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Feb 13, 2020, 12:04 PM
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E-Challenged's Avatar
Consider electric power FF scale using cutoff timer as used in electric powered control line flying.
(Brodak Models website).
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Feb 13, 2020, 04:30 PM
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MKellyvich's Avatar
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That's in the back of my mind, after trying to get it to fly with rubber power.

Mike
Feb 16, 2020, 07:41 PM
Balsa Flies Better!
Hi Mike

Umm, I think Tom's formula of 0.5 g/sq. in. is really intended for smaller airplanes, i.e. Bostonian/Embryo size. Wing loading is really more a function of volume than area, so I think you can bump up your weight some- which is good, because a 56" airplane at 100 grams is going to be as pretty delicate! I'd ask Tom what's his weights are- I'll be those Mega airplanes are probably between 8 oz and 20 or so- just guessing. 3 decades ago, I did the Comet T-craft- think it was 8 oz, although maybe I'm remembering wrong. I did get a few flights on it- but boy, it was not easy to handle! I've still got the prop I carved.... My B-26 at 30" weighs in at around 100 grams- and that's not a sled by any stretch, so I'm thinking that a couple of hundred grams might not be far out for the size airplane you're contemplating.

I keep thinking that one way to handle such short nacelles is to come up with gearing or cable so that the motor is in the wings- things like Estes rocket tubes come to mind. There have been some ARF rubber ships that do that trick.

Sam

But you're a braver man than I- I'd hesitate to build that airplane even with electrics.
Feb 16, 2020, 09:07 PM
B for Bruce
BMatthews's Avatar
Consider that the later days of the 200 square inch Wakefield category models were required to weigh a minimum of 8 oz. Yet they still flew well. Both Korda's winner and Fred Bower's second place 200 sq inch were 8 oz or more when they flew in 1939. Yet such models flew pretty darn well.

Based on that figure the wing area of the Commodore. I'll bet it's easily going to be up around 300 sq inches. So TECHNICALLY it could be as much as 12 oz with rubber on board and still fly at least as well. Actually a little better thanks to those pesky Reynolds numbers.

But at 12 oz with all those struts it may well end up with the old "weak rock syndrome". I think it was Leon Bennet that coined that phrase in his articles in the FAC newsletter years ago.

But even at 6'ish oz and with up to 3 oz potentially of rubber it would still be a pretty darn good for that size 9 oz. Or possibly a touch less if you can get by with less rubber.

What'cha think of those numbers?

Mike, speaking of weight what were you thinking of for the functional and rather important struts and rigging?
Feb 17, 2020, 05:30 PM
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MKellyvich's Avatar
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I agree that sub-200g flying weight is probably optimistic, but hey, that's what goals are for... Taken off the upscaled plan the wing area is 325 sq in. I want to keep the airframe as light as possible for three reasons:

1 - to require less rubber (and therefore less obtrusive motor booms and less flying weight)
2 - so it will fly slower - I'm hoping for that majestic big-airplane look in flight
3 - to reduce energy conversion in the inevitable impacts (back to your "weak rock" issue)

I went back and looked at Tom's NB-3 videos but didn't see mention of the weight in any of them. I plan to follow the plan structure as much as I can, but will make liberal use of lightening holes in ribs and formers and to try and select the lightest wood that seems strong enough to do the job. Not having built a free flight model of this size it will be a learning experience...and that's part of the fun. Worst comes to worst I can patch up the dings and have an impressive ceiling ornament for the building room.

Regarding the struts, I'm thinking bamboo for the struts running from the fuselage and wing center section to the outrigger floats and robust balsa for the remainder.

Now that the Broussard has flown I'm cleaning up the shop, going through my balsa and getting ready to start eating the elephant. I plan to start construction with the outrigger floats.

Mike
Feb 17, 2020, 08:46 PM
B for Bruce
BMatthews's Avatar
200gms is just shy of 7 oz. A worthy target goal for a flying weight.

With functional struts and rigging to secure the hull to the wings the struts will also serve to support and strengthen the wings at the same time. One of those situations where one thing does two or three jobs. And that sort of thing is always good for reducing some weight from what might be otherwise.

It'll be fun watching along for sure.
Feb 18, 2020, 05:17 AM
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slowmatch's Avatar
Cool subject Ralph O'Neill's book, A Dream of Eagles is an interesting read.

Regarding wingloading - as mentioned bigger models can tolerate more. This is why so called 'cube' loading can make a better estimate for actual flying behaviour as you change the scale.

https://www.hippocketaeronautics.com...?topic=11881.0

Jon
Feb 18, 2020, 10:14 AM
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MKellyvich's Avatar
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That's a very interesting tool Jon, thanks much for the link. It pre-dates my entry into HipPocket and I'd not discovered it yet.

Punching in the numbers for the Commodore it looks like (assuming 25% of flying weight is consumed by the motors) I could get a cube loading of 3 at an airframe weight of ~215g (not including rubber), and a cube loading of 4 at airframe weight of 288 grams. The comparison chart shows the Starleaf Dash 8 flew at a cube loading of 4 - sounds like I should have a little more room to play with than I initially thought.

This keeps getting more interesting...I may have to plug in my list of models and try to correlate observed flight characteristics to the calculated cube loading. All of my current models range between 13-28" span, but they also span a large range of wing loadings (.43-.84 g/sq in including 25% rubber weight), so it should make for some interesting comparisons.

A little extra margin would be great, as I think I'm going to have to make the outer wing panels removable in order to have a chance of fitting the model and the family's luggage (and the family!) in the truck for the trip to Geneseo this summer...

