for strength was that since we are capable of ten
    G maneuvers that the mount should withstand at least 12 to fifteen
    times the weight of everything we were hanging on it and you should
    be able to attach that kind of weight and drop it and let it hang
    without damaging the mount.  In normal vibration isolation the lowest
    HZ rating is usually determined then the piece to be vibration isolated
    is isolated to that frequency, this in turn isolates everything
    above that frequency.  That is , as it turns out, not entirely
    necessary and we have been tinkering with harder mounts that isolate
    above 5000 rpms with less movement of the motor for pattern guys.
    
    Up Elevator!
    
            
    

This brings up a question that I've been wanting to ask about
soft mounts.  One of the things that is always warned about in
engine mounting is to be sure that the engine is bolted tightly
and the mount is secure. This is to prevent engine damage from
vibration.  Doesn't a soft mount, by definition, lead to an
insecure engine mounting -- and resultant engine damage?

    Note 393.* in this file details some very successful experiments
    in noise suppression via homemade custom mufflers.
    
    	-Bill

    A solid mount typically transfers 100 to 110 G's of vibration force
    into the firewall.  If you have a loosely mounted solid mount the
    inertia of the vibrating motor is allowed to increase and a major
    pounding of the firewall occurs.  A soft mounted motor using the
    correct lord mount drops the g force to around 10 to 12 G's and
    allows for no pounding of the firewall.  The engine hits on nothing
    and is not subject to damage.  

    well last weekend my homebrew muffler blew out.  no, the rtv didnt
    let go, but a 1 inch diameter hole just blew out of one end of the
    soup can in mid-flight.  it wasnt a backfire, just fatigue i guess.
    for now i'm going to just rtv a patch over the hole, but it appears
    i've got to find a tougher kind of can.
    
    kevin

    Maybe one of you design guys and figure out how to put this idea
    into practice. Wouldn't it be great if we could laminate a piece
    of solid rubber between a Dave Brown style mount and a piece of
    aircraft grade plywood using only adhesive? This method (if possible)
    would be marketable and simple to use for most modelers.
    
    
    Tom

    I have heard the name "lord mount" can someone explain exactly what
    it is and what it looks like.
    
    
    							Dave
    

    On the serious side, Lord is the name of the company and they make
    vibration isolators used in all sorts of industrial applications.
    They are primarily a rubber company, I believe.  The Lord mounts
    that I have seen drawings of look like a cylinder with a machine
    screw sticking out each end.  THe cylinder is made of rubber and
    the screws are cast into the rubber ends.  You mount one end to
    the firewall and the other to your engine mount back plate or right
    to the engine backplate. (I don't like this because the engine
    backplate is not designed to support the engine and is not strong
    enough to do so.)  The rubber provides isolation between the mount
    and the firewall.  The proper disign of vibration isolators is not
    simple.  You must know the masses of the objects and the disturbance
    frequencies envolved to determine the size and material type of
    the isolators.  If you get lucky and get them right, the reduction
    in vibration transmitted to the fuse will be great.  If not, they
    will just add weight and have no benefit.
    
    Charlie
    

    Lord Corp. does spend a lot of time making their mounts do exactly
    as claimed- damp vibration and sound transmission.  They also make
    flex-shaft couplers to do the same things on power drive shafts.
    
    They're very real, very hard-working, and they deliver an excellent
    product for a very reasonable price.
    
    To choose a mount, you need the weight and moment of inertia of
    the object you wish to damp, and the frequency of the disturbing
    force.  So, changing throttle or prop will tend to make the selection
    slightly "wrong", but most of the Lord mounts have a reasonably
    wide "band" of absorption.  But- there's no reason to get something
    that won't work.
    
    Let me check my McMaster-Carr catalog for the mounts and specs.
    I'll report back anything useful.
    
    	-Bill

                Effect of  soft  motor  mounts on a four stroke.  This is
        Dan Snow's plane.

                Engine: OS 48 Surpass
                Prop: Master Airscrew 11x7
                Airplane: Balsa USA Easy 100
                Fuel: Red Max 10%

                db Measurement: 90db
        
                Engine is set  up  with  stock  silencer, and was running
        approx 9.5-10K

                      _ 
                     / |
        |  _====____/==|
        |-/____________|
        |    |        o \
             O           \ 
                          O
         Hang in there! o_|_
                          |
             Anker      \_|_/

    My wife has a company called R/C Noise Reduction Specialties.
    
    We have done a lot of work with Lord Corp engineers coming up with
    the correct mounts for R/C aircraft engines.  She supplies them
    to several R/C products distributors.
    
    I would gladly supply them to noters for cost, which is less than you
    can get them from Lord for since my wife buys them by the thousand.
    This offer is good for noters only.
    
