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Nov 25, 2013, 09:47 AM
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Build Log

BBCC 3 - Bristol Type 138 - Old Pilot


Guys.....

I've screwed up......again

I drew the He 71a, ran the aeronautical calc's and discovered that:
1. It had a higher wing loading than the Mk-1 based on similar AUW. (less wetted surface wing area)
2. Required a higher airspeed to maintain flight. (NOT what I was trying to do)
3. Would be a pain in the butt to build (tapered/non-parallel spars, semi-elliptical wing.
4. Tail feathers would have to be increased in area to satisfy tail volume calc's so much, that it would no longer be even close to a stand-a-bloody-way-off scale aircraft.
5. It looked way to much like the Mk-1 in profile (I overlaid the 3-views).....might as well build the Mk-1 again.......NOT.

A little more research and reference led me to a Bristol Type 138 High Altitude Research designed in 1936...only one was built....surprise, surprise !
Here's a pic and some small scale plans I found on the Vintage site.......Scale Rubber powered.

I plan to build at 1/12 scale (66 Ft WS to 66 In WS and so on).

If it's not to much trouble, I'd like to change my second entry from the HE 71 A to the Bristol.

I'll discontinue posting to the Heinkel 71A thread and start another one for the 138...that and I'll post the finished drawings to OuterZone when I'm done

From Wiki:

Design and development:

The Type 138 was born of a period of intense competition between aviation manufacturers in the 1920s and 1930s. There was a great deal of prestige, as well as technological progress, to be gained from breaking any of the major aviation records: airspeed, distance and altitude. By the 1930s absolute speed and distance records were beyond the resources of individual companies, and required the involvement of national governments.

Bristol were well placed to compete in this field but in the event they found themselves lagging. Between 1929 and 1934, there were a number of altitude records established by rival machines including a Junkers W.34, a Vickers Vespa and a Caproni Ca.113 biplane, as well as the first flight over Everest by a pair of Westland Wallaces in 1933; all these aircraft used Bristol or Bristol-designed engines.

Sensing Air Ministry interest resulting from the success of the Everest flight, Barnwell proposed a purpose-built high-altitude research aircraft in November 1933. This, the Type 138, was a large single-engine, single-seat monoplane with retractable undercarriage and a supercharged Pegasus engine. Nothing came of this until Renato Donati set a new record in April 1934; public opinion demanded a new, government-sponsored record attempt.[1] In June, the Air Ministry issued Specification 2/34 for two prototypes capable of reaching 50,000 ft (15,030 m). Barnwell revised the Type 138 to produce the Type 138A. This was of the original size and configuration but had a special, two-stage supercharged, Pegasus and, although remaining basically a single-seater, had provision for an observer's cockpit to be fitted if required. Weight-saving was a priority; the airframe other than the steel tube engine mount was a wooden monocoque and the undercarriage was replaced with a lightweight, fixed assembly.

The supercharger installation, with detail work by Clifford Tinson, was a two-stage system; the first-stage compressor was permanently engaged but the second-stage was engaged by the pilot at the correct height. The system employed an intercooler between first and second stages.

Considerable research was carried out by the Royal Aircraft Establishment and National Physical Laboratory both to establish the most efficient design for the aircraft and to develop a reliable pressure suit for the pilot. Sir Robert Davis of Siebe Gorman and Professor J.S. Haldane were instrumental in developing the helmet.

The airframe was complete in early 1936 and the Type 138A was flown for the first time on 11 May with a standard Pegasus IV driving a three-bladed propeller, piloted by Cyril Uwins, who had flown the Vickers Vespa on its world record flight. Two more flights were made at Filton before the aircraft was delivered to Farnborough, where tests of the pressure helmet were made before the aircraft was returned to Filton for the special Pegasus and four-blade airscrew to be fitted, being returned to Farnborough on 5 September.

Operational history

Bristol 138 in flight:

Squadron Leader F.R.D. Swain was selected to pilot the high-altitude flights. On 28 September 1936, Swain took off from Farnborough and climbed to an indicated 51,000 ft (15,440 m), engaging the auxiliary supercharger at 35,000 ft ( m). Swain ran low on oxygen on the two-hour flight and had to break the window of his pressure helmet once he had descended to a safe height. The data from this flight was homologated by the Fédération Aéronautique Internationale as a world record of 49,967 ft (15,230 m).

After this flight, further development of the aircraft resulted in a number of small modifications. In this mildly revised form, the 138A made six further flights to around 50,000 ft (15,240 m). During this period, Italy increased the record to 51,364 ft (15,656 m). In response, on 30 June 1937, Flight Lieutenant M.J. Adam undertook a 2¼-hour flight in which he achieved a record altitude, later homologated at 53,937 ft (16,440 m). Despite the cockpit canopy cracking, Adam was saved from injury by his pressure suit. After this, research flights continued but no further records were attempted.

