HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Jun 22, 2012, 03:15 PM
Shameful Warbirdaholic.
Aros's Avatar
Maple Valley, WA
Joined Apr 2006
8,407 Posts
Full Video (ground footage as well).

Be sure to watch to the end. It's the best part.

SR-71 Blackbird #8 Maiden (Full Version) (2 min 51 sec)
Aros is offline Find More Posts by Aros
RCG Plus Member
Sign up now
to remove ads between posts
Old Jun 24, 2012, 02:56 PM
Shameful Warbirdaholic.
Aros's Avatar
Maple Valley, WA
Joined Apr 2006
8,407 Posts
Sortie #2 was a success!

I went to the local school with bravery in my heart. I decided I was going to have to hand launch it myself at some point so today was as good as any day. I quickly realized that the little holes I cut out by the landing gear bays - while helpful - is still really not good enough for a good healthy hand toss. I gripped her best I could, throttled to WOT and chucked her. Even though she felt like she wanted to leave my hand with all that wattage, it still wasn't enough to keep her from nearly plastering into the ground.

She left my hands and immediately dropped...I wish I had this on video...It was literally inches off the ground and I managed to give her just the right amount of elevator (enough to keep her nose up and gain altitude without pitching straight up into a stall...Something we know is very easy to achieve with this bird). I didn't panic...She scooped herself upwards once some airflow was going through her and off she went.

I even rolled her once. The roll-rate was predictably slow. I have such limited aileron throw. She was still wing rocking pretty good but otherwise was perfectly fine. I definitely need to install thrust tubes though. I think I was around 80 top speed. Thrust tubes put me in the 90+ range easily, especially with my setup.

I dead stick her in on final. I just have to keep some down so her nose doesn't go skyward on me ending in a very unpleasant stall. She came in and landed ever-so softly on the soccer field grass.

I flew maybe 3 minutes tops. The battery (my other Nanotech 4000) was fine.

It's funny how I can fly some of my planes with such finesse, loops, rolls, inverted you-name-it, and it's no big deal. Yet if I fly this one around in the pattern and land successfully I am as giddy as a school girl.

Aros is offline Find More Posts by Aros
RCG Plus Member
Old Jun 24, 2012, 03:02 PM
Registered User
Joe 1320's Avatar
United States, FL, Sebring
Joined Feb 2006
7,488 Posts
Quote:
Originally Posted by Aros View Post
Sortie #2 was a success!

It's funny how I can fly some of my planes with such finesse, loops, rolls, inverted you-name-it, and it's no big deal. Yet if I fly this one around in the pattern and land successfully I am as giddy as a school girl.

LOL..... I know the feeling.
Joe 1320 is offline Find More Posts by Joe 1320
Reply With Quote
Old Jun 25, 2012, 09:26 AM
On the Edge of Space
dLdV's Avatar
Exit 4, South Jersey, USA
Joined Dec 2007
1,511 Posts
Awesome all over the place! Way to go, Aros!
dLdV is offline Find More Posts by dLdV
Reply With Quote
Old Jun 26, 2012, 07:43 PM
Shameful Warbirdaholic.
Aros's Avatar
Maple Valley, WA
Joined Apr 2006
8,407 Posts
Whenever I get a new SR-71 airframe and I put it all together and it's ready to fly, I stare at it for a few minutes. It's a little ritual I do. It's like somewhere, deep inside, I know that she won't remain in that pristine, unbroken condition for long.

#8, like all the ones before, took her final flight today.

I couldn't even get 3 whole stinkin' flights out of her.

I went to the school for sortie #3. I cut out an extra hole to the center and aft of the two main holes I cut out for launch. This allowed for a more confident grip. I was going to fly with my new Turnigy 4S 3300...Substantially lighter than my Nanotech 4000. I had to put the battery as far forward as she would go to meet the CG. Winds were variable, nothing too drastic.

I gripped her good, jammed the throttle to full and gave her a good toss. She left my hands and moved straight ahead on rails. Didn't drop an inch. By far, it was the best hand launch I've ever had with her. Sweet! Then suddenly as I am giving her elevator to gain altitude, I hear more of those sounds like you could hear in the maiden video...This time they were rapid and smoke came out of the exhaust port and she torque-rolled straight into the ground. I was helpless to do anything but watch the inevitable.

The front half was destroyed with the back half almost untouched. Which is the crucial section any ways. Still, the port engine inlet was crumpled and the inlet spike was crushed. I plugged the battery in and tried throttling up to see what was going on. In the aft motor immediately started smoking with that electrical smell. Time to unplug.

