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Author Topic: DFH-21 Pattern Airplane- build  (Read 479 times)
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Sundance12
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MAAC #25680, VE4BDF (amateur radio callsign)

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« on: March 02, 2019, 10:11:36 AM »

Type Model RC Sport Pattern.
Wingspan 42in.Lowwing.
Engine Glow .15
Control 4 channels.
Designer Bengt Lundstrom.
Magazine MB January 1979

I have been steadily working on a sport pattern dessign from 1979, called DFH-21, published in Model Builder, by Bengt Lundstrom. I wanted to test out a wing jig building system that I have been developing. Pictured below is this design. My modifications are few, mostly a. 28 engine instead of the original  . 15 so that is significant enough for now. There are other construction features that intrigued me as well.
Build wil now follow.

Sundance12

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« Last Edit: March 02, 2019, 11:13:32 AM by Sundance12 » Logged

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« Reply #1 on: March 09, 2019, 02:09:21 PM »

Wow, this does look rather modern for its time 1979. I was flying a Don Dewey, New Area III  at the time. This looks much like the more modern ships like the Dick Sarpolus, Hammer. I notice it has top driven wing surfaces and flaps. So is what is shown a 5 channel model? Do the plans show micro retracts?

I flew the New Area on a Cox/K&B Conquest 15. I think I propped the engine for 22K rpm on a reworked Top Flight 7x5 prop.
One needed to use the one piece "racing" connecting rod. The knock on the Conquest was that the engine produce power up to 24K rpm but can apart at 22K rpm. I was using the straight pipe extractor. The Cox Muffler cost over 2K rpm.

 While I subscribe to the idea that little replaces displacement other than cubic dollars. I'm curious as to how you plan to set up the Mecoa Sport 28.
« Last Edit: March 09, 2019, 02:19:27 PM by Konrad » Logged

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Sundance12
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« Reply #2 on: March 09, 2019, 05:45:16 PM »

This airplane is number 21 in a line of designs by the designer and is a small replica of the 10cc airplanes that he was working in competition. The airplane used aileron throws that were progressive, more at the tip. I dwcided not to use this feature on such a small plane. I will be detailing the build and how I adjusted the airplane for the  .28 cid and the presentation of a wing construction method that I have usex for many years, mostly in CL  stunt.
I will run the  .28 stock for the time being. Construction weight is now close to what was published, I dont expect to be going beyond 1450 grams.

Thanks for the help Konrad, I am learning. This engine has traditional numbers. I will not be modify it.

Sundance12
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« Reply #3 on: March 09, 2019, 06:06:55 PM »

MECOA .28
Exhaust
open at 115 deg
Close at 257 deg
Total 147
ball bearings.
ABC i think
Weight I have to measure yet.
could not find published rpms.
I think it is K&B family  heritage but seems more like OS FSR than K&B
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« Reply #4 on: March 09, 2019, 06:47:45 PM »

I haven’t held a Mecoa 28ABC BB in my hand, but it doesn’t look like a K&B nor does it look like an OS based engine. It does show some lineage to Kazuhiro Mihara (THE noted OS engine designer).  The OS 4cc to 5cc (.25- .32 cid) FSRs had a skewed cylinder so that the gudgeon pin would not be exposed to any ports.  When Mr. Mihara left OS, out of disgust for their engineering/manufacturing direction, for ThunderTiger he moved his new designed cylinder back to its normal side exhaust orientation.

On conventional piston and sleeve timed 2 cycle designs the opening and closing point for the ports are the same distance from TDC. An exhaust of less than 150° results in a very mild (flat) power band suitable for most mufflers. This should allow you to run larger props into the 8” range. Maybe even some flat pitched 9" props.

Back to your DFH-21 I like that top fairing. Is that shaped out of wood? I normally made mine out of a very dry epoxy micro-ballon mixture. Always admired the guys that could make a nice wood fillet!

All the best,
Konrad
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« Reply #5 on: March 09, 2019, 10:00:35 PM »

Thanks for the info and history.
The fairings are wood. The whole thing is wood. Top deck is planked.
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« Reply #6 on: March 09, 2019, 10:31:01 PM »

Nice nice nice! I really like these old designs that actually bring the fuselage sides to the spinner ring. The use of an ABS or fiberglass cowl never blends in well. There is always that part line!

