Wicked powered SR-71 - Page 79 - RC Groups
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Jun 20, 2012, 04:21 PM
Shameful Warbirdaholic.
Aros.MotionRC's Avatar
According to the specs, the plane's weight is around 34 ounces (and that's not including the Afterburnerz kit so add a couple ounces there) and my 4s 4000 45C I have been using is 17 ounces. So the quick math is approximately 53 ounces which according to redlite puts my last few birds as fat cows...No wonder a short shelf life!

So clearly I need to dump some weight. The only thing I can do is move to a lighter, smaller battery...Maybe a 2200? But I would think that's good for one, MAYBE two WOTs in the pattern and then back on the ground?
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Jun 20, 2012, 05:42 PM
Registered User
stacker's Avatar
I used a 30-40C 3300 4S on mine. My electric retracts had to weigh 4-5 ounces also. I thought mine came in around 43-44 ounces but I need to weigh again.

You hand launches are a lot braver then me. I don't mind doing it with the little 50mm jets but I'd throw my back out with this spear. Every time I hand launch I think of the guy that got so excited on his first hand launch that he threw his transmitter, not the plane ;-)

Jun 20, 2012, 06:37 PM
Shameful Warbirdaholic.
Aros.MotionRC's Avatar
I'm not sure if it's bravery or stupidity. LOL

Probably a little of the former and a ton of the latter.
Jun 20, 2012, 06:55 PM
Registered User
max-nix's Avatar

The More The Better

Simplify, then add lightness.
Colin Chapman

Lots of luck (Good luck that is).

Jun 20, 2012, 07:25 PM
Shameful Warbirdaholic.
Aros.MotionRC's Avatar
I just bought a 4s 3300 30C turnigy. Several ounces lighter than my 4000. That will have to do.
Jun 20, 2012, 07:48 PM
Registered User
Joe 1320's Avatar
You can also lengthen the harness for the afterburnerz to put the control module closer to the front. This will help keep the CG with a lighter lipo.
Jun 20, 2012, 08:00 PM
Shameful Warbirdaholic.
Aros.MotionRC's Avatar
Good idea...Thanks!
Jun 20, 2012, 11:45 PM
"Because I was pǝʇɹǝʌuı"
bighead93's Avatar
Last weekend Redlite and I completed officially flight number 50!! Winds were tough and I will tell you this bird doesn't like the wind! I earned it on the landing! Coming in crabbing down the runway is tricky and wouldn't be possible without the rudders! Also had a partial collapse of a thrust tube which didn't help! Still hit 76 mph on the 3S 2100's set-up, such a fun plane to fly!
Jun 21, 2012, 01:08 AM
Shameful Warbirdaholic.
Aros.MotionRC's Avatar
That's amazing. 50. I think I've had 50 flights with all of my 7 previous Habu's put together, lol.

Congrats on 50 flights. I truly think that's an impressive feat with this particular RC jet. Hope you get 50 more and then some!
Jun 21, 2012, 02:27 AM
redlite's Avatar
Weight is the issue with ANY airplane. With less than 2 square feet of wing area, (this is a rough estimate, but close) we are limited to the weight that can go up with this model. Someone should do an accurate estimate of the wind area so we can get a better handle on the model. If all your planes are the normal 8 to 15 ounces per square foot and you hop in the 25+ ounce model, where did you get the expertise to fly the heavy wing loading? Very tough to fly this wing loading, it won't forgive the slightest mistake, especially during the take off and landing phase. Goof around with the throttle during the flight and see what happens...it won't be pretty.

The delta wing platform can carry huge loads, which is why we are getting off the ground in the first place, but it will be limited in its performance (maneuverability) envelope. Slow down and it's lights out. But you have to slow for landing...here belly scratchers rule because they have a huge 'runway' so landings can be perfect. But the takeoff part is best done with wheels, for obvious reasons. Lots of things to consider with this kind of model.

We opted for dual three cell packs to save a ton of weight, but now we are limited to 90 mph flights. And a plane that is somewhat manageable. The four cell system should open the limit to above 100mph, but only if weight was managed properly. This is predicated on no steep dives, but a shallow dive to achieve the desired speed, using honest engine power.

Bighead also flys 3-D planes, so this should explain the fifty flights. His take is this model requires the same level of concentration, but the demands are quite different. It's the wing loading, first and last with this model. We are at 23 ounces per square foot, and that's not good. But with the proper flight plan and constant inspections, we've been able to dodge the bullets...our days are still numbered, though, nobody gets out alive here. Almost half the production run was lost on the full size...most had less than a thousand hours on them when they went in and this model is no different.

And that B-58 is very cool, stack...I have KS's kit waiting in the wings...

