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Old Oct 20, 2004, 01:57 PM
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Anderson, SC
Joined Oct 2004
13 Posts
1941 Class B FF "ANSWER'

[FONT=Times New Roman]I have the plans for Gordan Murray's 'Answer' which I would like to build as an electric sport flyer. It was powered by a .19 ignition, spans 44" with 310 sq.in. "Ritz" wing.Iknow nothing about electrics. Would someone please tell me what size motor, battery, etc. would be appropriate.? I would use a "Cannon mini" radio init.
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Old Oct 20, 2004, 05:42 PM
Balsa Flies Better!
Stamford, CT
Joined Oct 2000
6,704 Posts
Brave soul- building that wing. Anyhow, I think I have the big brother- a Super Skorpion, but it's not going to do you a lot of good. Conversely, I just finished off a slenderized Matador- around 325 squares, originally flown on an 09 diesel or so- 1947 design if memory serves. I'm going to assume that the construction of the Answer and the Matador is pretty close, so the power system I have in the Matador should work. oh yeah, my all up weight is 20 oz.

Anyhow- I'm using a Speed 480BB (I think it's the race version) in a MP Jet 3.5:1 gearbox swinging a 9.5 x 4 Graupner cam folder. Power is from a pair of 3 cell 1020 li-poly packs. It's maybe 65 watts or so, I haven't tached the prop. The climb is adequate- not rocketlike, but this is a gentle airplane to fly. Low and slow is phenomenal.

Assumptions- on that radio- you're on a budget. I hope you're prepared to do some connector soldering, most current stuff uses very different connectors.

Reason for choices- the motor is overkill in terms of weight and power- it should easily handle double the current load. However, you may be running it for extended periods of time and it'll last forever doing that. Also- it's heavy by an oz or two- and that airplane has a short snoot. You want a gearbox for these airplanes- I'll take a brushed motor, gearbox and big prop over a brushless outrunner any day. There's very little point to a brushless motor on these airplanes- you need the weight.

MP Jet gearboxes are OK- they're not wonderful, but they work well in the $20 range. I only use the BB versions. Remember the prop adapters.

Speed control- I like Castle Creations stuff- works well and Pat Castillo is a nice guy. A Pixie 20 is overkill but if you decide you want a larger prop, you'll have the reserve.

Batteries- If you have no electric gear and can follow directions well- I recommend li-polys with a dedicated charger. Do read the li-poly threads at the top of the Batteries page- these things can bite if mistreated. People have parted with first generation Kokam 1020 3 cell packs on a fire sale basis- you'll need to parallel them (use a harness and charge each pack separately!), but they work fine in this application. Alternatively, any 8 amp rated 3 cell li-poly pack will work fine as well.

Batteries- try Dave Skokan at www.lightflightrc.com

I'd call Kirk at New Creations for the rest of it- and he'd be happy to sell you batteries too, but he'll be a bit steep on battery cost. (936) 856-4630 Make sure that they know you have nothing- there are a whole bunch of doodads you'll need.

Sam
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Old Oct 20, 2004, 06:24 PM
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tim hooper's Avatar
Telford, UK
Joined Feb 2000
7,488 Posts
antiquepete,

Welcome to the RC Groups forums!

It just so happens that I have built both the Answer (in Scorpion44 guise) and the larger Super Scorpion for electric power. Both models are built from Ben Buckle plans.

Briefly, the Scorpion44 I have uses a 6v S400 motor, a Mini Olympus 2.33:1 gearbox and a 7x500AR cell pack. The prop is a 10x5 APC E. AUW is 23oz. The models ROG's with ease, and can be persuaded to perform a very sloppy loop or roll after a short dive.

More details here;

http://www.rcgroups.com/links/index.php?id=4300

tim
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Old Oct 22, 2004, 01:35 PM
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Anderson, SC
Joined Oct 2004
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Sam- Thanks for the help! I like the bigger motor idea for the Answer because the nose is short. I wonder if I need to imbed a 1/4" square spar in the wing for strength? I'll have to get the motor and gear on hand to determine if the cabin is large enough - pretty narrow on plans.
In re radio -what servos would you recommend for this old Cannon rig? Also, what is the advantage of the BEC? Pros & cons?
Thanks again for the help.
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Old Oct 22, 2004, 01:50 PM
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Anderson, SC
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Tim - Thanks for the quick help! I'm curious about the Scorpion - who designed it and when? Your info on building the Scorpion was very helpful, particularly the no firewall fuselage and Litespan covering. I'm thinking of substituting 1/8" spruce for longerons and cabin area uprights - 3/6" sq. on the original - balsa. I also like the laminated stag TE and LE.
Your pics from Old Warden were great - been there but not when flying was in progress. Got to go there again.
Thanks for getting me onto the proper site.

Pete
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Old Oct 22, 2004, 02:00 PM
Balsa Flies Better!
Stamford, CT
Joined Oct 2000
6,704 Posts
Hi Pete

You're on your own in terms of the wing- if I remember it was a Ritz? section- basically a solid block from which you carve cutouts for the ribs? I don't know how often these airplanes folded the wings, but if they had a penchant for doing so, then a little extra beef might not go amiss.

