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byoungplumbing(at)gmail.c
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 Posted: Mon Apr 05, 2021 8:30 am Post subject: Prop thrust |
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Rick said and it got me thinking....The CG is forward of the position of the prop butalso fairly close to the CG. Press down on the prop and the plane pitchesup, lift and it pitches down. .
Still trying to get this figured out in my mind. So ive gone to the extreme. I've seen a photo of a low wing plane with the engine mounted on top of the rudder... The prop is mounted 7 feet behind and above the cg and center of drag (cdrag) much like the kolb but much further back, in my mind the physics remains the same but on a larger scale than the kolb. Because the thrust is higher than the cg /c drag there will still be some nose down tendancy with increased power. The following examples may (i hope) shed some light on how to neturalize it.
Now this is where i am taking a good size leap, and if i am wrong I'll apologize in advance.
Now in the following examples if the angle changes don't register in your mind the pressure changes to the aircraft... Draw 2 pictures. One of the aircraft ive described, and one of an engine and prop. Place the second pic on top of the first, and if needed over exaggerate the thrust angle even more,,, say 15 and 30 deg. If you ask why i suggested this, ive done it.
Let's assume that the aircraft is flying straight and level, and the engine thrust line is parallel to the direction of travel. The thrust is above the cg/cdrag so there will be an amount of nose down tendancy that can be compensated for with a small amount of up elevator. When the power is cut, the nose down tendancy (forces) go away and the nose of the plane goes up.
I will exaggerate to make a point....
If we raise the front of the engine 2 deg. The thrust above the cg/cdrag will have a tendancy to push the nose down adding the up pressure from the prop thrust angle on the tail will require even more up elevator. When the power from cruise is cut the higher than cg/cdrag is removed also the 2 deg. uplift on the tail is removed. This should cause a much greater nose up situation.
If we lower the front of the engine 2 deg the majority of thrust is still above the cg/cdrag and the nose down tendancy is countered by down pressure on the tail from the prop pushing down 2 deg. No elevator is needed and when we cut power there should be no tendancy for the nose to drop or raise.
Try again with 4 deg of down angle ln the engine.(lower the front of the engine).. The thrust line is still above the cg/cdrag so a small nose down moment is still present, but now there is much more down force from the prop thrust angle. This additional down force to the rear of cg(tail) should require down elevator to compensate. If we remove power. The nose drops.
Now I have used a high tail mounted engine in the example. But would imagine that the same principals apply to the kolbs. The moment arms are much smaller on a kolb, and because of the shorter moment arms, some of the angles would be greater.
Burt Rutan is reported to have said that on a bird, every feather makes a difference in how it flies.. not accounted for in my explanation is relationship between cg and c lift, Moving cg with fuel burn, changes in attitude at different flight speeds, climbing or descending, the exact relationship (arm)between cg/cdrag and the thrust line of the engine prop. I did however assume the thrust line stayed above the cg/cdrag.
Please remember that the large angle changes in the narration were used to induce thought, not to be used in real life trials.
Again if i have it wrong, please , someone HELP me understand.
Boyd Young mkiii Utah
Sorry this got overly long
Do not archive
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Jerry-TS-MkII
Joined: 23 Aug 2017
Posts: 79
Location: Indiana
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| Posted: Mon Apr 05, 2021 9:27 am Post subject: Prop thrust |
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Just to add a few comments about this whole discussion... (and yeah, all
of this could be wrong too)
You might need a super computer to do the full blown flight simulation..
but the big factors (I believe) are the following:
You have the engine thrust vector.. how much it is pitched up or down.
Considering it's location, I doubt that is the primary contributor, if
changes are slight.
You have the drag of the wing, as profile AND the dynamic drag (due to
producing lift, which changes with speed and angle of attack).. AND there
is a pitching moment caused by how different airfoils react at different
angles of attack.
You have the total aircraft profile drag, which is WELL BELOW the wing,
engine, and thrust vectors.. ie, all of the drag from the fuse, pilot,
landing gear. And all of that changes with the square of the airspeed.
But I would suspect it doesn't change much with the attitude of the
aircraft, (if right side up).
You have the center of mass, which may be only slightly higher than the
center of drag.. considering again, undercarriage, pilot weight, fuel (in
most, behind the pilot but low), and the mass of the fuse
And you have the mass distribution over the whole of the air-frame. If
all the heavy stuff is close in to the center of mass, it's different than
if it is further out. (See ice skaters spinning.. arms in = faster, etc).
We'll HOPE that's not changing while we fly.. but it does have an effect on
how easily the plane pitches, with power changes applied.
