Mar 13, 2003, 05:22 PM Registered User Joined Apr 2000 2,300 Posts eConversions: a reality check. An O.S. .32 SXH delivers 1.2 HP @ 18,000 RPM. This translates to 895 watts in a 7 pound heli or about 130 watts per pound for a maximum power climb if all that power is used. A commonly seen number is that of 50 watts per pound for hovering. The power to double (2) hover pitch without changing rpm, takes the hover power multiple to the 3/2 exponent or about 142 watts per pound (50x2^1.5) . These numbers are pretty close, differing by only 9 percent. Probably the 50 watt hover number is a bit too high. 45 watts per pound is probably more accurate. This is the power that the battery pack has to deliver to the ESC in an eConversion. HankF Last edited by HankF; Mar 13, 2003 at 05:26 PM.
 Mar 13, 2003, 06:52 PM Registered User USA, CA, Martinez Joined Feb 2001 1,668 Posts Interesting... where did u get the 1.2 hp value for the O.S.?
 Mar 13, 2003, 08:52 PM Registered User Joined Apr 2000 2,300 Posts
 Mar 14, 2003, 06:19 AM Registered User Joined Oct 2002 349 Posts so what is that gonna mean? that os nitro setup is more effecient than e-setup in general?
Mar 14, 2003, 11:40 AM
Victim of Gravity
Cary, NC
Joined Sep 2001
814 Posts
Quote:
 This is the power that the battery pack has to deliver to the ESC in an eConversion.
Is that 1.2HP at the drive shaft?

If so, then that means the e-conversion has to deliver 45 W/lb at the motor shaft. If you figure 95% motor efficiency (probably a little high) and 85% ESC efficiency (an educated guess), that'd be 56W/lb from the battery. Still darn close to that 50W/lb rule-of-thumb.

By the way, Hank, I think it's really cool that you're constantly applying math to all of the "rules-of-thumb." You take some abuse (ie. criticism) for your implementation now and then, but I still think it's great.

KC
 Mar 15, 2003, 08:56 PM Registered User Joined Apr 2000 2,300 Posts Thanks, KC. I try to keep things simple so even I can understand them. Of course a constant power per unit of weight can't literally be true, only if the rotors are turning rpms proportional to the rotor diameter (probably something like 2600/diameter(in feet)^1/2). Even if you set the collective pitch at the zero lift angle, the profile drag of the airfoil and therefore the power required will increase with rpm for the same amount of weight. So the 50 watts/lb assumes similarity in geometry and proportionality in rpms and therefore should be taken as a rough rule of thumb. Hank
Mar 16, 2003, 08:09 AM
Registered User
Joined Dec 2002
5,609 Posts
HankF:

Finally another heli modeller who likes numbers !

A number of years ago I developed a spreadsheet using the standard lift-drag equations

Lift = 1/2 x rho x C x V^2 x A

I have posted it in response to a number of threads. In case you haven't seen it, here it is again.

I have found it most useful for gas to electric conversions and watching what happens when parameters are changed.

Because of Ezone's protocol for file attachment I have renamed the file HELI.ZIP.

It is NOT a .zip file but an Excel .xls file.

Apply the heli parameters and motor constants and it will give a relatively realistic indication of performance.

In the old days there were only a few sizes of cells available and I have estimated battery weight and cell resistance according to cell size.

Because of the large variety of cells available today it might be best to manually introduce cell resistance and weight.

This may be done by unprotecting the relevant cells in the spreadsheet and entering battery weight and cell resistance in them.