Do I have this right, on wing loading, stall speed, and pitch speed?

[bratcher]

Hi all -

I had originally asked this in the Builders Workshop forum, but I think this one might be more appropriate...

I am in the process of building a Nasty Toes NT-104 , and I am trying to determine a) how much weight I can realistically support with this wing, and b) how much thrust and / or pitch speed I need to keep it in flight at it's maximum practical weight.

The wing span will be right at 74" including the fuselage and two engine nacelles. Total wing area will be about 608 square inches (this is not counting wing area that would be "inside" the fuselage or nacelles - comes out to 648 with those included). The airfoil is an Eppler 197. I anticipate the model alone will come out to just under 40 ounces when complete, but I would like to be able to carry a camera up...

So, I looked around, and found this (http://adamone.rchomepage.com/cg2_calc.htm) CG calculator, which also has a wing loading calculator. In addition to wing loading, it also calculates stall speed if you know the lift coefficient for the wing. I used the default of 1.0, Although it seems the Ep197 can go a little higher... Based on 648 sq. inches of wing and 54 ounces, I came up with a stall speed of 13.9 MPH.

Somewhere, I found a rule of thumb stating the prop ptich speed should be 2.5x the stall speed or greater - so a pitch speed of 34.75 MPH would be the minimum at 54 ounces (which is more than I plan on carrying, but might be a good estimate for maximum weight).

Does this sound right? Is there anything else I need to take into consideration? The props I am planning on using will give me a 39 MPH pitch speed (GWS 9x5 props powered by a pair of SK400XT's from Hobby Lobby). Power in will be about 266 watts - I am going to assume an efficiency of about 75%, which means 199 watts to the prop. Can I get off the ground with that? Assuming I wanted to pick a maximum weight based on those motors and props, should I drop down a few ounces? Or maybe I should consder a slightly smaler prop with a higher pitch speed?

Thanks,

Jay

[Sparky Paul]

The plane might very likely leap from the ground with that power.
Calculators are merely guides, and do not compel. :)

[slipstick]

266Watts, <3.5 pounds = 80 Watts/pound. Should fly with ease.

Everything else is subject to fine tuning when you get it flying. Calculators produce educated guesses. The real world often turns out different ;).

BTW you should count the area covered by the fuselage and nacelles when calculating wing loadings.

Steve

[adam_one]

According to my estimation each prop gives you about 21oz static thrust- 42 total, resulting in a thrust to weight ratio of 0.78 which is quite enough to take off the grass.
However, it seems that you are somewhat overloading the SK400XT's since Hobby Lobby recommends it to planes up to 16oz and current up to 10A.
:)

[vintage1]

Its a twin, adam...

[bratcher]

Thanks for the input guys...

Adam One - hmmm.... Would that be your calculator I am using? If so, I really like it, and your page - I haven't gone through all of them, but there is lots of good info there!

As for the 400XT, I suspect it will handle the 9x5 props ok. They will both get some bench testing first, and if it looks like too much based on heat, I will back down the the 8x6, which should be more sane (at the expense of a few ounces of thrust).

[adam_one]

Thanks for the input guys...

Adam One - hmmm.... Would that be your calculator I am using? If so, I really like it, and your page - I haven't gone through all of them, but there is lots of good info there!

As for the 400XT, I suspect it will handle the 9x5 props ok. They will both get some bench testing first, and if it looks like too much based on heat, I will back down the the 8x6, which should be more sane (at the expense of a few ounces of thrust).You're right Bratcher, I made that site during my spare time for a couple of years ago, and I'm glad if you find it useful.
I still do some updates from time to time…
As for the SK400ST, I don't know the details about this motor, so my comments were only based on the figures shown on Hobby Lobby site.
Since you mentioned 266 Watts total, it gives about 133 W to each motor resulting in about 12A if you use 3s lipos or about 17A if you use only 2s lipos.
It's wise if you do some bench testing first. ;)

[flieslikeabeagle]

I'm using two of those 400XT motors on a Multiplex Twinstar II, with the same GWS 9x5 HD props Bratcher plans to use. Full throttle current draw is about 19 A for both motors on a 3S, 10C, 2000mAh Apex lipo pack.

