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Author Topic: SR-71 Blackbird  (Read 5879 times)
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Tyrannt
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« Reply #50 on: December 01, 2016, 10:45:36 PM »

Flight report.   Well... ummm.  Right now she is resting in the hanger awaiting repairs.

After I got the fuselage covered and before the rudders went on, I started flight testing.  The balance was really close to the center of gravity.  The across the room flight on the bed went pretty well. So to I took her outside.  She can be a bit of a challenge to launch. Center of gravity is right at my hand holds.  She is just like to real thing. The faster she flies the better she flies.  When the speed drops off she does a nasty death roll to the left ending in a nose plant.

I put the rudders on and she is tail heavy, and I haven't put the propeller on the tail yet. Definitely going to need nose weight.  Still having the problem with the death roll to the left.  Then it hit me.  I balanced the plane front to back not left to right.  Need to add weight in the right wing tip.

Damage is not bad.  The first bulkhead behind the nose cone cracked from the nose plants.  I used 1/32" balsa for the stringers from front to back  on the bottom.  The first set of stringers behind the first bulkhead cracked or broke from the nose plants.  I have cut out the tissue And replaced the three bottom stringers with 1/16" square.  Picture is of broken bulkhead and stringers.

I thought about running some planking from the first bulkhead to the second on the bottom.  Is this a good idea or should I just leave the stringers.
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Copbait73
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« Reply #51 on: December 02, 2016, 10:38:24 AM »

Unfortunately if you chase reinforcement to survive the type of arrivals described you will have a SR-71 Blackbird shaped Jart. The best insurance is to never glide test without first accurately confirming C/G. Jets are easy, fabricate a wedge and check it out. Next, most of my "pointy nose" jets and especial my large XB-70 have a nose cone held on by magnets so it separates on arrival.
« Last Edit: December 02, 2016, 11:26:46 AM by Copbait73 » Logged
fuzzywill
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« Reply #52 on: December 03, 2016, 03:37:20 PM »

I've been following this project.  Gotta luv pushers.  Since you need nose weight, I don't see harm in planking.  I'm currently building a Henschel P-75 and I used 1/16 basswood solid formers for the 1st 2 bays and 1/32 for the rest.  By the way, if you live near Logan, UT, we need to get together.
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« Reply #53 on: December 07, 2016, 10:35:09 PM »

So sorry she "darted" on you.  Copbait73 is sooo right on the "finding the CG" before glide test.  Best of luck with the repairs and many happy landings in the near future!
Tom
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Tyrannt
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« Reply #54 on: December 08, 2016, 03:04:57 PM »

Thanks everyone for the support in this project.  The kudos really mean alot to me. 

I was like a kid at the candy store when I finished covering the blackbird and went to fly her.  After crashing her I held my head in shame because I knew better than to try it.  A great lesson has been learned by me.  Check everything out before you fly.  I was just lucky the damage was so minor.

Copbait73 I like the idea of a wedge.  Wish I had thought about that years ago.  I just used my finger tips to do it.  To easy to cheat that way and get a bad balance.  It's what I did on the blackbird. 

I also like the idea of magnets.  I think I may change the engine inlet cones to magnets.  They are starting to get wiggly. 

I decided to plank just the bottom between the first and second bulkheads.  I realized they take alot of stress.  I have never done that before and was surprised by how fast and easy it was.  I was also surprised by how much strength it added.

Next on my list is recover the bottom front.  Make some wedges for balancing it and check it front to back and left to right.  Do the windshield and the red stripe down the fuselage.  Oh yah, put the propeller on.  (That might be a good idea.)

Fuzzywill that would be great. I live in the Salt Lake valley.