A second machine was ordered as the Type 138B in 1935. This was to be a two-seater powered by a Rolls-Royce Kestrel S engine fitted with a similar two-stage supercharger installation and generating 500 hp (370 kW). The airframe was delivered to Farnborough Airfield in 1937 for completion but the engine installation was never completed.

Variants
Type 138 Not built. Type 138A One built Type 138B One built to use a Rolls-Royce Merlin engine, never flown and became a ground instructional aircraft

Operators

United Kingdom - Royal Air Force

Specifications (138A)

Data from Bristol Aircraft since 1910

General characteristics
Crew: 1
Length: 44 ft 0 in (13.41 m)
Wingspan: 66 ft 0 in (20.12 m)
Height: 10 ft 3 in (3.13 m)
Wing area: 568 ft² (52.8 m²)
Empty weight: 4,391 lb (1,996 kg)
Loaded weight: 5,310 lb (2,414 kg)
Powerplant: 1 × Bristol Pegasus P.E.6S radial, 500 hp (373 kW)

Performance
Maximum speed: 107 kn (123 mph, 198 km/h)
Service ceiling: 54,000 ft (16,500 m)
Rate of climb: 1,430 ft/min (7.27 m/s) at 40,000 ft
Wing loading: 9.35 lb/ft² (45.7 kg/m²)
Power/mass: 0.0942 hp/lb (0.155 kW/kg)

Endurance: 2¼ hours
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Nov 25, 2013, 04:43 PM
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Kiwi's Avatar
Here's another set of plans for you
Nov 25, 2013, 05:38 PM
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Kiwi's Avatar
and some drawings and info
Nov 25, 2013, 06:59 PM
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Charlie,
What is it with you and low wing types mate? You want slow and steady at close to 60" I have just the thing - even though you can't use if for this comp.

Pete
Nov 25, 2013, 08:58 PM
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Old_Pilot's Avatar
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Bristo 138A Info


Quote:
Originally Posted by Kiwi
and some drawings and info
Kiwi,

thank you so much......any and everything will help.....I'm working up the 66" version now. will post when I'm finished.....probably will be next week sometime...Thanksgiving Holiday festivities for the rest of the week.
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Nov 25, 2013, 09:03 PM
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Old_Pilot's Avatar
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Quote:
Originally Posted by PETERRAKE
Charlie,
What is it with you and low wing types mate? You want slow and steady at close to 60" I have just the thing - even though you can't use if for this comp.

Pete
Probably stems from my days learning to fly......my first lesson was in a Cessna 150, and it took two phone books and a whoopee cushion with the seat all the way up just to see over the frickin' instrument panel....from then on I flew a Beech Musketeer and a Mooney 201....the only high wing I liked was a clipped wing Cub used to teach aerobatics....Flew a Texan a couple of times......
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Nov 26, 2013, 06:35 AM
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Old_Pilot's Avatar
Thread OP
Quote:
Originally Posted by PETERRAKE
Charlie,
What is it with you and low wing types mate? You want slow and steady at close to 60" I have just the thing - even though you can't use if for this comp.

Pete
Actually, this may have something to do with it............
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Nov 26, 2013, 10:41 AM
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Bristol 138A 3-View


This is a PDF of the design 3-View....working on the details now
Had to make the stab a little bigger , but not much !
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Nov 26, 2013, 03:31 PM
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66" Bristol Type 138A


OK........Here's what I'm going to build.

I've worked out almost all of the details, at least enough of them to buy wood. The rest of them I'll tackle as they come up. I'm not going to spend a lot of time annotating right now, unless someone wants construction quality drawings. I will however produce a set of as-built's to send to SteveWMD at Outerzone......but that will probably be after Christmas.

Formers are all 1/8" balsa. Firewall is 1/8" ply. Wing ribs in the straight section are 1/8", doubled at the joints. Wing ribs in the tapered sections are 1/16". Main spar is straight, I-beam, 1/8x1/2 T/B and web. Wing rods are 1/8" MW. Dihedral braces are maple. Tail feathers are 3/16 built-up and or plank. Stringers are 1/8" sq. Stringer caps are 1/32 bass. Fuselage sheeting over the wing area is 1/32 bass with 1/8 balsa or 1/16 ply doublers. Wing shoulders are built-up balsa. High stress area are plywood as required. Servo/battery/ESC tray is 1/16 ply. Cowl is built up balsa. covering will be MonoKote ('cause I've got lots of it)...OD Green/Tan/Black with RAF roundels. Landing gear undercarriage is undecided... but will probably make one attempt at wire...then give up and make a formed aluminum U/C like the one on the Schtick. We'll see. Same idea with the canopy....

