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PostPosted: Sat Mar 02, 2013 5:27 pm 
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http://en.wikipedia.org/wiki/Shuttle_Training_Aircraft

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In a normal exercise, the pilot descends to 20,000 feet (6,000 m) at an airspeed of 280 knots (519 km/h), 15 miles (24 km) from the landing target. The pilot then rolls the STA at 12,000 feet (3,700 m), 7 miles (11 km) from landing. The nose of the aircraft is then dropped to increase speed to 300 knots (560 km/h), descending at a 20-degree angle on the Outer Glide Slope (OGS). The Outer Glide Slope aiming point is 7500ft short of the runway threshold, and uses PAPI's for visual guidance in addition to the MLS system. At 2000ft the guidance system changes to pre-flare and shortly after, at 1,700 feet (518 m), the pilot starts the flare maneouver to gradually reduce the descent angle and transition to the Inner Glide Slope (IGS) which is 1.5 degrees from 300ft onwards, using a "Ball-bar" system for visual guidance. The shuttle landing gear release is simulated at 300 feet (90 m) above the ground surface, since the STA main gear has been down for the whole simulation. The nose gear of the STA is lowered at 150 ft (46 m) AGL in case of an inadvertent touchdown with the runway surface.
If the speed is correct, a green light on the instrument panel simulates shuttle landing when the pilot's eyes are 32 feet (10 m) above the runway.



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PostPosted: Sat Mar 02, 2013 7:25 pm 
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Though I am not a flight sim type person at all, I was fascinated by very basic "simulator" of the space shuttle, which I found in a small space museum in Ohio. It just gave you the approach and landing. Of course I cannot say how representative it was to the real thing, but is sure was different! After some practice, I was able to fly an approach through to a presentable landing, and it was quite a ride. This was really good practice for power off approach, and energy management. It's not like you are trying to conserve any energy, you're really trying to change the direction of a whole bunch of energy in the very last moment before arrival on earth!

Hawker Harrier Test Pilot John Farley makes excellent remarks about forced approaches in his book "A View From the Hover". He suggests that the forced approach is sometimes better carried out as a faster than normal glide, while controlling (adding) drag at the bottom to enable the landing as desired. Retaining this excess energy until you no longer need it, could compare to carrying power into the approach. Just another thing to practice....



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PostPosted: Sat Mar 02, 2013 10:53 pm 
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I see nothing wrong with practicing power off approaches in training (or other times if you'd like) - any airplane can do a power off approach; gliders do it every time and so does the space shuttle, both with drastically different glide ratios!

This thread essentially started because of a remark I made in another thread that I hoped students weren't being taught to do power off approaches. I meant to say that they shouldn't be told that all their approaches during training and while flying cross countries and such should be down power off. There are a number of good reasons why, in the main, you should be flying a powered approach. Why do commercial airplanes always fly powered approaches? What happens to the descent angle when flying a power off approach in a powered aircraft?

I think it's pretty obviously why airplanes need to fly a powered approach while on an instrument approach so this mostly deals with visual approaches. It is generally easier to fly a stabilized approach while under power. In both cases, powered and non-powered, you can trim the aircraft for a certain speed and the airplane will do a nice stabilized descent. The problem is when you need to adjust that profile either because of a miscalculation on the pilots part when setting up the approach path or due to a change in wind. When the power is at idle, your only option is to add power - that becomes a problem when you have performance enhancing wind shear. You could argue that to fix the problem you could sideslip but that requires more effort than adjusting the power, and puts you and all your passengers at an awkward and uncomfortable angle. On a windy day, flying in and out of sideslips can be quite a bit of work and not that pleasant. While flying a powered approach you don't have that problem (as much) as you are able to reduce power to counter the wind shear. Or the case where you end up not being able to make the field and you need to add power - now you're doing a powered approach, unless you want to keep power on until you're too high and then cut it again...

How do gliders do it? Spoilers and speed brakes which are easy to actuate and control, and can adjust the flight path with precision. Generally, a well flown approach in a glider will have spoilers partially deployed for the majority/all of the approach. It's actually quite similar to the throttle in a powered airplane.

