Waht is the key differential does one look into before choosign an engine , i.e .:

1. Practical RPM .
2. Output - BHP / RPM
3. Bore .

Or is ther any other factor .

What exactly is torque & how does one calculate it ?

Thanks .

I look at the following when choosing an engine.

1). Bore (got to get the right size for the plane)
2). Price (got to stay in budget)
3). HP
4). Remarks from current owners

Torque, (T), in short, is rotational energy.

------------HP X 5252
T(ft-lbs)=-------------
---------------RPM

Rearranging the above equation yields:

------T X RPM
HP= ----------
--------5252

HP is dependent upon Torque and RPM. One engine may have slightly higher RPM figures than another while it's torque is a little less. Yet both engines can have the same HP.

Torque figures would be a much better way of evaluating an engine.
Servo manufacturers use Torque figures-why don't engine manufacturers???

I agree with Rcflier_gi except for #s 3 and 4. For me they go hand in hand.

The BHP/RPM figures engine manufacturers state are generally not very useful though they do give you an idea of what to expect from an engine.

Once you pick an engine for an application ask around before you buy it.

Hope this helps,
Jeff

The absolute best way to pick an engine, is post here what plane you are building/buying, and ask them their opinion on what they used to power this particular plane, and how they like it.

Jazzy,
It should be pointed out that it is possible to have torque without RPM. Steam engines and electric motors are capable of producing torque without RPM. HP is power, power is work over a unit of time, torque is work.

Guys thanks for your reply .

Much appreciated .

Would also like to know the following :

I am planning to purchase the Pilot Seduction Free style .40 - .53 engine . Also am looking for a second engine range of .60 - 90 .

have been flying for a year now.

1. Will it be a better proposition to invest in a 2 stroke or 4 stroke engine ?

2. How woudl the transaction be from flying a .46 to moving onto a 60 or 90 class engine ?

Woudl appreciate all ur help in this decision .

Thanks .

Guys thanks for your reply .

Much appreciated .

Would also like to know the following :

I am planning to purchase the Pilot Seduction Free style .40 - .53 engine . Also am looking for a second engine range of .60 - 90 .

have been flying for a year now.

1. Will it be a better proposition to invest in a 2 stroke or 4 stroke engine ?

2. How woudl the transaction be from flying a .46 to moving onto a 60 or 90 class engine ?

Woudl appreciate all ur help in this decision .

Thanks .

HP figures are dubious at best. They're calculated differently by different manufacturers, and don't translate into actual performance when you put a prop on the motor and fly it.

I consider the RPM figures folks around here post, their collective impression of the engine, and my own personal experiences.

.To expand on an above comment. It is possible to have HORSEPOWER without TORQUE. Torque relates to ROTATIONAL FORCE . Horsepower is the rate of doing work, Move a weight through a distance in a certain period of time and you can express it as HORSEPOWER,or WATTS or various other quantifications. The work could be a hydraulic cylinder lifting a weight or a dog pulling a sled.
For work to be done there must of course be movement through a distance. A steam engine exerting a force but not rotating ( ZERO RPM) is producing ZERO HORSEPOWER
In many aircraft applications the HORSEPOWER figures are very misleading. Very often a screaming 2 stroke making 3 Horses at the crank will not pull vertical as well as a 4 stroke making only 1.5 horses at the crank.
The difference is EFFICIENCY , smaller props at high RPM are not as efficient as larger ,slower turning props. This is where REAL horsepower measurement would calculate how fast a 10 pound plane climbs vertically with a two stroke and compare that rate of vertical ascent two the 4 strokes performance with the same plane at the same weight.
Not saying horsepower is meaningless ,just that the HP number doesn't always mean performance.

MikeL is shooting you the A answer here. HP ratings are USELESS on these engines. Consider:

Zenoah G38, 2.5 HP
OS 160FX, 3.7HP

Both turn a Zinger Pro 18x8 Prop right on 8000 RPM (seen many times and tached myself at the field).

So, what does the extra 1.2 HP of the 160 get you?

The fact that they turn the same prop at the same speed on the ground doesn't give you the whole picture. The FX quite possibly gets more HP at a higher RPM so it may do better with a smaller prop, also it may turn the same prop at a higer speed in the air. That would give it the same thrust if hovering, but better verticle at speed, and higher speed straight and level. Now you may not need the extra speed but someone else may.

Sportpilot, I completly agree with what your saying. My point was that by numbers the 160 makes 48% more HP than the G38. Mount them on an aerobat and see if the 160 makes 48% more thrust than the g38, won't happen. Like Mike L said, you have to look at what prop it swings well at what RPM's more than trying to make a HP judgement.

I don't think there is 1:1 relationship between HP and thrust.

Thanks Ladyflyer.
I was trying to keep it simple and short for an apparent I.C. engine newbie.
Either he now has a much better understanding of our little engines or is totally confused!

Arjunchopra, are you thinking of putting a .60-.90 class engine in a .46 sized Free-Style aircraft?
Four strokers have a much different sound, rpm range, and tuning process. They are a little heavier than the equivalent sized two-stroke but offer a great amount of torque without having to really crank the Rs. In other words they can swing a much larger prop.


Jeff

I was replying to the steam engine and electric motor stuff. There was a reference to WORK being an applied force.
Work is moving a force thru a distance ,POWER is the rate of doing work ! An applied force without movement is not WORK . The force can be the torque of the engine spinning the prop ,or the prop pulling the plane. Rotational force is far from the only measure.
Newbies only benefit from simplicty if it is also accurate. To pick up misinformation as a newbie is worse than having to exercise above the eyes a bit and get off on the right foot
I'm sorry if what I am trying to do differs from what you are trying to do

LadyFlyer,
You indeed have it correct. I incorrectly called torque work. It is rotational force. Horse Power and Watt is a measure of work Torque is a measure of force. Sorry I mistated that. Intresting that both Watt and Horse Power came from the same man James Watt. He would have quickly pointed out the differance.

Hi Sport-
I was sure you were aware ,I just wanted to make a bit clearer for the newbies.