Hobby Lobby Cessna 337 Skymaster Build Log w/Modifications - Page 2 - RC Groups
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This thread is privately moderated by gtfreeflyer, who may elect to delete unwanted replies.
Apr 12, 2007, 01:04 AM
Registered User
Jcopter's Avatar
Hey gtfreeflyer your close
I live in Downey, Where do yo fly?
Hey Guys I have 2 2408-21 Outrunners 1750 KV and 2 - 18 amp ESC what do you think will they work to fly this plane?
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Apr 12, 2007, 01:57 AM
Put a bigger motor on it!
gtfreeflyer's Avatar
I fly at Fairview Park in Costa Mesa. www.1hss.org

I don't know how much you know about sizing motors, but here it is anyway:

I like to size my planes at 100 watts per pound. This is plenty of power, and you could probably even get away with 60 watts/lb, but that's for slow trainers. Use 150+ watts/lb for 3D plane applications.

Let's say your plane weighs in at 48 oz (to make the calculation easy), that equals 3 pounds. I would go with 300 watts to power the plane. This may be overkill for a scale plane, but the motors I found are pretty light so there is no weight penalty, and extra power always comes in handy... and is a lot of fun too! So to get 300 watts, each motor needs 150 watts...easy right? If you use a 3 cell LiPo, you have 11.1 volts. Volts times amps = watts. You'll need to choose a prop for your motor that will cause it to draw 13.5 amps in order to get 150 watts out of the motor. Your 18 amp ESC will do just fine. You can check your motor's specifications and recommended prop sizes for 3 cell LiPos to see how many amps the motor will draw.

Now you need to make sure your LiPo will handle the current. If you use one LiPo to power both motors, then you need a LiPo that can discharge safely at 27 amps. Let's say you have a 2100 mAh LiPo (pretty common). Ah times C rating gives you max discharge. 2100 mAh = 2.1 Ah. 2.1C=27. Therefore, you'll need a LiPo rated at at least 12.85C... so say 13C. My 2100 mAh LiPo is 22C, so it can discharge at 46 amps. In another plane of mine, my motor draws 32 amps. The battery has no problem pushing out the amperage for this, so the LiPo barely even gets warm.

I think a 9" prop is the best for this plane. If you tip the plane backwards, the tail will hit the ground before the rear prop does. The rear prop comes about 1/2 inch from the ground, so I would definitely NOT go bigger than a 9" prop. A huge flare on landing would bust your prop if you do. I'd rather have the tail strike the ground first.

With all that said, keep in mind that I have not flown my Skymaster yet! However, I have had great results with other planes using the methodology I just described to you.

Hope that helps.

Apr 15, 2007, 05:17 PM
Put a bigger motor on it!
gtfreeflyer's Avatar
Alright, I'm back with a lot of progress to report. Cooling holes were cut. I have outlets for the motor (inlet is the hole already cut in the front), and I also cut inlets and outlets for my ESC in the front. Air seems like it will also flow into the front of the plane, through the fuselage, and out by the aft motor. This should take care of cooling the LiPo in the plane.
Apr 15, 2007, 05:20 PM
Put a bigger motor on it!
gtfreeflyer's Avatar
Elevator and rudder servos/linkages were set up like they came stock. I did not want to cut my servo wires so I just glued them down to the foam to keep them from flapping around. Also, I pulled apart the stock Y-harness for the ailerons and put each aileron on a separate channel. Now I have flaperons.
Apr 15, 2007, 06:03 PM
Put a bigger motor on it!
gtfreeflyer's Avatar

I went to fly yesterday, not expecting to actually fly this plane (but I brought it along anyway). I decided to warm up with my Super Sportster EP. It was a perfect day for flying. Light winds, and not too hot or cold. Just great really. I flew my Sportster around for about 6-7 minutes and brought her in. I took out my LiPo and it seemed a little warmer than usual... And then it hit me! My LiPo was puffed up! Goodbye $65. What a great way to start the day. I think it has something to do with me accidently shorting the wire for a micro second when I was soldering the connector onto it. I got about 20-25 cycles out of the battery. Now it's just sitting in my BBQ just in case. I haven't figured out what to do with it yet.

Oh well, now to the good part. I decided I could not pass up the perfect day for a maiden flight with the skymaster. Luckily I had another LiPo ready to go.

