Mefisto build (BLEJZYK)

[Konrad]

At the Aloft Hobbies sponsored Camp & Fly at Sunset Beach get together I was impressed with the performance of the Blejzyk models. In the light to mild wind conditions that we had, they gave up nothing to the high dollar composite molded ships.

It was painfully obvious that I did not have many ships suitable for the great flying conditions. So looking at the Blejyk models Aloft hobbies had in stock, I decided that the Mefisto was probably the best light wind ship that would fit my flying style.
https://alofthobbies.com/mefisto-1-9-meter.html

Looking inside the box I was impressed at how light the epoxy/glass fuselage was. I was equally impressed at the technology shown in the construction of the wings. I haven't seen such nice molded wood wing tips since my Simprop Sagitta from the 90's. Also the wing surfaces are "live" hinged. And add to that, the leading edge of the control surfaces fit into wing pockets when deflected downwards! One normally doesn't see this feature until one moves up to composite molded wings.

I'll be setting up my Mefisto to run on a 6v system. I'm thinking that I'll be using 6 of these servos (PDI 0902MG). Not that they need to be this thin, I think the wing can take 10mm thick servos.
https://alofthobbies.com/6-v-1300-mah-2-3a-pack-flat.html
https://alofthobbies.com/jx-pdi-0902mg-servo-19kg-264-oz-in-10sec-94-grams.html

Now on the Aloft Hobbies sales page there was mention of ballast tubes. What are you guys using for a ballast system on these small ships?

Also I have some questions about the 110° V tail. When I see a 110° V tail I think the designer may have taken a short cut and based this angle solely on the fact that Beech used it on the Bonanza. My understanding of the V angle, is that it is based heavily on the wing's aspect ratio and wing tip design. I would think with the long wing and short fuselage that an angle closer to 95° to 90° would be more appropriate. To be clear I did not see any hunting in yaw, of the models I saw flying at Sunset. Anyone have any experience bringing up the V tail angle closer to 90°?

Now I like to fly with the CofG further aft than most. This takes some of the load off the "stabilizer".

All the best,
Konrad

P.S.
Some of my set up values as of May 2019:
V Tail set at 99°
Empty weight: 735 grams (fully ballasted 1100 grams)
CofG: 77mm to 78mm (Most folks don't like it this far aft.)
Elevator: Up 11mm, Down 14mm ( very hard to snap empty, but will snap with full ballast)
Ail to flap: Up flap 10mm, Down flap 8mm (would like 10mm both way)
Ailerons: Up 12mm, Down 10mm
Crow: Ailerons Up 11mm, flaps down 17mm ( ailerons go down 2mm for roll)

Take a look at post#55 for notes and warnings.

 

[Wayne]

I like a broad V layout and have not been very impressed with v-tails that are at 90 degrees. Honestly, they just look wrong to my eye. I prefer a cross tail to a V, but that is just me. (They are available for this plane, and many others in the line up.) I would stick with the v-tail as designed in the plans, they work great and the simple linkage works great with just simple Zbends.

The Mefisto can be built up very light. Mine is a feather even though I added things like a ballast tube (not needed IMHO). I think I have flown it only once with ballast. In all honesty, I like this plane light, and when the winds pickup I rather toss out a quicker/heavier ship and keep my Mefisto nice and safe. She is a great flying plane.

If I was to build mine again, I would probably glass the wing center and not bother with the wing joiner setup. I can fit this plane inside my little hatchback without taking the wings off.

-Wayne

 

[Konrad]

Agree, the cruciform tail appears to perform better for me across a much wider set of flying conditions. That is why I'm always looking for "improvements" in the V tail, like wider chords, different angles and surface area.

With the short fuse, the Mefisto might make a fine one piece plane. I think I will make the wing one piece. I'll use the weight credit from the removal of the joiner to make a 4 layer laminated glass center section.

I'll have to give your advise some thought. I might leave her a light ship, and set up the "First Plus" 2 meter as my general all-rounder.
https://alofthobbies.com/first-2-meter.html

 

[Motowncali]

Keep the Vtail at 110. 90 degrees would make the rudder more effective, but you loose effectiveness on the elevator. At 110 you actually need a little differential throw in the rudder to keep the plane from pitching. Been a while since I messed with fine tuning a v tail for control.

