eConversions: a reality check. - RC Groups
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Mar 13, 2003, 06:22 PM
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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.

Last edited by HankF; Mar 13, 2003 at 06:26 PM.
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Mar 13, 2003, 07:52 PM
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mumblety-peg's Avatar
Interesting... where did u get the 1.2 hp value for the O.S.?
Mar 13, 2003, 09:52 PM
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Mar 14, 2003, 07:19 AM
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so what is that gonna mean?
that os nitro setup is more effecient than e-setup in general?
Mar 14, 2003, 12:40 PM
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K.C.'s Avatar
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.

Mar 15, 2003, 09:56 PM
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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.

Mar 16, 2003, 09:09 AM
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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.

After downloading, rename the file HELI.XLS and it may be used with EXCEL.

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.
Last edited by Martyn McKinney; Mar 16, 2003 at 02:27 PM.
Mar 16, 2003, 09:28 AM
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mumblety-peg's Avatar
Thanks Martyn-

Perfect timing. I was going to post a request for just some such info. I'm thinking about building a gui type app that does the same as the spreadsheet.
Mar 16, 2003, 04:25 PM
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Hi Martyn, I downloaded your spreadsheet a while back but haven't had a chance to look it over. In any case maybe we can exchange spreadsheets:

Apr 12, 2003, 11:53 AM
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mumblety-peg's Avatar

Thank u very much for the spreadsheet. I finaly got around to using it today. I wanted to see if my rather short ECO runtimes made sense. Right now I'm just at the hovering stage.

So in looking at the calcs, I see that blade angle of attack is a function of the lift constant. If u could explain to me what the number is I'd appreaciate it.

Also, as an alternative to the above, could I just measure the hover pitch?

Also, how is the power constant derived?

One more question, where can I find some in depth discusions of this numbers. Is there a book or a faq around.

Apr 12, 2003, 01:07 PM
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Hi Mitch:

For heli blades without twist, the angle of attack is essentially the same as hover pitch since the inflow to the disk has zero velocity.
The particular airfoil is represented by the zero-lift blade angle (pitch). symmetricals are zero, semis are typically around 2 degrees negative and highly cambered by up to -6 to -8 degrees like on the LMH models.

The constants in the spreadsheet are used to make sure that the results fit the actual data for a wide variety of heli sizes. I couldn't make them constant until I included Reynold's number effects on drag.

There are obviously some assumptions made to simplify the calculations. For example, I assumed that the tail rotor would draw the same fraction of power delivered to the rotor system no matter what the heli size. That seems to be a reasonable assumption since the power constant produces reasonable results for all heli sizes.

Apr 12, 2003, 01:21 PM
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mumblety-peg's Avatar
Thanks Hank-

Are there any books around that explain this stuff. I'm looking for something that doesn't require more than, lets say algebra, to understand. College math was a looonngg time ago.

I'm asking because I'm going to have a go at building a GUI calc program for helis. I don't know if I'll make it available to all at this point. A lot depends on how much longer the job hunt takes. If I don't start using my tech brain soon it's going to atrophy.


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