In my gliders I rig my flaps as ailerons to have about 1/3 the movement of my ailerons. This is done to improve roll rate and minimize adverse yaw. Is this what the designer was thinking?

All the best,
Konrad
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« Reply #7 on: March 11, 2019, 03:19:35 PM »

I have always liked cowled engines, the CL builders have this technique wired.

This designer was actually warping the aileron for more deflection at the tips for various roll control at slow speeds. I feel this is not necessary, certainly it is never done in full size airplanes of my day.

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« Reply #8 on: March 13, 2019, 12:32:45 PM »

I started this build because I wanted to trial a set of wing building jigs in this session as well as build a pattern plane to introduce me to some building techniques found in the pattern planes of the period. Late 1970's and early 1980's was a period for pattern plane development that presented fast airplanes and smooth maneuvers. This design is actually number 21 in the designers stable, after hosting larger 10 cc (.60) sized in the years previous to this particular model. DFH-18 and the DFH-20 are large, and fast pattern designs that Bengt flew in a number of FAI championships. In this design, it's smaller size and portability was the showcase in the article. It utilized the hot 2.5 cc engines of the time. COX .15 Conquest was such an engine with high rpms and small props. 23-24, thousand RPM on a 8 inch prop. I was not able to obtain such an engine at this time so I altered the engine choice and went to a MECOA .28 operating a 9 inch prop. I have not benched this engine so I am not sure it's peak performance. It is new and should serve the purpose as I am not a 120 mph flier. However, I think that the mover to slower pattern criteria that later developed is where this airplane will be. I will have my hands full regardless. The construction methods in this design are all similar to the sister ships and now that I have done one, I can look forward to the larger designs in this series with no problems. The features in construction that grabbed my interest was the way the wing is built sparless. The sheeting in this wing is part of the construction attributes. Fuselage shapes in this series of airplanes is oval, and I chose to plank in some areas of the top and bottom decking. I enjoy wood construction and find that if one plans properly, that balsa can be arranged into any shapes that one desires. This model so far is really light, and the one presented in the article was set for a weight of 1360 grams or so and will be very close to that weight for sure.

Wing construction is set up on a flat bench where my jigs are screwed to the bench on top of the plan. Jigs are set on the outward edge of the leading and trailing edges and the alignment blocks inside of the leading and trailing edges. Leading and trailing edges are prepared to get inserted into the jig blocks and leveled with a sight level of some sort. I use a phone app for that which worked fine. The leading edge needed some small mods on the front side to align the leading edge on the edge so I made stand offs that do this well. They will be removed later in the shaping process.
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« Reply #9 on: March 13, 2019, 01:50:33 PM »

This stage is important, to get the leading and trailing edges level with each other and level with a reference. That reference is actually the concrete floor. My level App is able to zero out so I used the concrete basement floor as a zero reference and then the leading and trailing edges are level to that reference. The workbench is a support structure. I was able to measure the spars between themselves and also to the level reference. This takes a bit of extra double checking and also ensures that the wing is really level in the jig. I found that the level App was more than close, with in .003 of a degree. At first I was concerned that even that was not enough, but the concern was unfounded. Now the next stage can begin.
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« Reply #10 on: March 13, 2019, 01:58:12 PM »

Wing rib construction process.
This technique is traditional, and know by many as the stacked rib method. I use this method alot, it's fun, I like to carve and sand. It's also quick for me. Some additional time spent on templates made out of 1/64th or 1/32 ply with some holes for bolts are the extra work. These templates become tools, perhaps for another project. I had to make both a left wing and right wing set. I was pleased with the results. Ribs were deliberately made long and needed final sanding and trim to length during the installation of each rib using the cut and try technique.
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« Reply #11 on: March 13, 2019, 02:35:17 PM »

Installation of the wing ribs was a smooth process, one rib at a time, test fit, trimmed and placed then removed, glue added and replaced, all one rib at a time. This jig is quite rigid, and supports the wing well during the rib placing process. I needed to revisit a bit of level from time to time but found things stayed in alignment fine.
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« Reply #12 on: March 13, 2019, 08:56:10 PM »

Interesting approach SD. Levelling from a reference probably has more in common with full size aircraft construction rather than models but the advantage would be that the building board has little effect on the accuracy - as long as it is rigid.
I'm still curious as to how you actually approached the levelling. Did you use a laser type device(or similar) to set the LE and TE parallel to the floor?
It will make an interesting sports type model SD.

