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SR-71 prop jet project - 8/7/2007 3:06:07 PM   
funkworks



Posts: 418
Joined: 3/24/2003
From: Houston, TX, USA
Status: offline 		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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RE: SR-71 prop jet project - 8/7/2007 5:54:18 PM   
Lomcevak Duck



Posts: 972
Joined: 2/22/2004
From: Warner Robins, GA, USA
Status: online 		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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RE: SR-71 prop jet project - 8/7/2007 6:57:43 PM   
funkworks



Posts: 418
Joined: 3/24/2003
From: Houston, TX, USA
Status: offline 		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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RE: SR-71 prop jet project - 8/8/2007 12:21:40 AM   
SJN



Posts: 4390
Joined: 9/28/2002
From: Copenhagen, DENMARK
Status: offline 		I have an unbuilt Werner Kranz kit from germany.


Found some completed pics of it on the net.

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RE: SR-71 prop jet project - 8/8/2007 1:20:45 AM   
Mike Connor



Posts: 1384
Joined: 1/10/2002
From: Tulsa, OK, USA
Status: offline 		Where is the CG on something like that? Do you consider the long nose when calculating the CG?

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RE: SR-71 prop jet project - 8/8/2007 3:30:46 AM   
soarrich



Posts: 2375
Joined: 2/1/2002
From: Middletown, NJ, USA
Status: offline
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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RE: SR-71 prop jet project - 8/8/2007 12:32:11 PM   
smchale



Posts: 843
Joined: 6/19/2002
From: Malvern, PA, USA
Status: offline 		Cool Project...cool plane!

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RE: SR-71 prop jet project - 8/8/2007 1:34:32 PM   
combatpigg



Posts: 9806
Joined: 11/22/2003
From: arlington, WA, USA
Status: offline 		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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RE: SR-71 prop jet project - 8/8/2007 4:12:04 PM   
evan-RCU



Posts: 2677
Joined: 2/4/2002
From: Charlotte, NC, USA
Status: offline 		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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RE: SR-71 prop jet project - 8/8/2007 6:05:17 PM   
funkworks



Posts: 418
Joined: 3/24/2003
From: Houston, TX, USA
Status: offline 		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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RE: SR-71 prop jet project - 8/8/2007 8:34:17 PM   
funkworks



Posts: 418
Joined: 3/24/2003
From: Houston, TX, USA
Status: offline 		...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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RE: SR-71 prop jet project - 8/9/2007 2:40:35 AM   
combatpigg



Posts: 9806
Joined: 11/22/2003
From: arlington, WA, USA
Status: offline 		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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RE: SR-71 prop jet project - 8/9/2007 3:14:27 AM   
Lomcevak Duck



Posts: 972
Joined: 2/22/2004
From: Warner Robins, GA, USA
Status: online 		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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RE: SR-71 prop jet project - 8/9/2007 11:38:56 PM   
SJN



Posts: 4390
Joined: 9/28/2002
From: Copenhagen, DENMARK
Status: offline
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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