Earthstar Forums

Mark:

Why is it that the electric engines on aircraft are not very quiet ?

Is it mainly due to the speed of the tip of the props ? or,

is it mainly due to the electric engine ?

Jaime Sada

Jaime,
I can tell that from the ground, when Mark fly's over, most of the noise is from the prop. There is a soft whine of the motor, but nothing like the prop sound, which is about the same as a 4-stroke with a super-quiet muffler and it's prop noise. The noise seems to be quieter on a tractor, as compared to our pushers. I still want one, and my flying buddies are still mostly poo-pooing electrcic flight, until I remind them that most of our flights originate and terminate at the same local airport, and are for an hour or less.

kb

Good news !

Then, if that electric engine in particular, can put out its max power at
very slow rpms ( many can) , then an interesting development would be to
install a multi blade propeller, such as a 4 bladed french Arplast I have (
for a 4:1 gearbox and a Rotax 582 ) if the E-Gull can take a 66" prop

The difference in noise on my aircraft was very impressive with this set up
as compared to others with other gear ratios

Tractors will be quieter ( and arguably, more efficient) due to the fact
that on a pusher, the relative wind coming from above the wing vs the
relative wind coming from the underside of the wing strike the prop blades
at different speeds with different pressures. This is more dramatic at
higher angle of attacks than lower angle of attacks. Exhaust gas pulses
also affect the angle of attack of the relative wind striking the airfoil
blades

It would be really nice to fly in an aircraft that is far less noisy and
with less vibration, than the internal combustion types

Mark: Please correct me if I am wrong or missed a point

In a message dated 11/10/2011 5:32:00 AM Pacific Standard Time,
jaime.g.sada@gmail.com writes:


Mark:


Why is it that the electric engines on aircraft are not very quiet ?


Is it mainly due to the speed of the tip of the props ? or,


is it mainly due to the electric engine ?


Jaime Sada





Hi Jaime
It is mainly due to the speed of the prop.
While flying inside the plane it is really quiet.
Happy Flying,
Mark

Hi Mark:

At what rpms of the electric engine you are using can you get max power ?

Would you expect any noticeable improvement by using propellers designed
for slower rpms than with IC engines ?

Regards

In a message dated 11/11/2011 6:07:13 PM Pacific Standard Time,
jaime.g.sada@gmail.com writes:

Hi Mark:


At what rpms of the electric engine you are using can you get max power ?


Would you expect any noticeable improvement by using propellers designed
for slower rpms than with IC engines ?


Regards






Hi Jaime
68 in dia @ 2100 rpm.

Absolutely.

Happy Flying
Mark

Then, try the 4 bladed Arplast or two sets of 3-bladed IVO props

In a message dated 11/10/2011 10:22:20 AM Pacific Standard Time,
jaime.g.sada@gmail.com writes:

Good news !


Then, if that electric engine in particular, can put out its max power at
very slow rpms ( many can) , then an interesting development would be to
install a multi blade propeller, such as a 4 bladed french Arplast I have (
for a 4:1 gearbox and a Rotax 582 ) if the E-Gull can take a 66" prop

The difference in noise on my aircraft was very impressive with this set
up as compared to others with other gear ratios


Tractors will be quieter ( and arguably, more efficient) due to the fact
that on a pusher, the relative wind coming from above the wing vs the
relative wind coming from the underside of the wing strike the prop blades at
different speeds with different pressures. This is more dramatic at higher
angle of attacks than lower angle of attacks. Exhaust gas pulses also affect
the angle of attack of the relative wind striking the airfoil blades


It would be really nice to fly in an aircraft that is far less noisy and
with less vibration, than the internal combustion types


Mark: Please correct me if I am wrong or missed a point



Hi Jamie
I was talking to the Cessna engineer that designed the 337 Skymaster and
the 2 seat pusher that never made it to production. He told me a story that
when the Skymaster push pull twin was flown on the front 300 hp engine. it
would not maintain altitude while the 285 hp engine in the rear would climb
at 500 fpm at gross weight with the front engine shut off. there are a
nombre of reasons that he gave for this differential but the main reason he
gave was that the air going in to the prop is not as important as the air
going out. If you can have a relatively un obstructed exit you get more thrust
per dolor also the airplane that rides behind this excelerated flow of air
has 2 to 3 times the drag of the ambient flow due to its speed and
turbulent flow. This is seen as extra drag on a tractor airplane and less thrust
due to blockage of the outwash. He said that the reason that they did not
produce the 2 seat side by side pusher was that the lawyers made a case that
the pilots are not adequately concerned about weight and balance and that a
side by side pusher needs ballast to stay in the CG envelope if there is a
light pilot and no passenger or if there are 2 heavy occupants and no
baggage. They reasoned, correctly that there would be a lot of accidents due to
CG problems. This was wen I was in high school and I had designed a side by
side pusher. I scraped that for a tandem so that the passenger would be on
the CG. Since than I have taken Burt Rutans advice to stager the side by
side seats, and ben really happy with the results ever since.
The pusher configuration is only more drag to the
Tractor fundamentalists that are not willing to look at real world tests.
Each configuration has its one characteristics to study but I would put the
tractor and pusher neck to neck in performance. I really like the
unobstructed view of the pusher, so that creates a bias for me.
Happy Flying
Mark

