Auto conversions & gear boxes

On 23 Feb 2004 09:32:01 -0800, (Jay) wrote:

For props, bigger is better for static thrust (look at a helecopter)
but what about for top speed, a more desireable figure of merit for
fixed wing aircraft? I seem to remember hearing somewhere that for
top speed there is an optimal prop length that is not infinite. You
need to generate a stream of air that is going faster than the speed
that you want to fly.

You just described the reason no piston engined WWII fighter ever flew
faster than the speed of sound. The prop needed to produce enough
thrust to pull the airplane into supersonic speed, but the prop was
running into the wall of drag as the tips neared supersonic speed and
the fuselage was producing enormous drag too.

The prop tips would have to go supersonic if the airplane was to go
that fast too, and props of that era were not designed for supersonic
speeds.

Even going straight down at full power, just too much drag. They went
fast enough to scare the bajeebers out of a number of pilots though.
:-) They also went fast enough to lock up the controls and in some
cases, cause the destruction of the airplane... and the pilot.

Corky Scott

of one would resonate with the third harmonic of the other. But a
propeller and an engine are nowhere close to identical; their resonant
frequencies don't have anything to do with each other in the first place,

What resonance characteristics does a propeller have, and why?

so there's no point in trying to do anything to throw them out of match
with each other. (Unless you get unlucky; if resonance problems show up
in testing, then you can try re-machining one of the belt pulleys to a

Corkey

The P-40 wouldn't go supersonic. Had a few hours in bird and took one
to about 20K and rolled over at full thottle. Huffed and puffed on way
down and I started my pull out about 10K and was level about 3K.

Bird did not have a Mach meter, only a ASI. As I remember only got a
little over 400 mph max (high drag, low power).

That one dive conviced me it was a subsonic aircraft )

Big John

On Mon, 23 Feb 2004 19:45:15 GMT,
(Corky Scott) wrote:

On 23 Feb 2004 09:32:01 -0800, (Jay) wrote:

For props, bigger is better for static thrust (look at a helecopter)
but what about for top speed, a more desireable figure of merit for
fixed wing aircraft? I seem to remember hearing somewhere that for
top speed there is an optimal prop length that is not infinite. You
need to generate a stream of air that is going faster than the speed
that you want to fly.

You just described the reason no piston engined WWII fighter ever flew
faster than the speed of sound. The prop needed to produce enough
thrust to pull the airplane into supersonic speed, but the prop was
running into the wall of drag as the tips neared supersonic speed and
the fuselage was producing enormous drag too.

The prop tips would have to go supersonic if the airplane was to go
that fast too, and props of that era were not designed for supersonic
speeds.

Even going straight down at full power, just too much drag. They went
fast enough to scare the bajeebers out of a number of pilots though.
:-) They also went fast enough to lock up the controls and in some
cases, cause the destruction of the airplane... and the pilot.

Corky Scott

Corky,

Do you have any details available about your engine test stand, such as how you restrain it, instrumentation, cooling?

Also, a buddy of mine was talking about an engine build he did, and how he used water to match each header tube volume,
old news I'm sure...

--
Dan D.



..
"Corky Scott" wrote in message ...
On 20 Feb 2004 14:19:16 -0600, Barry S. wrote:

On Fri, 20 Feb 2004 18:08:35 GMT,
(Corky Scott) wrote:


Auto engines are tiny when compared to direct drive airplane engines.
Take a 180 hp Lycoming. It's cubic inch displacement is 360. They
turn the prop at around 2600 to 2700. The Ford V-6 in airplane trim,
puts out 180 hp also. It displaces 232 inches and makes it's power at
4800 rpm. No prop will work at that rpm. To harness the power, it
needs to be turned slower. Enter the prop speed reduction unit.

Speaking of Fords! How's your project coming?

__________________
Note: To reply, replace the word 'spam' embedded in return address with 'mail'.
N38.6 W121.4

Slowly. I have the engine assembled and is currently mounted in the
airframe. But there's everything else to do. The airframe has yet to
be blasted and painted. I think that can happen this summer. On the
other hand, we are planning some major kitchen redo's and trust me,
ALL of my attention had better be on that.

