Re: Re: [indoor construction] VP or VD

From: Nick Ray <lasray_at_gmail.com>
Date: Mon, 23 Oct 2006 23:27:39 -0400

Dr. Drela,
With the 9cm max blade cord, would you use a lower pitch prop than the
typical 32-36" range seen with F1Ds? Also, is a Symmetrical prop still
the most efficient? Lastly, how would gradual change in diameter
verses a prop folding at set intervals of torque effect the
efficiency?
Thanks,
Nick Ray

On 10/23/06, markdrela <drela_at_mit.edu> wrote:
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> --- In Indoor_Construction_at_yahoogroups.com, "Slobodan Midic"
> <slomi_at_...> wrote:
> >
> > 1. There are two ways for reducing prop RPM
> > in "low ceiling" space in the beginning of flight:
> > VP and VD. From the point of prop efficiency
> > what method has more advantages than others,
> > and why?
>
> Theoretically, the VD prop is better, by far.
> The VD prop reduces climb mainly by releasing the rubber's energy more
> slowly during the power burst, via a greatly reduced rpm. The VP prop
> reduces climb mainly by throwing away most of the rubber's energy
> during the power burst, via blade stall.
>
> Out of curiosity, I just briefly looked at the VD design problem.
> Here's are some first impressions:
>
> 1) The baseline minimum-diameter design wants to have a smaller
> diameter than usual, with signficantly wider blades. For F1D, a 20 cm
> radius with 9 cm max blade chord looks pretty good.
>
> 2) The pitch wants to be progressively smaller towards the center, by
> about 2-3 degrees, so that the design blade cl's might increase from
> 0.3 at the root to 0.5 at the tip. This will mitigate the "bad" twist
> which results from the blade being moved outward at high torque. (A VP
> prop wants to have a roughly uniform baseline pitch).
>
> 3) The smaller baseline diameter allows one to use a larger max/min
> diameter range for a given wing. For F1D, increasing the radius from
> the 20 cm to 27 cm looks pretty good. It's essential that the blade
> angles also increase about 7 degrees at the same time, so that the
> blade cl's are still good. The innermost 1/3'rd of the blade will now
> be stalled, but the outer 2/3'rds will be close to max cl/cd, so that
> the efficiency is still pretty good. (In contrast, a VP prop must be
> fully stalled at high pitch, with terrible efficiency).
>
> I calculate the following for the min-D, mid-D, and max-D points, to
> match level-flight thrust requirement:
>
> R = 20 cm
> delta pitch = 0 deg
> V = 0.67 m/s
> Thrust = 0.0036 N = 0.367 gf
> Torque = 0.000579 N-m = 5.9 gf-cm = 0.082 in-oz
> RPM = 56.0
> efficiency = 71.0%
>
> R = 23 cm
> delta pitch = 3 deg
> V = 0.67 m/s
> Thrust = 0.0036 N = 0.367 gf
> Torque = 0.000798 N-m = 7.9 gf-cm = 0.113 in-oz
> RPM = 40.9
> efficiency = 70.6%
>
> R = 27 cm
> delta pitch = 7 deg
> V = 0.67 m/s
> Thrust = 0.0036 N = 0.367 gf
> Torque = 0.001187 N-m = 12.1 gf-cm = 0.168 in-oz
> RPM = 29.1
> efficiency = 66.6%
>
> Note the small torque at cruise at min-D, and the very large allowed
> torque increase (roughly twice!) at max-D without climbing. So this
> prop will want a long motor, wound to nearly max turns even in lowish
> ceilings. The initial climb rate at 0.5 in-oz will be similar to that
> of a fixed-pitch prop at 0.25 in-oz, and with a vastly lower climb
> RPM to boot.
>
> The duration potential is best deduced via an energy argument:
> Since you're taking off with nearly max energy, and not dumping it via
> blade stall like with VP, theoretically you should be able to get
> high-ceiling durations with only a fraction of the total climb altitude.
>
>
Received on Mon Oct 23 2006 - 20:32:19 CEST