What about flaring? If you have a private license you should know that coming in either too shallow or too steep makes it more difficult to flare properly. There is a nice sweet spot for most aircraft that's right around the 3 degree slope, plus or minus a bit. Flying a power off approach at the proper speed requires you to do an abrupt flare at a very precise distance above the runway - slightly too high and you'll smack it on, slightly too low and you'll smack it on; quite hard usually in both cases. A way around this is to carry higher speed during the approach so that you have enough energy to pre-flare, shallowing out the approach path, so that you can make a normal flare. This is nice and fun to practice but requires more skill and attentiveness to land smoothly and precisely - it is easier to mess this approach up than a powered approach on a 3 degree slope.

Another problem with power-off approaches is engine spool up time in the event of a go-around. That's a big reason why jets don't fly power off approaches, even with the improved spool up times of modern turbofans. In pistons, if you're too aggressive with adding power you can get the engine to cough and sputter and depending on the conditions and the engine, it might quit. Adding power properly (smoothly) solves the problem and this usually isn't a big factor on piston airplanes.

If you are to change the trajectory (direction component of velocity) of an object, you must accelerate that object. Force causes acceleration and they're all related to inertia and energy. Essentially, it takes more energy (and time) to change the trajectory a greater number of degrees - should seem obvious. When flying a power-off approach you will generally be at a significantly higher angle than 3 degrees (unless you're a glider). If you're required to do a go-around, you will require more energy, time, distance, and altitude to be able to get to any point on the go-around compared to an airplane flying a powered approach around 3 degrees. I could do the calculations to see how much more energy is required (since your engine only has a limited amount of power, that extra energy comes from time, which means extra altitude and distance being used) but I think that'd be a bit much. Just consider that this is not a linear increase in energy required, it's an exponential increase - Kinetic energy = 0.5*m*v^2 - a change in 'v' requires an exponential change in kinetic energy. This boils down to being a safety factor. At most approach speeds you'll need to add power as the first action of the go-around - this accelerates you towards the danger that you're trying to avoid by doing a go-around. That's 'undesirable' but if you were to give in to your fear you might pull up first before adding power and depending on a number of factors, you might stall - in any case, you've increased the probability of stalling. By flying a 3 degree powered approach you have more margin for error in a go-around and you're less likely to screw it up.

Another reason to avoid flying power-off approaches is shock cooling. Need I say more?

Flying power-off approaches is a nice way to 'switch it up' and/or improve your flying skills but there is a time and a place for it, and I don't believe it should be taught as the approach to be flown under "normal" operations.



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PostPosted: Sat Mar 02, 2013 11:34 pm 
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Quote:
Essentially, it takes more energy (and time) to change the trajectory a greater number of degrees - should seem obvious.
While not commenting on the gist of your post, I think you're getting tied up in knots trying to produce a "physics" explanation here. Remember that accelerating a body only requires energy if the acceleration has a component in parallel with the velocity: a trajectory change that changes the direction of motion and not its magnitude doesn't require any energy input at all. So while changing a three degree descent to a three degree climb might take less time than changing a six degree descent to a three degree climb there's no change in kinetic energy in either case so long as the airspeed doesn't change.
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Just consider that this is not a linear increase in energy required, it's an exponential increase - Kinetic energy = 0.5*m*v^2 - a change in 'v' requires an exponential change in kinetic energy.
and there you have it correct: no change in the magnitude of v, no energy change.

I'll let you off the otherwise unpardonable sin of claiming that a v^2 relationship is exponential when it's not; of course it's a parabolic relationship. To be exponential it would have to look like some constant raised to the power v (k^v) - with v the exponent, whence the name. I think possibly you meant to say a superlinear relationship instead.


How do we feel about the shock cooling thing? Has anyone found any convincing evidence of its existence yet?



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PostPosted: Sat Mar 02, 2013 11:59 pm 
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photofly...

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Remember that accelerating a body only requires energy if the acceleration has a component in parallel with the velocity: a trajectory change that changes the direction of motion and not its magnitude doesn't require any energy input at all.