Aircraft designs can use a lifting stab.. such that with increased
airspeed, (producing more lift).. the flying stab tends towards keeping it
level, but still climbing. (HP above what is required for level flight,
produces ROC). A positive angle of attack on the flat stab of the Kolb has
a similar, but reduced effect. And the size of the tail area * tail arm =
the amount of control authority. Kolb tail feathers are really small in my
opinion, esp compared to those of my Drifter for example.
But the one thing which comes to my mind, is that by having the engine
mounted above and behind both the cg and center of drag and mass, is that
if near stalling, the quick application of power, will A) pull more air
across the wing, reducing the stall point, and forcing the nose down,
reducing the angle of attack. Both good things. But none of the Kolbs are
designed by aero-space engineers (or that I am aware of).. so it may well
have some characteristics which require trim, or human reaction to huge
power change inputs. Flying with my dad (old hat pilot).. and knowing a
little bit about engines in general.. huge/fast power changes are not
ideal, if they can be avoided.. either to accelerate or decelerate! And
with our non-certified engines, if they are running, may be more important
than anything else.
Making changes to a EXP? No problem there, if done carefully and with
some research. And yeah, engine placement is rather easy to edit. If you
are going to re-engineer it, there are a lot of factors, and just as many
things which could benefit from being revised. But it's not a simple
puzzle.
On Mon, 5 Apr 2021 10:30:07 -0600, B Young <byoungplumbing(at)gmail.com>
wrote:
| Quote: | Rick said and it got me thinking....The CG is forward of the position of
the prop but
also fairly close to the CG. Press down on the prop and the plane
pitches
|
| Quote: | up, lift and it pitches down. .
Still trying to get this figured out in my mind. So ive gone to the
extreme. Ive seen a photo of a low wing plane with the engine mounted
on top of the rudder... The prop is mounted 7 feet behind and above
the cg and center of drag (cdrag) much like the kolb but much further
back, in my mind the physics remains the same but on a larger scale than
the kolb. Because the thrust is higher than the cg /c drag there will
still be some nose down tendancy with increased power. The
following examples may (i hope) shed some light on how to neturalize it.
Now this is where i am taking a good size leap, and if i am wrong Ill
apologize in advance.
Now in the following examples if the angle changes dont register in your
mind the pressure changes to the aircraft... Draw 2 pictures. One of
the aircraft ive described, and one of an engine and prop. Place the
second pic on top of the first, and if needed over exaggerate the thrust
angle even more,,, say 15 and 30 deg. If you ask why i suggested
this, ive done it.
Lets assume that the aircraft is flying straight and level, and the
engine thrust line is parallel to the direction of travel. The thrust
is above the cg/cdrag so there will be an amount of nose down
tendancy that can be compensated for with a small amount of up
elevator. When the power is cut, the nose down tendancy (forces) go
away and the nose of the plane goes up.
I will exaggerate to make a point....If we raise the front of the engine
2 deg. The thrust above the cg/cdrag will have a tendancy to push
the nose down adding the up pressure from the prop thrust angle on the
tail will require even more up elevator. When the power from cruise
is cut the higher than cg/cdrag is removed also the 2 deg. uplift on
the tail is removed. This should cause a much greater nose up
situation.
If we lower the front of the engine 2 deg the majority of thrust is
still
|
| Quote: | above the cg/cdrag and the nose down tendancy is countered by down
pressure on the tail from the prop pushing down 2 deg. No elevator
is needed and when we cut power there should be no tendancy for the nose
to drop or raise.
Try again with 4 deg of down angle ln the engine.(lower the front of the
engine).. The thrust line is still above the cg/cdrag so a small nose
down moment is still present, but now there is much more down force
from the prop thrust angle. This additional down force to the rear of
cg(tail) should require down elevator to compensate. If we remove
power. The nose drops.
Now I have used a high tail mounted engine in the example. But would
imagine that the same principals apply to the kolbs. The moment arms
are much smaller on a kolb, and because of the shorter moment arms,
some of the angles would be greater.
Burt Rutan is reported to have said that on a bird, every feather makes
a
|
| Quote: | difference in how it flies.. not accounted for in my explanation
is relationship between cg and c lift, Moving cg with fuel burn,
changes in attitude at different flight speeds, climbing or descending,
the exact relationship (arm)between cg/cdrag and the thrust line of the
engine prop. I did however assume the thrust line stayed above the
cg/cdrag.
Please remember that the large angle changes in the narration were used
to induce thought, not to be used in real life trials.
Again if i have it wrong, please , someone HELP me understand.