 

[Konrad]

Thank you.

That is what I'm expecting, or looking for. I often see the V tail dutch rolling far too easily. I'm thinking of trading stabilizer power (elevator authority) for more rudder (vertical fin) authority. I find that with an aft CofG I don't need as much stabilizer as when I have the CofG forward.

What I don't want to get is having too much vertical fin and developing spiral instability issues.

I too have noticed that adding a bit (3% to 5%) of "down" elevator to the rudder input helps. I assume this is what you mean by differential throw in the rudder. My understanding of this is that the adverse roll from the rudder inputs slows down the ship. Just what I don't want in a tight slow thermal turn. So to help keep out of the stall (tail drops) I have set up my rudders for down differential ( the downward flipper moves more than the upward flipper).

 

[Motowncali]

You have it exactly Konrad. I would actually suggest 100 degrees as closer to optimal.

 

[Konrad]

Running some numbers it looks like 95° might be closer to what I want. Now this was done with a lot of assumptions and gross generalization. At Sunset Beach I never saw any flight issues with these set at 110°. So it might be prudent for me to back off a bit and go with 100°. But as an engineer I really want to let the numbers dictate the design!

 

[Konrad]

I have to ask if there is a convention as to how to interpret the dihedral call outs? Per the manual; "

I like a broad V layout and have not been very impressed with v-tails that are at 90 degrees. Honestly, they just look wrong to my eye. I prefer a cross tail to a V, but that is just me. (They are available for this plane, and many others in the line up.) I would stick with the v-tail as designed in the plans, they work great and the simple linkage works great with just simple Zbends.

The Mefisto can be built up very light. Mine is a feather even though I added things like a ballast tube (not needed IMHO). I think I have flown it only once with ballast. In all honesty, I like this plane light, and when the winds pickup I rather toss out a quicker/heavier ship and keep my Mefisto nice and safe. She is a great flying plane.

If I was to build mine again, I would probably glass the wing center and not bother with the wing joiner setup. I can fit this plane inside my little hatchback without taking the wings off.

-Wayne

Using masking tape hold the two wing halves together (with 50mm dihedral), making sure that both halves match each other". I read this to mean raise one wing tip 50mm, with the other wing down on the datum, and don't twist the wings.

But the sketch shows the dihedral as 30mm per side. I assume that as a result of flight testing the dihedral was dropped to 25mm per side (25mm x 2 = 50mm per the manual).

Then again the manual might be dated as I find this; "Ailerons can be attached with a clear tape. Start at the top surface and then turn the aileron flat on the top of the wing. Tape the bottom surface". The current production run of these kits have a live hinge. I see no need to add tape.

Back to the dihedral, is the spec 50mm or 60mm? I'm leaning towards 50mm as slope ships are usually flown crabbing against the wind

 

[Motowncali]

More dihedral will make the rudder more effective for turning a glider. If you are going to be thermaling then shoot for a little more. If slope flying, it doesn’t need any. Set where it makes you happy.

 

[Konrad]

Yep, that is a concern with these hybrid models. There are more differences between a thermal or slope ship than just the airfoil.
https://alofthobbies.com/mefisto-1-9-meter-thermal.html
https://alofthobbies.com/mefisto-1-9-meter.html

I really am not concerned with rudder authority (turning power). What I want is stability, actually closer to neutral stability (go where you point her). But I don't want dutch roll or spiral instability.

Looking at the 1.5m and 2m slope ships that I like, they have rather flat, along the top, dihedrals (dihedral come from the airfoil thickness tapper). Now the up turned tips add effective dihedral.

Thinking I'll deviate from the plans and drop the dihedral to 15mm. This and the added vertical area with the 100° to 95° tail should help the ship cut a nicer line on the slopes.

 

[Motowncali]

Add a foam sketch to the bottom of the fuse...

 

[Konrad]

??? You mean make the tail a Y-tail?

 

[Motowncali]

Yeah. Just a couple inches will make a huge difference. I added the lower vertical stab to my cub on floats. I was amazed at how much difference it made.