Really? So no energy needs to be added into a system to change its momentum? Tell that to my physics professor!

Quote:
and there you have it correct: no change in v, no energy change.


Velocity is a vector quantity meaning it has a magnitude and a direction. Changing the direction will change the velocity.

Quote:
I'll let you off the otherwise unpardonable sin of claiming that a v^2 relationship is exponential when It's not of course


Yes. Should have said it's proportional to the velocity squared... but lets not get into proportionality again! Superlinear would work as well except that hardly anyone has a clue what that means. Even though exponential increase is technically incorrect, it does communicate well usually to the 'lay' person. I like to be technically correct though so that's a bit of a conflict I have.

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How do we feel about the shock cooling thing. Anyone found any convincing evidence of its existence yet?


It is quite controversial but I haven't seen any scientific evidence proving it or disproving it.



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PostPosted: Sun Mar 03, 2013 12:13 am 
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dr.aero wrote:

Really? So no energy needs to be added into a system to change its momentum? Tell that to my physics professor!
Yes, really.

Your physics prof would remind you that no energy is required to change the direction of the momentum vector. Which is how I can whirl an object around on a string without the string doing any work, and how the earth orbits the sun without any change in its energy despite the considerable change in its momentum from one side of its orbit to the other.
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Velocity is a vector quantity meaning it has a magnitude and a direction. Changing the direction will change the velocity.
Correct. However the velocity can change without a change in the KE. Lets use bold for vector quantities. Then kinetic energy is proportional to v.v/2 and if you take the time derivative of that expression you'll get v.(dv/dt)

That is, the rate of change of KE is proportional to the dot product of velocity and the acceleration. If the acceleration is at right angles to the velocity the dot product is zero and there's no change in the KE.

Alternatively your physics prof will remind you that KE depends on the speed v = |v|, not the direction in which an object is moving.



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PostPosted: Sun Mar 03, 2013 6:11 am 
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How do we feel about the shock cooling thing? Has anyone found any convincing evidence of its existence yet?


Yes.

Though with a well planned and executed power off approach, the damaging effects can be mitigated. It's still not "good" for the engine though....



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PostPosted: Sun Mar 03, 2013 6:23 am 
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Cracked cylinders are annoying, but I think most
people would be surprised as to what causes them,
and it ain't "shock cooling", which I presume is what
happens to a C185 floatplane's engine after it flips
over inverted after a long, extended takeoff run and
the engine is dunked in the cold water.



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PostPosted: Sun Mar 03, 2013 7:08 am 
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PilotDAR wrote:
Quote:
How do we feel about the shock cooling thing? Has anyone found any convincing evidence of its existence yet?


Yes.

Though with a well planned and executed power off approach, the damaging effects can be mitigated. It's still not "good" for the engine though....

Go on then, smite me with this evidence.

Why should I not be worried about shock heating, too?

Colonel, I bet you do some really radical power reductions in your Pitts while flying a routine. How's your cylinder longevity working out?



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PostPosted: Sun Mar 03, 2013 9:19 am 
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What we do with the Pitts would blow your mind in
terms of power and airspeed changes.

Four S-2B's on the field and not a cracked (Lycoming)
jug. Ever. That's because shock cooling/heating from
airflow isn't what cracks cylinders.

One of my airplanes has it's original 1967 TCM IO-360
in it. That engine has a terrible reputation for cracking
cylinders at 400 hrs. They never make TBO - ask any
Seneca or Mooney owner that has one.

Except my TCM IO-360 cylinders are all original. Not a
crack after 46 years. Because all the hours on it were
put on by my father, my son and I - and we all know
what cracks cylinders. And it isn't shock cooling/heating
from airflow.

If shock cooling/heating from airflow changes cracked
cylinders, every aerobatic airplane would need a new
engine every 50 hours.



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PostPosted: Sun Mar 03, 2013 10:21 am 
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Quote:
Really? So no energy needs to be added into a system to change its momentum? Tell that to my physics professor!


