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Old Jun 20, 2012, 06:44 AM
Registerd Beaver
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Originally Posted by conrad907 View Post
Phil,
Does the 6X4 pitched prop have better pull out of the hole, than the 6X3 pitched prop? Also, does a 3 bladed 6 in prop of the same pitch drag the motor down resulting in less thrust than a 2 bladed version of the same pitch?
Conrad
Hi Conrad, yes the 3 blade takes more work by the motor to rotate.
rpm would drop a bit but amps will raise to compensate for the higher load.
Less thrust is probable because the revs are down.

A basic rule of thumb is that a 6x3x3B prop will have the same load on the motor as a 7x3x2B or a 6x4x2B. The extra blade basically adds the same load as an inch in diameter or an inch in pitch (but not both)

the usual disclaimers apply here as it's all about what speed the model needs to fly as to what pitch is required.
This model at 30oz will fly comfortably at around 20mph (35kmh)
If you used a prop of 4" pitch at 10,000 rpm WOT it roughly equates to 40 mph pitch speed (this is the speed of the prop-wash not the plane)
The plane will fly at around 60% of propwash speed which then tells me that it will fly at ~24mph = right where you want it.
Of course if your motor can produce 15000 rpm on full load with a 4" prop then you have tons of power up your sleeve.

The lower the pitch, the more low speed thrust the plane has.
Like starting off in your car from a stop sign in 1st gear versus 4th.
1st gives you great thrust to get momentum going but you run out of useful thrust at 20mph - you need to thrash the engine to get more speed.
If the freeway speed is 60mph, you will fail to keep up and this is undesirable (if it were an aircraft you don't have enough airspeed to fly and you stall.)

Now try 4th gear, you will have to nurse it up to speed, avoid burning out the clutch (= overload on an electric motor) but you can do it and then you can fly at the correct speed and be kind to the engine & clutch.

Seaplanes need that first gear ability to get moving, to get air over the surfaces for control to kick in early, then (because we can't change the pitch on models) they need high rev capability and sustainability to be able to cruise at a respectable speed.


Complicated? sounds like it but you're in the ball park to get it airborne and stay there I think.

Phil
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Old Jun 21, 2012, 12:07 PM
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Everyone's patience and answers to my novice questions has really been a help on this plane. I have re-weighed everything, and it looks very possible I will come in at about 28 ounces finished, including my estimating a battery at approximately 5 and a half oz.. It looks like the boss will let me return home by the 25th, but I know I will not make the end of the month deadline unless I cut corners on the finish assembly.

I will have it in the air by sometime in July though, and I am feeling confident that with the help I have received here for the power system, it should be a flier.

I am really looking forward to see your plane when it's finished Phil, it's a real beauty!

Conrad
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Old Jul 20, 2012, 06:34 PM
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Been a while since I updated. I lost a week to fishing when my brother came for a visit, plus the week at a time out of town work schedule slows my build down, but I am making some progress.
Servo's and controls are all in, tail section is mounted motor mounts fabricated. Dual ESC's are in, still need to make extended leads for the motors and wire the battery side of the ESC's to a single battery input. Some work remains on the wing, and some other small details require finishing.
Many things are coming out good, but weight is still the nemisis.
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Old Jul 20, 2012, 09:15 PM
Mustang Fever
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Lookin good, Conrad.
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Old Jul 21, 2012, 01:29 AM
Big gov never Works
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Quote:
I lost a week to fishing when my brother came for a visit
Nothing wrong with that!

Very clean work!

Fuzz
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Old Jul 21, 2012, 08:17 AM
It flew once before...
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Looking great
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Old Aug 18, 2012, 06:16 PM
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Been a while since I posted an update. I was hoping to have more progress before doing an update. My replacement plastic arrived from Guillow's, so I have installed the rear pylon fairing now, and finished the pylon to match the shape of my semi-elliptical Davis airfoil. I had to modify the plastic cowls quite a bit as well as reinforce them some. Getting the motor mounts to come out just right at about two degrees positive incidence yet having the wing at 5 degrees positive incidence is finally done. The elevator is about 2 degrees positive incidence as well. I measure incidence from TE to LE.

Many little details to wrap up still, but getting close to a finished plane. I fear it will be heavy. I will be lucky to come in at 32 ounces. The final choice of motors, and speed controllers with 13 ga. wiring blew my weight budget. I want to buy a programmable transmitter and program in differential speed control.

What a learning experience it has been! Look forward to building a simple single engine model after this. I have been slow to post updates, but appreciate the encouragement and help I have gotten along the way, I would have been lost without the help of this forum.
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Old Sep 05, 2012, 10:44 PM
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A few updated photos of my PBY. Finally getting down to small details. The bad news is the weight. Plane came out at 32 ounces without the battery, so it will probably be a hanger queen. Still have detail painting to do, decals, cowl mounts, and canopy mounts left to fabricate. I just taped the engine cowls and canopy on for the pictures. I also need to re-install the aileron linkage arms.