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fuzzywill
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« Reply #55 on: December 19, 2016, 12:44:10 PM »

I have to be in Salt Lake on January 11 and 12.  Perhaps, that could be a good time to get together?
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Tyrannt
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« Reply #56 on: April 23, 2018, 03:15:41 PM »

Just a quick update on things here.  I have finished the blackbird. 
  I got my red stripes on.  It was actually kinda scary for me.  Stripes don't usually turn out well.  This time I used a glue stick on some red tissue and folded it in half.  Then I used a steel straight edge with a brand new blade in my knife and I cut out several strips out 1/8" wide.  Then cut to size and glued using the same glue stick.  It went really fast and worked great.
   I then worked on the windows.  Since this is not for competition, I wanted mirrored windows.  Long ago I saw a picture of the blackbird coming in for an air to air refueling.  The sun was glinting off the windows in gold.  One of my favorite pictures.  To get that look, I used the lens out of a pair of busted sunglasses.  The four windows installed can be covered up by a dime.
  The propeller is a six inch pusher prop from a old model I had. (I think it was called the joy.  Foam wings and balsa body.). I am using a 21" loop of 3/16" rubber.
  Flight report.  First real throws showed the plane wants to fly.  Death roll to the left was fixed by putting some balsa in the right wing tip untill balanced.  With the new propeller in the balance point moved back a ways.  The plane then nosed dived in to the grass.  It flew about 20' before crashing in.  ( I did get a four second video of it.  Only one second of it flying.) I tried 200 and 300 turns.  It only flew about 30' then nosed dived in.  When the speed drops off, the wieght in the nose causes it to nose dive.  I did add some clear plastic to the trailing edge for adjustment.  Even with them almost straight up, they could not over come the nose wieght.  By this time I had crushed the nose in to the first bulkhead.  Have to rebuild the nose, and reduce the nose weight.
  I will post some pictures later showing the strips and windows.
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« Reply #57 on: April 23, 2018, 04:04:16 PM »

That is a cool model of one of my favourite aeroplanes. Please keep us updated about progress. I would love to see a photo of it flying.
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Tyrannt
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« Reply #58 on: April 23, 2018, 10:09:41 PM »

Pictures that I promised.  Flying pictures to come later.  Need to fix the nose first and take out some of the nose weight.

I am also considering of making a new 7" prop.  Even with 300 winds I don't feel like I'm getting enough thrust.  The current prop is adding thrust but not much.  I know I can get alot more winds in with this loop.  It just feels small.
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« Reply #59 on: April 24, 2018, 09:36:56 AM »

Don't know the weight, but with that nose, I question the need for any nose weight.  Also, peanuts are hard to trim with big props.  Suggest a 5 inch max for starters; 4 would be better.  Easier to start small and add more diameter later.  If it takes 3/16 rubber, it's too heavy.  That airplane is very difficult to build light though.  Lots of shapes.
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« Reply #60 on: April 24, 2018, 09:58:53 AM »

Great post. I would love to see this thing fly. I don't understand why it lawn darts when the power runs out if the glide is good. Next time you fly please post a vid, no matter what happens. I have a very finicky prop powered Mig-15 that when its good its very good but when its bad it flat out sucks! Roll Eyes
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« Reply #61 on: April 24, 2018, 10:38:44 AM »

just a "monkey thought".. maybe at low rpm the prop acts as a "rear air brake", causing the dive by leverage  Huh Huh
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« Reply #62 on: April 24, 2018, 10:38:56 AM »

Just had a thought after reading Crabby's post.  It may lawn dart when it runs out of power and the prop fails to free-wheel.  Acts like a drogue chute.  Definitely post a video for Crabby to look at.  He is a proper authority on these matters, having learned from a true expert.  Thee Old Man.
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Tyrannt
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« Reply #63 on: April 25, 2018, 03:02:03 PM »

I wanted to get a weight before posting but I discovered that my scale is broke.  My guess on weight is about 30 grams.  The frame came in at 20 grams.  And I think I remember that covered was around 25 grams.

The reason for the bigger rubber is about weight and wing area.  The wing area is around 56 square inches not including the engines or fuselage.  When you add that in it pushes it up to over 70 square inches.  With this in mind I am treating it more like an embro class airplane.

After reading your posts, I hadn't thought about the prop acting like an air brakes or drag chute.  It makes alot of sense to me. Lots of thinking to do now.
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Tyrannt
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« Reply #64 on: June 03, 2018, 10:10:41 PM »

Well... Last weekend, I took the blackbird out to fly.  I just new it was going to fly and get that dream flight I always wanted.  Took her out to a local soccer field for the flight.  I was a little enthusiastic about the flight and launched her hard.  She flew about seven feet and noses darted in the field.  The pictures are what happened after the third flight.  Second flight did most of the damage. Just had to try again. Been licking my wounds since.

