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Aug 25, 2015, 09:14 AM
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Build Log

40" Besson MB411 Peter Rake Prototype


I’ve decided to do a quick balsa build as a little distraction from my big Mavis and I saw that Peter put out a call for prototype builders for a few of his plans. Of course once I saw that there was a little floatplane I couldn’t resist.

So here I am, making a start on Peter Rake’s Besson MB-411 float plane.

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I know that this is a beta prototype, with Lake flyer having built the alpha prototype back in 2010. I thought I’d have a go and try and work out the kinks by learning what I can from his build and see if I can produce a sweet flying version of this plane.


Usually I would start a build log with a bit of background of the original plane, but if you have a look through Lake flyer’s thread there is a bit of discussion about it there. Suffice to say that is an odd little French design (but odd should pretty much be expected when talking about French planes) that was designed as a reconnaissance scout for the attack submarine Surcouf. Its peculiar shape appears to be largely a product of its submarine hangar, with the underslung tail, sub fins and removable wings.


In looking through the various images and photos of this plane there seems to be some variation in the shapes, particularly of the horizontal stabilizer, which is rounded in some photos and square in others. Given that this is a prototype build and its purpose is to prove the design, I will stick with the shapes Peter has drawn.

Peter, you have also stipulated as part of the prototyping process that you would like decent photos to accompany the log. If there is a photo of any detail that I miss and you would like me to take let me know. Likewise on the quality/lighting in the pictures.

-Mat-


EDIT:
Scale decal sheets for this model can be found in post 92 and are available through Callie graphics

Update
The Besson is complete and succesfully flown

Besson post-maiden flying (2 min 16 sec)
Last edited by Wormboy; Aug 18, 2016 at 02:54 AM.
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Aug 25, 2015, 09:14 AM
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Challenges


It appears that the previous build had a couple of challenges to overcome that I have been mulling over and will propose solutions to as I go. Peter and others, please feel free to add in whatever advice you like and we can discuss before I incorporate it as needed. As stated above, the purpose of this build is to produce a sweet flying aircraft and to prove (and perhaps amend) Peter’s design for a production plan.

Peter, if you object to any proposed changes please sing out and I will go back to your design as intended.

#1 The alpha prototype appeared a little overweight. I’ll be looking to shave weight where I can. We’ll see how I go with this and only time will tell. No radical changes here, except I may substitute some of the 1/8th ply for 1/16th where appropriate.

#2 On the plan the angle of the main float to the aircraft datum seems far too low-in-the-nose for my liking. The video of the alpha prototype appears to suggest that this was part of the problem on takeoff, so I’ll be adjusting this angle to something more moderate.

#3 Lakeflyers assessment of his maiden flight
Quote:
It seems to be dynamically unstable, ie, when it starts to bank it wants to bank more, when it noses down it wants to nose down more , etc .
I suspect the tail surfaces are a not working to their full capacity, given that the underslung tail would have had all kind of interference from the fuselage and the float struts. This is also in my mind because apparently Marcel Besson thought the same thing and added in two sub fins to compensate on the original. Maybe at this small scale it needs a little more help. I’ll be trying the vertical surfaces as Peter has drawn them first, but keeping in mind Lake flyers assessments of the flight characteristics.

I decided to cut my own kit for this plane. Although Manzano offers what I’m sure is a first rate kit, by the time I added the postage to it and calculated the exchange rate, it became easier to sit down a cut it by hand. The kit isn’t so complex that cutting by hand was that difficult. Also, I figured I’d probably have to make a few tweaks given it’s a prototype and probably end up cutting half of the pieces again anyway. Sorry Charlie, next time!

On to the build…

-Mat-
Last edited by Wormboy; Aug 25, 2015 at 09:43 AM.
Aug 25, 2015, 09:19 AM
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Fuselage #1


Never having built a Rakian (is that the correct adjective?) design before I was happy to see that the construction techniques used in this plan are pretty straightforward. This is helped further by the simplicity of the original plane, which had a very utilitarian and minimalist look about it (AKA boxy and weird-looking). No compound curves or rounded wingtips to deal with here.

The sides have gone together easily and the electronics tray squares everything up nicely. I thought a bit about the bend at the front of the fuse in the 1/8th box sheet and Peterís suggestion to make cuts to aid the bend. In the end I took to it with a hacksaw and removed about Ĺ the material from that section to make the kerf bend nice and easy.

One more evening on the fuselage and then on to the main float I think. Iíll do all of the sanding in one hit once I have the main sub-assemblies done.


