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SR-71 prop jet project

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Old 08-07-2007, 09:06 AM
  #1
funkworks
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Default SR-71 prop jet project

OK... FINALLY! It's time to finally get this bird outta my computer and on my work table. I've revised my drawings six times and I think Im almost happy now with the result. I'm about to have a kit cut soon after I finish creating the parts templates for the kit cutter. It's time to just build the thing. Here are the design specs:

wingspan: 52 inches
lenght: 80 inches
est. weight: 14lbs (dry)
est. wingloading: about 23oz/sq ft.
power: (2x) Thunder Tiger Pro .46 engines (tractor config. at the front of each nacelle)

Id like to see if anyone has flown anything similar. I know there is a pusher prop jet version floating around somewhere. One thing Im curious about is the use of a gyro for pitch control and possibly another for the rudders. Below are pics of a previous attempt to build this bird. I gave up on it after realizing I could build a much better model if I started over using what Ive learned.


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Old 08-07-2007, 11:54 AM
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Default RE: SR-71 prop jet project

Alright! That's the first model of the Blackbird I've see that looked half worth building. Retracts? What airfoil are you using?
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Old 08-07-2007, 12:57 PM
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Default RE: SR-71 prop jet project

Thanks bro.

It will have retracts. Im not sure exactly what the airfoil is. Ive basically blended the formers to create a thin semisymetrical airfoil with a thickness of about 6%. This one will look better than the first one too. Ive done more blending where the outter wing panels meet each nacelle. On the first one, the outter panel was basically just 1/4'' balsa sheet. The new one will be a sheeted structure that will incorporate a slight dip or twist to help delay any tip stalls. Ive seen pushers but I really think having the engines in front of the nacelles is the way to go. It will help make the model easier to balance and keep a good flow of air over the surfaces. The insides of the new one are totally different from the first. The model will separate where the delta wing meets the forward fuselage to make it easier to transport... I think Im going to rig a rear chute to help slow the thing down after touchdown...

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Old 08-07-2007, 06:21 PM
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Default RE: SR-71 prop jet project

I have an unbuilt Werner Kranz kit from germany.


Found some completed pics of it on the net.
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Old 08-07-2007, 07:20 PM
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Default RE: SR-71 prop jet project

Where is the CG on something like that? Do you consider the long nose when calculating the CG?
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Old 08-07-2007, 09:30 PM
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Default RE: SR-71 prop jet project


Quote:
ORIGINAL: Mike Connor

Where is the CG on something like that? Do you consider the long nose when calculating the CG?
Just in front of the main LG.
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Old 08-08-2007, 06:32 AM
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Default RE: SR-71 prop jet project

Cool Project...cool plane! [sm=thumbup.gif]
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Old 08-08-2007, 07:34 AM
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Default RE: SR-71 prop jet project

My guess is the forward cabin contributes something to the wing area, so the CG ends up slightly forward compared to a sport delta. I've always figured a decent flying sport model is doable on glow power if the wing area is fudged and if the model is built as light as possible.
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Old 08-08-2007, 10:12 AM
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Default RE: SR-71 prop jet project

I built a self design scratch build a few years ago, had the same proble figuring out CG. I ended up making a half scale model of the model out of cardboard and used that CG on my model. Flew good for a couple of flights until I had a high speed stall and it feel out of the sky. I still have the crashed airframe....
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Old 08-08-2007, 12:05 PM
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Default RE: SR-71 prop jet project

I think I have an idea where the CG will end up being but I would like to make sure beforehand. I think I will make a 1/2 scale version of mine out of foam and play with it for a while.
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Old 08-08-2007, 02:34 PM
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Default RE: SR-71 prop jet project

...well. Today has been kinda slow so I started drawing out a lil blackbird to test out my CG calc and some mixing options. I think Im getting close to something and maybe I can start building soon. So far.. Ive got a hand launched model that spans 32 inches and is almost 50 inches long (more like throwing a spear than a plane ). It will be powered by two .25's at the front of each nacelle. My target weight will be 5.5lbs which will give me a wingloading of about 23oz/sq ft. I chose the .25s because I already have one that is new in the box that I planned to use for something else. I decided against a pusher because I wanted to see the effects of prop wash over the surfaces... I can send a copy of my lil blackbird drawings if folks are interested in building along with me...
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Old 08-08-2007, 08:40 PM
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Default RE: SR-71 prop jet project

Sounds like a good plan of attack, one thing I would consider doing is leaving off the gear and launching it off a dolly. Every oz of unneeded weight will make flying something like this more work. The amount of plane that exists ahead of the CG has a destabilizing effect, too. The only way I know of to offset that is with more fin, your mock up model should reveal any problems.

Isn't there a drug store foamy SR-77 that flies pretty good?