Change in momentum is equal to impulse which is equal to force times delta time. Where does that force come from? Are you saying that no energy from anywhere is used to produce that force?...

Quote:
However the velocity can change without a change in the KE.


Yes, agreed. I believe talking about kinetic energy in this case wasn't the best way of describing what I was trying to. Impulse would have been better.

Colonel...

Quote:
Because all the hours on it were
put on by my father, my son and I - and we all know
what cracks cylinders.


What cracks cylinders?



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PostPosted: Sun Mar 03, 2013 10:44 am 
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Quote:
Change in momentum is equal to impulse which is equal to force times delta time. Where does that force come from? Are you saying that no energy from anywhere is used to produce that force?...
Energy and force are different things. The work done by that force (the energy change) is the magnitude of the force multiplied by distance moved in the direction of action of the force.

That's an absolutely critical point for you to understand. You can push as hard as you like on something, but unless the point of application of the force moves some distance in the direction of the force the force doesn't do any work. It's counterintuitive but completely correct.

That's why:

- the lift from the wing doesn't do any work on the aircraft (for clarity, I choose the reference frame in which the air is still)
- a hovering helicopter is 100% inefficient
- planets in a circular orbit don't need a source of power to continue to orbit, despite the graviational force
- my butt presses down on my desk chair and my desk chair presses up on my butt but neither does work on the other. Even while I move the forces, by rolling sideways around the floor.

I think you want to tell me that the aircraft has to expend energy to generate lift; and so it does. But the energy used up to create the lift is done against the drag force which acts against the direction of motion through the air. It's the lift force, not the drag, that changes the descent into a climb, and the lift force doesn't do any work to make that change happen.

Quote:
I believe talking about kinetic energy in this case wasn't the best way of describing what I was trying to. Impulse would have been better.
Not just better, but actually correct.



Last edited by photofly on Sun Mar 03, 2013 10:58 am, edited 1 time in total.

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PostPosted: Sun Mar 03, 2013 10:56 am 
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Quote:
What cracks cylinders?


There are many things that are bad for an
aircraft piston engine.

In no particular order:

1) not flying it. This is probably the worst thing
you can do to an aircraft piston engine, and is
generally the cause of most private engine
problems and overhauls. Internal corrosion.

2) not preheating it. With some ether, you
can start an engine in very cold temps. But
you're going to see a lot of metal-on-metal,
both because of lack of lubrication and and
decreased tolerances due to differential
metal expansion and contraction rate with
temperatures (think of a bimetallic thermometer)

3) overheating it. Now we're getting back on
topic with respect to cracking cylinders. The
hotter you run an engine, the shorter it's life.
This is not hard to understand. A lot of people
think you should "baby" an engine which is totally
wrong. It does not hurt and engine to produce
it's rated torque at rated RPM. Not in the least.
What hurts an engine is running it hot. You run
CHT's over 400F, the cylinders are not going to
last as long as if you kept them under 400F. This
is a matter of cowling and baffles and seals. I
don't like running hot oil temps, either. I've seen
260F at central america airshows and I don't
like it.

4) shoving the mixture in. Everyone loves to
pull the power back in a descent and then shove
the mixture in, which shoots cold fuel into the
cylinder and cracks it, according to the engineers
at TCM. This is a very bad idea with a TCM-360,
520 or 550 but everyone does it anyways, and
then wonders why their cylinders are cracked.


Let's look at the opposite of above. If you

- frequently fly an engine
- always preheat it
- keep the CHT's below 400F
- lean in a descent

Then your engine will go far beyond TBO. Most
people either can't bother to do the above, or
they object to it on some kind of artistic creativity
basis, and trash their engines.

There was an airplane that sat at my airport for
almost a year. This is very common when an
owner gets busy, sick or dies. I told the son of
the owner that the 540 Lycoming in that airplane
cost $50,000 from Lycoming, and it was being
destroyed by sitting.

He replied that $50,000 wasn't very much money
to them.

What would I know, about operating piston aircraft
engines compared to him?



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PostPosted: Sun Mar 03, 2013 11:23 am 
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photofly...