So far so good.
Last edited by Wormboy; Aug 25, 2015 at 09:36 AM.
Aug 25, 2015, 05:57 PM
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Hi Matt, I am glad you are taking up this model. I have been collecting information on the MB411 when I come across it ever since Lake Flyerís build thread appeared. So when you mentioned the angle of the float on Peterís design I did a quick review of the photos and drawings I had on hand. You have probably thought this through already, but I am attaching a couple of drawings that summarise the differences that I can see. I am not sure what it all means but there are many differences in geometry.

Basically, the photos and 3-view drawings show that the fuselage upper surface datum, the propeller thrust line, the tail plane and the top surface of the main float are all at the same angle. However, Peterís float top surface is set 2 degrees nose down. Some of the 3-views disagree on the angle of the planing bottom of the main float just ahead of the step. Some show that the bottom has the same angle as the top of the float as Peterís plan shows and others show that the planing surface is about a degree nose up relative to the float top. Photos tend to agree with the nose up angle. So Peterís planning angle seems much more nose down relative to the aircraft than the other sources show.

There is also a difference in wing incidence due to the angle of the bottom of the fuselage plus the zero lift angle of the aerofoil section (assuming that the 3-view aerofoil section used for comparison is accurate Ė it looks accurate to me). So Peterís wing is set about 2.2 degrees higher incidence in flight than the 3-views and full size aircraft, but more important during the take off it is set at 9.4 degrees relative to the planning bottom vs 4.1 degrees for the 3-view geometry. I think what this means is that Peterís geometry will cause the aircraft to lift off much earlier in the take off run Ė maybe even before the aircraft gets up on the step. (If you look at the video of Lake Flyerís 3rd take off, the aircraft seems to pop out of the water before the float is properly planning Ė the video ends as the aircraft is going vertical). So I think you are right about the angle of the float but I think the situation is worsened by the wing incidence setting and the planing bottom angle.

One other thing, Peterís step is located midway between the struts but the photos and 3-views all seem to show it further aft, in line with the rear strut, aft of the CG, providing yaw stability when the aircraft is running on the step. If the model were to run at high speed on the step it would be more likely to snake or even ground loop due to side forces that arise from sideslip relative to the water, especially if the pilot were holding the stick forward to prevent premature lift off.

There is also a German build thread on the MB411 at 6th scale. This model seems to follow the 3-view geometry and exhibits excellent behaviour in flight and on the water, both on and off the step.

http://forum.rc-wasserflieger.at/thr...8ce0607f9ed570

Anyway, this is just the result of comparisons of bits of geometry and data that allow some tentative conclusions to be drawn, but it doesnít say how you would directly design a seaplane to take off and land smoothly. There must be a simple set of rational criteria to base the design on. Iím looking forward to seeing how you resolve the geometry. The model is looking good so far. Regards John
Aug 25, 2015, 07:00 PM
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Mat,
Well mate, that's a great start to the thread. The build doesn't exactly seem to be going slowly either.
By all means bring up any points you feel I may have got wrong, then we can sort them out for all to see and hopefully end up with a model that flies better than it looks.

Just so we all know, I know nothing about floatplanes. I've nowhere to use them, there aren't really any that appeal to me so I never bothered with what makes them work. I just based the drawings on what Lake Flyer told me he'd need - including the angle of the float -
That said, if you take off the floats and fit a real u/c there's nothing about this model that makes me believe it would fly any differently to a conventional small low wing type. Add in the extra weight and drag and all bets are off - Although it did appear to fly okay in the end.

As regards the step on the real one being further aft, to me it looks as if the whole float should move back further - the perils of using a rubber power model for inspiration. Combine that with a reduced negative angle and you aren't too far off the 3 view illustration. So, a minor rework on the float mounting and you're getting pretty close.

So, keep the suggestions coming. Including that for me to stick to what I know if that's how you feel. I'll ignore it anyway, so you might as well.

Pete
Aug 26, 2015, 12:04 AM
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Welcome aboard guys,

Peter, I'd never be so unappreciative as to tell you to go back to the stuff you know. Always moving forward so they say. All my criticisms are designed to be constructive and to aid your design.


RPFJ, you've hit the nail on the head with your assessment I think. I'll post a bit of my musings in the next post showing my rationale for what you have come to intuitively. Yes, you're right, there are some simple rational rules for designing and setting up floats. More to follow.

