Scratch-built "Cuda"

[gsjames]

I became interested in RC Rocket Gliders about a year and a half ago thanks to a couple of local Tripoli North Texas rocketeers who are also expert RC pilots and builders. As a long-time RC pilot and pylon racer myself, the idea of a rocket powered rc glider interested me, so to that end I made a scratch-built Aerotech/Bob Parks "Phoenix" and a clone of Dave Schaefer's Me-163 I-powered rocket glider. I recently acquired a NOS Kevin McKiou "Cuda" from Launch Lab Rocketry, but rather than build this pristine old kit I have decided to make and fly a clone of it as well. I'll document the build in this thread. All of the parts are easily available and today I cut out the 1/16" and 1/8" balsa parts on the laser at the local library makerspace. Here is a photo of the original kit parts and my copy for comparison. Other than a few scorch marks, they look pretty good. I still need the airfoil coordinate file and a C-30 fiberglass tube (from a kite supplier) and I should be good to go.

 

[mikec]

Looks great, have fun. FWIW I built a Cuda a few years ago that got destroyed in a control loss (make sure you have the battery tied down well, I think the way the hatch attaches is pretty marginal!), and rather than building another one I bought an Art-Hobby Zuni with a pre-skinned foam wing because I found building the Cuda was quite a chore. I hope you have better luck than I did.

 

[burkefj]

Why was the komet the first and last flight?

 

[gsjames]

"Why was the komet the first and last flight?"

I flew it for the first time at Airfest this fall. The launch and flight were perfect until the landing approach. I had to "thread the needle" between 2 rows of trees on the final approach. (The Airfest site is not optimal for airplane flying) My depth perception was a bit off and I clipped a tree with about 3 inches of the right wingtip. It destroyed the wings/canopy/cowl but the fuselage was intact. Thus, first and last flight.

HOWEVER... since that time I have rebuilt it and built my own launcher ( I copied X30Pilot's design) so it's good to go again.

 

[gsjames]

Basic pod glue-up. I'll have to wait a few days for some components to arrive before proceeding much farther. I agree with MikeC that the attachment method for the forward pod is rather questionable. I did it like the plans, but in retrospect, I think that I should have used rare earth magnets.

 

[burkefj]

"Why was the komet the first and last flight?"

I flew it for the first time at Airfest this fall. The launch and flight were perfect until the landing approach. I had to "thread the needle" between 2 rows of trees on the final approach. (The Airfest site is not optimal for airplane flying) My depth perception was a bit off and I clipped a tree with about 3 inches of the right wingtip. It destroyed the wings/canopy/cowl but the fuselage was intact. Thus, first and last flight.

HOWEVER... since that time I have rebuilt it and built my own launcher ( I copied X30Pilot's design) so it's good to go again.

Great, Dave S and Tom Blakeney are great guys, and pilots, they have both bought and flown a couple of my R/C rocketplane kits too

 

[gsjames]

Yes, they are both great guys, (FW Thunderbirds and North Dallas RC Club) but knowing them is going to cost me a lot of MONEY... ;-) I will no doubt be purchasing some of your kits in the near future as well... I'm looking forward to it!


It's starting to look pretty...

 

[gsjames]

I installed the servos. I used 9gm servos. It's REALLY tight in there. Kevin used Sullivan #507 Gold-N-Rod cable-type pushrods. After playing around with it for awhile, I chose to use .025 solid wire pushrods in the sleeves that came with the #507s. They seems to be a bit smoother and less friction/binding. With tiny servos like this, less friction is good. The servos have much longer connection wires than needed and frankly, they are in the way. I may cut them shorter but my track record with making servo pins/plugs isn't particularly good so I am leery of doing so.

I understand why Kevin chose to use the canopy latch method that he did. In his writings he said that 1gm of weight is equal to 2 meters of boost height or 6 seconds of glide time. If that's accurate, that's HUGE in a competition aircraft. His canopy method is probably as light as possible. I think that I would have just used a piece of thin tape.

 

[mikec]

I understand why Kevin chose to use the canopy latch method that he did. In his writings he said that 1gm of weight is equal to 2 meters of boost height or 6 seconds of glide time. If that's accurate, that's HUGE in a competition aircraft.

Much depends on where your radio is, how big your battery is and how it's secured. FWIW, I was using a very small 2S lipo, an ESC, and a Hobbyking knockoff Spektrum receiver. I needed to add weight to balance and things were tight. The hatch coming off in flight (motor hiccup) and the battery getting ejected really adversely affected my flight time.