Boyd Young mkiii UtahSorry this got overly longDo not archive
|
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neilsenrm(at)gmail.com
Guest
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| Posted: Mon Apr 05, 2021 10:28 am Post subject: Prop thrust |
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1957
Very informative but even reading between the lines I missed the conclusion.
Rick Neilsen
Redrive VW Powered MKIIIC
On Mon, Apr 5, 2021 at 1:30 PM <1957grnchev(at)bluemarble.net (1957grnchev(at)bluemarble.net)> wrote:
| Quote: | --> Kolb-List message posted by: <1957grnchev(at)bluemarble.net (1957grnchev(at)bluemarble.net)>
Just to add a few comments about this whole discussion... (and yeah, all
of this could be wrong too)
You might need a super computer to do the full blown flight simulation..
but the big factors (I believe) are the following:
You have the engine thrust vector.. how much it is pitched up or down.
Considering it's location, I doubt that is the primary contributor, if
changes are slight.
You have the drag of the wing, as profile AND the dynamic drag (due to
producing lift, which changes with speed and angle of attack).. AND there
is a pitching moment caused by how different airfoils react at different
angles of attack.
You have the total aircraft profile drag, which is WELL BELOW the wing,
engine, and thrust vectors.. ie, all of the drag from the fuse, pilot,
landing gear. And all of that changes with the square of the airspeed.
But I would suspect it doesn't change much with the attitude of the
aircraft, (if right side up).
You have the center of mass, which may be only slightly higher than the
center of drag.. considering again, undercarriage, pilot weight, fuel (in
most, behind the pilot but low), and the mass of the fuse
And you have the mass distribution over the whole of the air-frame. If
all the heavy stuff is close in to the center of mass, it's different than
if it is further out. (See ice skaters spinning.. arms in = faster, etc).
We'll HOPE that's not changing while we fly.. but it does have an effect on
how easily the plane pitches, with power changes applied.
Aircraft designs can use a lifting stab.. such that with increased
airspeed, (producing more lift).. the flying stab tends towards keeping it
level, but still climbing. (HP above what is required for level flight,
produces ROC). A positive angle of attack on the flat stab of the Kolb has
a similar, but reduced effect. And the size of the tail area * tail arm =
the amount of control authority. Kolb tail feathers are really small in my
opinion, esp compared to those of my Drifter for example.
But the one thing which comes to my mind, is that by having the engine
mounted above and behind both the cg and center of drag and mass, is that
if near stalling, the quick application of power, will A) pull more air
across the wing, reducing the stall point, and forcing the nose down,
reducing the angle of attack. Both good things. But none of the Kolbs are
designed by aero-space engineers (or that I am aware of).. so it may well
have some characteristics which require trim, or human reaction to huge
power change inputs. Flying with my dad (old hat pilot).. and knowing a
little bit about engines in general.. huge/fast power changes are not
ideal, if they can be avoided.. either to accelerate or decelerate! And
with our non-certified engines, if they are running, may be more important
than anything else.
Making changes to a EXP? No problem there, if done carefully and with
some research. And yeah, engine placement is rather easy to edit. If you
are going to re-engineer it, there are a lot of factors, and just as many
things which could benefit from being revised. But it's not a simple
puzzle.
On Mon, 5 Apr 2021 10:30:07 -0600, B Young <byoungplumbing(at)gmail.com (byoungplumbing(at)gmail.com)>
wrote:
> Rick said and it got me thinking....The CG is forward of the position of
> the prop but
> also fairly close to the CG. Press down on the prop and the plane
pitches
> up, lift and it pitches down. .
>
> Still trying to get this figured out in my mind. So ive gone to the
> extreme. Ive seen a photo of a low wing plane with the engine mounted
> on top of the rudder... The prop is mounted 7 feet behind and above
> the cg and center of drag (cdrag) much like the kolb but much further
> back, in my mind the physics remains the same but on a larger scale than
> the kolb. Because the thrust is higher than the cg /c drag there will
> still be some nose down tendancy with increased power. The
> following examples may (i hope) shed some light on how to neturalize it.
> Now this is where i am taking a good size leap, and if i am wrong Ill
> apologize in advance.
>
> Now in the following examples if the angle changes dont register in your
> mind the pressure changes to the aircraft... Draw 2 pictures. One of
> the aircraft ive described, and one of an engine and prop. Place the
> second pic on top of the first, and if needed over exaggerate the thrust
> angle even more,,, say 15 and 30 deg. If you ask why i suggested
> this, ive done it.
