Jet Glider Sparrowhawk

"Eric Greenwell" wrote in message
...
Mark James Boyd wrote:

Eric Greenwell wrote:

Mark James Boyd wrote:
At 500 lbs, a notional Sparrowhawk with two AMT-450s

would accelerate at .12 g's (2 knots/second), to 40 knots
in 20 seconds using 1200 feet of runway. Assuming wheel
friction and drag make this figure double, under 2400
feet of runway is still respectable.

Wouldn't that be (500/(45+45) = 0.18 g's ? Or 3.4 knots/second to 40
knots in 10.4 seconds, and about 330 feet? So doubling is only 600 feet.


I avoided the math and safe-sided the heck out of it and
got a way too big figure. Thanks for the extra work


Using one engine, 0.09 g's gives 1.7 knots/sec, 21 seconds to 40 knots,
and 640 feet, doubled to 1280. With all due respect to Mike B, I might
be happy with one engine.
Eric Greenwell


For self-launch, perhaps, but for 100 knot level flight,
or 500 fpm climb, 200 Newtons (45 lbf) may be not enough.
I haven't done the disciplined math for this, nor
do I know the actual drag in Newtons of the Sparrowhawk.
I suppose this can be calculated rather than SWAGed based
on weight and the shape of the polar, eh? Any takers?

That's an easy one. Basically, the weight/(L:d) at the speed of
interest. Sticking with the 500 pounds weight, the drag at best L (36)
is 500lb/36=14 pounds, leaving 30 pounds thrust to climb. 500/30=17 L
climbing; climb rate is [59 knots at best L]/17=3.5 knots 350 fpm. Not
great, but interesting. For 500 fpm climb, a 56 lbf unit would do it.

For a 2000 foot climb:

- 2000'/350fpm = 6 minutes
- 6 min x 460 grams/min = 1200 grams, or almost a gallon liquid.

So, carrying 5 gallons would give you one launch, a 4000 foot climb out
of a big holes, and some travel towards home at 100 knots.

The L at 100 knots ~ 12, so 500/12=41.5 lbf for level flight, so that
goal is met with one engine.

OK, these are a bit optimistic, because I assumed the engine added no
drag. It does show one engine is close to being good, but a 55-60 lbf
engine would be "ideal".

--
-----
change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

Have you math whizzes figured out how far the wing runner will have to run
before this dog whistle is going fast enough to have aileron control?

Bill Daniels

Bill Daniels wrote:


OK, these are a bit optimistic, because I assumed the engine added no
drag. It does show one engine is close to being good, but a 55-60 lbf
engine would be "ideal".

Have you math whizzes figured out how far the wing runner will have to run
before this dog whistle is going fast enough to have aileron control?

Wing runner? On an 11 meter motorglider? We don't need no stinking wing
runners! This is _Self_ launching! Put wing tip wheels on it and let it
roll! Actually, letting it slide on it's plastic pads would work fine,
since there is so little weight on the tip. The wing will come up when
it's ready. In crosswinds, put the downwind wing down, to help keep it
straight. The ultimate answer is a tailwheel with just enough steering
ability to keep it straight, since it doesn't need to taxi.

--
-----
change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

Bill Daniels wrote:

Have you math whizzes figured out how far the wing runner will have to run
before this dog whistle is going fast enough to have aileron control?

Well, we've got a name for the project!

Project Dog Whistle. LOL

And Bill rightly points out that if the thrust
is behind the CG, any yaw at low speeds means
a ground loop on takeoff unless the tailwheel is
firmly tracking.

One more reason why the twin Cri-cri version had it
right, with the two engines well ahead of the
CG, and with exhaust away from anything that might burn.

In article 4004fb99$1@darkstar,
(Mark James Boyd) wrote:

Bill Daniels wrote:

Have you math whizzes figured out how far the wing runner will have to run
before this dog whistle is going fast enough to have aileron control?

Well, we've got a name for the project!

Project Dog Whistle. LOL

And Bill rightly points out that if the thrust
is behind the CG, any yaw at low speeds means
a ground loop on takeoff unless the tailwheel is
firmly tracking.

No it doesn't. The thrust is always directed through the centre of mass.

It's position of the main wheel with respect to the CM that matters for
a ground loop...