Mike
Feb 18, 2020, 10:46 AM
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slowmatch's Avatar
Quote:
Originally Posted by MKellyvich
...I may have to plug in my list of models and try to correlate observed flight characteristics to the calculated cube loading. All of my current models range between 13-28" span, but they also span a large range of wing loadings (.43-.84 g/sq in including 25% rubber weight), so it should make for some interesting comparisons.
Yes, that's the way to gain a practical understanding of the numbers. And I would really appreciate seeing that list

3-5 is a reasonably range for outdoor scale. 3 is really pretty good (think Bostonian or Embryo) with 5 still giving good performance. Of course plenty of subjects fly just fine at the higher weights still but duration will suffer. And of course the weight may be especially important if you have erm... 'sub-optimal' motors due to scale constraints, or particularly draggy airframes.
Feb 18, 2020, 03:52 PM
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MKellyvich's Avatar
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Alright, I'll bare my heavy-building soul here. Assuming I translated your spreadsheet's math correctly into my master model spreadsheet, here's the cube loading for each of my models calculated for 15, 25 and 30% motors (by weight). The results correlate reasonably well to my flying observations - the Wicko (3.35 at 25%) flies really well, and the Megow Skyrocket (7.36 @25%) struggles to get 30 seconds air time. The models under 6 cube loading have all flown over 1 minute, the models under 5 have approached maxes (90-120 sec) and two of the four models under 4 have maxed (120+ sec, the Navion has some spiral instability and the Broussard is early in trimming).

When comparing the numbers keep in mind that the Waco is a biplane, the P1052, SB2A-2 and Commodore haven't been built yet and the SB2C was a friend's model that I don't have flight experience with.

All of this suggests that Bruce and Sam's 200-300g airframe weight for the Commodore is reasonable and likely to produce a flyable model.

Thanks again for sharing this approach Jon.

Cheers,

Mike
Feb 18, 2020, 05:00 PM
Registered User
Hello Mike,
However you look at it this is a bold project. I guess if I was bold enough to attempt this it would be electric power and R/C, but I realise that is not where you want to be. Electric power would be easier to control both in the air and on the ground and R/C would make trimming simpler, maybe......
I would also think seriously about using carbon rod and tube for some of the struts to better anchor the centre section and reduce the rigging wire element. Increased strength and reduced weight.

Watching with a lot of interest.

Richard.
Feb 19, 2020, 05:32 AM
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slowmatch's Avatar
Thanks Mike, great info and thanks for sharing. It sounds like we are on the same page when it comes to translating the numbers into something meaningful for practical flying. I think the numbers in my spreadsheet are before rubber though so they look more favourable The sweet spot between scale detail and good duration seems to be about 4 for me.

Looking forward to your build.

Jon
Feb 19, 2020, 11:32 AM
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MKellyvich's Avatar
Thread OP
Thanks Jon. Here's the numbers including airframe-only cube loading. Results line up well with the guide in your spreadsheet - models around 3 are competitive, around 4 are fun and satisfying, over 5 should be flown for personal entertainment...

Enough bookkeeping, time to cut some balsa!

Mike
Feb 19, 2020, 05:57 PM
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slowmatch's Avatar
Quote:
Originally Posted by MKellyvich
Enough bookkeeping, time to cut some balsa!
Haha nice one. Look forward to seeing your progress.
Feb 19, 2020, 09:24 PM
B for Bruce
BMatthews's Avatar
Hey, I found this cubic wing loading calculator online. It's a nice one since it's got versions for both metric and imperial.

I stuffed the figures in for some of my models to get a feel for the WCL values vs how I know they fly. It's not much of a list because I'm rather low on models at the moment thanks to way too much time on other hobbies over the past 15 years. But such as it is here we go.....

Miss Canada Sr.- WCL= 3.5 A definite floater at 3.2 oz with a modest size motor.
Sparky - WCL=3.7 Good performer at 2.6oz, I'd say it just barely qualifies as a "floater".
Slicker 42- WCL=5.6 The model in my avatar. Not a floater by any means. Catches lift just fine but it is dead set on speeding along and arriving on the ground with gusto.
Sig Cabinaire- WCL=4.6 Nice glide and flies well but not what I'd call a floater.
Canadian Wake Winner WCL=3.7 at 6 oz, WCL=4.9 at 8 oz. Sort of floats at 6 and glides fast and non floaty at 8.
O/D Coupe d'Hiver WCL=2.3 This one WAS a true floater. Like it parked up there. Didn't even want to come down. Only have the wing left at this point.

So I'd call this a rather educational exercise. I'd also say that if we want a model that seems really floaty like it's hanging up there from an invisible thread that we're after a flying weight that gives us no higher than a WCL of 3.5. And something in the lower 3's would be better.

Mike I see your table (which is WAY more OCD than my meager offering ) shows your Commodore listed with some motor options. You know your flying better than I of course. But I tend to prefer more glide at the cost of height and then rely on being decently good at picking lifting air. As a result even my smaller contest models tend to use somewhat less than 30% motors. My Miss Canada motor is an example. The model is 2.56 oz empty with a 0.65oz motor for a total of 3.2 oz. 0.65/3.2 = .202. So just 20% motor. This gives me a climb of around 200ft and a roughly 90 second flight in dead air. If I had a scale model that flew like that I'd be more than happy. So I'm thinking that you don't even need to look at the 30% motor figures. You'll more than likely find happiness with the 15 and 25% options and perhaps even find the "baby Bear's just right" option at 20%.

Plus less motor means less weight for the motor sticks you'll need to withstand the torque.


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