I've built an engine test stand that will allow me to wheel the engine
outside and run it, with the prop installed. I'd like to get some 30
or so hours on the engine before it gets it's final installation onto
the airframe. I decided this after listening to a crusty old DAR
speak at a local EAA meeting. It sounded to me like he'd be REALLY
unhappy with such an engine unless I could show him that it had been
thoroughly tested.

At this point, I'm being educated about headers. I was going to just
bend up a bunch of tubes, weld them to be what I need, get them jet
coated and call it good. Then I started doing some research.

It turns out that the diameter of header tubing is critical to the
performance of the engine. Larger diameter is not necessarily better.
In fact in almost all aircraft type applications, bigger is virtually
for sure not better. The exhaust header flange has openings that are
1.75" in diameter. This matches the exhaust port opening in the head.
But the tubing diameter should be 1.5", or possibly even 1 3/8" in
diameter. Also, the length of the runners should be at least over 30
inches, and 36 would be better. In addition, each tube should be as
close in length to each other as possible. Finally, the collector
needs to be about 1 78" diameter and it should be 18" long.

Reality is rearing it's ugly head. The lengths I mentioned literally
won't fit without welding the headers into loops. Not going to
happen.

I think the best I can do is get the runners as long as I can make
them and make sure they are of equal length, and get the proper
collector as that also has a huge affect on engine operation.

Why is it so important to have the runners be the same length?
Because different length runners cause different scavenging effects
within the combustion chamber. You will end up with an engine that
does not respond to ignition adjustments nor mixture adjustments as
some combustion chambers will run rich and some lean. "A series of
single cylinder engines flying loosely in formation." Quote from John
Deakin.

Many builders of the Ford V6 have complained that their engine ran
rough at maximum power. Huge effort was made to modify the intake
manifold to correct the problem. But I have not seen a single picture
of an exhaust manifold where the effort was made to create equal
length exhaust headers of the proper diameter.

I talked with a header manufacturer who told me he had heard of Dave
Blanton because a bunch of builders had asked him about headers. He
told me they all wanted to ignor his advice about tubing diameter.
They all wanted to use bigger tubing than was dictated, because they
all thought bigger was better. It's not.

Why is it so important to have the proper diameter tubing? Because
the bigger the diameter the slower the velocity of the gasses inside
it, and visa versa, up to a point. Eventually you can have exhaust
tubing in a diameter too small such that exhaust flow is restricted.
Large diameter tubing tends to cause the engine's power to peak at
extreme rpms. The smaller the diameter of the tubing, the more low to
midrange power you have.

But everyone wanted to use 1.75" tubing because that's what the
exhaust port was. 1.75" tubing would be what you would use if you
wanted flash horsepower from the engine at 8,000 rpm, like at the
dragstrip.

The header manufacturer also had a lot to say about "Zoomie" type
headers. These are headers without collectors, basically straight
pipes. Not only are these tubes also usually too large a diameter,
they leave off the collector which is crucial to the proper design of
the header system.

So with all this information, I'm taking my time with the header
design. Obviously something so important to the proper running of the
engine is not something I'm going to throw together without using
proper design criteria.

Corky Scott

If I remember correctly, the nose down full throttle was a tactic used to escape the zero's. They knew they would not
break the plane and the enemy often did. Same was true of the Wildcat, no limitation, plane would reach terminal
velocity...

--
Dan D.



..
"Big John" wrote in message ...
Corkey

The P-40 wouldn't go supersonic. Had a few hours in bird and took one
to about 20K and rolled over at full thottle. Huffed and puffed on way
down and I started my pull out about 10K and was level about 3K.

Bird did not have a Mach meter, only a ASI. As I remember only got a
little over 400 mph max (high drag, low power).

That one dive conviced me it was a subsonic aircraft )

Big John

On Mon, 23 Feb 2004 19:45:15 GMT,
(Corky Scott) wrote:

On 23 Feb 2004 09:32:01 -0800, (Jay) wrote:

For props, bigger is better for static thrust (look at a helecopter)
but what about for top speed, a more desireable figure of merit for
fixed wing aircraft? I seem to remember hearing somewhere that for
top speed there is an optimal prop length that is not infinite. You
need to generate a stream of air that is going faster than the speed
that you want to fly.