If you've got a stash of material for this plane I'd love to see some of it, especially scale markings. There is a little green shield shaped marking on the rear of the fuselage that I cant get a good look at, perhaps you could shed some light on what it may be.


-Mat-
Aug 26, 2015, 12:05 AM
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Righto.. a bit of text here

The dynamics of floats and flying boat hulls has been discussed many times and are fairly well understood now amongst those who want to know about such things. In particular Andy Lennons book “Basics of RC Model Aircraft Design” give a fairly in-depth review of the subject. There is probably more information in there than the average float flyer need know.

Although there are planes that break these rules and fly perfectly well, the rules of thumb that I can distill out of Andy Lennon’s book as well as other discussion here on RCG are these:

The step should be behind the CG, on a line about 10 deg off vertical from the CG.
Float length should be about 70% of the fuselage length and stick out in front of the prop by about Ĺ the diameter of the prop I think it was
There should be a straight section of the hull in front of the step, this is the planing surface and this is the important surface when considering angles. (The angle of top surface of the float is largely irrelevant; it doesn’t affect any hydrodynamic or aerodynamic lifting forces)
The angle of the rear keel to the planning surface keel (Sternpost angle) can be shallow but it needs to be an angle to allow the float to rotate and to get over the hump. I think 6 deg is Lennon’s recommendation.
Chines and spray strips are good for keeping water out of the prop. Needed on the front of the float, not so much behind the step.

The situations that you most want to avoid is:
Swerving and looping on takeoff, this is usually diagnosed as the step being too far forward, and maybe even ahead of the CG. I think we’re OK here looking at the plan.

Taking off too early before you have enough speed. This is the one I think we need to consider as this is what I think happened to the alpha prototype.
What will happen is that you can be ploughing along in the water at low speed and to get up on the step you have to go up and over a hump of water. While you’re doing this, the planing surface of the float will go from 0 deg up to whatever it needs to go over the hump and then back to an angle that is right for the wings for takeoff. That maximum rotation is dictated in part by the length and angle of the hull behind the keel. In getting over the hump you really want the lifting forces of the wing to help lift and stabilize you. If your float rotates you too far back then your wing is completely stalled and you get no stabilization or lift. If you’re in this situation and you’re underpowered then you just wont be able to get up on the step you’ll just keep ploughing. If you’re overpowered then you will shoot up into the air with a completely stalled wing. Bad news.

So considering these in the context of the Besson
I won’t mess with the angles of the flight surfaces as Peter is more than capable of designing a plane that flies well. We’ll stick to adjusting the float parameters.

So if we take the aircraft datum as 0deg, I make estimate angle of incidence of the wing is about +2-2.5deg. That about right Peter? The sternpost angle is fin, I estimate it at about 6 deg. Float length is good and location of step is acceptable. So I’ll leave the structural elements of the float as it is. The only build comments I have is that the profile behind the step does not need to be concave, but it shouldn’t hurt so I’ll leave it as is. Also, I’d consider making former 4 out of 1/16th ply because it will take the compressive forces of the landings.

So long story short I reckon all we have to do is tip the float up at the front until the planing surface goes from -2 to about +1 deg. We'll see about the step location once we get it over the hump and planing
Aug 26, 2015, 04:52 AM
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Thanks for that, reference noted. John
Aug 26, 2015, 04:54 AM
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Mat,
Since FL4 is glued to FL5, do you think making it ply might be overkill?
Measured t.e. centre to l.e. centre wing incidence is just a hint over 2 degrees - or 1 degree along the flat bottom of the wing.
What do you mean by sternpost angle? I should know more about boats and matters watery, my grandfather was a boatbuilder and my father a ship's pilot. Both built model boats. Me, I build planes and joined the army instead of the navy. I've yet to see a tank that floats very well.

Pete
Aug 26, 2015, 12:15 PM
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Hi Mat, I have had a quick look at what I had in the file and I think the badge is a simplified version of the ship's badge of the French submarine Surcouf on which the aircraft was based. It looks like it consists of a shield and a suspended medal in the form of a gold cross. The illustrations come from a Russian site and relate to a resin kit of the MB411 made by Dujin (DUJIN RESIN 1/72 BESSON MB-411 FLOATPLANE SURCOUF). The 1938 version of the ship's badge appears in the first photo. I Googled (google.fr) and came up with a copy of a drapeau containing the badge in colour. I presume that since the badge is superimposed on a Cross of Loraine that it applies to the time the ship served with the Free French, but I guess it gives an indication of the colours of the version of the badge on the aircraft.
Aug 26, 2015, 08:46 PM
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On floats and markings


Peter, I’m just following the nomenclature set out by Lennon so I really don’t know how common the term Sternpost Angle is. I suspect it is only really useful when talking about flying boats and floatplanes so that would make it pretty obscure. I can’t see it having much relevance to properly nautical people like your grandfather and father.

