2-Stroke, 4-Stroke, Petrol, Diesel, Glow etc, etc ad infunitum

2-Stroke, 4-Stroke, Petrol, Diesel, Glow etc, etc ad infunitum

Post by Ian Maclaughl » Sat, 10 Jul 1999 04:00:00



Quote:

>By the way if anyone has a way of calculating torque output (lbs.ft) I would
>be glad to know the equation.
>Similarly one for Horsepower though I think the one I use is OK, giving the
>same answers as a friend's copy of Barry Hobson's program ThrustHP.

Hi Chris-
Since power is simply torque X rpm (by definition), you can easily
work it backwards to determine torque, given power.  Just watch out
for the units conversions, e.g., 1 hp (brit units) = 550 ft-lb/sec =
33000 lb-ft/min, or 1 hp (metric) = 542.5 ft-lb/sec.
And BTW, get your own copy of Barry's program - it's a freebie.  The
HP calculation is pretty accurate - agrees well with the canonical
formula for power absorbed by the prop and power coefficients that I
have been able to deduce from correlations with dyno gathered data.
Wouldn't put much stock in the thrust estimate though - it doesn't
vary with prop pitch, and that doesn't model the real world
observations very well.
Ian
San Diego
 
 
 

2-Stroke, 4-Stroke, Petrol, Diesel, Glow etc, etc ad infunitum

Post by C.J.E.Hind » Sat, 10 Jul 1999 04:00:00


I would just like to say how amazed I am at the diversity of the knowledge
that the people in this newsgroup have.  Myself I fly Diesels, Glow (2 &
4-Stroke) and soon Petrol (Gas to those in the US).  For reference I noticed
the discussing relating to large Diesel (that is Road Diesel) vehicles.
Over here in Britain Diesel power Cars are very common (about 23-25% of the
market) and these engines suffer nothing like the same narrow power bands.
Take for example the car driven by my father (the family car, one and only).
This is a Volkswagen Passat.  The engine is a 110Hp Direct Inject 4-Stroke

3000rpm.  This engine has a flat torque curve due to a variable geometry
turbo (to keep good boost all the time) and as a result out accelerates a
Golf GTi 125HP up a hill.

The answer to all our problems as far as models are concerned is larger
propellers.  In order to swing a larger propeller we may not prop anywhere
near the max power output but we sure as hell will get a quieter models that
goes as well if not better.  How should we achieve this?  Well the easy
answer is to swing a larger prop.  Over in the US I gather the 'ideal' prop
for a Thunder Tiger 42Gp is a 9.5x6 or 10x6.  On my Thunder Tiger 42Gp I
fitted a cheap mini pipe to gain 1200rpm and a 12x6 propeller (balanced).
This combination gives 9900rpm and is very quiet.  I have not had to shim
the head but I do not use Nitro so this to some extend compensates for the
increased loading.  At present I am working on an idea of reduction gear for
an engine (only really practical on a big model I guess due to weight) to
increase torque.  Combined with a Variable Pitch Propeller (such as those
which I believe are supplied by Gerrad in the US? & made by Syntec) this
should give an ideal combination of quietness and power.  If you were to
step up to really big Petrol engines (yup Gas) such a  3W Powermaster 150iB2

diameter full size variable pitch propeller.

No what about strapping that 110Hp VW Diesel with reduction gear to the
front of a 1/3rd scale Spitfire or Racing Mustang?

By the way if anyone has a way of calculating torque output (lbs.ft) I would
be glad to know the equation.
Similarly one for Horsepower though I think the one I use is OK, giving the
same answers as a friend's copy of Barry Hobson's program ThrustHP.

Chris Hinds
www.mjp.co.uk/erfc/


 
 
 

2-Stroke, 4-Stroke, Petrol, Diesel, Glow etc, etc ad infunitum

Post by Brian Morri » Sat, 10 Jul 1999 04:00:00


Quote:

> <snip>
> Wouldn't put much stock in the thrust estimate though - it doesn't
> vary with prop pitch, and that doesn't model the real world
> observations very well.
> Ian
> San Diego

 I used to think that pitch should be considered until I began taking
measurements of thrust and comparing results with theory.  The variation in rpm
as pitch is varied maintains the calculated value in line with the measured
thrust.

--
Brian

"Diesels dont deadstick"

 
 
 

2-Stroke, 4-Stroke, Petrol, Diesel, Glow etc, etc ad infunitum

Post by Paul McIntos » Sat, 10 Jul 1999 04:00:00


HUH?  Does that mean that if you change pitch then the RPMs will compensate to
provide the same thrust?  I think NOT.  Check out the actual measurements on my
site for several .46 class engines

http://www.tcfb.com/desertsky/compare.html

Quote:


> > <snip>
> > Wouldn't put much stock in the thrust estimate though - it doesn't
> > vary with prop pitch, and that doesn't model the real world
> > observations very well.
> > Ian
> > San Diego

>  I used to think that pitch should be considered until I began taking
> measurements of thrust and comparing results with theory.  The variation in rpm
> as pitch is varied maintains the calculated value in line with the measured
> thrust.

