This is why metal props are not a good
choice for acro.
He did put on a pretty good show for the
crowd, though - as long as he didn't hit
anyone with the prop :o
Nicely done
-
ScudRunner-d95
- Posts: 1349
- Joined: Thu Feb 13, 2014 5:08 pm
why would a metal prop be worse for acro? vs a wood one or maybe composite?
-
cgzro
Metal propellors usually exert a larger side-ways force on the crankshaft than a composite propellor when the propellor disk plane is changed. I.e. gyroscopic.
Three factors are involved. 1) Usually the weight of the metal propellor blades and hub is larger than the hub and blades of a composite prop, hence more gyroscopic (side-ways) force on the crank. Also, 2) with a metal prop there is more mass further from the centre of rotation (polar moment of inertia) which translates into stronger gyroscopic force for a given weight. Also 3) there is more vibration due to the larger polar moment of inertia of a metal propellor which can do damage all over the plane, but the crank has to absorb it all and can suffer accordingly.
Essentially if you remember your high school physics demonstration where you spin a bicycle wheel while holding it by the axle, then try to tip it, it fights back. That force you feel fighting back gets exerted 90 degrees out of phase from the orientation of the force. Do that enough times and the crank can crack.
Hence you are more likely to brake a crankshaft doing rapid gyroscopic in an aerobatic plane. The worst maneuvers are things like a tumble but even a hammer head creates a fair amount of load. If there is a high frequency vibration then that can have also a long term detrimental effect on the crank, case etc too.
Three factors are involved. 1) Usually the weight of the metal propellor blades and hub is larger than the hub and blades of a composite prop, hence more gyroscopic (side-ways) force on the crank. Also, 2) with a metal prop there is more mass further from the centre of rotation (polar moment of inertia) which translates into stronger gyroscopic force for a given weight. Also 3) there is more vibration due to the larger polar moment of inertia of a metal propellor which can do damage all over the plane, but the crank has to absorb it all and can suffer accordingly.
Essentially if you remember your high school physics demonstration where you spin a bicycle wheel while holding it by the axle, then try to tip it, it fights back. That force you feel fighting back gets exerted 90 degrees out of phase from the orientation of the force. Do that enough times and the crank can crack.
Hence you are more likely to brake a crankshaft doing rapid gyroscopic in an aerobatic plane. The worst maneuvers are things like a tumble but even a hammer head creates a fair amount of load. If there is a high frequency vibration then that can have also a long term detrimental effect on the crank, case etc too.
-
ScudRunner-d95
- Posts: 1349
- Joined: Thu Feb 13, 2014 5:08 pm
Dam I knew physics would have something to do with it.
-
Colonel
- Posts: 3450
- Joined: Wed Apr 29, 2015 10:31 am
Any maneuver involving a high rate of pitch
or yaw will place enormous stresses on the
crankshaft, with a metal prop.
These maneuvers include tailslides, tumbles,
torque rolls, spins, etc.
It will pound the bearing halves into the
clamshells, and crack the crankshaft, leading
to the complete failure and departure as
shown in your video.
This is actually not a big deal. What is a
big deal is when the prop hub itself fails,
and one metal blade departs. This happened
to a friend of mine, Floyd Brown.
The resulting imbalance tore the engine entirely
off the aircraft, and he started tumbling out of
control, with a massive rearward shift in his C
of G. At 1000 feet. Somehow, he got out and
pulled the d-ring. I flew that aircraft a few
years before that, at an airshow in Alabama.
Gently.
There is absolutely no paper on this. It is
up to the aerobatic pilot to understand the
physics behind this, which is polar moment
of inertia.
Light (composite) blades massively decrease
the PMI, and thus the stresses not only on
the crankshaft, but also on the rubber motor
mounts and tube frame fuselage clusters.
There is a recurring AD on Pitts S-2B's which
was caused by four wankers snap-rolling
over Vne, which broke the rear cabane clusters.
One of them, Nancy Lynn, managed to kill
herself in an Extra later. This did not surprise
me.
I do wish people spent more time learning
about physics instead of regulations.