I performed the all-too-familiar autopsy when I got home. Removing all of her guts and inspecting each for obvious damage, shorts or other. The only thing of note is that while you could still spin the motors and fans freely without rubbing against the fan shroud, you could plainly see that tell-tale marks of the fan where it had been rubbing around the housing. Clearly out of balance.

Since I have no idea what is wrong, Don, I may ask if you would allow me to send the electronics back to you. I would need someone who knows what they are doing take a look and see if it's repairable or the electronics are bad.

What a shame. Just when I seemed to have solved the weight issue, had a fantastic hand toss, a catastrophic unstart.

#9 and feelin' fine?
Aros is offline Find More Posts by Aros
RCG Plus Member
Old Jun 26, 2012, 09:22 PM
Build'm,Fly'm,Crash'm
redlite's Avatar
Sacramento CA
Joined Oct 2009
629 Posts
Quote:
Originally Posted by Aros View Post
Just when I seemed to have solved the weight issue, had a fantastic hand toss, a catastrophic unstart.

#9 and feelin' fine?
Did you ever get a final AUW with the smaller batt? I don't recall seeing it in your earlier discussions.
redlite is offline Find More Posts by redlite
Reply With Quote
Old Jun 26, 2012, 10:25 PM
Wonderfully Wicked
The Don's Avatar
Willoughby, Ohio
Joined Jan 2002
13,280 Posts
Sorry for the loss of #8. A 4S 3300 is pretty small for a 120 amp load in my opinion. Did you run it static at all with that lipo to see how it held voltage after say 15 or 20 seconds? You may be flirting with LVC threshold. I can take a look at them. Scrubbing on the housing is fairly common so that scrubbing may have been their all along.
The Don is online now Find More Posts by The Don
RCG Plus Member
Old Jun 26, 2012, 10:46 PM
Shameful Warbirdaholic.
Aros's Avatar
Maple Valley, WA
Joined Apr 2006
8,407 Posts
redlite- I did not get an AUW as I don't have a scale I can read with it on there. It was much lighter though and based how it was the easiest hand toss I think I've ever had, it had to have been closer to the accepted range.

Don- I didn't bench test the battery as I didn't think there would be anything to worry about other than landing within 3 minutes. Is it a C rating issue when you mention amp load? How can a 4S 4000 be okay but not a 3300? Wouldn't we be talking about less flight time? Sorry I'm confused.
Aros is offline Find More Posts by Aros
RCG Plus Member
Old Jun 26, 2012, 10:58 PM
Wonderfully Wicked
The Don's Avatar
Willoughby, Ohio
Joined Jan 2002
13,280 Posts
Yes C rating is what I am talking about. A lipo may be rated 40C but when you see the lipo discharge curve voltage may be really low at 40C.

Here is a post showing how the voltage drop as you discharge at higher rates to give you an idea or what happens.

http://www.rcgroups.com/forums/showp...8&postcount=11
The Don is online now Find More Posts by The Don
RCG Plus Member
Last edited by The Don; Jun 26, 2012 at 11:08 PM.
Reply With Quote
Old Jun 26, 2012, 11:23 PM
Shameful Warbirdaholic.
Aros's Avatar
Maple Valley, WA
Joined Apr 2006
8,407 Posts
So my 30C-40C 4S 3300 was not enough "discharge curve voltage" which could have caused instant LVC even on a fresh charge due to the 120amp load? What about my 4S 4000 45C battery that still would hit LVC after only a minute in?

What size is sufficient?

What would be the safest bet with this setup in your opinion? The Sky Lipo at what C rating?

Wow. Even when I thought a crash wasn't my fault, maybe it still was? That's really unfortunate if a brand new battery I thought was sufficient killed my latest.
Aros is offline Find More Posts by Aros
RCG Plus Member
Old Jun 26, 2012, 11:26 PM
Shameful Warbirdaholic.
Aros's Avatar
Maple Valley, WA
Joined Apr 2006
8,407 Posts
So would this have been a better choice? I see the continuous is 160A.

http://www.hobbypartz.com/77p-sl4000-4s1p-40c-4444.html
Aros is offline Find More Posts by Aros
RCG Plus Member
Old Jun 27, 2012, 12:11 AM
Registered User
stacker's Avatar
Salt Lake City Utah
Joined Mar 2004
3,364 Posts
Did you remove the afterburners? If not did you add a UBEC to help take the load off the built in BEC's? Wonder how much current the after burners draw?

Did you add thrust tubes? If not you prob. got about as much thrust as you would have with 3s and 80 amps. At 30C the battery would have been taxed at 90 amps and hurting and you are asking it to put out 120 amps. On 3s you might have been ok with good ventilation.