Jun 21, 2012, 03:24 AM
Shameful Warbirdaholic.
Aros.MotionRC's Avatar
I am the first to admit that I do not yet fully possess or comprehend the technical aspects and knowledge of aeronautics and aerodynamics that some do here. Especially when it comes to mathematical equations or formulas (wing loading/oz. per sq. ft, how it all translates in English). I've learned an immense amount of information about aviation and the mechanics of flight over the past several years, but I still have so much to learn and comprehend.

I know a heck of a lot more about this particular air frame than I did seven years ago but I don't speak in equations or formulas. I am a visual learner. With ground school, I need visual aids to go along with the concepts or I'm already thinking about what's for dinner or my client's project demands. Wing loading is still a fuzzy topic to me just like the dance of the proper electrical power setup for each plane in this hobby is.

I "see" things just fine. I see a small delta wing surface swept at the far end of a long protrusion. That alone spells trouble. There's no fat cambered airfoil with tons of wing surface with a giant propeller to guide it in the air with minimal effort. This is a missile. It's also my favorite plane in all of aviation. I love that it's so challenging and hate that it's so challenging. I've had many successful flights over the years to suggest that it's not all dumb thumbs.

Sometimes she bites because she can.

She's the mistress I can't seem to quit, no matter how hard I try. Deep inside, I guess I'm trying to tame her. To understand her. To find that perfect balance of symmetry. Like I get with all of my other planes, and yes, even some with heavier wing loading.

Whatever that means.
Jun 21, 2012, 11:41 AM
Registered User
stacker's Avatar
Do you have a link for the KS B-58 kit? I've been laying out a much larger version for 64mm fans but time keeps me from getting to it. Got to love the B-58.

I checked last night on where my Max version CG was. It was around 3 1/2 (88-89mm) from intake lip back. This was being ballanced from bottom. I believe I used same throws as Don. I did reduce the up trim to almost zero and will double check to see what final trim ended up at. My power setup was pretty light at 350-400 watts per motor on 4s. I prob. could have gone with a 4s 2700 and saved several more ounces but wanted a little more time.

Take care -- Stacker
Jun 21, 2012, 04:07 PM
Shameful Warbirdaholic.
Aros.MotionRC's Avatar
My good hand tossing buddy was able to come out today to hand launch her for me so I seized the opportunity. CG was set at 78MM and I went with my 4S 4000 45C brick as that was all I had available. The winds were swirling but we found the prevailing winds from the Northwest, pointed the nose in that direction, and he tossed her like a champ and off she went.

I could tell I needed some up and right trim but I was more focused on keeping her from stalling and so I chose to just "stay on the sticks" from start to finish. I recorded on-board footage with my key cam (video below) facing aft, but for some reason the lens was blurry. I have no idea why. So sorry for that in advance. At the 37 second and 1 minute mark, you can hear what the guys at the field thought was LVC with a hard cutoff. I came in to land the first time but I was too hot (so nervous not to stall her) so I went around again. When I gun it to go around (1 minute mark) you really hear the issue that was going on.

This is unedited video with sound so you can hear the throttle management. I tried to stay close to if not full WOT for most the flight, but with the LVC(?) sounds I had to back off the stick a few times. I ended up dead-sticking the landing (downwind landing, I was not about to reverse the pattern I had established) and kept some down stick all the way to the ground. I eased up right before touchdown and it actually was one of my better landings.

The battery was PUFFED beyond belief! I had no choice but to throw it away. Not sure if it was not getting enough airflow with all the tape I had over it to stay put or if it was overamping or what. One theory one of the guys thought is that the afterburnerz could be drawing too much from the system and I should invest in a good external BEC. The other guy thought it was hitting "hard cutoff" or LVC from potentially a battery not up to task.

The important thing is the maiden - as short as it was - was a success. She came home in one piece. I need to cut a hole to allow more air to cool the ESCs. I may also remove the afterburnerz kit and put it in a twin EDF that isn't as testy as the Habu.

Anyhow, I'm hoping some of you much smarter than I am with electronics can listen to the video and offer your thoughts. Much appreciated.

Maiden of my LX SR-71 Blackbird (#8) (1 min 43 sec)
Jun 21, 2012, 04:30 PM
Registered User
stacker's Avatar
Good news that your SR-71 is still in one piece ;-)

What kind of amp draw are you seeing to puff up a 40C 4000 battery? Even with little air movement that seems crazy. Can't remember if you posted your motor setup? My 3300 30-40c pack was just warm at 50+ amps. I think my motors were in the 3500Kv range but not sure as they were re-wound.

Again, great you are still in one piece -- Stacker
Jun 21, 2012, 04:34 PM
Registered User
Joe 1320's Avatar
Congrats on the maiden!

I would be interested in how well matched the ESCs and motors are. For example, I have two identical motors and fans that are dynamically balanced. One fan and motor draws 2A more than the other. That's on the same ESC so that rules out ESCs. Did you ever check each fan independantly for draw? Are you running a seperate UBEC?

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