Question for Kibort, Dangler and other aviation guys...
11-01-2009, 07:06 PM
#32
Still plays with cars.
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Mark K - all this stuff about thrust vectoring - sheesh, why make it complicated. No airflow over the wings, no lift. If the prop had enough power (engine too) to create lift, the first thing that would happen is the aircraft would flip on its side because of the torque. That's why helicopters have a tail rotor after all.
Best,
Best,
11-01-2009, 07:43 PM
#33
Rennlist Member
Prop hanging has a pure vertical vector from the prop thrust, and yes this can be done. however the closer to horizontal the prop is the less effective the lift is. Once the prop reaches 45 degrees only about 70% of the prop's thrust is lifting the plane and at 30 deg you are down to 50%. Assuming the string is in the horizontal plane and not aiding in lift or increasing down force. Looking at a tail dragger the prop may be at (guessing here) 10 degrees you get about 17% of your thrust in the vertical direction.
I don't think lift off is very likely. Lets take for instance a C180 (its a tail dragger) with a max take-off weight of 2800 lbs. This will require that the engine is capable of producing 16500 lbs of trust. So in essence you would need to mount something similar to a Pratt JT8D (16K-20K lb thrust engine on the MD80) to the front of your C180 to get it off the ground using only the vertical component of the trust.
As for the lift generated at the wings. There are a couple issues with trying to generate a large amount of lift from the flow generated by the prop thrust. There will be a great deal of turbulence generated in the airflow sent over the wing. Another issue that you will encounter is that due to the rotation of the prop the airflow sent back over the wings tends to spiral. This in turn decreases the effective angle of attack on the wing on the side of the plane with the descending prop blades, and in some cases may actually cause that wing to generate zero or negative lift even if this is offset by the increases effective angle on the other side it will induce a roll moment that has no means of resistance. During normal flight this roll moment is off set through addition of a counter roll moment generated by a slight input to the ailerons. However with no airflow at the wing tips the ailerons will be ineffective.
I don't think lift off is very likely. Lets take for instance a C180 (its a tail dragger) with a max take-off weight of 2800 lbs. This will require that the engine is capable of producing 16500 lbs of trust. So in essence you would need to mount something similar to a Pratt JT8D (16K-20K lb thrust engine on the MD80) to the front of your C180 to get it off the ground using only the vertical component of the trust.
As for the lift generated at the wings. There are a couple issues with trying to generate a large amount of lift from the flow generated by the prop thrust. There will be a great deal of turbulence generated in the airflow sent over the wing. Another issue that you will encounter is that due to the rotation of the prop the airflow sent back over the wings tends to spiral. This in turn decreases the effective angle of attack on the wing on the side of the plane with the descending prop blades, and in some cases may actually cause that wing to generate zero or negative lift even if this is offset by the increases effective angle on the other side it will induce a roll moment that has no means of resistance. During normal flight this roll moment is off set through addition of a counter roll moment generated by a slight input to the ailerons. However with no airflow at the wing tips the ailerons will be ineffective.
11-01-2009, 07:50 PM
#34
Shameful Thread Killer
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Lets take for instance a C180 (its a tail dragger) with a max take-off weight of 2800 lbs. This will require that the engine is capable of producing 16500 lbs of trust. So in essence you would need to mount something similar to a Pratt JT8D (16K-20K lb thrust engine on the MD80) to the front of your C180 to get it off the ground using only the vertical component of the trust.
11-01-2009, 08:54 PM
#35
Rennlist Member
I know there was no STC for a turbo thrust C180. My use of this was to say that you would need a thrust equivalent on a C180 equivalent to that of a passenger jet’s engine to get the 180 to lift off purely due to engine thrust.
this is not necessarily the case. We'll use a non-aviation example here. You have a 10 lb brick with two 1 foot strings tied to it. one string is anchored to the floor with the brick pulled taught. You hold the other end. if you pull on the rope with 10 lbs the brick will lift off the floor correct? Well that all depends on the direction your pulling. if you pull straight up yes it will but is you pull in the horizontal plane you could pull with infinite force and the brick will never lift off the floor. The lifting force on the brick is determined by the vertical component of the pulling force. This lifting force can be determined by taking the magnitude of the pulling force and multiplying it by the sine of the angle as measured from the horizontal plain. for instance if you pull on the rope at a 60 angle from the horizontal you will need to pull with 11.54 lbs... 11.54 x sin(60) = 10. if you pull at 45 degrees you will need to pull with 14.14 lbs. by the time you get to 10 degrees (the assumed angle of the prop axis with the C180 setting on the ground) it will take 57.59 lbs to lift the brick.
2800 lbs of thrust generated by an engine on a C180 would only lift it off the ground if it where already standing vertical.
2800 lbs of thrust generated by an engine on a C180 would only lift it off the ground if it where already standing vertical.
