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Sep 18, 2020, 07:54 AM
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Zephyr41's Avatar
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Build Log

HS/BAe 748


I've been fairly forward with my opinions in ruff1's Martin 4-0-4 thread so I guess it's well past time to put my money where my mouth is and document an airliner build on this forum. I have a part-finished build of a Breguet Deux-Ponts but the enthusiasm is just not there to finish it at the moment. One day...

For a while I've been trying to choose between the Fokker F-27 and HS 748, today I got my copy of 'Aeroplane' Sept 2020 with a feature on the 748 which made up my mind. Maybe I'll follow up with an F-27 because I've gathered some references and it was the first airliner I ever flew in.

The 748 has nice proportions for a model and I've chosen 1:15 scale because it allows me to build a number of aircraft that I really like in a common scale without anything getting too big and expensive or too small to fly well.

The references I have so far are the old Aeromodeller 1/144 scale drawing of the prototype Avro 748 from 1960, the HS 748 Srs2A maintenance manual from the mid-70s which conveniently has dimensions, incidence angles, dihedral angles, and station diagrams, and the 3-view drawing from the Aeroplane article.
The maintenance manual is the authoritative reference but the other drawings are very useful.
The Aeromodeller drawing shows some features that weren't carried through to the actual flying prototype but it has some good points, in particular the engine nacelles which you can't extrapolate particularly well from the station diagrams.

The model will be predominantly balsa although it might have a balsa-veneered foam wing, we'll see as I thrash out the details. I want to try using the scale airfoils which flies in the face of conventional wisdom but... if it really doesn't work out I will just make a new wing with a more appropriate airfoil. The wing root is NACA 23018 which is rather thick but the tip is the NACA 4412 which is fine. I think the scale airfoils will be OK. A compromise would be to use the 23018 at the root and transition to something more suited to a small model at the engine nacelle.

Unlike some build threads I don't have a head-start and some photos up my sleeve but I'll try to keep up a steady pace and post some progress every weekend. It's just a hobby and life does tend to get in the way!
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Sep 27, 2020, 06:13 AM
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Not exactly off to a great start but I have made some progress.
I had a close look at the Aeromodeller drawing of the 748 and it's spot-on with regard to every dimension I can find.
The only aspect of the drawing that doesn't match production aircraft is the wing tips and aileron outboard ends as it shows the 95' span wing used, maybe, on the first prototype but changed for every subsequent Srs1 aircraft as far as I can tell.
It shows a straight aileron hinge line from root to tip but the aircraft actually has a large balance area ahead of the hinge at the tip.
Since the maintenance manual has the rib stations it's easy enough to draw the aileron accurately and also add the approx. 21" wingtip extension used on the Srs2 and 2A.
Images of the wing root and tip airfoils attached. That root airfoil looks less ridiculous the longer you look at it...

I was considering balsa-sheeted foam-cored tail surfaces but now I have them drawn at 1:15 scale it looks as if they would be rather heavy.
Not decided yet but I'm leaning towards an entirely built-up structure.
The Aeromodeller drawing shows the horizontal tail to be around 12% thick which would make for a fairly large chunk of foam.

At this point I have templates to cut the foam tail using CompuFoil and a wood tail just about finished in devWingCAM2.
The tail airfoil I have chosen is NACA 63A-012. It could be NACA 0012 or just about any symmetrical profile, or a flat plate for that matter, but since I intend to build an F-27 with scale airfoils and it uses the 63A-014 I thought I'd try it on the 748.
Sep 27, 2020, 08:26 AM
A Day @ a Time - Matt. 6:25-34
ruff1's Avatar
Hey Rich, glad to see you jumping in on a project. The HS 748 is a great subject.

Do you have estimates of wing area and all up weight? If I read your posting right, 1:15 scale applied to a 95 foot wingspan puts you at 6.4 feet. My biggest mistake was applying the same down-scaling for all aspects of the model, including wingspan. Big mistake! The wing area must be reduced by a much-smaller proportion. I ended up having to cut the wing in half and extend the chord two inches to obtain a wing area suitable for flight.

Many calculators exist to determine wing loading and wing cubic loading. This is a quick way to do a check on the wing area you calculate with expected weight, and the wing cubic loading will tell you how the aircraft will fly (e.g. Trainer, Scale, Racer, etc. ) Only after adding two inches to the chord (122 square inches) was I able to end up in the scale range. It would have been much better had I managed the weight effectively; I wanted it to be in the high-trainer range, so she'll end up flying like a fighter!

Subscribed and looking forward to this build!

P.S. Test pilot is in Florida for an unknown time, so when he returns we'll put the Martin 4-0-4 up again. She is ready!
Sep 30, 2020, 12:01 AM
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Hi Kevin, it's great to have you on board!