Thanks for your comments
I do have some observations

I purposely wrote the word "arguably" due to these considerations you are
writing about, as well as some that he, and you, did not mention because
the matter is really much more complex
With pushers your main concern is the air going in, while with tractors,
your main concern is the air going out
I am not debating what he told you, for starters. I am adding other
considerations
With tractors, it is very easy to see that if the engine behind the prop
was so big as to exceed the propeller diameter by much, there would be no
thrust at all. You can easily test this with any RC engine and a flat board
behind the propeller. If the board is big enough, thrust can be negligible
or even non existing. You can even go to the extreme of reversing that
thrust, as in a commercial jet thrust reverser by diverting that flow
forward with an appropriately designed large obstruction. The equation F =
MA must be satisfied and whatever the direction of the acceleration УAФ of
the mass УMФ is, that direction will become the direction of УFФ


By the same token, in the case of pushers, we must ask ourselves where is
the mass of air being accelerated coming from ?

Part of it comes from the mass of air parallel to the fuselage. That sucked
in mass is being accelerated towards the pusher propeller so it is also
creating several times more friction on the skin of the aircraft ahead of
the propeller. However, it is very true that a lot of air coming into a
pusher propeller is being sucked from above and below the fuselage line as
this other mass requires less effort to be accelerated than the one close
to the fuselage due to this difference in friction. But accelerating this
mass (sucking it sideways) towards the propeller disk requires energy
expenditure too, because there will be an additional displacement (the
lateral one) as compared to the mass coming along and parallel to the
fuselage. This sideways coming mass will not impinge on the propeller at
the desired right angle of attack, also. Propellers are airfoils and
therefore, they have an optimum angle of attack to achieve their best
efficiency (L/D)

What I am stating here can easily be tested, again, by putting a large
obstruction ahead of RC aircraft propeller and comparing its static thrust
vs another one with as little obstruction ahead of the propeller as
possible. The propeller set up with the least obstruction ahead of it, will
produce higher thrust. If we put a very large obstruction, very close to
the pusher propeller, we will get very little thrust. What lies ahead of
the pusher propeller does matter, and very much

In the case of the Skymaster, and those findings that I am very aware of,
since I flew several Skymasters as a pilot, as my late father was a Cessna
dealer in the 50s Ц 70s, and did those tests then for curiosity, we must
take into account that when the only working propeller is the forward one,
the airflow will separate at the rear abrupt closure of the fuselage, right
ahead of the stopped rear prop, creating a lot of drag, while if we do the
reverse, stopping the tractor prop and working the rear engine only, the
air molecules at that fuselage rear closing position, will remain attached
due to the rear propeller "all around" sucking effect.

I am not pulling this out of my imagination. Air tunnel tests have proven
this. This, and not any other theory, was found, to be the main cause of
this puzzling difference in performance

The main reason for your staggered seating arrangement remarkable
performance vs a Rans S-12, is really the less energy consumed in 2 aspects

1.- Lateral air displacement (cross section ) is much lower in a staggered
arrangement than a side by side arrangement. While it can be argued that a
staggered (or better yet, a tandem) arrangement will have more skin surface
(wetted area) this additional friction requires less energy spent than
accelerating air molecules sideways (form drag). Time and again, tandem
seating high performance sailplanes have shown to be far more efficient
than side by side designs.

2.- In a Rans S-12 (side by side) they have exactly the same problem as the
337: Air molecules can not accelerate fast enough and close around the much
wider rear fuselage termination and separate, creating drag and turbulence
and inefficiency of the rear propeller. The Odyssey has much better rear
ending fuselage ( less abrupt closure)

You have designed a much tighter УpackageФ than Rans, with much less
friction areas, less form drag ( smaller cross section ) and slower closing
rear ending fuselage. This and other additional factors result in a much
more efficient and light design by comparison

As you say, there is nothing better than enjoying the view while flying a
pusher configuration

Happy flights to you too



Hi Jamie
I was talking to the Cessna engineer that designed the 337 Skymaster and
the 2 seat pusher that never made it to production. He told me a story that
when the Skymaster push pull twin was flown on the front 300 hp engine. it
would not maintain altitude while the 285 hp engine in the rear would climb
at 500 fpm at gross weight with the front engine shut off. there are a
nombre of reasons that he gave for this differential but the main reason he
gave was that the air going in to the prop is not as important as the air
going out. If you can have a relatively un obstructed exit you get more
thrust per dolor also the airplane that rides behind this excelerated flow
of air has 2 to 3 times the drag of the ambient flow due to its speed and
turbulent flow. This is seen as extra drag on a tractor airplane and less
thrust due to blockage of the outwash. He said that the reason that they
did not produce the 2 seat side by side pusher was that the lawyers made a
case that the pilots are not adequately concerned about weight and balance
and that a side by side pusher needs ballast to stay in the CG envelope if
there is a light pilot and no passenger or if there are 2 heavy occupants
and no baggage. They reasoned, correctly that there would be a lot of
accidents due to
CG problems. This was wen I was in high school and I had designed a side by
side pusher. I scraped that for a tandem so that the passenger would be on
the CG. Since than I have taken Burt Rutans advice to stager the side by
side seats, and ben really happy with the results ever since.
The pusher configuration is only more drag to the
Tractor fundamentalists that are not willing to look at real world tests.
Each configuration has its one characteristics to study but I would put the
tractor and pusher neck to neck in performance. I really like the
unobstructed view of the pusher, so that creates a bias for me.