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Old Oct 29, 2012, 07:40 PM
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Hi Nick, I think you may get a better GE run with the Polaris than the WBT.
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Old Oct 30, 2012, 03:02 AM
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Thanks, Captain. However, foamies in my stable generally have a fairly short season. The Polaris was recycled some time ago, though I would recommend it to anyone. Many people on the Polaris thread have built a dozen or more of them.

The idea of the WBT was to test out a lifting body and as that it works well. Trying it with a canard seems another interesting idea. An all-moving one can be set up to improve control out of GE without necessarily providing any lift at all. Elevons will be useful as an addition to the existing straightforward elevator and ailerons at the rear. In GE I hope to provide lift and, with the gyro, to correct for the sudden pitch up on transition.

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Old Oct 31, 2012, 07:44 PM
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A New Idea

Please check the Icon A5

http://rcelectricflyer.com/parkzone-icon-a5-review/

My canard Twin flys well with the central CG and the motors positioned just aft of it. Wanting the same performance without the hassle of two motors, a plan was drawn to reduce the twin by 7% and to place a single motor at the center of the fuselage by cutting it down similar to the Icon A5. The model will be built light weight with a high revving motor turning a 5 inch prop. The canard will be high as it is on the Twin but the rear wing will be low with a tail dragger gear. It will run on a 2200 three cell battery and should weigh less than two pounds. The wing construction may be built with only front LE and rear spars since the front third will be sheeted. The side view plan is drawn. Any interest or ideas for such a model?

Charles
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Old Oct 31, 2012, 08:59 PM
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Nick do you still have the polar stuntcat? That one should do fine. The large inner wing of the WBT will cause to much of cp shift IGE. The polar stuntcat may work even better than the Polaris.
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Old Nov 01, 2012, 05:39 AM
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Charles, you didn't specify watts for the motor, but inferring from your battery size, I suspect that a 5" prop is going to be way too small for that much power, especially if roughly a fourth of it is blocked by the fuselage. If you want equal performance from approximately equal power, you need to get the prop running in clean air, and with roughly the same total disk area as your twin. That's likely to be difficult in the layout you propose, without getting into thrust line issues. However, if you don't, you are likely to find that your power system is very effective at turning watts into noise, but not much else.
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Old Nov 01, 2012, 02:03 PM
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Hi Don, always a pleasure to read ur posts
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Old Nov 01, 2012, 10:12 PM
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Don, Thanks for your help. The motor is the Grayson Hobby 2212-06-V2 which claims to be like a Speed 480. It will stand 20 amps at 11.1 volts. It has a long shaft which will put the 5.5 APC prop 1.5 inches aft of the motor mount. If the motor is mounted as an exposed pusher, it would only block about 1 inch of the prop center from the airflow. The fuselage will have to be tapered to a point below the motor to facilitate air flow. Your input led to the above thoughts. The 20k plus rpm could be irritating but to keep the thrust line low it will have to do. A picture of the motor/prop assembly will follow soon.

Charles
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Old Nov 02, 2012, 07:14 AM
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That's potentially 222 watts, although that will depend on what the prop can absorb. Two things could happen. Either the prop could reach the limits of what it can handle, and at that point it stops loading the motor enough to pull the full wattage (and the efficiency will be too low to make much thrust from what it does pull), or else it will pull the full wattage, but without making much thrust from it.

The GWS prop we're referring to is probably the 5-4.3, same as the ones we use on our Roadkill Series models at about 6 watts, or else the 5-3, which would pull less watts, but be a better fit for the RPM. I am not aware of a GWS 5.5" prop, the next size up is 6". At 20K RPM (which I doubt that either of those props can really handle without becoming a "zero-bladed prop"), that's a pitch speed of 119 feet per second (81 mph) for the 4.3, or 83 FPS (57 MPH) for the 3" pitch.

At 222 watts it's also a Cp ("power coefficient") of 0.148, which is impossibly high (0.03-0.05 is more typical). Assuming your plane flies at about 30 MPH, the angle of attack on the blades is certainly enough to stall them, which that ridiculously high Cp would also indicate.

Any way you cut it, it's not going to be an effective (or safe) way to turn those kinds of volts and amps into thrust.
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Old Nov 02, 2012, 10:21 AM
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Don, Super discussion!
The prop will be changed to an APC 6-4. They claim about 70 mph on a Park foamie but I feel that with my drag, it should slow down appreciably. The camera needed a charge so the pictures will come later. My changed drawing shows the prop without blocked air except for the canard's influence.

Charles
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Old Nov 04, 2012, 02:48 PM
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Hi Charles
Who'd have thought that cutting back to one motor would have made life more difficult than with the twin?

How's about lifting the single motor up about an inch? Shouldn't ruin the visual look you appear to be after, but it will allow you a little more leeway in the prop and motor selection dept.

Be a real bummer to build such a nice model and then find out you couldn't get enough power down the prop blades to fly it decently.