In competition most people who know how to fly (not me) max out in thermals anyway, so I'd rather use a bit more weight for a more secure setup. But YMMV.

 

[gsjames]

Thanks for the input Mike!

ESC? Motor? It sounds like you did an electric conversion to this design. ( I fly a lot of electric RC ) This one will be D/E rocket motor boosted, with a FrSky X4R receiver and a 300 mAh LiFe (6.6v) battery. I'm a pretty good RC pilot (even if I do say so myself) so I don't expect it to be much of a problem after flying a Stratoblaster, a Phoenix and a 60" Me-163 on an I-161 (a copy of X30Pilot's model)

I'll just put some tape over the canopy like Bob Parks said to do to secure the Phoenix canopy. That way I know that it won't come off. The battery will be secured with a Drone battery strap.

Of course, the Cuda was designed and competed in the late 1990's (I think) when the FAI S8e rules were different from what they are today. According to those that I have asked, it is no longer even remotely competitive in S8e-P or S8e-D as they have gone to molded composite airframes from modified DLG's. I am building this strictly as a sport plane for the fun of it. It uses small enough motors that I can fly it at AMA fields.

 

[mikec]

ESC? Motor? It sounds like you did an electric conversion to this design.

Nope, motor as in rocket motor -- Aerotech E7 for the flight where the battery got ejected. I was using an old ESC with integral BEC to get 5V from the 2S battery, just something I had lying around.

FWIW, I just found the little pin and tube arrangement for the hatch a PITA, and would have been better off using tape as you suggest.

 

[gsjames]

Ah, using the ESC for its BEC, that makes sense now. I agree, the pin and tube is a PITA. I'll still use the tape as backup.


All "lathered up" now with lightweight spackle and awaiting final shaping/sanding.

 

[mikec]

Two more bits of my experience:

1) I built mine with only dihedral on the outer panels, not the full polyhedral, so I had two fewer wing breaks to deal with. It seemed to fly OK that way.

2) The inverted V-tail is prone to break off on bad landings. If I built another I might be tempted to not invert the tail, but it's pilot error if you do break it.

 

[gsjames]

Smooth as a baby's behind! Now to decide if I want to glass it or not. Since it's for sport, I probably will



I have sanded the inverted V tail to shape and am awaiting the arrival of some 1/2" fiberglass tape for the hinge per the plans.

 

[mikec]

Oh yeah, the fiberglass hinge. I just couldn't make that work, I must just not know the trick. I'd suggest using CA hinges or Monokote.

 

[burkefj]

I'd just go straight 1s, a sport flyer like this will work fine, lots of dlg's and other sailplane guys use 1s, a lot of servos will run fine on 3.8-4.1v and the receivers will run down to 3.5v, a small 1s will be more than enough.

 

[mikec]

I'd just go straight 1s, a sport flyer like this will work fine, lots of dlg's and other sailplane guys use 1s, a lot of servos will run fine on 3.8-4.1v and the receivers will run down to 3.5v, a small 1s will be more than enough.

Totally hear you, Frank, and yes, a lot of people do it and it seems to work fine. As an avionics engineer, though, running like this just makes me too nervous. I've got a little boost regulator that will take 1S up to 5V for my next build. For this class of glider, you can use a heavy battery or you can use a light battery and some ballast, so saving total weight may not really be an option anyway.

 

[burkefj]

Totally hear you, Frank, and yes, a lot of people do it and it seems to work fine. As an avionics engineer, though, running like this just makes me too nervous. I've got a little boost regulator that will take 1S up to 5V for my next build. For this class of glider, you can use a heavy battery or you can use a light battery and some ballast, so saving total weight may not really be an option anyway.

I understand I'm a retired electrical engineer but I've got thousands of flights with this setup with zero issues so I'm confident in it and you could just run a higher capacity one s battery to get the same amount of weight but I understand if it doesn't cost you anything and you're happy why not

 

[gsjames]

The 1/2" x 0.005 fiberglass tape arrived a couple of days ago, so I hinged the V-tail per the method shown in the manual. It turned out better than I anticipated. I also glassed the fuselage pod with some very lightweight 0.5 oz/sq.yd. glass cloth and ACP laminating resin. I am sanding it down now. Because the glass is so thin, I have to be very careful not to sand through it. Also much of the sanding must be done "by touch" and with a low-angle backlight to be able to see the imperfections.
View attachment 20221223_093311[1].mp4

 

[gsjames]

I have not been able to find the coordinates for Kevin McKiou's "KM7" airfoil section. He no longer has that data. On the plans, he lists it as being 8% thick. Thus I will use the AG40d-02f airfoil section from Dr Mark Drela of MIT. Prof Drela is a champion level competitor in DLG and this section (8% thick) was used on one of his 3m sailplanes. It should perform well.

