Mini-Pearl

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  • #40438
    Dean McGinnes
    Participant

    I am beginning on my Mini Pearl for One Design at this year’s NATS, and my grandson will also be building one to replace his vernerable 1/4A T-Bird.

    The One Design is flown to Nos Gas rules.

    I would like to start a discussion on suggested mods, structural and otherwise that those of you out there have found to work on the Mini-Pearl. Albert and I will both be using Tee Dee .049/.051 engines.

    TIA

    #42941
    Dean McGinnes
    Participant

    There doesn’t seem to be much traffic on this thread πŸ˜• πŸ™



    #42942
    George Reinhart
    Participant

    Dean,
    I worked from the “official plans”, recently purchased.
    I sheeted the center two rib bays, 1/8THth dihedral break ribs and tips.
    No other changes. 2 function Tomy timer from John Clapp, new engine from Cox successor (couldn’t find a used one),. Covered with Microlite synthetic. Added just a little left thrust per Marvin Mace, and made the nose length per plans. It came out tail heavy so I had to put the tracker in the middle of the fuselage extension that supports the rudder to put the C.G. at the location shown on the plans. Only other changes I would make is to go to 3/32 or 1/8 on the rudder because it’s not very stiff and I would build a tank compartment into the front end.
    Finished weight was 156 grams (more than my target).
    Cheers!

    #42943
    George Reinhart
    Participant

    Dean,
    OOPS!!!
    I meant to say it came out TAIL heavy.
    Cheers

    #42944
    Dean McGinnes
    Participant

    Pete,

    Tell me more about the 2-function Tomy timer. What two functions does it do?

    #42945
    George Reinhart
    Participant

    Dean,
    I use it for Engine cut off and D/T
    You will have to modify it slightly for a latch or launch switch.
    I used it because it’s very light at about 5 1/2 grams.
    Would nhave preferred Texas Timer but weight was critical.
    Cheers!

    #42946
    Dean McGinnes
    Participant

    You must be referring to The MAX-III 2-function timer. It is fairly heavy for a 1/2A model. I use their Micro which is built on a Tomy-type mechanism.

    It is one function, with a very positive fuel shutoff. I think it is around 5 grams weight but I am not sure. I will weigh one of mine tonight.

    It can be rigged for quick DT very easily. You will need a separate DT system for normal flying.

    They are extremely accurate. While some of the larger timers are affected by engine vibration, and really must be tested with the engine running, the little Micro units I have used seem impervious to vibration. You can dry-set them on the ground, and they seem to perform very accurately in flight. For sure, the performance is very repeatable.

    I have used three of them in various models and find them excellent units.

    I will post the weight later this evening.

    I have no interest in Texas Timers other than as a satisfied customer. πŸ˜€ πŸ˜€ πŸ˜€ πŸ˜€

    #42947
    BOB STALICK
    Participant

    I thought I would give you the benefit of my experience with the Mini Pearl. The new one I have just trimmed out is my 4th. All have been powered by a T.D. They have all weighed in around 6 ounces and are covered with Japanese tissue. Although I have used pressure tanks, the new one uses an old Cox Red plastic tank on suction. Works fine. I typically use 35 – 50% fuel and an APC 5.5x 2.5 prop, which turns the engine at around 22K.
    My current model has one washer of downthrust (under the top tank mount holes), and it has one washer of right thrust. The model has a tendency to slide to the left under power toward the end of the engine run, and I am gradually shimming the stab t.e. up to get it to groove to the right. A bit of right tab (t.e. stock) is now on the fin.
    It is very nearly ready to compete in the one design event. It is also a pretty good airplane to use in Classic Gas.
    These are forgiving models and easy to build and fly.
    Good luck with yours,
    Bob Stalick

    #42948
    Dean McGinnes
    Participant

    Bob,

    Do you have any warps in the wing? I am referring to the washout in the tips and washin in the right main panel.