A little history since my last report.  I sent Crabby an message.  A big thank you for the help.  He suggested that it was way too nose heavy, and possible tail weight.  He also suggested bigger strips on the tail for adjustments, and an adjustable prop.

What I did. 
Step 1.  Rebuilt the nose and took out the nose weight I had added.  4 grams of nose weight.  Big surprise. Wow.  I did not add any tail weight after I had removed the nose weight. Mistake #1.

I did not make a new prop. Mistake #2.  Been scared to make a prop.  Never had any luck building a props, but I see the wisdom now and now have a 2 liter bottle for one.

Mistake #3.  Trying to fly alone.  Going to fly next time with someone much better qualified.  Never been able to fly well. Bad part of being self taught.

Mistake #4 forgot to add bigger and more tabs on the tail for adjustments.

Oh a little extra knowledge.  Got my scale to work. Contrast display was all the way down.  Total weight after removal of nose weight.  Total 45 grams.  Ouch.  Plane 37 grams. Prop assembly 2 grams.  Rubber 5 grams.

Do you think it's too heavy to really fly?  Is it worth rebuilding?
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« Reply #65 on: June 04, 2018, 03:09:02 AM »

Sorry to see that Tyrannt. No, I wouldn't guess it's too heavy to fly at all. It'll just be a trimming 'journey' I should think. I would surely repair it and yes if there's someone who might have more instinct than you about how it needs to be set up for flight, take 'em along.

Stephen.
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yagua
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« Reply #66 on: June 04, 2018, 08:38:43 AM »

 Cry Cry
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« Reply #67 on: June 04, 2018, 09:07:42 AM »

  That was bad luck, I keep hoping to see a flight video but good to see you haven't  given up.   I agree with Stephen, it should fly at that weight probably just not for a long time.   I may have missed it but I don't see any where that you have done any test glides.  I think this is important in an unconventional model, once the glide is sorted( CofG and elevator) then you can gradually work up the power and thrust angles. Start with your small prop and increase the rubber size until it starts to extend the glide and you are on your way.  Try to find some soft grass at least for the test glides.

             Good luck with your testing
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Crabby
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« Reply #68 on: June 04, 2018, 09:28:04 AM »

Tyrant, keep up with the Blackbird, I would strongly suggest that you install elevons at the trailing edges of the wings, and also build a profile test model from sheet, and get your glide tests straight. I have seen an indoor version of this plane at Johnson City TN, it was a no-cal and it was mesmerizing to watch. The weight? well it was a piece of goose down.
« Last Edit: June 04, 2018, 09:39:54 AM by Crabby » Logged

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« Reply #69 on: June 04, 2018, 09:47:56 AM »

Hi All,
this is an interesting project and would be a challenge even for those of us who have been flying free flight scale all our lives. I will throw my hat in the ring here, please disregard anything that repeats what other sages have already offered.

What I would do:
1.Find a field with some long grass. Getting ff models to fly is always a bit of a thrilling challenge and this is especially true of an unusual subject. You need to have a chance to make errors and deal with the unexpected arrivals. Long grass that gently captures your missile will give you the chance to make adjustments and learn. Soccer field are always inviting but they are actually pretty poor ff fields because the grass is kept short and the substrate is packed hard so that it holds up to the efforts of the cleated footballers. Soccer field are ok for well-trimmed reliable performers but not for an untrimmed tricky prototype.

2. Now at your new grassy field (it can even be a smallish field) on a calm morning or evening, spend some time figuring the CG out. I would remove the prop and balance the model roughly where it does when you have prop and rubber installed. Make sure the model is warp free with neutral rudders and elevators. Now with a blob of clay handy I would start test gliding the model. You are looking for a smooth steady glide with a slight turn. If the nose is up and the model rocks from side to side I would add some nose weight (just a bit at a time). If the model continues to rock it may need more rudder area (just add some rudder extensions with paper or tape). If the model glides but the slows and dives quickly it is probably stalling (I can imaging that this design would have a vicious stall...though I could be wrong) .... more nose weight. Given the weight I could see this ship gliding straight and fast for a fairly short distance.If you can get it to this point you are probably ready for the next step. Video would definitely help us to diagnose any issues.  Note: as you add or remove clay DO NOT mess with the rudders or elevators etc. One change at a time so that you know what your clay is doing.