My Twinstar II weighs 33 ounces (factory specified weight is a full 20 ounces more, with heavy inefficient ferrite can motors and a boat-anchor NiMH pack). The two 400XT motors give it very ample power - it hand-launches with a little push, climbs out at 80 degrees, and with a fresh charge on the battery, can be coaxed (once its already moving) into climbing vertical for a short distance, hanging on the props for a second, and then falling off to one side.

I think you'll find the twin 400XT's do what you want. Be prepared for a set-your-teeth-on-edge shriek from the motors, though. The 400XT's have many virtues, price being a major one, but not even the most charitable could honestly describe them as quiet.

-Flieslikeabeagle

[bratcher]

I'm using two of those 400XT motors on a Multiplex Twinstar II, with the same GWS 9x5 HD props Bratcher plans to use. Full throttle current draw is about 19 A for both motors on a 3S, 10C, 2000mAh Apex lipo pack.

My Twinstar II weighs 33 ounces (factory specified weight is a full 20 ounces more, with heavy inefficient ferrite can motors and a boat-anchor NiMH pack). The two 400XT motors give it very ample power - it hand-launches with a little push, climbs out at 80 degrees, and with a fresh charge on the battery, can be coaxed (once its already moving) into climbing vertical for a short distance, hanging on the props for a second, and then falling off to one side.

I think you'll find the twin 400XT's do what you want. Be prepared for a set-your-teeth-on-edge shriek from the motors, though. The 400XT's have many virtues, price being a major one, but not even the most charitable could honestly describe them as quiet.

-Flieslikeabeagle


Thanks for the info, Beagle - I had been planning on using these for a while, but seeing your Twinstar posts gave me some reassurance that these tiny motors could pull this off. I have been following the 400XT thread for some time, and I am very impressed with the one I had in my E-Starter. Incidentally, mine was actually pretty quiet - quieter in fact than any other motor I have, except for a Potensky outrunner I have on an Uzi - that motor is also very quiet.

The only thing I can think of regarding the sound is the ESC - I have been using an old BP ESC (Tower Pro, I believe). I haven't spun up the new motors for my twin yet, so I don't know if the one motor is an oddball or what - the current draw and thrust seem to be in line with Kiwi's numbers, so I really think it must be a function of the ESC.

Anyway - my new twin should be done in a week or so, at least to the point that I can give it a test flight. I still need to run some wires in the wings, attach some servos, install landing gear, and coat the bottom of the plane...

[flieslikeabeagle]

Bratcher, curiouser and curiouser, as Alice would have said!

I had a 400XT on an E-Starter with an EP Brushless ESC - it screamed loud enough to deafen you when tested in the house, and loud enough to be heard plainly when the model was flying 300 ft from me at the field if there was no trafficy nearby.

I now have two 400XT's on the Twinstar II, connected to two Thunderbird 18 ESC's. The pair of them shriek so loudly that I cannot run them up indoors without earplugs, and at the flying field the TS-II sounds a bit like a full-size turboprop, what with the screaming motors and the sound of the twin counter-rotating props (I recently switched out the GWS 8x6 HD props for a pair of pusher/puller Graupner 8x6 mirror-image props from Hobby Lobby).

All three of these are the "old" 400XT's, by the way, with very strong cogging, which I think turns my foamies airframes into giant sounding-boards.

Perhaps your quiet 400XT is the new version? Red/Blue/Black wires, and noticeably less cogging?

Cool to hear of someone using the Nasty Toes bits and pieces, by the way. I've been thinking for a while that I should build something out of their parts. I wish they sold symmetrical airfoil section wing sections!

-Flieslikeabeagle