Yep, that cabin is skinny- I'd mount the servos during construction- well before I add the acetate for windows. In terms of servos- you really don't need much. I'll bet Hitec HS-55s are plenty. There are cheaper servos- but Hitec's warranty is hard to beat (you break em, they fix 'em for free- just may take a bit.) I think the Cannon rx should work fine (as should Cannon servos if you have any- but the Hitecs are smaller, lighter and cheaper.) but do be prepared to change the connectors on the rx yourself.

In terms of BEC- I haven't used a separate rx pack in years- I find it wonderful. In this power range- no reason not to use it- simpler lighter and more convenient.

An alternative to spending money on power systems at this point- you might start building and when you get closer to complete- check the weight of the airframe and do a little guesstimating. Tim's Skorpion came out a bit heavier than mine, and consequently he went to a fatter motor. If your airplane comes in really light- we can juggle power systems to match.

Sam
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Old Oct 22, 2004, 02:55 PM
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Telford, UK
Joined Feb 2000
7,488 Posts
Pete,

I'm glad you got here allright - I was beginning to fear that my message had gone astray..........

"First published as a freeflight design in the August 1940 issue of Model Airplane News in the US under its original name of the ‘Answer’, the design is credited to American pilot Gordon “Scotty” Murray V.C, an American-born RAF pilot. Murray was a Spitfire pilot, which may explain the elliptical planform of the model’s wing. Sadly Murray was killed in action in 1942.

Kitted for a short time as the ‘Topper’, the model was then redrawn by the noted Albert Hatfull, before being kitted by Keil Kraft and marketed as the ‘Scorpion’.

In 1975 Joe Travers enlarged the design by 50% to become the Super Scorpion, and in 1978 Mike Whittard converted the model to radio control. Ben Buckle made some revisions to the tailplane and wing-section in 1982, and the Buckle company has supplied both kits and plans to the present day."

I wrote the above a while ago, and I'm reasonably sure of its accuracy - although I expect that someone will tell me otherwise.

I'm not familiar with the structure of the original Answer, but it's worth remembering that it was built to carry the weight of a sparkie engine and its batteries and survive uncontrolled landings. Our models can be lightened considerably! My Scorpion44 has 1/8 sq balsa longerons, with 3/32 sq stringers. Nice and light!

tim
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Old Oct 28, 2004, 07:30 PM
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Anderson, SC
Joined Oct 2004
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I took a plunge into the wonderful world of electrics at Randy's Hobbyshop in Mt. Pleasant, SC this week. Well-stocked shop with very knowledgeable people, and here's what I acquired for the "Answer":
Master Airscrew S400 7.2V. motor
MPI 3.1 gearbox
Pixie 7P ESC
GWS 10X4.7 prop
7 cell NIMH 730MA battery
HobbyZone Peak charger with 12 V. input.
Whaddayathink?

Charged up my old Cannon (1989) xmitter and batteries were dead; opened up and found a corroded mess in the battery box. E-mailed SR batteries for replacement pack and got outstanding service - order acknowledgement in 20 minutes.

Laid out all these components on the plans and will have enough room in spite of the narrow crosssection at bottom longerons.

The wing is constructed of 1/4" sheet with the center cut out, wet down and formed into an airfoil on a jig. Ribs are stripped from 1/16" on 1" centers - 36 in all. I like tim's idea of shimming the construction board to build in washout, and I have a sheet of 1/4" Masonite for forms for laminating tail outlines. Tim -how is the Skorpion wing built. The Answer wing is called a Ritz wing - from another Brooklyn modeler named Jerry Ritz, who pioneered it.

Comments?

Pete
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Old Oct 28, 2004, 08:18 PM
Balsa Flies Better!
Stamford, CT
Joined Oct 2000
6,704 Posts
Hi Pete

I'm sorry to say that I'm not impressed with their choices-

1) Speed 400 7.2V motors are best used on 10 cells or so. Sounds like the clerk thought that the motor nomenclature meant something- so he matched a nominal 7.2V battery with a motor that says 7.2V. Problem is that the hotter wind Speed 400 6V motor is what people use on 7 or 8 cells, and 10 cells is pretty common for the 7.2V version. People in the game would know this. Yes, it's annoying. Spare us the observation that we need standards- we don't- we already have plenty to choose fromhttp://www.rcgroups.com/forums/editpost.php?do=editpost&p=2824790#
Big Grin.
2) Speed 400 7.2V motors should be able to handle up to 10A. While the Pixie 7P is a great little controller, you may really need to be pumping 10A through your motor to get some climb- especially if you come out a smidge heavy. Alternatively, if you need to stay with a low number of cells- a commonly used power upgrade is to swap a 6V motor which handles 12A or so- definitely a bit much for the Pixie 7P. However, a Pixie 20P is within $5 of the 7P- weighs an insignificant amount more in this size airplane, and will handle a 480 BB Race if you really want a power upgrade.- A very poor choice in Speed controller unless you told him you were a weight fanatic.
3) Batteries- 7 cell NimH packs of around 720 mAH typically handle about 6A or so- maybe more with newer cells but I'm out of date. This translates to about 45 watts max- within the limits of the controller, but the motor can take more. I'd be worried that you're really on the light end of power- if this airplane weighs more than 16 oz, you'll probably find it marginal in power.