At 1:15 the wing area is 30.3dm.sq (470sq.in).
That's just the main wing panels. The small centre-section wing stubs and the extended tips on the Srs2A 98'6" wing will add a little more area.
I may well increase the chord slightly over scale.

As for weight, 2kg (70.5oz) gives a wing cube loading of 12 and a wing loading of 66g/dm.sq or 21.6oz/sq.ft.
Based on that I'd like to try to keep it down to 2kg!
Since I haven't chosen motors or a battery yet it's hard to tell if it's a realistic goal.
Sep 30, 2020, 12:46 AM
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I decided to cut a foam core for the horizontal tail from some Knauf foam just to see how much it would weigh. I don't have a weight per volume for this particular foam but it seems quite similar to blue foam.
It turned out OK. I had a little trouble with the cutting wire running span-wise which caused the grooves near the trailing edge. The groove nearer the leading edge was caused by me trying to nudge the bow sideways. It's been a while since I cut such a short, tapered panel and I've obviously forgotten the finer points of setting up my cutting bow... The next one will be better!

The foam core weighs 14.8g. The span is 350mm and I don't think it's unreasonably heavy.
I'll go ahead and skin it with 1mm balsa, might get away with a veneered foam wing and tail after all.
Sep 30, 2020, 06:08 AM
A Day @ a Time - Matt. 6:25-34
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Quote:
Originally Posted by Zephyr41
Hi Kevin, it's great to have you on board!

At 1:15 the wing area is 30.3dm.sq (470sq.in).
That's just the main wing panels. The small centre-section wing stubs and the extended tips on the Srs2A 98'6" wing will add a little more area.
I may well increase the chord slightly over scale.

As for weight, 2kg (70.5oz) gives a wing cube loading of 12 and a wing loading of 66g/dm.sq or 21.6oz/sq.ft.
Based on that I'd like to try to keep it down to 2kg!
Since I haven't chosen motors or a battery yet it's hard to tell if it's a realistic goal.
Good stuff Rich. WCL of 12 is a bit under the Martin 4-0-4, and others in my club have told me to not be afraid of that number; just to be aware to keep airspeed up as it will require more than your average trainer. I missed the part in your posting about the wing tip extensions, that should give you some good wiggle room in case you build a bit heavier. I think you can make 2 kg if you do a better job at controlling weight than I did; somewhere along the way I tacked on nearly 1 kg on the Martin 4-0-4 and never really figured out where the weight came from!

I just had a look at the Wikipedia page, and I didn't realize this aircraft is so similar to the Martin 4-0-4 in a lot of ways. Most notable is the upward angle of the outboard wing sections.

A difference you can work to your advantage is the mounting position of the RR Dart engines. This should allow you to swing a bigger prop, which has been an issue for the Martin as the shaft line for the P&W R-2800 engines is even with the leading edge and ground clearance has been an issue. I can only run about an 8.5 inch diameter prop.
Sep 30, 2020, 08:54 AM
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The scale prop diameter is around 9.6 inches I think, but a 10-inch prop will still clear the fuselage and the props will be quite well protected from ground strikes on landing. It's good to have a bit of leeway to choose the most suitable props.

With the thick wing and high-ish cube loading I really don't know what I'm getting into- for a job I fly highly loaded UAVs (up to 180g/dm.sq or approx. 60oz/sq.ft) and at cruise speed they're fine. You don't want to get them too slow though and they are optimised for the desired performance envelope, unlike a scale airliner with an 18% thick wing...

I have to admit I hadn't considered the dihedral in comparison to the 4-0-4 but they're quite similar. The 748 has 7 degrees of dihedral measured on the airfoil datum at 25% chord.
When you get the 4-0-4 shaken down and trimmed out it will be interesting to see how effective the ailerons are.
Both aircraft should turn pretty well on rudder only...
Oct 05, 2020, 06:16 AM
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Since I have 3 different types of foam on hand I decided to weigh them for a comparison.
They are Knauf 'general purpose' foam, Owens-Corning Foamular 250, and blue Dow XPS.

To my surprise they're all the same density at 32kg per cubic metre.
The Foamular 250 seems softer than the other two and I expected it to be less dense. All three cut and sand differently.
The blue XPS and Knauf foam give an excellent surface when hot-wire cut and sanded.
The Foamular 250 is more grainy and the hot wire cuts best at a slightly lower temperature.

I have a lot of the Foamular 250 so I ended up using it to cut a pair of horizontal tail cores.
They're both 15.3 grams which is the same as the Knauf foam for practical purposes.
The 1mm balsa skins are 8g each and I think the foam tail will be very similar in weight to a built-up tail.
Got to be careful with the adhesive of course, I'll be using epoxy.