Another story. There is a video of Sean Tucker
giving aerobatic instruction in a 2-metal blade
Pitts. And, the crank broke and departed. Sean
calmly tells the woman student he has good
news and bad news.
The bad news, Sean says, is that they have
lost the prop.
The good news, Sean says, is that you are flying
with the best pilot in the world.
And despite the massive change in the flight
characteristics of the aircraft - never mind the
C of G shift, he has no drag from the prop - Sean
makes a perfect forced landing on the airport
below, as shown in your video.
Sean is your typical modest airshow pilot. And,
there is a lesson there about doing acro over
an airport, which I strongly prefer, despite objections
from wiser, non-acro ground pounders.
or yaw will place enormous stresses on the
crankshaft, with a metal prop.
These maneuvers include tailslides, tumbles,
torque rolls, spins, etc.
It will pound the bearing halves into the
clamshells, and crack the crankshaft, leading
to the complete failure and departure as
shown in your video.
This is actually not a big deal. What is a
big deal is when the prop hub itself fails,
and one metal blade departs. This happened
to a friend of mine, Floyd Brown.
The resulting imbalance tore the engine entirely
off the aircraft, and he started tumbling out of
control, with a massive rearward shift in his C
of G. At 1000 feet. Somehow, he got out and
pulled the d-ring. I flew that aircraft a few
years before that, at an airshow in Alabama.
Gently.
There is absolutely no paper on this. It is
up to the aerobatic pilot to understand the
physics behind this, which is polar moment
of inertia.
Light (composite) blades massively decrease
the PMI, and thus the stresses not only on
the crankshaft, but also on the rubber motor
mounts and tube frame fuselage clusters.
There is a recurring AD on Pitts S-2B's which
was caused by four wankers snap-rolling
over Vne, which broke the rear cabane clusters.
One of them, Nancy Lynn, managed to kill
herself in an Extra later. This did not surprise
me.
I do wish people spent more time learning
about physics instead of regulations.
Another story. There is a video of Sean Tucker
giving aerobatic instruction in a 2-metal blade
Pitts. And, the crank broke and departed. Sean
calmly tells the woman student he has good
news and bad news.
The bad news, Sean says, is that they have
lost the prop.
The good news, Sean says, is that you are flying
with the best pilot in the world.
And despite the massive change in the flight
characteristics of the aircraft - never mind the
C of G shift, he has no drag from the prop - Sean
makes a perfect forced landing on the airport
below, as shown in your video.
Sean is your typical modest airshow pilot. And,
there is a lesson there about doing acro over
an airport, which I strongly prefer, despite objections
from wiser, non-acro ground pounders.
-
ScudRunner-d95
- Posts: 1349
- Joined: Thu Feb 13, 2014 5:08 pm
Interesting discussion which raises a question, Why on earth if this is "common" knowledge to aerobatic pilots do we see metal props still attached to their planes? did they not get the memo?
-
Colonel
- Posts: 3450
- Joined: Wed Apr 29, 2015 10:31 am
Good question. I suspect the answer is that the
vast majority of aerobatics flown is of the "old
man" variety: light, positive G. Gentle. You
know, loops, rolls and perhaps an occasional
hammerhead or two.
The RV is the poster child for this. I have
removed the "no-aerobatic" restriction on
many RV homebuilts, and I remember on an
RV-8, the fantastic data logging showed that
at the end of the aerobatic evaluation, the
maximum G I ever pulled was +3.9G. And
no negative G, of course - not only does the
engine hiccup due to fuel interruption, it
barfs copious amounts of engine oil out the
breather onto the fuselage.
So the answer is that if someone only wants
to fly "old man" aerobatics - gentle, like Bob
Hoover - they can use a metal prop if they
are willing to deal with the unpleasant gyroscopic
effects at slow speeds. For example, in a
hhead with a metal prop, stick forward into
the right corner. This is especially the case
in the 450 Stearman, and the 700hp Harvard
with the enormous 3-blade metal prop.
However. If someone is going to fly hard,
they must install a composite prop, otherwise
they are going to break their airplane. Even
with a composite prop, if you fly hard enough,
you will break anything and everything. That
is the case of Skip Stewart. He has broken
everything: his gas tank, seat, canopy, you
name it.