I set her up on the ground, pointed into the wind. I jammed the throttle full and the plane pulled hard to the right and pulled the front prop into the ground. No damage. I did it again, but this time I pushed the throttle up very slowly (about 3 seconds from idle to full). It was a nice scale takeoff roll. After about 50 feet the plane just came off the ground itself and then rolled hard to the right and wanted to dive. I made one complete circle while holding left aileron and up elevator for the entire flight (trimming the plane was not going to be enough), and brought it in to land. I dropped the throttle all the way down on final approach and felt I was going to lose her. I had forgotten that EVERYONE has said that this plane must be landed with power. I now definitely agree! I added a little more power before I stalled and the landing was quite nice actually.

I walked over to the plane, and adjusted my sub-trims on the radio. The ailerons were now obviously offset with the new configuration.

I set her up to take off again. I slowly applied full throttle again, and this time I was able to notice that my motors supplied PLENTY of power for this plane. I climbed very steeply to get to my comfort zone quickly. It still wanted to roll right a little bit, but I trimmed it out. The elevator now seemed to be in the correct spot. On the second round around the field, I made my right turn and the plane started to lose altitude very quickly. I was approaching the tall grass way too quickly. I was slowly pulling up and the plane came within inches of the grass before I started gaining altitude again. The guys behind me shouted "What was that?", and I just replied, "I don't know!".

I went around the field about 3 or 4 more times. I guess I figured out how to fly the plane after I almost crashed it as the rest of the flight was noneventful, and I actually liked the way the plane flew! I was cruising around at about 1/2 throttle the whole time, and I pushed it up to around 3/4 throttle for the turns.

The next landing approach was nice and the plane flew straight. When I was about 2 feet off the ground, I reduced my sink rate and dropped the throttle to about 1/8. When I was inches off the ground, I cut the throttle all the way and let the plane bleed off the rest of it's speed... BAD IDEA! The landing gear is very springy, and the plane just started hopping around. Since I had cut the power, the plane was now in a stalled condition and I had no control over it. I just had to let it bounce around until it came to a stop. 3 or 4 prop strikes later. The plane stopped. Again, no damage except for a few scuff marks on the prop.

I have now learned, and would like to pass on to you... Keep the throttle on for the entire landing process! Only once the plane is on the ground and stops bouncing, then you should drop the throttle all the way back and let the plane roll to a stop.

I would agreee with every other post about this plane: This is not for beginners! After 2 flights with this plane, I have noticed that it requires a lot more throttle management than anything else I've ever flown... other than helis. It stalls very easily. I looked at the wing and it seems like it has backwards washout! The tips are at a higher angle of attack than the roots of the wings. This means your tips will stall before the roots. No wonder they discontinued this plane. When my plane almost crashed, I think it went into the dive because my wing stalled on the turn. This is why I was adding power before making my turns. Also, the plane does not glide. Zero throttle will give the plane the same results as a rock without wings. I believe it is necessary to keep the power on so that you are always blowing air over the tail with your aft motor, giving you pitch control. I did test out the rudders, and they seem to do nothing. The tail hardly sways at all when using the rudders. I don't like the idea of not having rudders, but if you want to save weight on your Skymaster, then don't even bother installing a rudder servo. I spent so much time putting mine in and adjusting the linkages that I'm just going to leave it in the plane, but I doubt I'll ever use it again in flight.
Apr 15, 2007, 06:09 PM
Put a bigger motor on it!
gtfreeflyer's Avatar
Alright, so I got home and tried to figure out why my plane rolled to the right so much, and required an unusual amount of subtrim.

Another look at my front motor answered my question. My motor was mounted way beyond 2 degrees right thrust. It was closer to 10 degrees I think! This would blow more air over the left wing, and also cause the left wing to advance more than the right wing (although I didn't notice too much yaw), all causing the left wing to have more lift than the right wing... a.k.a. plane rolls right.

Today I shimmed the motor and now I have maybe 1 or 2 degrees of right thrust. The aft motor seems to be fine.
Apr 15, 2007, 06:14 PM
Put a bigger motor on it!
gtfreeflyer's Avatar
Today, I also finished up my "cosmetics".