>
> Lets assume that the aircraft is flying straight and level, and the
> engine thrust line is parallel to the direction of travel. The thrust
> is above the cg/cdrag so there will be an amount of nose down
> tendancy that can be compensated for with a small amount of up
> elevator. When the power is cut, the nose down tendancy (forces) go
> away and the nose of the plane goes up.
>
> I will exaggerate to make a point....If we raise the front of the engine
> 2 deg. The thrust above the cg/cdrag will have a tendancy to push
> the nose down adding the up pressure from the prop thrust angle on the
> tail will require even more up elevator. When the power from cruise
> is cut the higher than cg/cdrag is removed also the 2 deg. uplift on
> the tail is removed. This should cause a much greater nose up
> situation.
> If we lower the front of the engine 2 deg the majority of thrust is
still
> above the cg/cdrag and the nose down tendancy is countered by down
> pressure on the tail from the prop pushing down 2 deg. No elevator
> is needed and when we cut power there should be no tendancy for the nose
> to drop or raise.
> Try again with 4 deg of down angle ln the engine.(lower the front of the
> engine).. The thrust line is still above the cg/cdrag so a small nose
> down moment is still present, but now there is much more down force
> from the prop thrust angle. This additional down force to the rear of
> cg(tail) should require down elevator to compensate. If we remove
> power. The nose drops.
> Now I have used a high tail mounted engine in the example. But would
> imagine that the same principals apply to the kolbs. The moment arms
> are much smaller on a kolb, and because of the shorter moment arms,
> some of the angles would be greater.
> Burt Rutan is reported to have said that on a bird, every feather makes
a
> difference in how it flies.. not accounted for in my explanation
> is relationship between cg and c lift, Moving cg with fuel burn,
> changes in attitude at different flight speeds, climbing or descending,
> the exact relationship (arm)between cg/cdrag and the thrust line of the
> engine prop. I did however assume the thrust line stayed above the
> cg/cdrag.
> Please remember that the large angle changes in the narration were used
> to induce thought, not to be used in real life trials.
> Again if i have it wrong, please , someone HELP me understand.
> Boyd Young mkiii UtahSorry this got overly longDo not archive
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Jerry-TS-MkII
Joined: 23 Aug 2017
Posts: 79
Location: Indiana
|
| Posted: Mon Apr 05, 2021 11:47 am Post subject: Prop thrust |
|
|
Like the first line, it says, "to add a few comments".. So two conclusions
could be drawn. Each plane is different, from configuration, loading, W&B,
HP and prop, etc. And "Your results may vary".
On Mon, 5 Apr 2021 14:28:26 -0400, Rick Neilsen <neilsenrm(at)gmail.com>
wrote:
| Quote: | 1957
Very informative but even reading between the lines I missed the
conclusion.
Rick NeilsenRedrive VW Powered MKIIIC
On Mon, Apr 5, 2021 at 1:30 PM wrote:
Just to add a few comments about this whole discussion... (and yeah,
all
of this could be wrong too)
You might need a super computer to do the full blown flight
simulation..
|
| Quote: | but the big factors (I believe) are the following:
You have the engine thrust vector.. how much it is pitched up or down.
Considering its location, I doubt that is the primary contributor, if
changes are slight.
You have the drag of the wing, as profile AND the dynamic drag (due to
producing lift, which changes with speed and angle of attack).. AND
there
is a pitching moment caused by how different airfoils react at
different
|
| Quote: | angles of attack.
You have the total aircraft profile drag, which is WELL BELOW the wing,
engine, and thrust vectors.. ie, all of the drag from the fuse, pilot,
landing gear. And all of that changes with the square of the airspeed.
But I would suspect it doesnt change much with the attitude of the
aircraft, (if right side up).
You have the center of mass, which may be only slightly higher than the
center of drag.. considering again, undercarriage, pilot weight, fuel
(in
most, behind the pilot but low), and the mass of the fuse
And you have the mass distribution over the whole of the air-frame. If
all the heavy stuff is close in to the center of mass, its different
than
if it is further out. (See ice skaters spinning.. arms in = faster,
etc).
Well HOPE thats not changing while we fly.. but it does have an effect
on
how easily the plane pitches, with power changes applied.
Aircraft designs can use a lifting stab.. such that with increased
airspeed, (producing more lift).. the flying stab tends towards keeping
it
level, but still climbing. (HP above what is required for level
flight,
produces ROC). A positive angle of attack on the flat stab of the Kolb
has
a similar, but reduced effect. And the size of the tail area * tail
arm =
the amount of control authority. Kolb tail feathers are really small
in my
opinion, esp compared to those of my Drifter for example.
