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Author Topic: SR-71 Blackbird  (Read 4128 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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Sky9pilot
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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!
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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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piecost
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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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flydean1
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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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Crabby
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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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yagua
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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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flydean1
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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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