One more reason why the twin Cri-cri version had it
right, with the two engines well ahead of the
CG, and with exhaust away from anything that might burn.


--
Alan Baker
Vancouver, British Columbia
"If you raise the ceiling 4 feet, move the fireplace from that wall
to that wall, you'll still only get the full stereophonic effect
if you sit in the bottom of that cupboard."

Alan Baker wrote:

Project Dog Whistle. LOL

And Bill rightly points out that if the thrust
is behind the CG, any yaw at low speeds means
a ground loop on takeoff unless the tailwheel is
firmly tracking.

No it doesn't. The thrust is always directed through the centre of mass.

It's position of the main wheel with respect to the CM that matters for
a ground loop...

Alan Baker

Correct. I'm mixing apples and oranges. In our Sparrowhawk design,
the engine(s) thrust is through the center of mass, so this
doesn't matter.

I wonder about the Genesis mock-up, where this may not be the case.

In article 40057c28$1@darkstar,
(Mark James Boyd) wrote:

Alan Baker wrote:

Project Dog Whistle. LOL

And Bill rightly points out that if the thrust
is behind the CG, any yaw at low speeds means
a ground loop on takeoff unless the tailwheel is
firmly tracking.

No it doesn't. The thrust is always directed through the centre of mass.

It's position of the main wheel with respect to the CM that matters for
a ground loop...

Alan Baker

Correct. I'm mixing apples and oranges. In our Sparrowhawk design,
the engine(s) thrust is through the center of mass, so this
doesn't matter.

I wonder about the Genesis mock-up, where this may not be the case.

How can it really be otherwise?

And what's more, a ground loop is cause when the main wheel -- ahead of
the CM -- develops a sideways force which causes further yaw, which
causes larger sideways force, etc.

Even mounted off the CM laterally, and engine would still be fixed to
the fuselage and hence the yaw force would be *constant*.

--
Alan Baker
Vancouver, British Columbia
"If you raise the ceiling 4 feet, move the fireplace from that wall
to that wall, you'll still only get the full stereophonic effect
if you sit in the bottom of that cupboard."

"Mark James Boyd" wrote in message
news:4004fb99$1@darkstar...
Bill Daniels wrote:

One more reason why the twin Cri-cri version had it
right, with the two engines well ahead of the
CG, and with exhaust away from anything that might burn.

But introduces the possibility (certainty?) of asymmetric thrust! I
saw a Cri-cri (piston) lose power on TO one year at Oshkosh, it crashed
almost at the crowd line.

Vaughn

Vaughn wrote:

One more reason why the twin Cri-cri version had it
right, with the two engines well ahead of the
CG, and with exhaust away from anything that might burn.

But introduces the possibility (certainty?) of asymmetric thrust! I
saw a Cri-cri (piston) lose power on TO one year at Oshkosh, it crashed
almost at the crowd line.

Vaughn

Probably because the Cri-cri is such a terrible glider...
A lot in common with the BD-5: rotation speed = Vne = Va =
Vmc = Vs... (or nearly so). ;( The Gruman Yankee also had
critical speeds close together, and the Speed Canard had a
real high stall speed too. One of the reasons I LOVE gliders
is the generally low stall speed.

Some full size regional jets, and the RC models that use the
little turbines, have placed the twin turbines very close
together and near the rear with no blast towards surfaces.
It would be great to put turbine(s) on the tail of the Sparrowhawk,
but the CG would never, ever work...

Alas, back to the drawing board...

"Mark James Boyd" wrote in message
news:40057f22$1@darkstar...
Vaughn wrote:

One more reason why the twin Cri-cri version had it
right, with the two engines well ahead of the
CG, and with exhaust away from anything that might burn.

But introduces the possibility (certainty?) of asymmetric thrust! I
saw a Cri-cri (piston) lose power on TO one year at Oshkosh, it crashed
almost at the crowd line.

Vaughn

Probably because the Cri-cri is such a terrible glider...

Probably so, but I see I was not clear in my above post. The Cri-cri
apparently lost power in one engine only, the one towards the crowd. We
were sitting at the crowd line, but at the other end of the field, so we
were not among the menaced. Looking at the Cri-cri, the engines look close
enough together that you would think that asymmetric thrust would not be a
problem; apparently not so.

Vaughn