There's one on display at Boeing, that plane looks awesome from every angle.
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Old 08-08-2007, 09:14 PM
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Default RE: SR-71 prop jet project

I go over to the AFB museum here in town at least once a week to worship at the feet of our SR-71. #71958 and one of the D-21 drones) For good reading, check out a book called "SR-71 Revealed" it's written by a retired Blackbird pilot, Col. Richard H. Graham, and has some great information for us aviation history nuts.
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Old 08-09-2007, 05:38 PM
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Default RE: SR-71 prop jet project

Quote:
Where is the CG on something like that? Do you consider the long nose when calculating the CG?
Heres a picture of the CG on the instruction page

"S" marks the spot

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Old 08-09-2007, 10:23 PM
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Default RE: SR-71 prop jet project

Thanks for posting that, Sonnich. Looks like the CG is a little bit forward of where a sport delta CG would be. Whoever drew that can handle a pencil pretty good!
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Old 08-10-2007, 07:50 AM
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Default RE: SR-71 prop jet project

yup...thanks for posting that Sonnich. I can compare that to what I calculated for mine. Looks like Im close...

I just finished the drawings for my mini blackbird. Im going to start cutting parts for it this weekend. If anyone would like to build along, let me know. I can send the drawings in .DXF format. Here is a sample of the drawing.
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Old 08-10-2007, 08:59 AM
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Default RE: SR-71 prop jet project

Looks right up CP's alley[8D]
I dare ya!
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Old 08-10-2007, 09:39 AM
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Default RE: SR-71 prop jet project

Vic, I'm thinking about it......a pair of Cyclon .15s would do a nice job on a medium size model. Or Cyclon .061s on a miniature. That long nose sure is awkward for a small model to aerodynamically counter balance. I've built similar planes [with lots of area in front of the CG] that just always were looking for an excuse to spin. A big challenge would be to "design out" what we know won't work well at small model flying speeds without loosing the look of the original. Vaccuuforming anything that isn't part of the wing itself might be a good way to keep the weight way down and the time consuming carpentry to a minimum.
The power situation....the most power for the least weight. That is why I think world class combat engines are hard to beat here, plus you can get a pair of .061s for less than $300. The fuel system is a slam dunk, a single high pressure bladder with a line coming out of each end, it can be set anywhere on the plane, like at the CG. A pair of Cyclons running in the mid 30s should give the plane a pretty cool sound to go along with it.
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Old 08-10-2007, 11:23 AM
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Default RE: SR-71 prop jet project

I think it's funny that the Cyclon Electric motor banner is right above your post as I read this.

FW, have you ever read the article in RCM on the design of Mr. Mikaloso's Blackbird? He goes into a nice explanation of how the nose acts as a wing on it's own and the unique slo flight characteristics it gives.
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Old 08-10-2007, 01:37 PM
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Default RE: SR-71 prop jet project

Vic, Mikulaskos' ideas about that are flawed. To illustrate a point, lift a big frying pan from somewhere around it's CG and take note of how much energy it took to accomplish that. Now lift it from the very end of the handle and keep it perfectly level as you raise it up, you will notice that it takes a little more effort and the pan isn't as stable as when you lifted it from the CG. That forward area on the Blackbird will contribute more to instability in both pitch and yaw than it will to lift. The best cure of all is to keep the weight at the extremities as low as possible, it acts like unsprung weight in a car. I might be all wet and I am just speculating based on appearances, there is no substitute for trying the real thing.
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Old 08-10-2007, 01:43 PM
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Default RE: SR-71 prop jet project

I take it as confirmation that you read the article[sm=75_75.gif][8D]

The comments went far beyond what I feel like typing. But he flew his.
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Old 08-10-2007, 01:46 PM
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Default RE: SR-71 prop jet project

No I haven't read that article. I'd be very interested in seeing it if you still have it. The effect of the front part of the fuse is what is giving me the most stress. I think Im close in my calculations but on some models close isnt good enough. That's why Im going though the trouble of building this test rig. I'll post some pics of my progress. Hopefully, I can start building over the weekend.

CP- There are fewer Blackbird pilots than astronauts... You know you wanna build one .
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Old 08-10-2007, 02:06 PM
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Default RE: SR-71 prop jet project

He lacks the spine it would take to do such a cool project[>:]
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Old 08-10-2007, 02:31 PM
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Default RE: SR-71 prop jet project

Vic, I've got the spine to build it, but you have to be the one to do all the pitting and to fly it....

FW, this will be one of the most interesting threads to follow.
I know a couple of things electric power is great for, building fast prototypes and doing multi engine set ups. The problem with too small of a prototype is that the wingloading won't be proportional to the Reynolds numbers [and all that other scientific stuff] so comparing the results of a miniature can be misleading. The old saying of "bigger flys better" could be hotly debated with this model, I'll bet.
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Old 08-10-2007, 02:50 PM
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Default RE: SR-71 prop jet project

Quote:
Vic, I've got the spine to build it, but you have to be the one to do all the pitting and to fly it....
Ahhh[sm=teeth_smile.gif]
the money shot

I am like Raymond, I'll try anything once. If you did a small one lekky would be the ticket, but up in the .25-.40 size there are enough glo engines around to do several prototypes.
Like CP I am watching closely.
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