Idle/Braking (was Re: Winter / Focus / Fuel/Electric.)

[davel322 at comcast.net]

As most modelers, I don't have math to substantiate it, but a clean widebody does not add much drag.  There have been prior posts to this list noting that a cleanly shaped widebody fuse contributes a small percentage of drag to the overall airframe.

I found Amir's post interesting - the 1.2 number was derived (if I read his post accurately) from actual experiments.

>From all the re-pitching of APC propellers I have done (with an old Prather pitch gauge), I can tell you without a doubt that the reading on the Prather does not match the stated pitch of the prop - my understanding from APC is that the true pitch of the prop is the centerline of the prop airfoil - which rarely matches the flat side (bottom) of the prop.  The bottom of the prop is still a good reference tho.

Idle speed - I'm pretty comfortable stating I can tell a difference in 200 RPM (give or take 10% of idle speed) on a pattern plane I am familiar with.  Similarly, I can tell the difference in braking with a 1" pitch change on the prop (also give or take about 10%).

Regards,

Dave Lockhart
DaveL322 at comcast.net
> I ask this question each time I see the opportunity. One of these days some
> one is going to answer it with the math, I hope!
> 
> Flight training standards for full scale teach that a windmilling prop has
> more drag than a stopped prop. Training execises sometimes refer to setting
> power to "zero thrust" rather than idle.
> 
> I suspect that the difference is small, I am just curious. Nearly all of my
> personal flight time in full scale aircraft was in high wing Cessnas or
> Beech Bonanzas. The Cessnas are just too draggy in general for me to tell
> much about the question.
> 
> In the case of the Bonanza I could argue the question both true and false.
> For a give airspeed the Bonanza has two different flight attitudes, one
> requiring considerably more power than the other. Bonanza pilots refer to
> this as "being on the step". Most training manuals deny the condition, most
> Bonanza pilots take advantage of it. The books are not always right.
> 

> There does seem to be better braking in the Bonanza with a little power but
> the airplane is so clean that a slight change in attitude masks tiny power
> changes.
> 
> Contrary to earlier practices, I never felt it wise to totally shut down the
> engine in flight. Aside from the obvious risk of failing in restart the
> sudden chilling of the cylinders shows up as expensive welding repairs on
> the next overhaul.
> 
> IF there is an optimum power setting on a pattern plane in regard to braking
> that is the setting we should be using rather than the slowest idle.
> 
> In the current period of Sumo style airplanes the effect is probably
> insignificant.
> 
> John Ferrell
> 6241 Phillippi Rd
> Julian NC 27283
> Phone: (336)685-9606
> johnferrell at earthlink.net
> http://home.sprintmail.com/~johnferrell/dixiecompetitionproducts/
> NSRCA 479 AMA 4190  W8CCW
> "My Competition is Not My Enemy"
> 
> 
> ----- Original Message ----- 
> From: "Adam Glatt" <adam.g at sasktel.net>
> To: <discussion at nsrca.org>

> Sent: Wednesday, December 03, 2003 11:16 PM
> Subject: Re: Winter / Focus / Fuel/Electric.
> 
> 
> > John, experimentation or theory, though I have yet to hear anyone
> > qualify the statement that a slow turning prop causes more drag than a
> > stopped one with an explanation.
> >
> > John Ferrell wrote:
> >
> > >As I am sure you are aware, full scale pilots have been taught for many
> > >years that a windmilling prop creates more drag that a stopped prop.
> > >
> > >How do you determine what rpm generates the most drag?
> > >
> > >John Ferrell
> > >6241 Phillippi Rd
> > >Julian NC 27283
> > >Phone: (336)685-9606
> > >johnferrell at earthlink.net
> > >http://home.sprintmail.com/~johnferrell/dixiecompetitionproducts/
> > >NSRCA 479 AMA 4190  W8CCW
> > >"My Competition is Not My Enemy"
> > >
> > >
> > >----- Original Message ----- 
> > >From: "Doug Cronkhite" <seefo at san.rr.com>
> > >To: <discussion at nsrca.org>
> > >Sent: Wednesday, December 03, 2003 4:28 PM
> > >Subject: RE: Winter / Focus / Fuel/Electric.
> > >
> > >
> > >
> > >

> > >>Well.. you fix that by setting a good idle speed before you take off.
> You
> > >>never let the motor stop in flight for a few reasons.. 1) as you
> > >>described, it looks weird.. 2) a stopped prop is not the best solution
> for
> > >>downline breaking.. 3) the start/stop is hard on the gears..
> > >>
> > >>-Doug
> > >>
> > >>
> > >
> > >
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> > >
> > >
> >
> >
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> 
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