    If so, how much?





    #42949
    BOB STALICK
    Participant

    Hi, Dean:
    Yep, I build in 3/32″ washin in the right main panel and 3/32″ washout on each tip. When covering, this washin/washout usually increases a bit, but it’;s not a serious problem.
    One other item: I don’t use the pointed airfoii leading edges. I round them off and upsweep the l.e. a bit from the bottom. It’s more like a Clark Y type than a Lindy type.
    Good luck with this model. They are fun to fly.
    Bob Stalick

    #42950
    DAN BERRY
    Participant

    Hey Bob,
    Regarding the Mini-Pearl sliding to the left at the end of the motor run— You are describing a classic symptom of a wingtip washing out as a result of lack of torsional rigidity. Adding diagonals would help alleviate the problem, but would disallow the M.P in the OneDesign event. Extra Jap tissue on the mains might help,. The washing-out effect at speed isn’t really indicative of a problem with the tips. They are the end of a lever which includes the mains. Stiffening the mains would most likely stop the plane going off-pattern. A different prop might also help. Get up to a given speed and stay there. If you are still accelerating at the end of the run, the rigidity issue will affect the plane. You didn’t mention the elapsed time for the problem to show. Maybe just shorten the run a bit. 9 secs for 2 minutes seems a bit generous.

    #42951
    BOB STALICK
    Participant

    The issue is not wing washout, as best I can tell, but a lack of incidence. My models usually slide to the left at the end of the power run regardless of which model it is. I cure it by adding incidence under the stab t.e. until it continues the right turn all the way to the end of the run. If it doesn’t transition correctly, then I add a bit of right tab. The end of the climb trim and transition is a combination of incidence and tab.
    Thanks for the thoughts, though.
    Bob Stalick

    #42952
    DAN BERRY
    Participant

    Indeed, they are just thoughts. Trimming is always an adventure. I guess that’s why so many fellows go to the Dark Side with auto-surfaces.

    #42953
    JLorbiecki
    Participant

    Bunt? Did I hear bunt????Or, if you could put a little less structure in the wing it could be a folder…..



    #42954
    BOB STALICK
    Participant

    Hi, Dean:
    Re: The Mini Pearl. I have built a few of them over the past few years, and here are some suggestions for you:’
    1. the fuselage can be made stiffer by adding 1/16″ x 3/38″ diagonal strips running inside the fuselage between the upright formers. I usually set them at about 4.0 inch intervals. This will keep the fuselage from bending.
    2. I use a rear mounted fuse on my models because the pressure of a rubber band will bend the fuselage and give unwanted left rudder effect. The fuse is located in an angled snuffer tube which exits the fuselage immediately behind the stab hook, which is angled out from the center of the stab t.e.
    3. I outline the 3/32″ fin and subrudder with 1/64″ x 3/32″ basswood to keep it from warping and to protect it from dings.
    4. I change the spar structure in the wing by using thinner wood; eg: where it calls for 1/8″ thick spar stock, I use 3/32″ etc. There is plenty of spar mass in this wing. The stab uses the stock spar setup.
    4. I change the leading edge of the wing and stab by using thicker wood, and changing the pointed style airfoils to a more typical rounded (Clark Y) style. The wing airfoil is a bit thicker than needed, and I thin it out a bit…down to 10% or so. The shape remains the same, and the high point is in the same location, it’s just thinner by about 1/16″
    5. These changes will improve the glide a bit. These models are fast in the climb and in the glide. Anything to slow down the glide is an improvement. You should try for a model weight R-T-F of 6.5 ounces or less.
    6. Use a hot T.D. 049 or .051 engine and an APC 5.5×2.5 prop. I use suction feed on mine (with a Cox red tank), but I have also used pressure bladders in the past. The suction is just easier to handle, and the performance seems about the same.
    Good luck.
    If you have trouble understanding anything I have written here, drop me an email.
    Bob Stalick



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