I took some plastic spoons and cut off the front half of the spoon to make air scoops.

Check it out. Awesome!
Apr 15, 2007, 06:21 PM
Put a bigger motor on it!
gtfreeflyer's Avatar
For the exit vents, I went to Wal Mart and bought a wire mesh desk organizer for $1.50. I cut it to size, installed it as shown, and dropped some CA onto it to hold it in. I like the way it looks!

That sums up my progress until now. I'll take it out for another flight this week and report back here. Wish me luck!

Bye bye for now,

Apr 15, 2007, 06:36 PM
Put a bigger motor on it!
gtfreeflyer's Avatar
I decided to add one more screen after my last post. Here it is on the aft motor inlet.
Apr 15, 2007, 07:07 PM
Registered User
Jcopter's Avatar
Hey gtfreeflyer good read, Thanks for the info on the motor situation.
So, Did you move your tail servos or are they still in the stock place?
I have a new Hobby Lobby catalog and it has the Skymaster as a Mixmaster using a brushless setup does any one know any thing about this?
this is what I am going to do
Apr 15, 2007, 07:39 PM
Put a bigger motor on it!
gtfreeflyer's Avatar
Yes, the tail servos are still in the original locations. I had to cut out the foam a little bit since my servos are slightly larger than the stock ones.

There are people that got the MixMaster already. Do a search and see what they say about it. Hobby Lobby messed up one guy's order of his Skymaster and shipped him the Mixmaster instead. Lucky guy! He posted something about it. I think they did the same for me, but caught the error before they shipped it because they changed my order number before the package went out.

I forgot to write about the CG of the plane. I balanced it at 2.5 inches back from the leading edge. This is exactly where the metal wing struts attach, so it is easy to locate without measuring. The plane flew fine like this.

I moved my NiMH receiver battery from the battery hatch to just behind the forward ESC. This is where my receiver is as well (There is a nice pocket that both drop down into). The LiPo is located just above the receiver and battery. It is easily accessible through the front window. I currently hold the front window on with a rubberband wrapped around the fuselage. I'll do something nicer later on... maybe magnets. A rubberband is just too easy though so I may leave it for a while. No other weight was needed for balance. Everything worked out perfectly once I moved my NiMH battery from the battery hatch forward about 3 inches. My battery hatch is now unoccupied. When I get comfortable with the plane, I'll add 2 hinges and it will become a drop hatch...lol. Maybe I'll drop paratroopers.

Anybody know where I can get stickers of monkeys? In my favorite childhood movie, Project X, the monkeys fly away in a Skymaster at the end of the movie. I'd love to put some stickers of monkeys on the windows just for the hell of it. Preferably a sticker of a monkey giving the finger, since that's what happened in the movie as they flew away! Haha... it would be great.
Last edited by gtfreeflyer; May 21, 2007 at 11:57 PM.
Apr 15, 2007, 10:46 PM
RC Airplane Crasher
Wow what a crazy plane! I was tempted to buy one back when they first went on sale but it looked kind of intimidating. I'm glad I didn't buy one, I surely would have crashed it.
Apr 15, 2007, 11:53 PM
Put a bigger motor on it!
gtfreeflyer's Avatar
Okay, I couldn't resist. I left the house around 5:30 PM today after I made all the modifications and posted info about them, and I went out to the field.

The winds were about 8-10 mph, blowing perpendicular to my takeoff and landing direction. I put the plane on the ground and enjoyed taxiing around for 30 seconds or so. I still love how I can turn this plane on a dime. I decided to bite the bullet and I pointed as much into the wind as I could and I slowly pushed the throttle forward to full power.

The plane rolled a scale distance and took off the ground on its own when it was ready to (due to the angle of attack I set with the nose gear length).

Climb out was great, and a few clicks of aileron trim set the plane flying on a straight path. The plane flew great! Flying downwind was fun because of the increased speed. Flybys were nice and slow, and I brought it 10 feet off the ground for show. The people at the field really like the plane and they all gather around it when it's just sitting in the pit area. They enjoy the details I've built into it, and they like to ask questions about the upgrades. The plane looks and sounds great in the air with those twin engines.

After the flyby, I pulled the nose up 60 to 70 degrees and opened up the throttle. The plane pulled (and pushed) itself up with no problem at all. Next time I fly I'll be testing out its vertical capability.