I cut two sheets of 1mm x 1200 x 100 balsa into pieces that are 5mm longer than the tail half span and edge-joined them with balsa cement.
There are other ways to do it- I'm going to vacuum-bag the balsa skins to the cores and I know a lot of people simply tape the skins together for that purpose.
Usually it works but if epoxy squeezes out between the balsa sheets there will be hard ridges in the skins and they are extremely difficult to sand down without over-sanding the wood.
To underline this, I never use tape with no glue. As above, others have no problems with tape only.
Also, superphatic works just as well as balsa cement. Yellow wood glue is fine but balsa cement and superphatic can be sanded down more easily without leaving any ridge at the join.

I match the edges as closely as possible, trimming them with a knife and straight-edge if necessary, and then tape one side with plastic tape.
Then I fold the joints away from the tape side and apply a generous bead of balsa cement before closing the joints and weighing them down for an hour or so.

The skins are cut to the cores plus 1.5mm at the leading edge and tips. That's the actual core surface, not the projected plan surface!
If you use the projected surface (i.e. cut the skins exactly to the plan view) they will come up short! In that case the parts can be saved but it's easier to account for the curvature of the surfaces in the first instance,
The trailing edges are full chord (NOT the trailing edge of the core!) plus 3mm and I cut the foam blanks 3mm oversize at the TE as well.
This is to allow the TE to be slightly oversize and also fully supported by the foam off-cuts in the vac bag.
It's a bit of a balancing act as making the skins too much oversize can result in them being bent over and broken under vacuum.
I go with 1.5mm/1/16" but a bit more is probably OK.

The attached pic shows the 100 x 350mm balsa sheets edge-glued and weighed down.
I have some 0.5m long pieces of 20mm square aluminium tube filled with 000 shot as weights.
They have plastic tape on one side so they don't stick to the balsa.
Oct 06, 2020, 05:33 AM
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No issues bagging the tail halves except that I carelessly allowed them to touch and they got slightly glued together.
Luckily the overhanging skins allowed me to cut them apart without doing any damage.

They turned out quite nicely.
The trailing edges have been cut to the correct chord and I sanded the tips and leading edges flush with the cores.
Not sure of the weight at this point but I took care not to overdo the epoxy.
Oct 06, 2020, 07:19 AM
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Each tail half weighs 34g (1.2oz).
The cores are 15.3g each and the skins are 16g per side.
I put a strip of light glass cloth along the trailing edges as well.
That leaves around 2.5g (0.09oz) for epoxy.
Not too bad...
Last edited by Zephyr41; Oct 06, 2020 at 07:23 AM. Reason: Added imperial units
Oct 07, 2020, 08:18 PM
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There was one being built further along on this forum but it's been quiet all this year even tho it seemed to be close to being finished
https://www.rcgroups.com/forums/show...?2451513-Hs748

I spent about 12 years working on the full size ones
Oct 09, 2020, 08:32 AM
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Hi Kiwi, thanks for the link.
I searched this forum before I started this thread but somehow I overlooked the details of that one.
The weight is a bit worrying since it's slightly larger than mine and a very light structure...

Nice to know you worked on them- last time I saw one flying was in 2005 and it was a welcome sight. It was providing the daily mail service to the remote town where I was living at the time. The sound of the Darts is unique and I always stepped outside to watch the Budgie performing a very tight circuit at low level. Never knew why they did this actually, there was virtually no traffic and no other aircraft did such a low, close-in circuit.
The recent Aeroplane article had a report from a pilot that was rather dismissive- I felt it sold the aircraft short.
BTW "my" 748 had an NZ registration. Maybe it was an ex-Mt Cook aircraft...?
Oct 26, 2020, 04:17 AM
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Our aircraft were ZK-MCA, MCD, MCJ, MCF, CWJ, DES.
Oct 27, 2020, 10:01 PM
A Day @ a Time - Matt. 6:25-34
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GDay Rich. Progress looks great.

Had some logistical setbacks on the Martin 4-0-4. Test pilot had to go to Florida for about a month, this week I'm helping my son move to Phoenix Arizona (1600 miles away). He works for Piedmont Airlines (American Airlines subsidary) as a ramp agent and is transferring to Sky Harbor airport. Flying back on Monday.

Not really sure when we'll get out. November and December has some good days so maybe...
Nov 02, 2020, 05:55 AM
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Kiwi, Mt Cook Airlines is on my list of possible colour schemes, the others are Dan Air and Air North. The old "Mount Cook Line" colours with the all-blue tail from around 1980 is one of the nicest 748 schemes IMO. Simple and elegant.

Kevin, I hope you get some good weather!
Not a huge problem here in general but it will start to rain in the next month or so and you have to pick your flying days. The way this model is going we'll be in to the 2021 dry season before it's ready to fly though...


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