    Call me at (707) 429-0350, let me know what kind of motor you have
    and I'll send you a pack of four for $5.00.  
    
    I have 5 planes with lord mounted motors.  I attach at the firewall
    and at the motor mount.  I agree with the previous noter that the
    backplate is the wrong place to attach the mounts to.  I would be
    glad to answer any questions that come up on this subject.
    
    Will Beatty

    re .22
    	How long is the rubber part of these mounts?  I am thinking
    about using some rubber mounts on an existing plane and I can't
    space the fiberglass mount very far from the firewall without major
    surgery.  I saw Dan Snow's Davis mounts and they look slick!  They
    attach like wall anchors through the firewall rather than with studs
    on each end.
    
    Charlie
    

    Re .23
        There is a 1/4"X20 stud on each end of a rubber piece that is
    .62" long.  The diameter of the rubber part is .62" also.  
    
    I put 1/4"X20 blind mounts on the firewall and either bolt the engine
    side stud directly into the motor mount or into a 1/8" aluminum
    plate which I then secure the motor mount to.  Total distance from
    the firewall to the back of the motor mount is .62" if you go straight
    into the motor mount.
    
    If moving the firewall back in the plane presents major problems,
    I have cut out a square in the firewall and then built up the backside
    of the firewall with new plywood moving the mount attachment point back
    by the depth of the original firewall thickness.                  
    
    Regards,
    
    Will Beatty

    I have now used the Davis Diesel rubber engine shock mounts in my
    Super Sportster 20, and have had 4 flights on them. Here are my
    comments so far:

    I didn't notice any difference. They don't seem to have any effect
    at low frequencies (engine idling), I still get occasional
    rattling from the fuse/wing when I pick up the plane while it's
    idling - sitting on the ground, no rattle, pick it up, rattle.
    Unlike some of the bigger trainers that have built in drums in the
    wing and fuse, the SS20 never had noticable airframe noise. My
    bottom line is that I think the engine mounts have marginal
    effect.

    After my second flight on Sunday (which became my last flight that
    day because of this problem) I noticed that the spinner had been
    rubbing at the bottom on the front of the fuse. There hadn't been much
    clearance; now there was none. I thought perhaps the engine mounts
    had loosened causing the engine to droop a bit. I tightened the
    top mounts, but couldn't reach the bottom ones without removing
    the engine. I chose not to fly any more with the spinner rubbing.

    I think that the rubber washers compressed somewhat. This was
    compounded by the fact that the engine mount only covered part of
    the washer, and when tightened, the washer curled around the edges
    of the engine mount.

    My repair consisted of making a 1/16" ply backplate for the engine
    mount that was sized to cover the entire washer at all four
    corners. I also tightened it very tight, because the bolts did
    seem to have loosened. You can't use loctite on these bolts
    because the threaded washer part is inside the rubber shock mount
    and it would be impossible to remove the screws once the loctite
    set up - the rubber would just twist.

    So now I have a very stiff but slightly flexible engine mount. I'm
    not sure it has any value at all in this airplane.

    Dave Hughes

I understand what happens when I tighten them up hard - I defeat
a lot of the dampening, particularly at low frequencies. However,
if the alternative is to have the screws back out over time, I
consider that to be unacceptable too, because I have to remove the
engine to tighten the bottom bolts into the firewall. 

I think that the bottom line is that the effect of soft mounts on
my SS20 is negligible, because other components of the noise are
far greater than fuse/wing resonances due to engine vibration.
I'd have to first quiet the exhaust, prop, and probably intake noise
before it got quiet enough to notice any contribution that the
engine mounts make.

I'm interested in a bit of experimentation, but I have no intention
of making a career out of this. I want to fly the plane, not spend
all day messing with rubber mounts.

I am seriously considering whether or not to use these mounts in the
SS40-Bipe, since they don't seem to do much for me and are becoming
more trouble than they're worth. I would have tossed them Sunday nite
if I could have, except for the 1/4" holes in the firewall that I'd
have to do a lot of work to fix.

Dave

    Tonight in my club monthly meeting and I'm giving a little
    demonstration on rubber mounting of engines. As aprt of this
    I've been flying a "retrofit" rubber mounting scheme on a
    ScatKat. Its' quite simple. If you are using a bolt on beam style
    engine mount and you can move your engine back on these beams
    app. 1/4" then you can retrofit you plane with a rubber mount.
    Here is how...
    
    To start remove the engine and engine mount. Using a screwdriver
    remove the "t" blind nuts from the rear of the fuse. 
    