I flew basic pattern for 4 minutes and was testing out my flaps for the first time. I didn't notice too much difference at all. I only set them up to deflect about 10 degrees to start with. There was a little pitch down tendency that required a slight touch of up elevator.

When I decided to land, I made my turn to final and dropped the throttle to 1/4. I dropped the flaps and kept the plane in a shallow dive. Two feet off the ground I flared and held it about a foot off the ground. My initial thought was to cut the power, but I quickly remember not to do that. I kept it at 1/4 throttle until all three wheels touched the ground at the same time. There was no bounce at all and as soon as ground friction killed the speed, I cut the power back all the way. The noise of the engines winding down while the plane was rolling to a stop was something worth hearing. Very neat. Success!

Next time I'll bring the power down to 1/8 during my flare, just so that I can get that scale look of a nose high landing while touching down on the mains and then lowering the nose.

Readjusting the thrust line of the front motor was what made this such a successful and stable flight. I was expecting the worse when I took off based on the last 2 flights. My ailerons are now trimmed very close to their neutral positions. They are not perfectly neutral, but I'm not making any changes in that area based on my flight today.

For my next flight, I'll have about 20-30 degrees of flaps for landing. I'm still learning the plane, so I won't be doing anything fancy just yet. Today I also noticed that my carbon fiber tube (.188" OD x .116" ID x 24") fits PERFERCTLY into the tubes that are preinstalled in each wing half. For my next mod, I think I'll run my CF tube thru the center section of the wing. It will stick out about 4 to 5 inches on each end and will insert into the wings when they plug onto the ends. This will make me much more comfortable because right now, I still don't trust the way the wings attach to the center section. Once I get the CF tube on the plane, I'll start experimenting with inverted flight.

I did not use my rudder at all during this flight, except for nose wheel steering. There really is no need for rudder on this plane. And remember, I'm still saying this even with my crosswind landing today!

So all in all, 4 minutes of flying and 1 minute of taxiing around made my LiPo slightly warm, but nothing to be worried about. When I plugged it in to charge, the cells were all at 3.84V, down from the initial 4.20V. I put back 959 mAh into the battery, about what I had expected. Based on this, I probably should fly this plane no longer than 8 minutes. I'll probably keep it to 6-7 minutes just to be safe.

I'll have to get a larger capacity LiPo one of these days to simulate a scale range for the airplane! I'm sure that would be an expensive battery though.

My girlfriend called me today and said she found monkey stickers for the plane. Awesome! I'll see how they look and post some pics of them. Perhaps for my next flight, the monkeys will be piloting the airplane!
Apr 16, 2007, 11:47 AM
Women Fly Too
CrashCaley's Avatar
gtfreeflyer, Great to see you have flown the Skymaster successfully. I take it that you had more than enough power to climb out without fear of tip stalling. You mentioned that you disabled the BEC on your ESCs. How do you do this? Wish you could place a video up showing the performance. Would really be neat. Thanks for doing this thread and the providing the tips to making this a good flying airplane. Caley
Apr 16, 2007, 12:04 PM
Put a bigger motor on it!
gtfreeflyer's Avatar
CrashCaley, I asked tech support the same question: How do I disable the BECs?

I am copying and pasting the email reply from them:

"1.Remove the red wire from the ESC's receiver plug. To do this use a small flat bladed screwdriver to carefully raise the plastic tab holding the metal pin. Carefully pull the red wire out of the receiver plug and insulate the pin with heat-shrink tubing or electrical tape.
2. Plug the ESC into the throttle channel on the receiver.
3. Connect a separate receiver battery to a receiver switch harness and plug this into the battery slot on the receiver.
In most electric planes a 300-600mAh receiver battery will work well. If the ESC is used in an electric sailplane a 600mAh receiver battery is recommended.
4. To operate the receiver, first switch on the transmitter, then the receiver harness, then connect the motor battery to the ESC. Reverse the order to shut the receiver off."

Hope that helps. It worked for me!

As for the video, I really want to get one. It has been on my mind. I don't have a camcorder, but I'll have someone take video with my camera phone when I go out there later today. Hopefully the winds will be light so I can fly. I'm sure the video quality won't be great, but hopefully it is good enough for the internet.

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