SR-71 BlackBird

[AcendenceRC]

Hello all, for the past little while i have been working on a foam board SR-71 in the FT style. Although this process has been very slow, I have worked on 3 prototypes.
The prototype i will be posting here is the latest iteration of the bird. Keep in mind, I am not sure if it will fly because I have not figured out the CG just yet, it seems to have plenty of power but is very nose heavy. If anyone wants to beta test it or help me figure out the CG that would help me a lot. In the comments on my article about version 2, I said that I would release the plans once they work/are finalized, however, I seem to have hit a wall when it comes to testing.

(Some Pictures of version 3)


From the Pictures you can see that the EDFs are missing, this is because the EDF nacelles are damaged. This plane was to attempt to figure out the CG so that i could refine the design of the plane. As time goes on i will do more testing and make new models, however, I wanted to put these plans out as this is the closest thing I have to a flying version ( And it may fly with a better CG).

PLANS:

 

[AcendenceRC]

If you do decide to put this plane together you will need to cut a battery hatch. (There is no battery hatch in the plans because the battery hatch placement is dependent on the CG)

 

[veloxsouth]

You might want to approximate your plane's shape on here to find the aerodynamic center, choose your stability margin, and that should help you find the CG you want.
https://www.ecalc.ch/cgcalc.php

 

[Merv]

I would ask @PoorManRC to weigh in.

I believe you will need to model the entire bottom view of plane to find the CG. That is I believe the entire body is the lifting surface. I prefer Sketchup for this kind of work. When you have bottom view of the body, simply use the area function to calculate the CG. You want 25% of the area forward of the CG.

 

[PoorManRC]

I would ask @PoorManRC to weigh in.

I believe you will need to module the entire bottom view of plane to find the CG. That is I believe the entire body is the lifting surface. I prefer Sketchup for this kind of work. When you have bottom view of the body, simply use the area function to calculate the CG. You want 25% of the area forward of the CG.

I'm flattered! Especially since everything I've suggested before, has been passed by......

Unfortunately, this issue I can't really help with. 😞
Having never built or flown a Model of it, I couldn't begin to describe where the CG would be.

It is a Lifting Body, mostly. The Forward Chines provide some Lift, but not quite in the way a designed Lifting Body would...

I have mentioned, this beautiful Bird has the Gliding capabilities of a ROCK! 😜
Get as MUCH Thrust as you can get, and hold on!

I don't believe that this can be hand launched - unless you're an Olympic Discus Thrower!!! Maybe some sort of Slingshot, if you just don't want Landing Gear.

I'm GUESSING that she will have a MINIMUM Stall Speed of about 50mph.

The real one was on the edge of Stall, when Refueling with a 500mph KC-135!!

Takeoff Rotation was around 230-240 knots. The higher she got, the higher the Stall Speed.
This will be more of a challenge to FLY than to build...

But it will be an impressive feat!

 

[AcendenceRC]

hmm, do you think adding a KF-step would help? Like on the Ft delta?

 

[AcendenceRC]

Also Merv, I 3D modeled the bird for the plans, so I think ill do the area thing that you suggested.

 

[Merv]

Also Merv, I 3D modeled the bird for the plans, so I think ill do the area thing that you suggested.

First calculate the area of the entire bottom view. Then mathematically calculate the target area, I figure 25% & 30%. Then I measure down from a point on the plane that is convenient, I will mark off several 1/4 inch increments to test and find the marks closest to the target areas.

I set the CG at 25% for the maiden flight to get the plane trimmed out. They I will slowly move the CG aft until I get the instability I prefer, usually around 30%.

I’ve used many Kf airfoils, they are an improvement over a flat wing. I don’t think they perform as well as a folded FT wing.

 

[PoorManRC]

I agree, a KFM wouldn't be the best solution...
There's a little hope - I recently saw a Foam Board F-104 fly beautifully. It has the same sort of long Forward section, but even LESS Wing! 😳

I'm really interested in where the CG ends up on the HABU. That's one thing I really don't know.

 

[AcendenceRC]

@PoorManRC Just calculated it, seems to be 4.5 inches in front of the engine nacelles

 

[AcendenceRC]

I think an airfoil might help, but I'm not sure how to implement it.