Sorry to hear the plane went in.

--Stacker
stacker is offline Find More Posts by stacker
Reply With Quote
Old Jun 27, 2012, 12:38 AM
Shameful Warbirdaholic.
Aros's Avatar
Maple Valley, WA
Joined Apr 2006
8,407 Posts
My confusion is that I would think 4S trumps 3S any day of the week on this setup, even at a lower C rating...
Aros is offline Find More Posts by Aros
RCG Plus Member
Old Jun 27, 2012, 12:52 AM
--
Cobra68's Avatar
United States, WA, Ravensdale
Joined Jan 2011
155 Posts
Quote:
Originally Posted by Aros View Post
My confusion is that I would think 4S trumps 3S any day of the week on this setup, even at a lower C rating...
Aros,

Here's how i understand the whole battery thing -- and how to compute what's sufficient for your setup -- (anyone else please step in and correct me if I say something wrong).

Your "C-rating" is an indicator of how rapidly you can either charge or discharge your battery without damaging it. If you have a 2200mAh battery and want to charge it at 1C, you charge it at 2.2A. If you want to charge it at 2C, you would set your charger for 4.4A.

The discharge works the same way... a 2200mAh battery, discharging at 20C, is capable of supplying 44A. A 2200mAh battery, discharging at 40C, is capable of supplying 88A.

So, with your batteries, a 3300mAh battery that's rated for 30C can supply 99Amps... and when you are pulling more than 99A out of the battery, you run the risk of hitting the LVC on your ESC and/or damaging the battery. With your 4000mAh battery that's rated for 45C, you could theoretically run a load of 180A on that without damaging the battery. The hobbypartz battery that you linked to lists a max discharge rate of 160A (4000mAh x 40C = 160A).

So... to know what battery you should be using, you need to know how much power your particular ESC/motor combo is pulling. If you need to supply 120A, you could choose some combination of capacity (mAh) and C-rating that multiplies together to meet/exceed 120A.

Hope that helps clear things up...

-Sully
Cobra68 is offline Find More Posts by Cobra68
Reply With Quote
Old Jun 27, 2012, 01:19 AM
--
Cobra68's Avatar
United States, WA, Ravensdale
Joined Jan 2011
155 Posts
Quote:
Originally Posted by Aros View Post
My confusion is that I would think 4S trumps 3S any day of the week on this setup, even at a lower C rating...
I realized that I didn't mention anything about the differences between 3S and 4S.

Here's how I do my calculations -- first, you've got to look at your motors and the kV rating. If you have a 4000kV motor, that means that you are "supposed" to get 4,000rpm out of that motor for each volt of power supplied to the motor.

On LiPo batteries, a fully charged single cell sits at 4.2V. A fully charged 3S battery would max out at 12.6V and a 4S battery would get you up to 16.8V. Obviously, as soon as you throttle up the motors, that voltage starts to drop a bit... which is why you see Don's test data on some of his motors list the voltage that he took his readings at. He also is very clear with the specific fan that he tests his motors with -- and you see that the same motor may have higher Watt or Amp numbers with just a different fan.

Watts = Volts x Amps. So... a motor that's "rated" at 800W on a 4S setup, should pull no more than 47.6A (Amps = 800 Watts / 16.8 Volts) -- but the fan that's attached to the motor is a variable that will change that math a bit. Think of the fan as a drag device that's slowing the rotation of the motor... the number of blades on the fan, or the pitch of the individual blades can cause the motor to "work" harder to achieve the charted KV... which will result in higher power draw (increased Amp draw from the battery).

So... 4S should always result in a higher motor RPM than a 3S setup... but will also result in a higher Amp draw from the battery. With that said, replacing a 3S 2200mAh 20C battery with a 4S 2200mAh 20C battery isn't always going to work. Both batteries can supply 44A continuously... but the 4S battery will spin the motor faster (due to the KV rating of the motor), and increase the Amp draw. If the 3S setup was pulling 42A, and the 4S setup is now pulling 48A, the 4S 2200mAh 20C battery will not be big enough.

Does that help at all???

-Sully
Cobra68 is offline Find More Posts by Cobra68
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
SR-71 for EDF 50 (Closer than we might think) Thomas B Electric Ducted Fan Jet Talk 187 Feb 22, 2011 11:56 PM
SR-71 anyone? steve neill Electric Ducted Fan Jet Talk 71 Oct 08, 2007 08:34 AM
hobbylab sr 71 bushpilotx1 Sport Planes 7 Mar 06, 2002 01:53 AM
Electrics that can outperform the SR-71? SchiessCo Electric Plane Talk 4 Jul 30, 2001 09:44 PM