I'm knee deep in the installation of the turbines into my scratch built SR-71, and I've got a question to pose.

I know that if I were doing, say a twin B-25, with props pulling, I'd shim the engines with 1-2 degrees of down and 1-2 degrees of inward angle on the engines. Kind of a 'toe in' approach on the engines, if you would.

Now, ponder the SR-71 with turbines mounted very far forward in the nacelles. Tailpipes are 39 inches long. When standing aft of the plane, do you think you would want the pipes to:

A. Exit a bit down from center line = down thrust
B. Pipe exits pulled together 1-2 degrees
C. Pipe exits pushed out 1-2 degrees
D. Pipe exits straight back on centerlines
E. Any combination of the above
F. I have no idea, but think they should be pointed somewhere towards the back of the plane.

Any thoughts would be appreciated.

Lance

D [sm=thumbup.gif]

....drop and a photo if you have time[8D]

Toed in (pipes farther apart at the back) would help negate any effects from losing an engine.

Dennis

A. Not for this configuration.
B. Absolutely NO !!
C. Should help when single engine
D. See "C"
E. See "C"
F. ;-)
G. Consider a yaw gyro for assistance during an engine failure.
H thru Z: See G.

Best wishes !!

On a prop twin you shim the engines to point OUT away from the fuse so that when you lose an engine the good one is pulling outward and against the direction the plane is now yawing and/or rolling towards.
Jeff

[quote]ORIGINAL: Lance Campbell



I know that if I were doing, say a twin B-25, with props pulling, I'd shim the engines with 1-2 degrees of down and 1-2 degrees of inward angle on the engines. Kind of a 'toe in' approach on the engines, if you would.

twin engine:

left zero
right must have right thrust

case of one engine die ...get altitute and go for landing /on downleg shut off the secont engine too.

this is how i do it with props

Take the engine out senario out of the question, for the moment. (yea my brain was backward early this morning on the pointing prop engines inward, as I was using the 'toe-in' analogy)

Assume for the question that both engines are running solid, does a bit of outward angle on the exhaust pipes help with any stablity issues?

One that comes to mind, is for example throttle changes if one engine is a second or two behind or ahead of the other one.


Lance

Angle of the engines, , ,

I'm almost inclined to think that it wouldn't make much difference.

With the engine say, , , 7" or something down in the nacell:
- the intake comes in straight,
- then the exhaust gets kicked to an angle by the engine.
- there's some degree of swirling action down the rest of the pipe.

Although there's no fixed stator's down the tube, I think the general exhaust flow is still going to want to straighten out to some degree because of the pipe length and the inherent swirling effect.

I was think along the lines of maybe a small deflector of some sort at the very end of the pipe.

IMO highhorse has the hot tip: be thinking stability augmentation on the Z axis - that means a rate gyro on the rudder(s) to compensate for thrust vectoring.

Slight side note, of why the engine out issue won't be relevant..

I'm putting in a pair of JetCat 120's into it. JetCat has a unique feature that allows you to connect two ECU's so that if one engine has an uncomanded shutdown, it can shut down the other one as well.

If this was a centerline mounted twin, such as a F-4, F-14, or A-10, such a feature would not be relevant, as one engine could have a chance to bring back the plane.

As much as I'd like to have delusions of grandeur of bringing in a Blackbird on one engine, banked into the engine at 30 degrees and all, I'm too realistic. I flew my Yellow SR for 3 years, and had 3 engine failures (the first 3 flights with it). The first 2 landed without a scratch, as I could hear when one ducted fan engine died, and could immediately throttle the other one back. The 3rd time I wasn't as lucky and could not hear it when it happened (other jets in the air), and stayed with the throttle. Seconds later it was in a spin. I've also helped a few other guys fly their ducted fan SR's as well.

From all of that I've come to the conclusion, that if you have an engine failure, you've got 5-7 seconds to get the other engine brought back in power, or you'll have a spin on your hands. I even weighed the thought that, would I want the good engine brought to idle and I could throttle it up carefully, for a few pounds of thrust. Then after realizing a fire is the biggest risk of loss, I'd much rather have a traditional 'dead stick' with no power, than a spin into the ground with one engine still turning and a potential resulting fire. With that in mind, I considered the JetCat auto shut down of the other engine a real strong feature in my particular application.

Lastly, thanks for all the posted thoughts above.

Lance

I would mount them straight. If the engine out issue is not an issue then I would just go straight out the back.

Awesome project Lance.

IMHO, engine out performance isnt an issue unless BOTH engines are out. The crews had a difficult time responding in time and they were INSIDE the plane.
With a model, visual orientation would make it nearly impossible to even realize in time that this condition was occurring unless on takeoff or landing.

With respect to trim changes due to power settings, Im thinking that thrust becomes thrust at the point of exit, regardless of how far up the turbine is mounted, unlike a propjob which produces push very near the engine, unless of course, there is a shaft involved. assuming your turbines are mounted dead center of the vertical thrust line, I would also go with zero incidence mounting as Ianober has suggested, and expect to trim down at high power settings.

Good Luck and MORE POWER!

BELA