3. Prop in place, lubricated rubber installed, CG set to the exact spot you found using your glide tests. Now you can start winding her up; start with 300 turns and launch with some authority (pushers need a good shove to get air flowing over the wings). Hopefully your glide testing will pay off resulting in an extended powered glide. If something weird happens it should be there result of torque or thrust line issues, because this is the only variable that you have altered. I think I will leave things here re. trimming ... please report your results back to us.

4. From what I read you are a more or less novice ffer. As much as I would enjoy following your progress with this exciting design I fear it will prove to be a frustrating experience for you. I would therefore suggest possibly shelving this ship until you have built a few more conventional designs and gotten them to fly well (at that long grass field). With more experience and success under your belt you could revisit this bird in the future. Crabby's idea of building a profile ship is a good one. It could even be built out of deperon foam .... this would help you figure out what is needed to get the built-up model to fly.

my two cents.

BG
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« Reply #70 on: June 04, 2018, 11:45:43 AM »

Tyrant, I found the source of the indoor SR-71 I referred to a post or two back. It is in our plans section, and was designed by Dave Aronstein. I saw this thing fly at Johnson City. I am wrong about it being a no-cal. If you haven’t already, check this plan out
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« Reply #71 on: June 04, 2018, 11:56:07 AM »

Crabby could you find a reference to a CG on this plan? I couldn't. I still think a simple profile version would be useful.
BG
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« Reply #72 on: June 04, 2018, 12:43:37 PM »

Bernard, No I don't see anything, but check out Don S's reply#35, seems to make sense. Make and install a set of scale LG and then take a 15 degree angle and "X" marks the spot approx. I think most planes rotate on take off at or around the CG. Anyway that would get you in the ballpark. I am getting twitchy over a CLG model of this thing. Aronsteins SR-71 flew on the edge of stall, nose up like on a landing approach. His was only like 8"wingspan, but qualified as a peanut by means of fuselage length. He covered his with Gampi, I think. Whatever. It was almost lighter than air.
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« Reply #73 on: June 04, 2018, 02:33:23 PM »

That's a good idea for finding the CG, Crabby, although, I might warn that sometimes it's not going to work, like the B-52. Tail draggers probably work if you reverse the 15%.  The main gear typically needs to be near the CG so that the plane can easily rotate, for the case of a tricycle geared aircraft, or, to easily get up onto the main gear in the case of conventional geared aircraft.  I don't think a B-52 rotates, I think it just magically levitates somehow.

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Tyrannt
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« Reply #74 on: June 05, 2018, 01:11:43 AM »

Thank you everyone for all the help and encouragement.  I have decided to rebuild the blackbird.  I can't just let this one go.

In reply #19 on this thread sky9pilot posted a plan with the CG location on it.  When I went to fly it it was just a little ahead of the one showing on the plan. I think that's why I tried to fly it.  Been thinking about the flights and I think it was a launch error and too much nose weight.  To toss the plane in the air you have to hold it I front of the CG.  It makes it a little tricky sometimes.  I had no winds in the rubber just a dead stick toss to check trim.

I haven't figured out yet, how to toss the plane, and hit record on the camera and keep it all in focus at the same time yet.  I have been flying by myself.  That's the big reason for no flight videos.  The couple of times I tried to take video, you get to see my grass or my shoes more than the blackbird.

So for now I think I will start rebuilding the blackbird.  Step #1 put the elevons on the plane before I forget again.  I also want to replace the strips on the tips of the wing and make them a little bit bigger.  Step #2 rebuild the nose and try to keep it light. Step #3 build a better prop.  Step #4 fly with someone that knows how to trim an airplane.  In a field with very long grass next time.  (Alfalfa fields are looking real good about now.)

I have to admit that I'm 100% a novice, a rookie, or greenhorn, or what ever you want to call it.  You don't know how much I appreciate all the help.  No way this would have been possible without out  all your help.  Thank you.

Crabby.  I did see that plan. Thank you.  I actually studies them before I drew up my own plans.  It's good to hear it flew well. Gives me hope.  Are you thinking of a catapult launched glider.  That would be cool.

BG.  Thanks for all the info on trimming going to try it.

The only one I know around where I live that flies free flight lives two hours away from me.  But I am learning that it's a trip well worth the time.
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