HumanPoweredDesigner
Well-Known Member
I plugged some numbers into a program called PropDesigner. It gave me some nice numbers. I then did the calculations myself and got the same numbers.
It and I calculated that a 3 hp engine with a 75% efficient 1 meter prop can produce 15 pounds of thrust at direct drive 4600 rpm at 40 mph, and it would have a lot more static thrust than that. Mach number is about 0.72. Too loud?
So if I get a 5 hp engine and run it at 3 hp, it should last a while, and be super cheap.
To do this, I need to make an ultralight that only needs 15 pounds of thrust at 40 mph. I think with some streamlined wires, a good aspect ratio, and some rear farings, I can make an ultralight with an overall cruise L/D of 25, and 400 pounds gross weight (I weigh 145).
400/25/2.2*9.8*40*5280/3600/3.25/0.75/745 = 2.3 hp.
The only penalty will be in the climb rate. Suppose I want halfway decent 5 ft/s. Nah, lets make it easier: 2 m/s.
400/2.2*9.8*2/0.75/745 = 6.4 hp.
I don't want to exceed the 65mph speed limit, so 2.3*(65/40)^3 = 9.9 hp is needed to fly that fast level.
These number assume 40 degree air near 500 ft above sea level, so with less density, I'd probably need 13 hp to do all I want, which means an engine that maxes out at 15 for longevity. I would never want to fly 65 mph anyway. I just want the occasional climb, and the ability to fly in warmer weather. My typical speed would be 30 mph.
Now how hard is it to do L/D = 25 at a gross weight of 400?
Ultralight: At least 60 pounds.
Engine: 15 hp, maybe 40 pounds with prop and muffler, manual start, air cooled.
Gas: 35 pounds.
Pilot 150 pounds.
And I want it to be aerobatic too and be able to land on grass, and feather the prop for gliding, and adjust the prop for short take off distance with flaps.
I have many design ideas for this, and I can't wait to build it, ... after I build the helicopter.
So if I make a very efficient hp a, it will take forever for the pilot to climb to 1000 feet. Much easier to just fly fast without climbing.
It and I calculated that a 3 hp engine with a 75% efficient 1 meter prop can produce 15 pounds of thrust at direct drive 4600 rpm at 40 mph, and it would have a lot more static thrust than that. Mach number is about 0.72. Too loud?
So if I get a 5 hp engine and run it at 3 hp, it should last a while, and be super cheap.
To do this, I need to make an ultralight that only needs 15 pounds of thrust at 40 mph. I think with some streamlined wires, a good aspect ratio, and some rear farings, I can make an ultralight with an overall cruise L/D of 25, and 400 pounds gross weight (I weigh 145).
400/25/2.2*9.8*40*5280/3600/3.25/0.75/745 = 2.3 hp.
The only penalty will be in the climb rate. Suppose I want halfway decent 5 ft/s. Nah, lets make it easier: 2 m/s.
400/2.2*9.8*2/0.75/745 = 6.4 hp.
I don't want to exceed the 65mph speed limit, so 2.3*(65/40)^3 = 9.9 hp is needed to fly that fast level.
These number assume 40 degree air near 500 ft above sea level, so with less density, I'd probably need 13 hp to do all I want, which means an engine that maxes out at 15 for longevity. I would never want to fly 65 mph anyway. I just want the occasional climb, and the ability to fly in warmer weather. My typical speed would be 30 mph.
Now how hard is it to do L/D = 25 at a gross weight of 400?
Ultralight: At least 60 pounds.
Engine: 15 hp, maybe 40 pounds with prop and muffler, manual start, air cooled.
Gas: 35 pounds.
Pilot 150 pounds.
And I want it to be aerobatic too and be able to land on grass, and feather the prop for gliding, and adjust the prop for short take off distance with flaps.
I have many design ideas for this, and I can't wait to build it, ... after I build the helicopter.
So if I make a very efficient hp a, it will take forever for the pilot to climb to 1000 feet. Much easier to just fly fast without climbing.