You just described the reason no piston engined WWII fighter ever flew
faster than the speed of sound. The prop needed to produce enough
thrust to pull the airplane into supersonic speed, but the prop was
running into the wall of drag as the tips neared supersonic speed and
the fuselage was producing enormous drag too.

The prop tips would have to go supersonic if the airplane was to go
that fast too, and props of that era were not designed for supersonic
speeds.

Even going straight down at full power, just too much drag. They went
fast enough to scare the bajeebers out of a number of pilots though.
:-) They also went fast enough to lock up the controls and in some
cases, cause the destruction of the airplane... and the pilot.

Corky Scott

Dan

The P-38 had high speed dive problems. After a number crashed they
went back and retrofitted them with 'dive' brakes (narrow strips on
top of wing that could be raised to increase drag). These slowed the
bird down enough it could be pulled out

The P-47 also had some dive problems. It wanted to tuck at high speed
in a dive. To recover you kept full throttle on and when you got to a
lower altitude you slowed down in the thicker air and regained enough
control to recover.

The P-51 was red lined at 505 mph. I have had it up to that speed and
didn't have any problems recovering from dive.

I have friends who were in Europe and told of far exceeding the 505
red line when getting away from 109/190's. The 109 had a structural
problem in their tail and it would come off if they got too fast in a
dive (per word passed to our fighter jocks).

All of this in a time and land far away )

Big John

On Tue, 24 Feb 2004 11:58:03 GMT, "Blueskies" wrote:

If I remember correctly, the nose down full throttle was a tactic used to escape the zero's. They knew they would not
break the plane and the enemy often did. Same was true of the Wildcat, no limitation, plane would reach terminal
velocity...

Not with gears! But a belt can provide any ratio. Of course any
irrational number can be approximated arbitrarily well by a rational
number... but the idea is just to choose a number that can't be
approximated well by a ratio of small numbers, not one that's genuinely
mathematically irrational. (Indeed, the latter concept is almost
meaningless in engineering, which is why I felt free to use "irrational"
as shorthand for the former concept.)

The idea isn't complete nonsense. If you had two identical assemblies,
linked by a belt drive, it'd be exactly the thing to do. You wouldn't
pick a 2:3 ratio, for instance, since that would mean the second harmonic
of one would resonate with the third harmonic of the other.

I struggle with understanding how you will throw a dangerous harmonic down a
viscoelastic belt. I guess it could be done but it doesn't seem as if it
could be done easily. Metal gears or similarly high modulus materials will
have an extremely low tangent delta and therefore have good
transmissibility. Vibration along a broad spectrum of frequencies should be
efficiently transmitted in such materials. Rubber doesn't transmit
frequency very efficiently unless its a glass; then it doesn't bend either
(unless you bend it very, very, very slowly). More likely you select the
ratio that gets you your desired prop RPM at the desired engine RPM. The
rubber belt should be an excellent (not perfect) vibration decoupler.

Charlie Smith
KIS Cruiser 4021

Pete Schaefer wrote:
: Easy. Just use one gear with 1.4264 teeth in it.

: "Tim Ward" wrote in message
: ...
: Just out of curiosity, how would you get any ratio to be an irrational
: number?

Actually, that wouldn't work either, since 1.4264 = 14264/10000. Now, if you
found the mystical gear with sqrt(2) teeth... then you'd be in business...

-Cory


--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

"Big John" wrote in message

The P-38 had high speed dive problems. After a number crashed they
went back and retrofitted them with 'dive' brakes (narrow strips on
top of wing that could be raised to increase drag). These slowed the
bird down enough it could be pulled out

Wasn't part of that fix also a mass balancer on the elevator, that was a
blob raised up on an arm above the elevator?
--

Jim in NC


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wrote:

Pete Schaefer wrote:
: Easy. Just use one gear with 1.4264 teeth in it.

: "Tim Ward" wrote in message
: ...
: Just out of curiosity, how would you get any ratio to be an irrational
: number?

Actually, that wouldn't work either, since 1.4264 = 14264/10000. Now, if you
found the mystical gear with sqrt(2) teeth... then you'd be in business...

-Cory

--

Maybe we can ask Jim to design a pi detector.

That's a tough one,
A lot harder than the BS detector...