 

[Konrad]

That then has its own issues with landing. If changing the tail configuration I'd go with a cruciform tail. Adding area to the vertical fin is what I'm doing by going to a narrower V angle.

Floats add a lot of area ahead of the center of gravity. Hince the need for more vertical surface area, with its subsiquent "wetted area" surface drag.

 

[Motowncali]

I’m saying to fly it with your new tail angles. If you notice the tail wagging and want a little more vertical surface to add a piece of foam to see if you like it. If the foam breaks off, no big deal.

 

[Konrad]

That's a good idea, if need be.

Much like the adding of a dorsal fin to many of the fighter of the 30's and 40's when the planes were found not to be good gun platforms.

 

[Wayne]

As I recall a used less dihedral than the plans called for and I am very happy with the plane. I will have to see if I can remember to measure mine when I get home.

 

[Konrad]

After waiting for 9 months for an dihedral angle reading (just kidding). Actually I've been thinking of how to ballast the Mefisto. I resently found a thread by Wayne of how to make custom ballast tubes. So that jump started this build. Also I got, imported at great expense from Germany, some 15mm copper coupling. (thanks RalfH).

What I'd like to show is that there is little space for the shot gun style of ballast loading window. This forced me to go with the skewed side by side mounting of the servos. Actually I like the skewed mounting of the servo as it allows for push rods that are close to the same length. This means that the left and right V tail flippers will have close to the same push rod flex. It also moves the servo arm axis a little further away from the side than a normal side by side set up usually allows.

As most Blejzyk kits use a glued on nose this means it is easy to add some reinforcement. I've found that with my style of landings I like to have more fiber across the fuselage part line (seam) than what most modern lay up afford us. So I added some carbon fiber to both ends of the nose insert.

I also know that I'd rather lawn dart the landing rather than stall and cart wheel. So I added some carbon roving along the sides ( this is not needed, but). Now I went overboard and added this roving at the same time as I glued on the nose insert. This wet lay up was done to bring this added carbon all the was back to over lap the factory carbon stiffeners. (I'm here to say it was a pain to do and surely not worth the effort).

After fitting the push rods my receiver will rest on top of the ballast tube.

Some links that might be helpful.
https://forum.alofthobbies.com/inde...s-how-to-make-a-fast-easy-and-good-setup.566/
Sorry about this. (I hope the moderator doesn't delete this link).
https://www.rcgroups.com/forums/showpost.php?p=22450863&postcount=3


BTW: My dihedral will be flat along the top of the wing. Actually it is now at 6mm in the center, or 12mm at one tip.

All the best,
Konrad

 

[thenated0g]

thanks for the high quality pictures. will reference when i get around to doing my blejzyk build.

 

[Konrad]

Thanks, but these are just quick and dirty cell phone photos. I do have a set of diopter for my DSLR if you really want some detailed photos of something. But I've found that on the forums the cell phone with a steady hand is often more than enough to get the idea across.

Well, to show you all that this modeling god has feet of clay, I had a failure with my epoxy. I normally don't like hobby grade epoxy as they are often too easy to mix. (read I get sloppy). The other night I had mixed up some 3 hour epoxy to glue on the nose. Well it failed to set up properly. This forced me to try to do a post cure heat treatment. This is tricky as the previously cured epoxy (that is the nice epoxy work from Blejzyk might start to flow [warp]).

I placed the fuselage in an oven at 250°. * The problem was that my oven isn't large enough to fit the fuselage. So I was forced to leave the door open. Luckily the night was cold and I was able to tell the wife this was our new "central heating system"(I don't think she's buying that!!!) 8 hours in the oven and the epoxy has set. The take away is don't get lazy with measuring and mixing your epoxy. Cleaning up half set epoxy is a real chore!

Something I need to mention is that with slip on nose cones one needs to maintain the pilot key diameter for 360°. That is leave 4mm to 6mm of a ledge all the way around, to hold the nose cone in place .

* Make sure you have adequate ventilation when heating epoxy. I live in an 1880's Victorian that has more air exchange than the Cave Of The Winds

All the best,
Konrad