    You will need;
    			4   6/32 x 1 1/4" bolts 
    			4   6/32 lock washers
    			4   6/32 nuts (from hardware store, thin if
    possible)
    			8   flat metal washers
    			8   rubber (fauset) washers
    			2"  silicon fuel tubing
                                             
    
    	Using a 1/4" drill bit drill out the existion engine mount holes
    in the firewall. On one of the 6/32 bolts add; 1 metal washer,1
    rubber washer and a 1/4-5/16" length of the fuel tubing. From the
    inside of the firewall insert this assembly threw the firewall so
    that the bolt extends into the engine compartment. Add1 rubber
    washer and the 1 metal washer. Thread on the flat 6/32 nut. Tighten
    until the rubber washer begins to slightly deform. Repeat the process
    on the other three bolts. Now add the engine mount. Thread on the
    four lock nuts. It will require that you hold the bolt from inside
    the tank compartment. Once tight you are ready to redrill the engine
    mount. You may want to use a new mount.
    
    I've been flying this set-up for a week and it does work. How well?
    Don't know but it does seem to work. I didn't test the plane before
    the rubber mount but it reads 89.5DB now with a ASP 40 with stock
    muffler.
    
    
    Tom

    I had thought I had added a note here on the Paco Amber range of soft
    mounts, but anyway here is my experience with them so far.
    
    I have only used one to date, this is on my Acro-Wot, but I am just
    about to test a prototype for them. The Acro-Wot's engine is an OS91
    4S. After changing to the Paco mount I couldn't believe the level of
    noise reduction that I got. I was _really_ impressed. I am now totally
    for the soft mount and would very reluctantly go back to the hard
    mounted engine. As a matter of comparison I have a Wots-Wot with
    another OS91 4S on it. I haven't got around to replacing the hard mount
    on it , but anyway, I have compared the two and you would never think
    that they were the same engine. 
    
    The Paco Amber mounts are precision engineered and are supplied with
    all mounting screws, Allen keys amd bulkhead fixings. Everything is
    well made and a full line of spares are available. Very clear
    instruction are included. 
    
    The mount consists of a back plate. To this are attached the arms which
    are adjustable for a range of engines. The arms are circular steel
    stock and are appropriate length for the engine. The engine is attached
    by 4 rings (can't think of a better word) that are bolted to the
    engine. Alignment of the two rings on each side is done by sliding a
    (supplied - nice touch !) bar into the rings and the bolts tightened.
    The rubber bungs are then slipped into the rings and all is then slid
    onto the arms. Clamps fore and aft hold the engine in place. Difficult
    to explain. Picture is worth ... 
    
    I have pic's etc and installation instructions for anyone who is
    ineterested. Just mail me or reply here !
    
    They have quite a number of mounts which they advertise in the
    following classes :-
    
    	small for engines in the .11 to .46 cu. in. range (2S & 4S)
    	regular for engines in the .35 to .90 cu. in. range (2S & 4S)
    	large for engines in the .75 to .180 cu. in. range (2S & 4S)
    	contest for aerobatic models (2S & 4S)
    	special for other large engines.
    
    Prices are between 25-50 pounds sterling inclusive of VAT. But worth
    every penny ! They have a catalog that explains what is available,
    mount dimensions etc.
    
    The address is Paco Amber, 48a Fairlight Avenue, Telscombe Cliffs,
    Peacehaven, East Sussex BN10 7BS, England. Telephone (0273) 582786.
    
    I have always found them very helpful and knowledgable about particular
    engine mounting questions. I have only had one of the rubber insulators
    give way, but then they do say that you will get a reduction in the
    life of the rubber if you use an electric starter. I think I got 9
    months out of the rubber bung (one of the four). Anyway I replaced the
    single rubber and got a few spares which will last me a good few years.
    
    
    Eric.

    Eric,
    	These mounts sound interesting - and I have not seen them
    advertised in the US.  Do you know if anyone imports them?  They sound
    easier than most to retrofit in an existing plane.  Most of the soft
    mounts I have seen have the rubber part connected to the firewall - not
    easy if your plane is already built/tank installed, ect.  I have a
    couple of planes with OS91's on them and I love the engine!  I also
    agree that soft mounts are the way to go - but I have not gone for them
    on any of my planes yet.
    
    Charlie
    

    I am not aware of them being distibuted in the US. THe British mags
    advertise them occasionally.  I will forward you a copy of what I have
    for your information. Since you have a some OS91's I recall the mount
    being about 30 pounds stg - somewhere in the  $50 area. They are
    probably in the top end of what is available here. Since noise
    reduction is only becoming more important now I guess is that they will
    become more popular. If I was selling them I would have sold a good
    number by now ! Since most inquire and are impressed with the sound
    reduction. It makes me sometimes wonder if the engine is running at
    full power. 
    
    Regards,
    
    Eric.