I have generated the cut files for my CNC foam cutter using devWingFoam2 and will try to cut some appropriate cores using devCNCFoam software.

 

[gsjames]

I tried several times to assemble the inverted V tail the way that it says in the plans using the little pylon, but wasn't having much luck. So, I built myself a little "gluing jig" with a 120deg included angle to support the tail at the correct angle. It seems to have worked well. As luck would have it, this 120 deg is the same angle as a Q-500 RC Pylon racer so I already had it made up.

 

[mikec]

The 1/2" x 0.005 fiberglass tape arrived a couple of days ago, so I hinged the V-tail per the method shown in the manual. It turned out better than I anticipated.

FWIW, it seems to me that your video doesn't show travel in both directions. Mine cracked at the joint after a few cycles, which is why I was skeptical about this method, but hopefully you didn't make whatever mistake I did.

 

[gsjames]

FWIW, it seems to me that your video doesn't show travel in both directions. Mine cracked at the joint after a few cycles, which is why I was skeptical about this method, but hopefully you didn't make whatever mistake I did.

Nope, it works just peachy in both directions. I have cycled it many times testing it. The cracking you experienced may have been due to having a very brittle resin. The resin that I used (APC Laminating Resin) doesn't get all that brittle. BTW, we use a method similar to this on Q-40 RC Pylon racers.

 

[gsjames]

A simulation/proof cut of cutting the right wing panel.

 

[gsjames]

Preparing to cut the cores. The CNC machine is all set up.

 

[gsjames]

The CNC cutting of the cores is NOT going well. The problem seems to be excessive "kerf" melt-back of the tip section. Because there is a big difference in chord length, and because the chord is so small to begin with, I am having a lot of trouble getting an acceptable core. I have made 5 attempts varying the heat of the wire, speed of the cutter and kerf width. The last attempt is borderline acceptable, but I am going to try again when I get some more foam. I also will tighten the wire some more to help eliminate wire "sag". UGH!

 

[gsjames]

After a multi-month hiatus to finish up my Level 2 qual bird, I am back working on the scratch-built Cuda. I decided to use the best of the cores that I cut, they are not great, but I think that they are acceptable. Here are some pics of the cores in the vacuum bag, just after removal from the bag and with the leading edge glued on (and drying). I'll shape the leading edge tomorrow and hopefully get started on getting the polyhedral set up. I am hoping to fly it at the TNT Texas Shoot Out on Memorial Day weekend

 

[gsjames]

The wing is assembled and the polyhedral is done. I am now tapping the joints and will start the covering next

 

[TheAviator]

Looking good! The Cuda is an oldie but a goodie. Chad Ring had over a 30 minute flight with one on a D motor at NARAM-50.

I know you're already a good way through the build, but one of the weaker points of the Cuda is the tailboom stiffness. Not an issue if you keep the nose vertical, but can cause problems on an off-nominal flight. Something just to keep in the back of your mind. Replacing it with a cut-down Allegro Lite boom would not be unadvised, but would require some slight tweaking of the fuselage.

Also, as far as the high taper, cutting the cores in two pieces is a good option. It effectively cuts the taper in half.

 

[gsjames]

Looking good! The Cuda is an oldie but a goodie. Chad Ring had over a 30 minute flight with one on a D motor at NARAM-50.

I know you're already a good way through the build, but one of the weaker points of the Cuda is the tailboom stiffness. Not an issue if you keep the nose vertical, but can cause problems on an off-nominal flight. Something just to keep in the back of your mind. Replacing it with a cut-down Allegro Lite boom would not be unadvised, but would require some slight tweaking of the fuselage.

Also, as far as the high taper, cutting the cores in two pieces is a good option. It effectively cuts the taper in half.

Thanks for the hints. I am almost finished with this one but I may build another. I found the coordinates for Kevin's KM7 section and I also received a bunch of notes courtesy of Ez2cDave that would make the next one much better. Of course, this is only for sport flying. That's a good idea about cutting the cores in two pieces. It would help a lot with the "melt-back kerf" and the hot wire marks.