But the one thing which comes to my mind, is that by having the engine
mounted above and behind both the cg and center of drag and mass, is
that
if near stalling, the quick application of power, will A) pull more air
across the wing, reducing the stall point, and forcing the nose down,
reducing the angle of attack. Both good things. But none of the Kolbs
are
designed by aero-space engineers (or that I am aware of).. so it may
well
have some characteristics which require trim, or human reaction to huge
power change inputs. Flying with my dad (old hat pilot).. and knowing
a
little bit about engines in general.. huge/fast power changes are not
ideal, if they can be avoided.. either to accelerate or decelerate! And
with our non-certified engines, if they are running, may be more
important
than anything else.
Making changes to a EXP? No problem there, if done carefully and with
some research. And yeah, engine placement is rather easy to edit. If
you
are going to re-engineer it, there are a lot of factors, and just as
many
things which could benefit from being revised. But its not a simple
puzzle.
On Mon, 5 Apr 2021 10:30:07 -0600, B Young
wrote:
> Rick said and it got me thinking....The CG is forward of the position
of
> the prop but
> also fairly close to the CG. Press down on the prop and the plane
pitches
> up, lift and it pitches down. .
>
> Still trying to get this figured out in my mind. So ive gone to
the
> extreme. Ive seen a photo of a low wing plane with the engine
mounted
> on top of the rudder... The prop is mounted 7 feet behind and
above
> the cg and center of drag (cdrag) much like the kolb but much
further
> back, in my mind the physics remains the same but on a larger scale
than
> the kolb. Because the thrust is higher than the cg /c drag there
will
> still be some nose down tendancy with increased power. The
> following examples may (i hope) shed some light on how to neturalize
it.
> Now this is where i am taking a good size leap, and if i am wrong Ill
> apologize in advance.
>
> Now in the following examples if the angle changes dont register in
your
> mind the pressure changes to the aircraft... Draw 2 pictures. One
of
> the aircraft ive described, and one of an engine and prop. Place the
> second pic on top of the first, and if needed over exaggerate the
thrust
> angle even more,,, say 15 and 30 deg. If you ask why i suggested
> this, ive done it.
>
> Lets assume that the aircraft is flying straight and level, and the
> engine thrust line is parallel to the direction of travel. The
thrust
> is above the cg/cdrag so there will be an amount of nose down
> tendancy that can be compensated for with a small amount of up
> elevator. When the power is cut, the nose down tendancy (forces) go
> away and the nose of the plane goes up.
>
> I will exaggerate to make a point....If we raise the front of the
engine
> 2 deg. The thrust above the cg/cdrag will have a tendancy to
push
> the nose down adding the up pressure from the prop thrust angle on
the
> tail will require even more up elevator. When the power from
cruise
> is cut the higher than cg/cdrag is removed also the 2 deg. uplift on
> the tail is removed. This should cause a much greater nose up
> situation.
> If we lower the front of the engine 2 deg the majority of thrust is
still
> above the cg/cdrag and the nose down tendancy is countered by down
> pressure on the tail from the prop pushing down 2 deg. No
elevator
> is needed and when we cut power there should be no tendancy for the
nose
> to drop or raise.
> Try again with 4 deg of down angle ln the engine.(lower the front of
the
> engine).. The thrust line is still above the cg/cdrag so a small
nose
> down moment is still present, but now there is much more down
force
> from the prop thrust angle. This additional down force to the rear
of
> cg(tail) should require down elevator to compensate. If we remove
> power. The nose drops.
> Now I have used a high tail mounted engine in the example. But would
> imagine that the same principals apply to the kolbs. The moment arms
> are much smaller on a kolb, and because of the shorter moment arms,
> some of the angles would be greater.
> Burt Rutan is reported to have said that on a bird, every feather
makes
a
> difference in how it flies.. not accounted for in my
explanation
> is relationship between cg and c lift, Moving cg with fuel burn,
> changes in attitude at different flight speeds, climbing or
descending,
> the exact relationship (arm)between cg/cdrag and the thrust line of
the
> engine prop. I did however assume the thrust line stayed above the
> cg/cdrag.
> Please remember that the large angle changes in the narration were
used
> to induce thought, not to be used in real life trials.
> Again if i have it wrong, please , someone HELP me understand.
> Boyd Young mkiii UtahSorry this got overly longDo not archive
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===========
Links:
------
[1] mailto:1957grnchev(at)bluemarble.net
[2] mailto:1957grnchev(at)bluemarble.net
[3] mailto:byoungplumbing(at)gmail.com
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