Right sized engines
10-13-2002, 11:07 PM
#2
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Multi-engines
I don't consider myself an expert on twins yet, (working on it!) but here are some general guidlines for single-engine models that may help. Details about airfoils, props, drag, etc., are ignored here, assuming "typical" aircraft.
First of course, is that the performance you're looking for matters a lot. Using Andy Lennon's book, "The Basics of RC Model Design", here are some examples of "Power Loading" priniciples. (Completed weight of aircraft in ounces, divided by the engine displacement.) The abbreviation "cid" is for "cubic inches displacement" (Note the differences between 2-strokes and 4-strokes.)
> A 5 lb trainer with a .40 s-stroke gives a power loading of 200 oz/cid
> A 5 lb sport plane with a .46 2-stroke gives a power loading of 173 oz/cid
> A 8 lb. sport plane with a .60 2-stroke gives a power loading of 213 oz/cid
> A 10 lb. Pattern Plane with a 1.4 4-stroke gives a power loading of 114 oz/cid
> A 6 lb. 3D plane with a .91 4-stroke gives a power loading of 105 oz/cid
Most of us add a great deal more power than required for scale flight, as a safety measure. (and for extreme performance, like aerobatics) We typically add more power than needed to a twin, thinking about enabling the plane to fly on one engine. (Engine reliablity is the key, right?) I can't give you an exact figure, but with a twin, you can reduce the power needed for a same-weight single, due to the fact that there is more "prop disk" driving your plane. I'm usually surprised to find that the engines used in twin models are smaller than I expected. I am assuming for my own scale designs of twins, that a power loading of 200 to 250 oz/cid is about right. (i.e. 20 lbs. (320 ounces) powered by two .60-size 2-strokes (1.2 cid total) would give 266oz/cid.) The same 320 ounce twin, powered by two .70 4-strokes (1.4 cid total) would give 228 oz/cid.
You could also approach this from another point of view, which is to consider the weight of all your components, then design the airframe around them. If you assume that you'll need about 100 sq. inches of wing area per pound, for sport type performance, you'll end up with the same figures as above.
Using electric power, you could reduce the "displacement" even more, because in theory, you're not going to have an engine failure.
First of course, is that the performance you're looking for matters a lot. Using Andy Lennon's book, "The Basics of RC Model Design", here are some examples of "Power Loading" priniciples. (Completed weight of aircraft in ounces, divided by the engine displacement.) The abbreviation "cid" is for "cubic inches displacement" (Note the differences between 2-strokes and 4-strokes.)
> A 5 lb trainer with a .40 s-stroke gives a power loading of 200 oz/cid
> A 5 lb sport plane with a .46 2-stroke gives a power loading of 173 oz/cid
> A 8 lb. sport plane with a .60 2-stroke gives a power loading of 213 oz/cid
> A 10 lb. Pattern Plane with a 1.4 4-stroke gives a power loading of 114 oz/cid
> A 6 lb. 3D plane with a .91 4-stroke gives a power loading of 105 oz/cid
Most of us add a great deal more power than required for scale flight, as a safety measure. (and for extreme performance, like aerobatics) We typically add more power than needed to a twin, thinking about enabling the plane to fly on one engine. (Engine reliablity is the key, right?) I can't give you an exact figure, but with a twin, you can reduce the power needed for a same-weight single, due to the fact that there is more "prop disk" driving your plane. I'm usually surprised to find that the engines used in twin models are smaller than I expected. I am assuming for my own scale designs of twins, that a power loading of 200 to 250 oz/cid is about right. (i.e. 20 lbs. (320 ounces) powered by two .60-size 2-strokes (1.2 cid total) would give 266oz/cid.) The same 320 ounce twin, powered by two .70 4-strokes (1.4 cid total) would give 228 oz/cid.
You could also approach this from another point of view, which is to consider the weight of all your components, then design the airframe around them. If you assume that you'll need about 100 sq. inches of wing area per pound, for sport type performance, you'll end up with the same figures as above.
Using electric power, you could reduce the "displacement" even more, because in theory, you're not going to have an engine failure.
10-14-2002, 12:47 AM
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Right sized engines
The size of engines and amount of power required can vary quite a bit depending on what you want the plane to do. Some examples:
It takes more power to take off from a grass strip than from a paved strip.
It takes more power to take of with a short run than a long run.
It takes more power to climb swiftly than to climb slowly.
It takes more power to fly a plane with heavy wing loading than light wing loading.
It takes more power to fly a plane with lots of drag than a streamlined plane.
It takes a lot more power to fly fast than to fly slowly.
A plane with one engine out only has to be able to fly above stalling speed with extra drag due to yaw and a dead prop. This may or may not be controlling depending on what other things you want the plane to do. If you want unlimited vertical performance then that might be controlling. If you want to be able to fly at more than, say, 8 times the stall speed then that may be controlling.
So, what are all the things you want your plane to be able to do? What will it's wing loading be and how streamlined will it be?
You could pick a plane that is successful at all the things you want your plane to do and copy its power loading and wing loading.
It takes more power to take off from a grass strip than from a paved strip.
It takes more power to take of with a short run than a long run.
It takes more power to climb swiftly than to climb slowly.
It takes more power to fly a plane with heavy wing loading than light wing loading.
It takes more power to fly a plane with lots of drag than a streamlined plane.
It takes a lot more power to fly fast than to fly slowly.
A plane with one engine out only has to be able to fly above stalling speed with extra drag due to yaw and a dead prop. This may or may not be controlling depending on what other things you want the plane to do. If you want unlimited vertical performance then that might be controlling. If you want to be able to fly at more than, say, 8 times the stall speed then that may be controlling.
So, what are all the things you want your plane to be able to do? What will it's wing loading be and how streamlined will it be?
You could pick a plane that is successful at all the things you want your plane to do and copy its power loading and wing loading.