John

















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« Reply #13 on: March 14, 2019, 10:32:44 AM »

I set the LE and TE parallel to the bench, and this was measured in the center of the wingspan. Then I used a level APP on my Android phone for level referenced to the floor as described before. In previous wing builds years ago I used a small sight level, so keeping with the times and being a slave to my gadget phone, I found an app that worked. When a wing is in the jigs, it becomes more rigid as parts build in. I check for level from time to time as I find that I  take out the wing build and flip it over at various times. I was satisfied at the straightness of this wing. I am sure I could have got it deadder,...

I will be building another wing this way but for a different plane in the near future. In the meantime the build presentation continues, for the sake of the design.

SD12
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« Reply #14 on: March 14, 2019, 02:17:06 PM »

So I did a bit of research on the MECOA .28 engine. First image is the MECOA.  Second image is a OS .28F and I am not wondering if the MECOA is not a clone of this OS engine or not? They seem so similar it's uncanny. Anyway, I was looking for similar engine reports from the past to get a impression of some performance criteria. Any further info as to the heritage of the MECOA .28 would be very interesting, I know others have commented and I appreciate all points of view. In the meantime I reference this article for the time being.
http://sceptreflight.com/Model%20Engine%20Tests/OS%20Max%2025F%20and%2028F%20Heli.html





Ratz Edit: Attachment Adjustment
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« Reply #15 on: March 14, 2019, 03:18:53 PM »

Well they both have that skewed (rotated cylinder)! And if the Mecoa actually has a true ABC P&L set she should potentially be a better engine than the OS FSR with her ABCtype (ABN actually)P&L.

Don't see a Mecoa .28 ABC BB listed.
http://www.mecoa.com/mecoa/32/3201.htm

All the best,
Konrad
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« Reply #16 on: March 15, 2019, 11:29:07 AM »

The MECOA .28 ABC is not available any longer but it will be similar to the MECOA .32 now.
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« Reply #17 on: March 15, 2019, 03:25:30 PM »

Wing construction with the use of jig-its continues, and the ribs all are inserted. Various camera angles for the wing at this stage. Wing is lifted from the jigs and flipped over and re-inserted in the jigs. Next stage is the sheeting stage.
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« Reply #18 on: March 15, 2019, 03:50:21 PM »

While in the jigs, the wing is sheeted first on the bottom then the top. Sheeting was done in 8 sections, 4 bottom and 4 top. I used white glue, weights and pins and glue was applied to every rib to attach to the sheeting. Sheeting cut to shape with a template from the plan and then trimmed small bits at a time to fit. The wing becomes very rigid in the jig at this stage and I was quite confident in its shape.
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« Reply #19 on: March 15, 2019, 04:06:48 PM »

While in the jig, the center structure for the tank bay cutout, the servo tray supports for the ailerons was installed as per plan. Mostly measure from the plan, cut and try method. last image is the wing with the ribs, bottom sheeting and center section installed.
For a while I was concerned with the fact that this designer did not use any spars. I considered putting them in but the larger designs like this did not use spars either and there was mention in one article that this wing gets it's strength from the fact that the sheeting is glued to all ribs. So I made sure that all the ribs were glued as described. This designer has used this spar-less wing in many previous designs with no wing failures of any kind. Granted this construction feature was of interest to me as nearly all the sheeting and all the ribs are made from 1/16th sheet all over. This egg shell construction was quite unique and I will use it again in some other builds in the future.

In the next photo series I detail some aileron linkage in the traditional tab and torque tube method. I use tin and made meter hinge tabs that contain the wire that gets bent to shape to form a torque rod extension to the aileron. These are fastened to the trailing edge by these tabs. Both left and right sides, with the threaded sections in the center for aileron push-rods connections. Once they were installed, the top sheeting resumed.
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