Don't know about the charger or how good the gearbox is- except I had an MPI gearbox once- and by the time I was done, I wanted to take a sledgehammer to it. Might be a different one though.


I take it you decided that li-polys weren't worth the risk? A perfectly reasonable conclusion- but I'd have gone to a 10 cell NiMH pack then- and one capable of at least 8A and preferably 10A.

Sam
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Old Oct 30, 2004, 03:20 AM
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Telford, UK
Joined Feb 2000
7,488 Posts
Pete,

I agree with sam wholeheartedly!

The 7.2 motor is too 'soft', the cells won't supply the current, the ESC is too small as is the prop.

Check the list I gave in my previous post. I'll add that you'll want an ESC capable of coping with at least 15amps, and I'd recommend you switch to 1050KAN NiMH cells too, as they're capable of delivering the amps you'll pull.

Sorry.

tim
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Old Nov 01, 2004, 08:15 PM
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Anderson, SC
Joined Oct 2004
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Well, I got sold again - I once bought a Rogers 29 sparky - a slag engine with1/1 gas/ oil ratio!

I'm really confused by this motor volts designation and the actual voltage the motor will handle. Hooking 12 volts into a 7.2 volt motor seems to lead to short motor life.

Should I get a S400 6 volt and a 20 amp ESC? Would this do the job with my battery pack?

I think I have a use for what I have - a 1/2A with 270 squares and light. I 'll put off the Answer because I don't think I can build that Ritz wing strong enough and make the weight.

Thank you for your help and counsel- sorry I'm so ignorant.

Pete
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Old Nov 01, 2004, 08:18 PM
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Anderson, SC
Joined Oct 2004
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Tim:

Thanks for your help. What prop should I use with what I have?

Sorry I'm so ignorant - we colonists have our moments!

Pete
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Old Nov 02, 2004, 03:01 AM
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Telford, UK
Joined Feb 2000
7,488 Posts
Pete,

The voltage values attached by the manufacturer are purely titular as far as we're concerned;

A 6volt S400 will be quite happy on 7 or 8 cells (8.4 - 9.6 volts) for hundreds of flights, given some cooling air and a current draw of less than 10-12 amps.

A 7.2volt S400 will be similarly content on 12v.

My own standard set-up for a vintage or simple scale model of between 20 to 24oz is as follows;

6volt S400 motor
2.33:1 gearbox
ESC capable of at least 15 amps
9x5 to 10x5 prop
7 or 8 cells (KAN1050 NiMH for example, which are capable of sustaining a 12A current drain without flinching).

Assuming a 7-cell pack produces around 7 volts under load, and the motor eats 10 amps, then that's an input of 70 watts (amps X volts = watts) - plenty for a 1˝ slow-flying model!

As for the gear you've already purchased, I'm not sure I can think of an immediate application. However, swopping that 7.2 S400 motor and gearbox for a geared S280 might well be suitable for a smaller model; I flew a small (16oz) Aeronca on similar.

Gets complicated, doesn't it?

tim

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Old Nov 02, 2004, 07:56 AM
Balsa Flies Better!
Stamford, CT
Joined Oct 2000
6,704 Posts
Hi Pete

In an attempt to clarify Tim's post above- I'm going to muddy the waters a bit further.

Battery manufacturers are only slightly better than motor mfgs at providing useful descriptions of their products. Thus, as Tim pointed out= a 7 cell nickel pack (either nicad or NiMH) will probably hit around 7V under load- that's in use for e flight, but it's label will say its 8.4V since the battery mfg doesn't intend for the battery to be drained in 10 minutes or so- at a lower rate of drain, it will be closer to 8.4V. Thus, a 10 cell pack which may be advertised as 12V really delivers about 10V under load does that make you feel better about using the 7.2V motor with such a setup? I highly recommend the FAQ on this site which explains a lot of this stuff.

The one part of the combo that's going to be really hard to use in anything is the battery- an 8 cell 720 NiMH pack works very nicely with a Speed 300 6V motor though- but 7 cells needs a Johnson 250- and those don't last very long.

Sam
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Old Nov 17, 2004, 03:33 PM
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Anderson, SC
Joined Oct 2004
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Sam-Thanks for your advice and counsel. I bench tested the rig I bought- everything worked as advertized. Low voltage cutoff, BEC, throttle. Ran 9 1/2 minutes at varying throttle settings and cycling the servos. I charged the 700mah battery at 700 MA. What is thew max charge rate for this battery? The MPI red. gear is a noisy bugger!
I will backburner the Answer until I get more power and get my radios checked out. Meanwhile I'm scratch building a test airplane - 265 sq.in.- and testing my designing and building light skills. It's hard to not beef up the structure like IC R/C. I followed Tim's thread on the WACO and it scared me.
Thank you again for all your help.

Pete
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