I was looking at the profile of the fuse and it bears a striking resemblance to a Hawker Tempest Mk-V......C'est la vie.

I'll be back next week.......Need to help with preparations for the Thanksgiving feast.. think 16 are coming.

Happy Thanksgiving to all of you....whether you celebrate it or not

O_P
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Nov 29, 2013, 06:08 AM
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My board is lonely and sad


'Nuff said
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Nov 30, 2013, 06:26 AM
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Spar Material


I cut the spar material yesterday.... straight grain Sugar Maple. I know it's heavier than Spruce, but I'd have to use almost twice the material to get the same
load bearing capacities on such a long cantilever.....and I've got quite a bit of Maple. Not going to bore you with the beam stress/deflection calc's. Suffice it to say that the main spar was designed to carry all the wing loading plus a 20% safety factor.

Main spar 7/64 x 5/16 top and bottom.....3/4 x 7/64 web

I'll cut the 3/4 x 3/64 in half (3/8 x 3/64) and put them top and bottom at the wing third points...

Rough cut them on the saws using my new jigs, and planed/squared them using a router.

Spar will taper

TE will be balsa. Haven't decided what the LE will be.
Last edited by Old_Pilot; Nov 30, 2013 at 06:44 AM.
Nov 30, 2013, 08:50 AM
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davidterrell80's Avatar
It wasn't clear from the plan... are you planing a one piece main spar, tip to tip?
I'm interested in points of view on joining wing sections.
I've only had two in-flight failures.

One, was due to choosing spar flange material in a "C" section that was too soft and having the upper flange fail in high-G compression, in the space between root and first outboard rib.

Second was a failure at a joint similar to the one you have, between the rectangular center section and the trapezoidal outer section. I had built completely separate sections, and not built a solid spar. The failure was of the white glue joint.
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Nov 30, 2013, 10:26 AM
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Wings so far


Well, here's what I'm thinking right now....

I'll build the wing in three sections; the rectangular and the two trapezoidal sections. My building board is only 24"x48". However, it will be a one piece wing when completed . I think Mr. Rake is right.

The top and bottom flanges of the spars are continuous across the wing sections, the ribs being notched to fit. The web will be cut to fit between each rib except at the dihedral break. That brace will be full height/width from flange to flange, with shear webs on both sides of the flanges, forming a pocket to receive the brace. Ribs will be cut to fit each side of the shear webs in those areas. I've learned over the last three builds that I can control the dihedral much better with this method. And it satisfies the CIC (Charlie's Impact Coefficient).

One more thing......I can cut the dihedral brace profile from one piece of, in this case maple, and then cut that piece in half....the braces are identical
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Nov 30, 2013, 01:21 PM
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Thread OP
Quote:
Originally Posted by davidterrell80
It wasn't clear from the plan... are you planing a one piece main spar, tip to tip?
I'm interested in points of view on joining wing sections.
I've only had two in-flight failures.

One, was due to choosing spar flange material in a "C" section that was too soft and having the upper flange fail in high-G compression, in the space between root and first outboard rib.

Second was a failure at a joint similar to the one you have, between the rectangular center section and the trapezoidal outer section. I had built completely separate sections, and not built a solid spar. The failure was of the white glue joint.
Mr. Rake and I had a conversation about the same thing....and me being an old Civil, as the saying goes, "ya gotta dance with the one that brung ya''.
Here's what he and I talked about:

Most of you probably know all this, but I'll post it anyway.

OK.. structural lesson for the day......here's an I-beam... I

When you bend it long ways with the top and bottom flanges oriented as shown, the bottom flange is in tension, and the top flange is in compression,
because the beam section will want to rotate about it's horizontal neutral axis (centerline of the web, halfway between the flanges). The compression flange is where a long span beam usually fails, especially a cantilever type. The stresses go up almost exponentially as you get closer to the root. The wing ribs restrain the web from crippling and the flanges from torsional movement...doesn't take much, and since the ribs on the Bristol are on about are on 2 1/2" centers (1 1/4 at the root), the beam becomes rigid about both axis.


And I have this thing about wing failure......remember Scrap I ?

Still working on the drawings !
Last edited by Old_Pilot; Nov 30, 2013 at 01:50 PM.
Dec 01, 2013, 11:06 AM
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Spars et al


Cut all the long pieces
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