Quote:
Energy and force are different things. The work done by that force (the energy change) is the magnitude of the force multiplied by distance moved in the direction of action of the force.


I know.

Quote:
That's an absolutely critical point for you to understand. You can push as hard as you like on something, but unless the point of application of the force moves some distance in the direction of the force the force doesn't do any work. It's counterintuitive but completely correct.


I understand. Same reason why THP is zero when the TAS is zero - even though your engine is producing 250BHP.

Quote:
I think you want to tell me that the aircraft has to expend energy to generate lift; and so it does.


BINGO-ish! Essentially the energy is coming from the fuel that's being burnt by the engine. It requires more fuel (energy) to... (fill in what I wrote about in my first post on this thread). When I wrote about kinetic energy that was wrong - I'll blame it on being 1am or so in the morning. The point I was trying to make initially still remains - you require more energy to do a go-around from a steeper descent path, which essentially turns into requiring more time, altitude lost (before attaining a positive rate) and distance flown to achieve the go-around.

Colonel...

Thanks for the write up. I've known about those 4 points you talked about being bad for the engine. It's sad to see that an overwhelming majority of planes hardly fly. And it bugs me every time a student slams the mixture in when doing the descent checks! :| Or is scared to lean the mixture while taxiing, one of the times you're essentially guaranteed to not do any damage! I think there's a lot of misinformation regarding engine care and people get paranoid about the mixture so they always err on rich side which is often too much.

For others on here, John Deakin has written a lot about these matters: http://www.avweb.com/news/pelican/182146-1.html



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PostPosted: Sun Mar 03, 2013 11:29 am 
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To keep this simple.

Thermal shock can cause metal fatigue if said shock is in a short duration.

I have the pilots handling manual for the C117 here in my stuff and it clearly explains thermal shock and the risk of cylinder choking caused by different metals expanding and contracting.

To keep this even more simple.

My reason for requiring pilots to be able to do power off landings from 200 feet during water training touch and goes for the PBY type ratings was to get them comfortable with energy management and precise height judgement during the flare and water contact.

If they could not add two and two but could fly the airplane to the standard I required then I signed off their type rating.



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PostPosted: Sun Mar 03, 2013 1:02 pm 
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Quote:
power off landings from 200 feet


For the newbies: There is a tremendous amount of
folklore surrounding radial engines and power-off
descents with respect to the danger descending with
the power off, and the prop driving the engine.

With a boxer Lyc or TCM (NOT geared), it is perfectly
acceptable for the prop to drive the engine, which
happens with the power off, and is more noticeable
with constant speed props. I do this in the Lyc-powered
Pitts and TCM-powered Maule all the time. Fantastic
source of drag. In the Maule, I never need to use full
flap or even sideslip. In the Pitts, I can approach at
ridiculous speeds on short final, pull the throttle back
and let the 3-blade prop stop me.

With a geared TCM (eg GTSIO-520) if you pull the
throttles all the way back 'way up high and do a power
off descent with the props driving the engines, you
will damage the gear drives. Don't do that.

Now onto radial engines. If you pull the power all
the way back on a radial engine - carefully, it is
supercharged after all - and let the prop drive the
engine, there is a danger that there will be a lack
of lubrication in the main bearing.

Back to Chuck. From 200 feet, you probably won't
do any damage because if the props do drive the
engines, it will be of extremely brief duration.



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PostPosted: Sun Mar 03, 2013 1:21 pm 
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Radial engines need smooth throttle and RPM changes when increasing or decreasing power.

Radial engines have counter weights which will change the rotational forces in the engine, therefore smooth, slow changes of power will help give the longest engine time before needing overhaul.

The counter weights are very noticeable when shutting down a radial engine, that is what that clacking noise is as the RPM drops to zero..

Quote:
Now onto radial engines. If you pull the power all
the way back on a radial engine - carefully, it is
supercharged after all - and let the prop drive the
engine, there is a danger that there will be a lack
of lubrication in the main bearing.


I have around fifteen thousand hours on radial engines and about forty years working on them as a mechanic.