I know a guy, broke the crankshaft on a
450 stearman he was snap-rolling and spinning.
The gyroscopic precession laid that big metal
blade prop down on the cockpit and decapitated
him.
Again, a gentle reminder that the answer to
all of the above is found in physics and
engineering. Not in paper from TC.
PS Peter alludes to vibration aka oscillation.
This is more of a problem with the four cylinder
Lycomings which do not have crankshaft
counterweights. You see yellow arcs on their
tachs, depending upon which prop is installed.
6 cylinder Lycomings have crankshaft counterweights
installed, which deals with the torsional resonance
problem. No yellow arcs there. Redline isn't much
of an issue, either. All the experimental/exhibition
guys run their 6 cyl Lycs to very high RPM.
As a side note, people over the years have
experienced problems trying to install 2-blade
composite props on pumped-up 4 cyl Lycs.
Best to go with a 3-blade in that instance
(high compression pistons, cold air intake,
helicopter camshaft, etc).
vast majority of aerobatics flown is of the "old
man" variety: light, positive G. Gentle. You
know, loops, rolls and perhaps an occasional
hammerhead or two.
The RV is the poster child for this. I have
removed the "no-aerobatic" restriction on
many RV homebuilts, and I remember on an
RV-8, the fantastic data logging showed that
at the end of the aerobatic evaluation, the
maximum G I ever pulled was +3.9G. And
no negative G, of course - not only does the
engine hiccup due to fuel interruption, it
barfs copious amounts of engine oil out the
breather onto the fuselage.
So the answer is that if someone only wants
to fly "old man" aerobatics - gentle, like Bob
Hoover - they can use a metal prop if they
are willing to deal with the unpleasant gyroscopic
effects at slow speeds. For example, in a
hhead with a metal prop, stick forward into
the right corner. This is especially the case
in the 450 Stearman, and the 700hp Harvard
with the enormous 3-blade metal prop.
However. If someone is going to fly hard,
they must install a composite prop, otherwise
they are going to break their airplane. Even
with a composite prop, if you fly hard enough,
you will break anything and everything. That
is the case of Skip Stewart. He has broken
everything: his gas tank, seat, canopy, you
name it.
I know a guy, broke the crankshaft on a
450 stearman he was snap-rolling and spinning.
The gyroscopic precession laid that big metal
blade prop down on the cockpit and decapitated
him.
Again, a gentle reminder that the answer to
all of the above is found in physics and
engineering. Not in paper from TC.
PS Peter alludes to vibration aka oscillation.
This is more of a problem with the four cylinder
Lycomings which do not have crankshaft
counterweights. You see yellow arcs on their
tachs, depending upon which prop is installed.
6 cylinder Lycomings have crankshaft counterweights
installed, which deals with the torsional resonance
problem. No yellow arcs there. Redline isn't much
of an issue, either. All the experimental/exhibition
guys run their 6 cyl Lycs to very high RPM.
As a side note, people over the years have
experienced problems trying to install 2-blade
composite props on pumped-up 4 cyl Lycs.
Best to go with a 3-blade in that instance
(high compression pistons, cold air intake,
helicopter camshaft, etc).
-
Colonel
- Posts: 3450
- Joined: Wed Apr 29, 2015 10:31 am
With the original Continental W670 engine,is Stearman certified for even spins
most certainly! The US military used it
extensively as their main primary trainer
in the 2nd half of WWII. It didn't have
much power, but it would do pretty well
all the acro you needed to do, to train a
military pilot back then.
However, if you put an engine with double
the power up front, that changes things
slightly!
Regardless of whatever paper is dreamed
up by some bureaucrat in a cubicle, I will
not snap-roll or spin (or tumble or tailslide
or torque roll) an R-985 Stearman.
That big, heavy metal prop will just eat
that poor Wasp, Jr crankshaft alive.
It's a terrible character flaw of mine that
I pay more attention to the physics and
engineering, instead of the paperpushers
with the big tummies that don't fly. I
will try to work on that.
-
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