56" electric BV 208/15 Build

[fred clarke]

As the nights draw in and the garage is getting too cold to work in I thought I would share my latest project with those who might be interested.

 

The project is a 56" electric powered Blohm & Voss 208/215 hybrid; the title gives it away really.

 

Firstly, the background....

 

Way back in 2002 there was a free plan issued for a 33" span BV 208 designed by Chris Golds. It was all sheet, designed for a Speed 400 and 8 cell Nimh. I duly built this but never tried to fly it following suggestions that it was too difficult to get to go. The model was, therefore, put into the archive (loft) and forgotten about.

 

Fast forward to Oct 11 and while looking in the loft for something else found the model (complete with very impressive warp!). I have recently adopted a 'fly or die' approach to models and thus elected to 'kill or cure'. Thus the model was taken out, re-engined with a 2812-06 and a 3 cell 1300 Lipo. After some interesting initial flights (more later) I was hooked. Also I decided that this winter's project would be a larger version 'designed' by scaling up Chris' plan.

 

First thing to decide was the size. Looking for inspiration I came across the canopy of a foamy EDF that I had rescued following a demolition by another member of the club. A quick measure against Chris Gold's plan came up with scale factor of 1.7 and a span of 56" - this seemed about right so a start was made.

 

I then stumbled on the BV 215 build thread on this forum and the thus this thread was born....

 

My basic spec is:

 

Wing span = 56"

with electric retracts leading to nosewheel steering and twin rudders

conventional (eg balsa) structure

flying surfaces covered in solartex and then painted

fuselage covered in lightweight cloth with WBPU and then painted

Use twin rudders as air brakes (if I can sort the mixing and if they are needed)

Power system TBD once I have a better idea of the weight.

 

Talking of weight, the little 33" model weighs 17 oz but not sure how I could use this as the basis of an estimate of the weight of the larger model - any suggestions gratefully accepted!

 

My plan is to post more info as I go providing there is some interest shown - if none I will quietly sink into the sunset! In my next post I'll explain the title and aircraft designation.

[kevin wild]

hi
nice subject

even building lightly I think you'll be over 4lb. I'm not an expert though so don't quote me on it

Kev

[fred clarke]

Kev

 

Thanks for the guestimate - 4lb would suit me fine.

 

I thought something like original weight*scaling factor squared = 1.1lbs * 1.7*1.7 = 3.2lbs add a bit (8 oz) for the undercarriage and extra servos = 4lb (ish). Would then look for something like 500 watts to give 120 watts per lb.

 

I'll weigh each bit as it gets finished.

[Stefan Hafner]

I'd have to agree with kenin on this, i reckon you'll be over 4lbs, couldn't say by how much but at a guess with sheeted wings and cloth i would estimate about 6 to 7 lbs. Have you thought about what sort of wing section you're going to use?

 

Should be an interesing build though

[fred clarke]

Stefan

 

You reply got me thinking. I have weighed another model I have that has flown succesfully. It is a 40" high wing trainer style, all solarfilm covered, box fuselage, 2 wheel undercarriage, built up wing and ready to fly weighs 41 oz. The build of this is the same as I am following for the BV with standard D box, cap strips, shear webs etc for the wings. On this basis adding 8 oz for the retracts etc gives a bit of leeway before I get to 4lb which I have now set as my target!

 

As far as airfoil sections, I use Profili to plot them out and have concluded as follows:

 

Fin/rudder - fully symmetric 6% NACA simply because it is thin yet looks right

 

'Elevator'/Elevon - fully symmetric 10% NACA as above

 

The rudders and elevons are built up with full sheeting top and bottom a few ribs and a sold chamfered leading edge - I will use tape to hinge.

 

Wing - obviously need to be thick enough to house the retracts. However with a root chord of 17" and a CG that is relatively far back, if you used a standard section it would be something like 2.5" thick at max depth. Thus my plan is to use a NACA 0012 section throughout the wing. I have drawn up the section using the constant chord outer wing as the starting point. For the inboard section this will be modified so that the front portion of the wing will follow the section as will rear portion by splitting the template at the max depth point. These two bits will then be joined together by a constant depth extension that will vary in length depending on the wing chord. Thus the wing will have a triangular flat section in the inboard panel that will disappear at the point that the wing planform changes from delta to constant chord. I'll put somthing together and post to show this better.

[Stefan Hafner]

Sorry, i misread the first post, i thought you were planning on using cloth on the wings aswell as the fuselage, in which case it'll be lighter.

