What was my prior position? I can't afford any of these planes, so any position I have is lightweight at best...

I'm just trying to scratch a little of the surface of the math, engineering, and physics that underlies the designers' choices in these aircraft.

You said that 2 tow trucks each with 500# of power was the same as 1 truck with 1000# of power, the only difference was drag.

Well now we have Cirrus jet with 1400lbs of thrust and Mustang with 2800lbs of thrust divided over 2 engines and the Mustang is only 40 knots faster even though it has 2X the thrust.

Mustang BOW 5600lbs
Cirrus Jet BOW 3700lbs

But a big portion of that extra weight on the Mustang has to be the 2nd engine and systems required to run it.

No props in this scenario so is the issue still drag?

I didn't look up the spec. for thrust one says 1800 lbs the other 1400 lbs. I'd say given the weight difference and from seeing a Mustang and pictures of the Cirrus Jet. It looks to me the frontal area is also larger on the Mustang. It was a much larger plane than I expected. So yes I think drag is still relevant.

Farther up the thread, I'll snip:

Mustang is 13% faster, which by itself would require 45% more thrust (1.13 ^ 3).
Mustang has 62% more thrust, which when you account for the 45% more thrust required for the speed, gives you 12% more thrust available. (It's geometric, not arithmetic.)

Nobody ever said anything about this. It's like you've gotten defensive because were talking Cirrus. Assume 2 identical airplanes. Manufacturer is not important. 1 has to small engines the other 1 large engine.

Anything out in the airstream is drag.

Anything out in the airstream is drag.

I don't care one whit about Cirrus either way. I like that the Klapmeiers (sp?) have breathed some life into general aviation with some significant competition. But, I don't care to attack nor defend them on their jet program.

Two small turbine engines will always be less efficient than one large turbine engine. I said that many posts above. Some of it is related to inherent inefficiencies as you scale down a turbine. Some of it is related to increased drag of having two discs out in the airstream. Some of it is the additional weight. I agree with you, but by all means let's keep discussing it...

Twins give you more redundancy. It's a pretty terrible way to get more performance. I agree to all of that.

Seriously? C'mon man. 40 knots is a small percentage of 300. It's not like adding 40 knots to a Cessna 172. 40 knots is nothing when comparing 300 to 340. Are you trying to be difficult?

Yes, I agree.

Thank you for offering your brain power to this. I've learned a lot.

I guess they need to come up with a single engine design that could deploy a second engine if needed. Otherwise it's tucked away. At that point, it may be easier to develop an engine that runs on anti-matter and just skip wings all together.

viewtopic.php?p=1189033#p1189033

All in good fun, of course...

I can imagine some kind of ducted approach, feeding two engines, with the ability to shut one engine down entirely in cruise. You could launch and climb on 2 (a good thing), cruise on 1, and land on 2.

Switch over between which of the two you use in cruise based on time and you might have a workable design. Of course, you'd also have the option of cruising 25% faster on double the cruise fuel flow by running both, and a lot of us would opt for that, if range wasn't a problem.

Interesting concept. I would think the easiest/most efficient solution would be a TP with a hidden tiny straight jet (or turbofan?)

I am picturing an S-tube to the rear jet with some aerodynamic covering or hiding of the inlet.

I saw a glider that had this. Glider with an engine that has a prop that folds away when not in use. I'll see if I can find on youtube.

Even better
http://www.gizmag.com/bonusjet-glider-w ... ine/16017/

http://en.wikipedia.org/wiki/Motor_glider

[youtube]http://youtu.be/zJV4vcuKww4[/youtube] is one.