All the radial engines I flew had direct drive oil pumps and as such the oil pressure is a result of RPM, not manifold pressure.


I never ever saw any reduction in oil pressure caused by reducing manifold pressure when approaching with throttles closed.



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PostPosted: Sun Mar 03, 2013 1:47 pm 
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http://www.precisionengines.com/pdf/oilBulletin.pdf

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for war birds and some other non-fleet aircraft, the power may be changed abruptly or reverse loading can occur. This greatly increases the loading on the master rod bearing momentarily. When a multi-grade oil is used and these changes occur, the load on the master rod can force all of the oil film out of the clearance between the bearing and the journal. When this happens, the bearing material can makemetal to metal contact with the crankshaft surface. This will start to hammer the soft bearing material



That article is actually about radial engines and multi-grade
oil (which is good in the winter, but not good in the summer)
but it gets the point across.

It also explains why I only run W120 in radial engines.



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PostPosted: Sun Mar 03, 2013 3:21 pm 
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Yes C.S. that is correct.

The reverse loading is partially cased by the counter weight chance of force......
.....

....HOWEVER ..slow smooth throttle and pitch changes avoid that problem.

just like slow smooth throttle and pitch changes are absolutely necessary in your geared C421....reverse loading of the gear box.

How many licensed pilots do you run across that ram the throttle from idle to full power on then take off so fast it is a blur?



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PostPosted: Sun Mar 03, 2013 3:55 pm 
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What I teach in the 421 is to find a clear patch of
pavement - no stones - then brakes on, and bring
the MP up to 25 inches. This spools the turbo-
chargers up.

Release the brakes, then smoothly bring the throttles
up to 39.5 inches. Hands a blur in the cockpit is almost
always bad news. A good pilot is always miles ahead
of the airplane, and rarely moves fast in the cockpit :wink:

Back to radial engines ... the master rod bearing is
under tremendous stress, and as such must be babied.
There's a whole lot of torque constantly being transmitted
through it!



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PostPosted: Sun Mar 03, 2013 6:14 pm 
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Just killing a bit of time I read this whole thread again and am puzzled by this post.

Dh8Classic wrote this.
Quote:

Looks like you found your golden arm.


I have no idea what that means???



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PostPosted: Sun Mar 03, 2013 6:21 pm 
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Chuck Ellsworth wrote:
Just killing a bit of time I read this whole thread again and am puzzled by this post.

Dh8Classic wrote this.
Quote:

Looks like you found your golden arm.


I have no idea what that means???
Chuck, on page 1 of this thread, CS posted that he'd be interested in hearing from any of the "golden arms" on AvCanada who routinely approach with turbines spooled all the way down. Your responding post noted the shuttle as an example of a power-off approach, and Dh8Classic was humorously pointing out to CS that he had just found his "golden arm", i.e. the shuttle pilots. It got a smile out of me.



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PostPosted: Sun Mar 03, 2013 6:46 pm 
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Ahhh.... Thanks.

Sometimes I don't connect the dots because they are to obvious. :mrgreen:

This one though I think I do understand.

Quote:
You're not. However, the argument that power-off provides more controllability is simply not true.





To increase controllability with no power all one needs is higher airspeed...

....for instance suppose you are approaching power off just above the stall speed and you quickly move the elevator to full nose up your biggest worry is stalling the airplane.

Conversely if you are approaching power off just below VNE and you quickly move the elevator to full nose up you will not have anymore worries because your control effectiveness will be so high you will break the airplane.



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PostPosted: Sun Mar 03, 2013 7:27 pm 
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Yes, with high speed - and the ability to turn on
and off large amounts of drag - one has plenty
of controllability, with no power!

For example, if you had speed brakes, spoilers,
a drag chute, and a tailhook with wires on a
12,000 foot long runway, you could approach
power off at tremendous speed.



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PostPosted: Sun Mar 03, 2013 7:54 pm 
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Yes C.S. that is why I could not let this comment stand without challenging it.

Quote:
You're not. However, the argument that power-off provides more controllability is simply not true.



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