> --
> Brian

> "Diesels dont deadstick"

--
Paul McIntosh
Desert Sky Model Aviation
(877) 311-3759 (toll free)
(602) 780-9005 (in AZ)
desertsky.goplace.com
 
 
 

2-Stroke, 4-Stroke, Petrol, Diesel, Glow etc, etc ad infunitum

Post by Paul McIntos » Sat, 10 Jul 1999 04:00:00


OOOPS, That one points to the RPM and HP measurements.  Use this one:

http://www.tcfb.com/desertsky/spreadsheet.html

Quote:


> > <snip>
> > Wouldn't put much stock in the thrust estimate though - it doesn't
> > vary with prop pitch, and that doesn't model the real world
> > observations very well.
> > Ian
> > San Diego

>  I used to think that pitch should be considered until I began taking
> measurements of thrust and comparing results with theory.  The variation in rpm
> as pitch is varied maintains the calculated value in line with the measured
> thrust.

> --
> Brian

> "Diesels dont deadstick"

--
Paul McIntosh
Desert Sky Model Aviation
(877) 311-3759 (toll free)
(602) 780-9005 (in AZ)
desertsky.goplace.com
 
 
 

2-Stroke, 4-Stroke, Petrol, Diesel, Glow etc, etc ad infunitum

Post by Brian Morri » Sun, 11 Jul 1999 04:00:00


Quote:

> HUH?  Does that mean that if you change pitch then the RPMs will compensate to
> provide the same thrust?

Hardly!  I mean that if you change pitch but not the diameter or the brand/style of
prop; put the prop back on the engine; run it up and measure the actual thrust;
record the rpm and make the thrust calculation based on the new rpm; the measured and
calculated thrust will agree.  At least they do with my equation at rpm's below 10k,
and with a fish scale having increments of 1/4 lb readable to 1/8 lb..

Quote:
> I think NOT.  Check out the actual measurements on my
> site for several .46 class engines

> http://www.tcfb.com/desertsky/compare.html

I'll look at your site, and I don't think I'll find anything unusual.  We are just
looking at the issue from a different perspective.

I don't mean to be offensive.
--
Brian

"Diesels dont deadstick"

 
 
 

2-Stroke, 4-Stroke, Petrol, Diesel, Glow etc, etc ad infunitum

Post by Jim Whi » Mon, 12 Jul 1999 04:00:00


Quote:


>> HUH?  Does that mean that if you change pitch then the RPMs will compensate to
>> provide the same thrust?

>Hardly!  I mean that if you change pitch but not the diameter or the brand/style of
>prop; put the prop back on the engine; run it up and measure the actual thrust;
>record the rpm and make the thrust calculation based on the new rpm; the measured and
>calculated thrust will agree.  At least they do with my equation at rpm's below 10k,
>and with a fish scale having increments of 1/4 lb readable to 1/8 lb..

so what exactly is that supposed to mean???  intuition tells me that
with an increase in pitch, the rpm's will drop, and the measured
thrust will agree with a calculation based on the lower rpm... so
what??... the question is, if you hold the same rpm, wouldn't you
expect the thrust to go up somewhat with an increase in pitch???  yet
the thrusthp program shows no increase in thrust...

Jim White
AMA 2466
WB2WOY
WPMPA/BCF/PFC
SouthShoreSoftware
Treasure Island, FL

 
 
 

2-Stroke, 4-Stroke, Petrol, Diesel, Glow etc, etc ad infunitum

Post by Brian Morri » Mon, 12 Jul 1999 04:00:00


Quote:



> >> HUH?  Does that mean that if you change pitch then the RPMs will compensate to
> >> provide the same thrust?

> >Hardly!  I mean that if you change pitch but not the diameter or the brand/style of
> >prop; put the prop back on the engine; run it up and measure the actual thrust;
> >record the rpm and make the thrust calculation based on the new rpm; the measured and
> >calculated thrust will agree.  At least they do with my equation at rpm's below 10k,
> >and with a fish scale having increments of 1/4 lb readable to 1/8 lb..

> so what exactly is that supposed to mean???  intuition tells me that
> with an increase in pitch, the rpm's will drop, and the measured
> thrust will agree with a calculation based on the lower rpm... so
> what??... the question is, if you hold the same rpm, wouldn't you
> expect the thrust to go up somewhat with an increase in pitch???  yet
> the thrusthp program shows no increase in thrust...

> Jim White
> AMA 2466
> WB2WOY
> WPMPA/BCF/PFC
> SouthShoreSoftware
> Treasure Island, FL

 My intuition says yes.  I haven't tested in that manner.  I let the engine do what it can
each time and rpm drops as pitch is increased.  But as rpm falls, thrust drops also.
Bringing the rpm back to where it was will certainly require more power.  More thrust than
before?  Maybe not.  I am not familiar with Barry's program but the equation I use is from
an engineering handbook and it doesn't have a pitch term.
--
Brian

"Diesels dont deadstick"