I’ve included a labelled picture from the book to show the various parts of the float etc. Sternpost angle is just the angle at which the afterbody keel rises from the step to the aft of the float.

My only concern with F4 being balsa instead of ply is the point of the step, which is unsupported by the F5 lamination. Perhaps if F4 was rotated on the laser cut sheet to run the grain vertically, then F5 laminated to it with horizontal grain? That would give you a very strong former (effectively balsa 2 ply) with somewhat better compressive strength at the unsupported point and zero added weight. Although it may then be prone to splitting vertically. I’ve actually almost completed the float now anyway and have used F4 with horizontal grain. We’ll see how it holds up. Pics tonight.

Thanks for the images and research RPFJ, that clarifies the markings a little bit. I’ve got a little while before I start having to make decals for this beast so I can do a bit more digging. If you come across other relevant bits feel free to post them up. I actually quite like the white livery in your images rather than the blue. Perhaps I’ll go for that one as it will be considerably easier to see in the air. Maybe I will have to round off those horizontal tail surfaces after all.

Once we’ve worked the kinks out of this design do you think you’ll be keen to give it a go? It’s a pretty easy and enjoyable build so far.
Aug 27, 2015, 08:55 AM
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Float construction


As promised, here are some photos of the float build.

I have made a couple of changes here but nothing too drastic. I'll run through each.

First couple of photos are self explanatory and are as per the plan.
note that I beveled and taped the two afterbody panels together along the seam and glued them as you would for edge joining sheeting. This made the join nice and tight. Then I glued them to the float as a unit while the seam was still wet. taped the whole lot down and left to dry (this was done with Titebond III to give me some working time).

I had a lot of mucking around with the forebody sheeting. I still don't know how lakeflyer managed to sheet these with the double curve. My balsa just didn't want to do it.

My choice was to either strip plank it or to iron out one of the curves. I chose the latter because I wanted to keep the bottom as solid as I could.

I have trimmed the outer edges of the sides and flattened the former curves to give me two flat panels. Once the sheets are on I'll glue a 1/8 x 1/16 strip on the edge and shape it into a spray strip with some filler and sanding. I used this technique with my Sikorsky and it seems to be doing the trick.

I also added in a 1/8 x1/8 keel with the top side beveled to give a keel point This was to keep everything straight and to give me more stability along that edge.

I then cut the sheets using the old greaseproof paper trick.. photos pretty much tell the story.

And that's where I'm at as of tonight.
So I've deviated from the plans a little bit but hopefully have not added too much weight. At least it will all be ahead of the CG.
Aug 27, 2015, 11:19 AM
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I suspect those images I posted from the resin kit instructions are simply black and white drawings with the markings coloured in. I don't recall any photos of an aircraft with a white scheme. I did come across a scheme with a dark blue/gray colour top and bottom as opposed to all bright blue or bright blue top and light gray under surfaces. The scheme in the first attached drawing shows an overall blue scheme with the shield marking, but there is a drawing showing an aircraft with the same fuselage code with no badge and the colour is a much darker blue/gray.The photos you posted seem to be a scheme with medium gray upper surfaces and light grey lower surfaces. Lots of choices, I am afraid. Regards, John
Aug 27, 2015, 07:42 PM
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There is a white one depicted here. It has the same burnished front on the cowling and exhaust as depicted in your illustration. What do you reckon, legit colour scheme?


http://www.airwar.ru/enc/other1/mb411.html
Aug 28, 2015, 04:51 AM
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They may all be legitimate but I think that the colour scheme on the cover of the book about the Surcouf would be the most certain - medium grey upper surface and light grey under. That would be a good camouflage scheme for northern European waters and it seems to tie into the photos of the aircraft on the ship. The destroyers in the background look British to me and so the photo may have been taken when the ship was operating out of Plymouth. It doesn't look a million miles away from the scheme that you posted if white is actually grey with lighter highlights rather than white with shadows. The scheme I fancy is the overall dark blue/grey one which I think would be the most visible, at least in a hazy northern European sky, but I haven't seen a corroborating photograph. Somebody must know about French Navy colour schemes but there doesn't seem to be much first hand information on the internet.


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