Since it is my first RC plane, it was a learning experience for sure. I know I could rebuild this kit and shave the needed weight after having done it once, but it is a very hard kit to build, IMO. I'll need to think about a redo for a while!

I will continue to finish off the little details while getting anew project going now.
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Old Sep 06, 2012, 04:54 AM
Registerd Beaver
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Looks fantastic Conrad.
hanger Queen? well maybe for a while.
I can tell you it will fly even with another 5 oz in it.
Mine was around the same weight.
It's a huge wing area and will have no difficulty.

I would suggest thought that you do shelve it and learn to fly a high wing monoplane first until all your reflexes are automatic.
This model is not at all a pig to fly - just unique.


Phil
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Old Sep 06, 2012, 08:48 AM
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Thanks Phil. The great job you and Fuzz did on these Guillow's Catalina's is what inspired me to try one for myself.I was reallly afraid I had gotten too heavy to hope for a chance at flying. I am encouraged greatly to hear you think it is possible at this weight.

I will do as you suggest, and make my next build a high wing monoplane to get some skills. I have been flying a Hobbyzone Super Cub trainer, but it is a very light foam plane. It can be a handful in even a slight breeze, really wants to float.

I think this hobby has gotten its hooks into me pretty good, can't wait to start on a new build. I will post some more pics when I get some of the other finishing details done on this.
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Old Sep 06, 2012, 04:07 PM
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No probs Conrad, that is actually a very good starting point.

When you master the Cub, start adding some weight to it to make it behave differently, then remove some and move the CG back just a few mm to feel how it changes the plane.

If you can get it to the the same wing loading as the PBY and a roughly equivalent CG point you will be as ready as ever to switch.

The PBY will never want to be flown near stalling point (it will not recover like the cub will) so learning to maintain flight at a similar wing loading will teach you a lot about rolling into turns while adding up elevator + increased throttle.
Next step is to practice landings - throttle is all that should be used - set up the approach, throttle down to minimum revs (does not allow flight but ensures airflow over the tail for elevator to have authority) and allow it to sink, as it is about to land, flair out and cut throttle.

That will help a lot and iron out any bad habits you may have (or not have) acquired yet.

Phil
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Old Sep 09, 2012, 11:07 AM
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Phil,
That sounds like a good idea. I will give that a try. The foamie cub has a pretty generous wingspan, about 45". It is a very forgiving plane for a beginner like me. Playing with weight and cg is something I hadn't thought of.

In the mean time, I also picked up and started a Guillow's 172 Cessna kit. I am more of a warbird guy, but as it is a high wing, thought it would be a good quick build plane. I will try and teach myself to apply film, (Doculam) this build.

Maybe this winter sometime will be a good time to maiden the PBY. Crashing on snow, if that is how it goes, might not be so bad. I think I could design some simple removeable skis for take off. The hull would handle snow good, maybe some simple skis to keep the wingtips from digging in? I'm thinking skis mounted about 2" out on each side roughly where the maingear would have been, only higher to allow the hull to ride in the snow. Just an idea going through my head.
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Old Sep 12, 2012, 04:29 PM
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Conrad, other than what Phil suggested, I suggest you keep the CG well forward, at this weight, which is not really that bad as you think. The problem arises in the small vertical area of the fin. Keep the nose level and no jerky up elevator inputs. Co-ordinate the rudder in the turns, to avoid the tail dropping, and resulting snap. Slight nose heavy will help.

Heck..I have had Guillows Hellcats flying well at 2#, and OS 15's!

Fuzz
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Old Sep 12, 2012, 04:37 PM
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Made many a take off on thick grass with float/seaplanes
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Old Sep 13, 2012, 04:46 AM
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I must admit to being greatly encouraged to hear from two masters at Guillows conversions that it is possible this plane can be a flyer! I learned some valuable lessons on this build. When the weight at the very end of the build, escalated, as I built and installed the wiring and ESC's and motors, it was clear to me I hadn't estimated what the last details would add to the plane. By that time I couldn't go back and change things, like sheeting all the airframe.

The Cessna I just started is gonna reflect a new approach on my part. Lessons learned.

I also ordered some good grade balsa today. Hard to find locally. Mostly heavier grades here. Guess I'm hooked on doing more builds.

I had started a Hellcat build prior to the PBY but shelved it. The Hellcat was going to be my first build, but I couldn't resist joining the Funbuild, even though I finished late. I will try as Phil suggested, get some high wing time in and work on my piloting skills. My foamie trainer doesn't have ailerons, so I can learn those on the 172, if I don't crash it too badly during intial flight testing! I will practice co-ordinated rudder/elevator/ aileron turns with it.

Everybodys build threads and advice means more than you know to someone starting out like myself, many thanks!

Conrad
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