 

[Merv]

Just calculated it, seems to be 4.5 inches in front of the engine nacelles

I built a number of RC Powers planes before I discovered FT. Based on that experience, I would have guessed the CG to be about at the engine nacelles. I would like to know where @kilroy07 had his CG during his test flights.

All of the RC Powers planes that I tried flew very well. They all used a flat sheet wing with the option to add KF airfoils.

 

[kilroy07]

Hello all, for the past little while i have been working on a foam board SR-71 in the FT style. Although this process has been very slow, I have worked on 3 prototypes.
The prototype i will be posting here is the latest iteration of the bird. Keep in mind, I am not sure if it will fly because I have not figured out the CG just yet, it seems to have plenty of power but is very nose heavy. If anyone wants to beta test it or help me figure out the CG that would help me a lot. In the comments on my article about version 2, I said that I would release the plans once they work/are finalized, however, I seem to have hit a wall when it comes to testing.

(Some Pictures of version 3)
View attachment 149063 View attachment 149064

From the Pictures you can see that the EDFs are missing, this is because the EDF nacelles are damaged. This plane was to attempt to figure out the CG so that i could refine the design of the plane. As time goes on i will do more testing and make new models, however, I wanted to put these plans out as this is the closest thing I have to a flying version ( And it may fly with a better CG).

PLANS:


View attachment 149070

AWESOME, another Blackbird lover!! I'll be watching your progress too.
I'll share plans of my design shortly (work has really put a damper on my "play time" lately!!)...
It looks like yours is coming along nicely! Great job!

. I would like to know where @kilroy07 had his CG during his test flights.

Here's a quick image with what I came up with, glide tests were great, maiden of Alpha build were good, just woefully under-powered/over weight (Beta should address both issues.)

 

[TEAJR66]

AWESOME, another Blackbird lover!! I'll be watching your progress too.
I'll share plans of my design shortly (work has really put a damper on my "play time" lately!!)...
It looks like yours is coming along nicely! Great job!



Here's a quick image with what I came up with, glide tests were great, maiden of Alpha build were good, just woefully under-powered/over weight (Beta should address both issues.)
View attachment 149353

I made a profile chuck glider and that is about where the CG worked for it.

AWESOME, another Blackbird lover!! I'll be watching your progress too.
I'll share plans of my design shortly (work has really put a damper on my "play time" lately!!)...
It looks like yours is coming along nicely! Great job!



Here's a quick image with what I came up with, glide tests were great, maiden of Alpha build were good, just woefully under-powered/over weight (Beta should address both issues.)
View attachment 149353

I made a profile chuck glider and this is where the CG worked for me.

 

[TEAJR66]

Here are a couple pics.

 

[AcendenceRC]

Just did a quick test with the new CG @kilroy07 said to use, and it works great! Gave me new hope! I like the simplicity of v3, so i am going to work on that a little more. The next set of plans will be the final set, so stay tuned, I will try to update you guys along the way.

 

[BATTLEAXE]

I like the profile of the bird you are building @AcendenceRC, I have seen @kilroy07 vids on his and this would be great to see it fly. It seems you already have the CG figured out. I was goin to say to chuck glide it with Playdoh as adaptable weight till it closes level on the pitch axis, unloaded of coarse, then plan where all the electronics go based on that CG. But you got it.

As far as the KFm wing goes I would suggest it, for a few reasons. It does create a considerable amount of lift. I was skeptical of it in the beginning until I tried it and it amazed me to how level the plane will fly even at slower speeds without going into a high alpha to maintain level flight. It also adds a little drag to help the plane slow down on landings, more scale. Plus it adds strength if done properly, which is super simple, to keep the wing from going soft in positive G's over time, or even on harder landings that we all experience.

Here is some video of the FT-22 with a KFm wing as opposed to the single sheet version

That's just my opinion, I will be following this one

 

[AcendenceRC]

I will definitely look into a KF step, just need to make it look inconspicuous...

 

[kilroy07]

I will definitely look into a KF step, just need to make it look inconspicuous...

Though not often seen in RC applications, the KF airfoil can also have the step on the underside.
https://en.wikipedia.org/wiki/Kline–Fogleman_airfoil

I also wanted to give you a heads up.
One thing I think many who try to model the SR don't realize that there is a negative thrust angle in the design.
I measured/used a 2.6 degree angle in my design (it was actually the one thing that gave me the most trouble...)

 

[AcendenceRC]

Is there a reason to use the negative thrust angle?