11-01-2009, 09:21 PM
#36
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Ok, you didn't put the angle in there. I assumed straight up, and you were talking about some angle of incidence that is the sine of the angle. I get it now.
11-01-2009, 10:34 PM
#37
Nordschleife Master
If it was possible, we wouldn't be speculating we'd be looking at YouTube footage.
IBTL!
11-02-2009, 12:17 AM
#39
Rennlist Member
Bob, Alerons could counter act that, in theory. (big alerons ! 
)
want proof? airplanes can hang on their props and not rotate.
)want proof? airplanes can hang on their props and not rotate.
Mark K - all this stuff about thrust vectoring - sheesh, why make it complicated. No airflow over the wings, no lift. If the prop had enough power (engine too) to create lift, the first thing that would happen is the aircraft would flip on its side because of the torque. That's why helicopters have a tail rotor after all.
Best,
Best,
11-02-2009, 12:25 AM
#41
Rennlist Member
good points.
I was talking purely theoretical. we will have to see if the model guys have done this . certainly a stock, or even pumped up cessna cant do it. but with enough thrust , it could levitate. I think you might be on the right track of how much.
mk
I was talking purely theoretical. we will have to see if the model guys have done this . certainly a stock, or even pumped up cessna cant do it. but with enough thrust , it could levitate. I think you might be on the right track of how much.
mk
Prop hanging has a pure vertical vector from the prop thrust, and yes this can be done. however the closer to horizontal the prop is the less effective the lift is. Once the prop reaches 45 degrees only about 70% of the prop's thrust is lifting the plane and at 30 deg you are down to 50%. Assuming the string is in the horizontal plane and not aiding in lift or increasing down force. Looking at a tail dragger the prop may be at (guessing here) 10 degrees you get about 17% of your thrust in the vertical direction.
I don't think lift off is very likely. Lets take for instance a C180 (its a tail dragger) with a max take-off weight of 2800 lbs. This will require that the engine is capable of producing 16500 lbs of trust. So in essence you would need to mount something similar to a Pratt JT8D (16K-20K lb thrust engine on the MD80) to the front of your C180 to get it off the ground using only the vertical component of the trust.
As for the lift generated at the wings. There are a couple issues with trying to generate a large amount of lift from the flow generated by the prop thrust. There will be a great deal of turbulence generated in the airflow sent over the wing. Another issue that you will encounter is that due to the rotation of the prop the airflow sent back over the wings tends to spiral. This in turn decreases the effective angle of attack on the wing on the side of the plane with the descending prop blades, and in some cases may actually cause that wing to generate zero or negative lift even if this is offset by the increases effective angle on the other side it will induce a roll moment that has no means of resistance. During normal flight this roll moment is off set through addition of a counter roll moment generated by a slight input to the ailerons. However with no airflow at the wing tips the ailerons will be ineffective.
I don't think lift off is very likely. Lets take for instance a C180 (its a tail dragger) with a max take-off weight of 2800 lbs. This will require that the engine is capable of producing 16500 lbs of trust. So in essence you would need to mount something similar to a Pratt JT8D (16K-20K lb thrust engine on the MD80) to the front of your C180 to get it off the ground using only the vertical component of the trust.
As for the lift generated at the wings. There are a couple issues with trying to generate a large amount of lift from the flow generated by the prop thrust. There will be a great deal of turbulence generated in the airflow sent over the wing. Another issue that you will encounter is that due to the rotation of the prop the airflow sent back over the wings tends to spiral. This in turn decreases the effective angle of attack on the wing on the side of the plane with the descending prop blades, and in some cases may actually cause that wing to generate zero or negative lift even if this is offset by the increases effective angle on the other side it will induce a roll moment that has no means of resistance. During normal flight this roll moment is off set through addition of a counter roll moment generated by a slight input to the ailerons. However with no airflow at the wing tips the ailerons will be ineffective.
11-02-2009, 01:38 AM
#42
Rennlist Member
First, in building R/C model airplanes you install the motor at an angle to the fuselage. This is to counteract the torque of the motor and swirling thrust, it is even called the thrust angle. You do this so the plane with fly as straight as possible with the least amount of control surface trim.
There is a whole world of aerobatics being done now where the plane stays in the air rolling, spinning, and tumbling about various axis and there is no aerodynamic principles holding it aloft. It's mostly deflection of the prop thrust. The term for these maneuvers is 3-D Aerobatics. It started in R/C planes and has moved into the freestyle performances of aerobatic pilots. It is done with a stunt plane with more than enough power to hang on the prop.
A simple example of a 3-D aerobatic maneuver is a flat spin where the plane gains altitude. The plane is horizontal to the ground and spinning around the center of the plane with the axis of the spin perpendicular to the ground. Kinda like you threw the plane like a frisbee only it hovers in one place slowly gaining altitude.
I've also seen a very talented R/C helicopter pilot make a helicopter tumble nose over tail many times over while hovering in one spot and not losing any altitude. Very Impressive.
