By continuing to use this site, you agree to our use of cookies. Find out more
Forum sponsored by:
Forum sponsored by CML

120" Majestic Major-General (electric)

A scaled up version of Ben Buckle Majestic Major

All Topics | Latest Posts

Search for:  in Thread Title in  
David Mellor17/12/2018 11:58:16
avatar
1276 forum posts
611 photos

I have started building an enlarged version of the Majestic Major.

The MM has a wingspan of 88", my enlarged version will have a wingspan of 120".

So the Linear Scale Factor is 120/88 = 1.36, or 136% if you prefer.

This won't be a detailed build log (I've already done that on the Majestic Major thread). But I will post some photos and report progress from time to time.

Here are the two fuselage sides in half inch square balsa. The balsa varies from rock-hard (for the engine bearers) at the front, through hard (front), medium (middle) to soft (rear strutting):

p1010632.jpg

And here is the MM-G part built fuselage sitting on top of my nearly completed regular size MM:-

p1010642.jpg

David Mellor21/12/2018 15:29:04
avatar
1276 forum posts
611 photos

I'm working to a target weight of 14 pounds (224 ounces). This is based on my Majestic Major (MM) build data.

The MM-G wing area will be 16.1 Square feet (scaled up from the 90" span MM).

So the cubic wing loading will be 224/(16.1^1.5) = 3.5 OPCF which is the same as the MM.

So my weight budget for this build is:-

1. Fuselage woodwork (inc. sub-fin) = 54 oz

2. Tailfeathers woodwork (wood only) = 17 oz

3. Covering, hinges, horns and fixings for tail feathers = 6.5 oz

4. Fuselage covering, all dowels & windshield = 6.5 oz

5. U/C, wheels & collets = 51.5 oz

6. Servos, linkages, supports & Rx = 10.5 oz

7. Power System (prop, motor, ESC, battery) = 78 oz

TOTAL = 224 ounces.

David Mellor21/12/2018 15:43:05
avatar
1276 forum posts
611 photos

As mentioned, my target weight and weight budget is calculated from data compiled during my Majestic Major build here:-

**LINK**

- which in turn was based on building a couple of Junior 60s, here:-

**LINK**

I think I'm in the right territory with these target weights as Warren B (on RCGroups) has posted a weight of 244 ounces for his IC powered 112" span Junior 112. I'm sure he won't mind me posting his RCGroups photo here:-

e541d433-bfe5-4e4f-8a81-60965f2d22ed.jpeg

fly boy321/12/2018 16:35:40
avatar
3454 forum posts
18 photos

Great project, upsizing has always concerned me Regarding larger wood etc,always wondering by how much to enlarge the balsa. lol. ie if you enlarge by say 50% does all the wood go up the same percentage. Cheers

David Mellor21/12/2018 17:29:33
avatar
1276 forum posts
611 photos

Thanks!

I think different people have different ideas on how to choose the wood sizes when enlarging a model.

For modest changes in size, it probably doesn't matter what method is used. But for large changes - say twice the wingspan - it will start to matter structurally.

There are basically two fundamental options when enlarging a conventional "stick built" model. One is to keep the wood the same and increase its cross sectional area. The other is to use a stronger wood and increase the cross sectional area only slightly (if at all).

Balsa is a nice material to work with because the strength roughly correlates with density, so you can do quite a lot by choosing heavier grades when scaling up.

One technical point worth being aware of is that to keep the scale-strength the same when going up in model size but using the same grade of wood, you need to increase the cross-sectional area of the sticks by the cube, rather than the square, of the linear scale factor. This can tend to make the structure look unduly bulky which is one reason why modellers will intuitively use stronger grades of wood or introduce new elements such as additional structure (more diagonal bracing, gussets, biscuits etc) or stronger materials (laminated timbers, plywoods and carbon fibre) in structure-critical areas.

Dave

Andrew76721/12/2018 20:40:45
avatar
801 forum posts
4 photos

David

Watching this with interest...Great idea.

Andrew

David Mellor21/12/2018 22:10:48
avatar
1276 forum posts
611 photos

Thanks. It will be a slow, steady build as it needs quite a bit of space.....

David Mellor23/12/2018 18:41:54
avatar
1276 forum posts
611 photos

With an AUW of 14 pounds, and a cruise consumption of 20 W/lb, the model will need a minimum of 300 Watts to take off, climb and cruise. To overcome crosswinds and headwinds it is handy to have nearer 60 W/lb. So the maximum power needed is 900 Watts.

I'll be using one of these two motors - both are Turnigy SK3 4250 motors, one is 500 kv (1300 Watts) and the other is 350 kv (1150 Watts). It will be a bit of trial and error to find the best match to the model.

p1010673.jpg

 

Since the weight budget for the power system is 78 ounces, and the motor, prop and ESC weights are also known (10.25 oz, 2.3 oz, 2.45 oz) I know that the weight budget for the battery is 63 ounces.

 

I'll be using 6S batteries and the budgetary allowance of 63 ounces for the battery translates into approximately 12,000 mAh.

 

Since the cruising consumption will be around 300 Watts, the current draw will be just under 14 Amps. A 12,000 mAh battery will, to an 80% drain, therefore provide 9,600 mAh. So the cruising time will be around 42 minutes. Which is just as well, since batteries this size are quite expensive (a lot more than the motor, ESC and prop combined).

 

 

Edited By David Mellor on 23/12/2018 19:02:02

Don Fry23/12/2018 18:53:43
avatar
3300 forum posts
39 photos

If you plan to cruise for 48 minutes, and descend, thereafter, have you any plans to have a butler, or bartender in attendance.

David Mellor23/12/2018 19:00:16
avatar
1276 forum posts
611 photos

 

I cocked up the duration estimate, Don (I blame Christmas spirits).

 

Sadly, it isn't 48 minutes, just a measly 42 minutes (I subsequently edited the post to correct it).

 

I usually give the butler the day off when I go flying, but I shall expect him to have packed a nice hamper and champers for me, which I shall demolish from the comfort of my favourite folding deck chair during the flight whilst admiring the view through window via one of the on-board cameras.

 

Merry Christmas to all,

 

Dave

 

Edited By David Mellor on 23/12/2018 19:26:18

All Topics | Latest Posts

Please login to post a reply.

Magazine Locator

Want the latest issue of RCM&E? Use our magazine locator link to find your nearest stockist!

Find RCM&E! 

Support Our Partners
Slec
Pepe Aircraft
Gliders Distribution
Wings & Wheels 2018
Motion RC
electricwingman 2017
Revoc
CML
Advertise With Us
Sarik
Latest "For Sale" Ads
Does your club have a safety officer?
Q: Does your club have a safety officer, or is the emphasis on individual members to each be their own safety officer?

 Yes we have a SO
 No, it's down to everyone

Latest Reviews
Digital Back Issues

RCM&E Digital Back Issues

Contact us

Contact us