Nothing to do with this model - but just to prove I do more about canard'ing than just read this forum, I cast an eyeball over my 'VSC' - Very Short Canard' - plan last week, with a view to building one this winter. It's basically a "Stand Way Off Scale' version of the Despretz Flash single seater canard - which, with the prop just ahead of the front and low mounted wing, sounds like some kind of trouble...

Good luck with the new baby duckling.

D
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Old Nov 04, 2012, 03:47 PM
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Dereck
Quote:
How's about lifting the single motor up about an inch? Shouldn't ruin the visual look you appear to be after, but it will allow you a little more leeway in the prop and motor selection dept.
Thanks Dereck, The little motor is maxed out with a 6-4 APC and has 480 type power. The main man at Grayson picked it for my job. I have found that this hobby is more fun to me when a task is risky. The challenge of past projects has kept me motivated to build and see the results.
I hope that you will build that Flash! Here are some shots from Andy Lennon's book.

Charles
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Old Nov 05, 2012, 03:31 AM
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Quote:
I have found that this hobby is more fun to me when a task is risky. The challenge of past projects has kept me motivated to build and see the results.
Quite right Charles.

How about a three blade prop? I've had acceptable results by putting two 5" props on a single motor (1800 Kv).

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Old Nov 05, 2012, 10:22 AM
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If you have too much power and not enough diameter (i.e.: a high "disk loading"), more blades can help, which in fact is why you often see more than two blades on full scale applications. Our disk loadings almost never get high enough (with the exception of ducted fans) to need more than two, and more blades is not as good as having enough diameter to begin with, but if you are forced into too-little diameter, more blades can help.

That said, you do need to be careful with four or more blades because of something called "reactionless modes". This is where half the blades are flapping one way and the others are flapping the other way, so there is no net vibrational force transmitted back to the motor and airframe, hence the name "reactionless". The airframe feels nice and smooth, no unusual level of vibration, but meanwhile the props are busy beating themselves literally too death!

You can get reactionless modes from very long and flexible blades (like some very slender helicopter rotor blades), but normally it only comes up when you have four or more blades. A four-way has one reactionless mode frequency, a five-way has two of them, three with a six-way, and so on. They tend to be at lower frequencies, usually below the normal operating range. On full scale props they can really complicate things. Stiffening up the blades to try to drive the resonant frequencies up above the normal operating range will often pull the reactionless modes up into the operating range. They often get triggered when on the ground, below normal flight RPM, especially for things like taxiing with a quartering tailwind.

I recall one case where a particular regional airline had a blade failure (one of the most serious things you can encounter on a full-scale aircraft, other than having a wing fall off), and subsequent inspections disclosed cracked blade shanks on a number of their other planes. On that particular type of engine there was an overspeed governor test that was part of the pre-takeoff runup check. This test would pull the prop RPM up enough to get into the reactionless mode RPM range, and so this test was specified to be done with the airplane parked facing into the wind, where the excitation of the reactionless mode would be minimal. To save time, these folks decided to do the overspeed governor check while taxiing downwind towards the end of the runway for takeoff. The tailwind condition dramatically raised the blade stresses during this check, resulting in blade failures. When you consider that the centrifugal force on one aluminum blade is typically around 40,000 to 50,000 pounds, a rotating 25 ton unbalanced rotating force on the end of the prop shaft causes a great deal of mayhem! Yanking the entire engine right off its mounts, or folding the engine 90 degrees to one side is a fairly typical result.

The CF on one of our blades is not normally that high, but 400 pounds or so is not unusual. Also, smaller props typically run at higher RPM's, so the CF on a blade on a small prop is typically not all that much different than the CF on a larger prop.

Bottom line: respect those props, even the little ones!
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Last edited by Don Stackhouse; Nov 05, 2012 at 10:27 AM.
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Old Nov 05, 2012, 11:26 AM
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The more I read of Don's words of wisdom, the less I want to go flying in real aircraft

Charles - oddly enough, my inspiration drawings for an ittybitty canard are the one's you suggest. I have Andy Lennon's book, plus that 3 view appeared in a magazine article many years ago. As we're having the condo painted, my 'shop' is now storage for about everything breakable/moveable in the rest of the place, and I can't actually see much of my dearly remembered hobby right now.

But if the world doesn't wobble too much, I shall really enter the canard world this winter.

On canards - anyone else see the cute one in the latest Model Aviation mag? It's in the two pager of AMA members' model photos. Basically, the builder took a moulded foam e powered soarer kit, with a pusher prop atop the wing TE. He altered it to a canard fuselage, that added a longer nose to the kit front end, put the kit tailplane at the front and mounted the kit fin/rudder centrally on the wing to clear the motor/prop, which looks to be where it normally would be. Flies good looks cute and confuses his club mates according to the caption.

He started with the pricey regular version of this soarer, but there are cheaper rip-off versions around if anyone's interest is perked up.

D
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