One of the funnest times I had flying R/C airplanes was on days the wind was higher than the takeoff speed of my plane, but steady and not gusty. It was great fun making takeoffs and landings either hovering with respect to the ground or with the plane traveling backwards with respect to the ground. I was only able to taxi the plane in that wind because I could flip a switch that was programmed to reverse the flaperons (ailerons that also work as flaps) to both go up making reverse flaps. That would reverse the airfoil making the plane stick to the ground instead of take off.
There is a whole world of aerobatics being done now where the plane stays in the air rolling, spinning, and tumbling about various axis and there is no aerodynamic principles holding it aloft. It's mostly deflection of the prop thrust. The term for these maneuvers is 3-D Aerobatics. It started in R/C planes and has moved into the freestyle performances of aerobatic pilots. It is done with a stunt plane with more than enough power to hang on the prop.
A simple example of a 3-D aerobatic maneuver is a flat spin where the plane gains altitude. The plane is horizontal to the ground and spinning around the center of the plane with the axis of the spin perpendicular to the ground. Kinda like you threw the plane like a frisbee only it hovers in one place slowly gaining altitude.
I've also seen a very talented R/C helicopter pilot make a helicopter tumble nose over tail many times over while hovering in one spot and not losing any altitude. Very Impressive.
One of the funnest times I had flying R/C airplanes was on days the wind was higher than the takeoff speed of my plane, but steady and not gusty. It was great fun making takeoffs and landings either hovering with respect to the ground or with the plane traveling backwards with respect to the ground. I was only able to taxi the plane in that wind because I could flip a switch that was programmed to reverse the flaperons (ailerons that also work as flaps) to both go up making reverse flaps. That would reverse the airfoil making the plane stick to the ground instead of take off.
11-02-2009, 07:30 AM
#44
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I hope that everyone understands that most R/C airplanes have a specific weight that has little to nothing in common with any modern aircraft.
Only the very early designs (wooden frame covered with muslin ) come close. But due to size even those had to use much denser wood aso.
This doesn't necessarely translates in much higher power to weight ratio, but especially in much higher flotability (might be using the wrong english term here, but you get the idea). Many R/C models can glide with the engine cut off at very low speeds aso.
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1979 Euro
Only the very early designs (wooden frame covered with muslin ) come close. But due to size even those had to use much denser wood aso.
This doesn't necessarely translates in much higher power to weight ratio, but especially in much higher flotability (might be using the wrong english term here, but you get the idea). Many R/C models can glide with the engine cut off at very low speeds aso.
.....................................
1979 Euro
11-02-2009, 08:25 AM
#45
Rennlist Member
Adrian, Sure, that's why NASA and Burt Ratan use R/C models to flush out new designs. There is this thing called the Reynolds Number that reflects the size of the airfoil compared to the size or air molecules. There is also the consideration of scale airspeed that makes them seem not quite so slow or floaty. There is a whole segment of R/C that builds true to scale R/C planes and they find they are very difficult to fly as the scaled down airfoil is just not up to the loading for the slower speeds needed for take off and landing the smaller scale planes.
There is a plane developed here in Oklahoma called the bush plane that amazingly flies at only 25 mph and can take off and land in only 75 ft.
I also think if you go look at the modern aerobatic planes (also developed and built here in Oklahoma, the EDGE) like those used in the Red Bull Air Racing you will find they are very light and "floatable" and have a very comparable power to weight ratio as R/C planes.
But when you go to indoor R/C planes, yes there are some foam r/c planes made for indoor aeribatics now that are insanely light and are flown in exhibitions in gymnasiums. I am anxiously awaiting the materials and radio controls to be sized down in both weight and size to be able to fly aerobatics slow enough to be able fly it around in my living room instead of a basketball court sized gymnasium. So far mini micro R/C are only rudder, elevator, and throttle control, no ailerons yet. They use the vibrator motors from pagers, watch batteries, and the radio equipment and servos weighs in at just 2 grams.
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There is a plane developed here in Oklahoma called the bush plane that amazingly flies at only 25 mph and can take off and land in only 75 ft.
I also think if you go look at the modern aerobatic planes (also developed and built here in Oklahoma, the EDGE) like those used in the Red Bull Air Racing you will find they are very light and "floatable" and have a very comparable power to weight ratio as R/C planes.
But when you go to indoor R/C planes, yes there are some foam r/c planes made for indoor aeribatics now that are insanely light and are flown in exhibitions in gymnasiums. I am anxiously awaiting the materials and radio controls to be sized down in both weight and size to be able to fly aerobatics slow enough to be able fly it around in my living room instead of a basketball court sized gymnasium. So far mini micro R/C are only rudder, elevator, and throttle control, no ailerons yet. They use the vibrator motors from pagers, watch batteries, and the radio equipment and servos weighs in at just 2 grams.
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Last edited by RKD in OKC; 11-02-2009 at 08:48 AM.