Generic List of IC HP?

Vassel

I guess it would be the easiest way to estimate how many watts a model requires to fly. Using 1 HP/750W, life would be easier.

Haldor

That would be true if the IC HP figures were true....
A typical .46 is claimed to have like 1,5HP at 16k rpm's yet with a 10x7 prop it will only do 10k - rather base it on that - how many watts do you need in order to swing a 10x7 prop at 10k.

Vassel

Dang, never caught on! If I first figure out the performance I want (I am thinking 50,70,90 W/LB, but I'll have to search to make sure), and just use the IC weight of the model. Multiply, I'll have what I want to know. If all else fails, they normally tell you the wing loading and the wing area, so I can figure out the model weight.

I just never made the connection. Life made simple...

[This message has been edited by Vassel (edited December 12, 2000).]

Dick Huang

I would be careful using watts/lb numbers.
Let us assume that the 0.46 IC numbers are correct. 16000 rpm at 1.5 hp but with a 10x7 prop it only runs at 10000 rpm.
to solve for the hp at 10000rpm the following relationship can be used:
HP1/HP2=(rpm1)^3/(rpm)^3
HP2=1.5*(10000/16000)^3=0.366 or 273 watts
Yours,
Dick Huang

jbourke

Dick,

Sorry to confuse the issue even more, but that would be watts out. Since we typically measure watts in, we have to take the efficiency into account.

Also, I think that a .46 will turn a 10x7 just a little faster than 10,000 RPM. It depends on the engine, the brand of propeller, etc.

But I think that 16 cells at 30 amps is about equal to a .46 if that helps.

Jim

Haldor

The .46 numbers I presented are just from memory so I wont claim any accuracy - but they make my point.

Vassel - if you're after .46 performance on a typical sport/pattern type of plane try to obtain actual rpm figures for them. Then calculate the watt output needed and go from there.

jrb

Agree that generic list of glow engines and the props they turn @ what rpm would be of some use.

Also if kits listed typical prop & rpms for as designed performance.

Which is better prop choice for a sport/aero plane -- 340 watts in /290 watts out:

12x10@6900 rpm = 4.1# static & 69 mph

10x6@10400 rpm = 3.2# static & 62 mph

Vassel

Actually, I was thinking along the lines of a 16oz plane that I wanted to be a sport's flier at 75W/LB would require a 75W power system. I'd want to design the power setup to come in at 75W or above. I wouldn't really need the IC HP anymore, just the weight of the plane. The watts needed would be adjusted as a more accurate airframe and RC weight was established.

If I understand things correctly, this is a good way to estimate the required power system. Who needs IC equiv?

hvberk

Here is a rule of thumb I use for initial rough estimate.

Take 1 Oz of cells for every 0.01 cubic inch of IC engine.
A .60 IC ==> 60 Oz of cells. this equals 30 SC cells
A .40 IC ==> 40 Oz of cells. this equals 20 SC cells
A .20 IC ==> 20 Oz of cells. this equals 10 cells
A .05 IC ==> 5 Oz of cells. this equals 7-8 smaller 500AR cells for speed 400.

This assumes a "normal" duration, like 25A on a SC cell.

Disclamer: Rules of thumb are no exact sience and have limited scope. They only give a starting point for further calculations or measurement.

I can second jbourk that 16 cells 30A is about a .46. I have a combination like this on a ARF40 trainer.

Andy W

Now THAT'S a good rule-of-thumb! For less efficient ferrites, it would be close, for Cobalts and Brushless, we're ahead of the curve, especially at "more than sport" current levels.

I like it, thanks for sharing!
..a

GWRIGHT

Interesting rules of thumb,.. but I go at it a bit differently. I've found motocalc to be invaluable in comparing power systems. With glow engines, you waste a LOT of power due to the little tiny/innefficient props. I wanted something equivalent to my OS32SX for a project. I know that that motor spun a certain prop at a certain rpm, so I plugged in a range of cells (10 up to 30 I think) and some motor, into motocalc. I was not interested in the current (ranged from like 30 to a couple hundred amps), I just wanted to get thrust and pitch speed numbers for a given prop and a given rpm. I then plugged in a couple electric motors, various gears and props, and my trusty 10-cell packs. Some combinations give me more thrust/a bit less speed, and vice-versa. These are with much larger props however, due to the ability to gear and swing bigger/more efficient props with the electric. For example,.. the 32SX on my dazzler spun a 10X5 MA wood prop at 13.2K, and when I get 10X5 at that rpm into motocalc (again, I don't care about current/cells/etc) I see thrust of 70 ounces, pitch speed of 62mph. Then I plugged in the trusty Endoplasma on 10 cells, set a filter of 45 amps (a bit above what I like to run it at), gears from 2 to 9.6 and 11 to 16 inch props in 10 pitch (apc "E" series range). 16X10 APC "E" at 7.4 gearing looked good. Pitch speed down to 46mph and I wanted to slow it a bit anyway, but thrust is predicted at 77 ounces (better than the 32SX). This gave me something to start testing with. I knew according to motocalc I needed to spin a 16X10 at 4800rpms so I geared the motor and charged a pack. Motocalc is usually a bit off, it predicted 38.5 amps, and I read 40 amps (probably cause the motor has timing set/advanced), and I'm spinning the 16X10 at 5000rpms almost exactly. If the equations in motocalc are near correct for thrust, I should have my 32SX equivalent power system, a bit MORE thrust, and a bit less speed,.. perfect for the situation. If the speed is a bit lower than I want, I can go to 6.4 gearing and a 15X10 which is right at the 1.5 to one pitch/diameter relationship so as not to be stalled statically. That setup actually looks a bit better, but at 44 amps on current. rpms are up to 5700 (measured with a tach)with an inch less diameter and different gear. The "E" dazzler will probably be flown at 1/4 throttle most of the time,.. just like my glow one was flown,..but with these figures I should be able to duplicate the performance,...i.e. hover and unlimited vertical. The HP of the 32SX is something around a bit over 1HP which would equate to 750~800 watts,.. but I can achieve the same OUTPUT at roughly 1/2 that input power with the electric due to the big efficient prop. I allready know the motor will survive cause it survives in my scout at that power level,..and on the dazzler it will be even more "out in the breeze". The only hurdle is going to be saving weight in the airframe, but after a couple hours going over the plans I think I have that figured out.