|Sep 08, 2002, 02:08 AM|
How do I determine model/engine size?
I'm stuck on a problem, and I could use some help. I'm planning on scratch building a FW 200 Condor over the fall (and possibly the winter as well). I have several 3-views, and access to a set of plans (from Bob Holman) that I can scale down, but I just don't know what size to build it, and what motors to use.
I know I want to use geared motors (because I think a geared motor sounds more like an engine), and that I'd like to keep the plane's wingspan down to under 6 feet. I see a lot of DD speed 400 planes, and a few geared ones as well, but these all seem a little big for me. Likewise the GWS geared motors (the small ones like in the TM) seem a little too small.
In addition, I'd like to use can motors instead of those nice brushless ones, because cost IS an issue.
Has anyone here built a 4 engine bomber (b-17, b-24, etc.) using geared motors smaller than speed 400s, and bigger than the GWS "A" motor? If so, then what size motor/props/battery did you use? What was your wingspan, and all up weight? Oh and did you feel it was underpowered, over powered, or just right?
I figure once I have an idea of size, then I can deside whether or not to add retracts.
Thanks for your help
|Sep 08, 2002, 12:29 PM|
My son built a FW-200 Condor from a Multiplex Cargo kit. You might want to do that first. I'll take a photo of it and post this evening.
|Sep 10, 2002, 01:49 AM|
Nice plane. Sort of a stand off scale, but your son definately captured the look, especially in the nose area. Thanks for the photos.
|Sep 10, 2002, 08:47 AM|
I would think a promising direction would be to use Permax 280BB's geared about 1:2 and propped to run at about 6A on 6 cells. The regular 280 is only happy at half the current and would not be so well suited.
4 geared 280BB's would give you about the same power as two 400's, so you would be in 'Twinstar territory' regarding size and weight of the model.
If you've not read this thread it may help :
|Sep 17, 2002, 02:01 AM|
ChrisP - A very warm thanks for the inclusion of the link. It was exactly what I've been looking for.
For the record. I've been doing some number crunching, and I think I'm going to go with a 72" inch wingspan. For a more scale-like flight performance I need a pretty low wing loading. If I can hit the 12 to 14 ounce per sq. ft. realm (or even under) then I'll be very happy.
I will not make the final engine choice until I have built most of the plane, and have a better idea of how much it will weigh. Then I can bother someone with motocalc, and do some real number crunching. I will be keeping all flight surfaces as close to the scale size and proportion as possible.
Here's a few numbers:
FW-200 at 72" wingspan
scale prop size is 7.5"
Wing area 564.5 sq.in. or 3.92 sq. ft.
With this wing area I figure wing loading at
12 oz./sq. ft. = 47.1 oz. or 2.94 lbs.
14 oz./sq. ft. = 54.8 oz. or 3.43 lbs.
I've looked at 4 speed 400s geared fairly high (3:1 or 4:1) each using a 4 blade 7" Vario prop, and all 4 powered by an 8-cell 2000 NiCd, or a 10-cell 2400 NiMh. Total weight for all electronics (motors, battery, radio, servos, props etc.) might be as much as 31 oz. This gives me an empty weight target of around 20 oz.
Ouch! that'll be tough.
At the above mentioned weights, each of the 4 motors will need to draw from 4.6 amps (10-cells, 2.94 lbs) to 6.6 amps (8-cells, 3.43 lbs) to attain a conservative 75 watts/lb.
I know from my Grumbler that a geared speed 400 can fly a 28 oz. plane, with a very large wetted area, quite well. This tells me that the plane will fly with an All Up Weight between 80oz. (5 lb.) and 100oz (6.25 lb.). I just would hate to go that heavy because the flight speed will only be increasing as the wing loading gets up that high. I'm NOT that good a pilot that I would want to fly (or land) a 4 motor rocket.
A slightly smaller version (59" wingspan, like I originally wanted) had such a disproportionally smaller wing area (378 sq. in.) that the all up weight would have to be an amazing 31.5 oz. (12 oz/sq. ft.) to 36.75 oz. (14 oz./sq.ft.). That would be a hard target with 4 electric motors, and a battery big enough to push them. Believe it or not, if I could hit this target weight, I could fly it on 4 GWS "D" motors swinging 4 blade props.
I even looked at a 96" wingspan version. A FW-200 at this size has slightly over 1000 sq. ins. which would give you pleanty of room to build. I vetoed this only because a plane this large won't fit into the trunk of my car (a Honda Del Sol).
BTW: Pat Tritle, designer of the Dare Engineering B-17 (72" span) was very helpful, and personally answered my many emails concerning the size, weight, etc. of his plane. He uses 4 geared speed 400s using the mini-olympus gear box, and 9x5 props. His AUW is 4.3 lb. with a 13.5 oz/sq.ft. loading. Power is from a single 8-cell 2000mAh NiCd, and flight times are in the 6 minutes range at 60% power.
My thanks to everybody for their help and info.
|Sep 17, 2002, 12:04 PM|
You're squeaky close to what I would do :
564.5 sq ins are fine (assumed net, i.e. without fuselage area)
57 ozs AUW gives you Twinstar wing loading@ 14.4 ozs/sq ft. I think that 12ozs/sq ft is unrealistic for an outdoor flyer.
Bare weight of model max 23 ozs (you can do it !!)
Gearing 1.1.85 with 7x4 props on 8x2400 mAh cells. 1:3 or 1:4 is too 'slow' a gearing.
The Revell 1/72nd scale plastic model is good as a reference source.
Go for it !
|Sep 17, 2002, 02:28 PM|
ChripP - didn't you quote the math for prop area on that other link? My math skills are a little fuzzy so I welcome any corrections.
My Grumbler flys an 11x7 on a 3.6:1 (or there abouts) MPJet gearbox on a speed 400. Without moving the nascels out I can't go with a bigger prop, and I'd rather gear higher, and fly slower. At least I think I got that right.
|Sep 19, 2002, 06:44 AM|
Nope, it wasn't me.
A prop with a 4'' pitch will have 4'' of pitch however many blades it has or how big the diameter is.
I do have a calculation program I wrote myself in Excel and it tells me that the model should be targeted to fly at around 15 metres/second (54 kmh or ca. 33 mph) for the sort of wing loading and power in mind.
If you gear up to 1:3 or 1:4 you will have to be going to props of the size 7x8 or the like to get enough pitch at the rpm's available. This high pitch would be necessary to match the speed of the airflow from the prop to the speed of the model in flight (plus the magic fudge factor of 20% for 'slippage').
You should take a look at the way the Multiplex Cargo is set up regarding prop sizes and gear ratios. Sorry I don't have the details myself, but it is definitely far away from 1:3 more like 1:2 max or even 1:1.5.
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