 

I'm assuming you'r planning on throwing it about a good bit with a symetrical section, so will you be using the elevons to add reflex to the wing? Most normal wings try to make the plane dive, thats why we have tails on planes, so you need to have both elevons trimmed up slightly to prevent this, or another option is to use a "zero pitching moment profile" as these will avoid this, but are more difficult to build, a selection to be found here. If you have profili why not just get ribs of the right size for the inner section too?

 

hope this is some help

[fred clarke]

Stefan

No worries re covering.

Yes the little one I have whizzes around and is very manoeverable and I hope to replicate a large part of that and thus the symmetric section. Clearly the power loading and wing loading will playy a big part.

 

The reason or modifying the centre section airfoil is to keep the maximum thickness down as I equate this to drag. To get the majority of the retract within the wing it needs to be around 1" thick about 12" from the LE at the root. If I used a fully plotted section this would give something like a 2-2.5" max thickness which I am not keen on.

On the question of reflex, Chris Golds designed his with the whole of the 'elevator' including the elevons (eg the anhedral bit outboard of the wing/fin/elevator join) set at -3 degs thus providing reflex. It works well on the little one and thus I will stick to that.

Grateful for any other suggestions you may have.

[Simon Chaddock]

fred

Just one point - all things being equal the weight goes up as the cube of the scale so 1.7*1.7*1.7= 4.9. Your weight would be 4.9*17 = 83oz or 5.2lbs.

 

Normally as planes get bigger you can save some weight as things like the covering don't scale and it is easier to make the small components 'just strong enough' rather than simply a convenient size but it does take some care and attention.

However on the plus side bigger planes are aerodynamically more efficient and so fly better.

 

Going bigger does eventually bring ever reducing returns but with modern electrics it seems the maximum practical size is.....er..... completely impractical for most of us!

Edited By Simon Chaddock on 10/12/2011 21:41:32

[fred clarke]

Simon

 

Got it!

 

Square = area

Cube = volume

 

Also a 9g servo weighs 9g whatver you have put it in. I guess the better guesstimate is to only consider the weight of the structure and then add in the hardware eg servos, battery etc.

I think my approach was that I am hoping to keep as close to the wing loading of the little one as it performs so well.

 

Ho hum - in the end the weight will be what the weight will be (hopefully not over 5lb). I plan to leave the selection of the power train until I have a better idea of the final AUW.

 

I think you have summed up modern electrics well. I take the view that the only limits are your imagination (or, in this case, someone elses!) and the depth of your pocket.

 

Fred

[Stefan Hafner]

You'll find that a bigger model will generally handle better than a small one, i suspect becuase the larger size will usually make it a bit less responsive to control input, even though that mainly depends on how you set it up. I tend to favour my bigger planes as they just "sit" better in the air.

 

As for keeping the wing loading the same, you'lll end up withe really heavy model doing that. Looking at the simple maths for doubling the span if a model as an example

Wing loading = Wing area / Weight

Now doubling the span will quadruple the area of the wing as when scaled the chord will also double. So so keep the wing loading the same you'll need to quadruple the weigh which will make it a very heavy plane.

 

I would suggest just build it and see how much it weighs, but as bigger planes generally handle better i think you'll end up with a good un'

 

[fred clarke]

Stefan

 

Not sure I agree with you re the increase in weight if the loading is kept the same.

 

For example, lets say we have a 2lb model with 3 Sq ft of wing ; this would give a loading in the region of 10oz/sq ft.

 

If you double the size of the model then the wing area would go to 3*2*2= 12 sq ft. To maintain the wing loading then the weight must be kept to 12 * 10 oz = 120 oz = 7.5lbs. This is the approach I have taken - the problem is can the model be built at this weight.

 

The point Simon was making was the weight may well go to 2lb*2*2*2=16 lb giving a wing loading of over 21 oz/sq ft.

 

I am now simply going to build using sensible material sizes (eg not going for something so light that hangar rash becomes an issue) and see where that gets me.

 

Fred

[fred clarke]

As promised above, a quick picture (on the next post) showing how I have made the wing ribs for the inboard tapered section. The outboard section is constant chord. As a reminder, the basic starter rib is the outboard rib of the inboard section which is the same as the inboard rib of the outboard section.

For those that are interested this approach is simply an extension of the process used to create ribs for control line combat wings with tapered palnforms.

 

I have had to put the drawing on a seperate post as I cannot get ot on this one no matter how I play with the settings - any suggestions on how to do thei gratefully accepted!!

 

Now that I have figured out how to add pictures etc (sort of!) I'll add a few more.

[fred clarke]

Drawing to go with post above

 

[fred clarke]

Sorry about previous post, this might be better.

[fred clarke]

Well, I have the answer to the weight question - the model weighs 4lb 8oz with just a litlle more needed to 'bling' it up eg a bit of splinter camouflage and decals etc.

The main specs are now:

Span - 56"

Length - 39"

Weight - 72 oz

Wing Loading - 18 oz/sq ft

Motor - EMax GT 2820

Prop - Aeronaut Cam 12.5*7.5 folder

Max Watts - 550

Watts per lb - 122

ESC - 60Amp Cont, 80 amp burst

Battery - 3S1P, 3600 maH

Having completed the model I decided to fly it before applying the colour scheme - just in case!!

The CG was taken from the original Chris Golds plan and was achieved without additional ballast. In fact, I had to move the battery back to achieve this.

Two flights (perfect conditions) have been completed (about 12 mins, battery at about 50% on landing) with a few things worthy of note:

The aircraft tracks beautifully with no evidence of a swing

As the aircraft sits slightly nose down, and with a fully symmetric wing, it has to be flown off. No tendency to leap into the air.

No trim change on retracting landing gear - not really surprising

No major trim changes on reducing power to zero

At 18 oz/sq ft - glides pretty well

Need to file flats on main LG legs to prevent rotation on landing! The 2 landing were like carrier landings without a hook or wires!

Now need to increase control throws, particularly 'aileron'. 'Elevator' is fine.

Need to play with prop sizes as managed to hit the prop on the 2nd take-off.

The beast is back in the workshop for the camouflage, decals, landing gear flats and some more watt meter tests.

Once this has been done I'll get some photos done and posted.

[kevin wild]

Hi

I know you're having trouble posting pics but can we please have some of the model

thanks

kev

[fred clarke]

Kev

I think I have solved the posting pics bit - all else had failed so I read the instructions!

At the moment the aircraft has about half the camouflage and markings on; I expect to post photos before next weekend.

Fred

[fred clarke]

Kev

Actually the current state of the aircraft will give you a good idea of how far I have got thus a few piccies to be getting on with. Note that the fuselage and wing are not fitted together - they are just sitting on the hi-tech, hi-cost ground support module.

Fred

[kevin wild]

Hi

very nice. that'll look fantastic on a fly by

do you intend to do a plan for it. I'm sure there's others that would like a go

 

kev

Edited By kevin wild on 22/05/2012 14:40:32

[fred clarke]

Kev

I don't really bother with a plan when I build my 'own' designs. For wings, tailplanes and fins, I just draw the front of the LE, the rear of the TE, the LE of any spars and the centre lines of the ribs. For fuselages, I tend to draw direct onto the wood of one side, add any doublers and make up the formers/hatches as I go along. Details such as the wing tip pods etc also are 'designed' as the build progresses. The drawings, such as they are, are on some seriously old fan-fold computer printout paper.

Having said that, more than happy to provide sufficient detail, with dimensioned sketches, for anyone interested. As I said above, the model is strictly traditional with lots of balsa and a little bit of ply, blue foam and fibre glass/WBPU so anyone who has built a model or 2 from a traditional kit or plan would have no problems.

By the way, I decided against glassing the fuse and it was simply covered in solartex, as were the flying surfaces.

Let me know if you do want some sketches.

Fred

[fred clarke]

Well, we suffered the same abysmal weather as the rest of the country and it was not until this weekend I felt the weather gods were smiling enough for the beast's 3rd flight.

Since the last flight I had finished off the blinging process, filed some flats on the main LG legs and tweaked up the aileron throws.

Attached are some photos of the final model , some airborne shots and the landing. The nose LG had hung up (see the last 2 photos) and I elected to land on the tarmac with the mains down. End result was a minor scuff on the nose that didn't even wear through the solartex. My trusty photographer (and co-pilot) has managed to catch the moment the nose touched down. Note that the prop is windmilling and not under power.

The flight was fine although the roll rate is still a bit low although all basic manoevres are readily achieved providing the speed is right.

So, more tweaking of throws etc and then to see if another prop can give a bit more top end speed.

All in all and enjoyable project with more fiddling to come.

Fred