How are you guys measuring thrust?
09-07-2005, 09:11 AM
#1
Senior Member
Thread Starter
My Feedback: (1)
Join Date: Nov 2004
Location: Shorewood, IL
Posts: 566
Likes: 0
Received 0 Likes
on
0 Posts
How are you guys measuring thrust?
I'm sure I'll end up buying a watt's up meter eventually but that'll just show watts, amps etc. How do you convert that info into calculating thrust? Or is there another way?
09-07-2005, 09:27 AM
#3
Senior Member
My Feedback: (11)
Join Date: Jan 1970
Location: Muskegon,
MI
Posts: 598
Likes: 0
Received 0 Likes
on
0 Posts
RE: How are you guys measuring thrust?
I tried measuring thrust with a digital postal scale, but found that the bigger the prop, the lower the thrust readings; this was not supported by flight testing; increasing prop size will increase amp draw, eventually reaching the point where the max amp draw rating of the motor is exceeded. Some electric motor performance predicting programs may include a theoretical thrust component. If your plane can carry 2.3oz payload, you can put your Wattsup meter on board and read out the in-flight numbers upon landing....Jim
09-07-2005, 10:05 PM
#4
Member
My Feedback: (3)
Join Date: Jun 2005
Location: Kansas City,
MO
Posts: 49
Likes: 0
Received 0 Likes
on
0 Posts
RE: How are you guys measuring thrust?
Xray:
I use watts for calculating thrust. As a very loose rule (and you will learn to tweak this with experience) 35-50 watts to the prop for every pound of plane is good for a glider assist, 50-75 watts/pound is OK for easy sport/trainer flying, 75-100+ watts/pound will give you a decent aerobatic plane, and 150 watts/pound will give you 3D. I have a 32 oz Stick with an AXI 2808, 3S pack, and a 9X6 prop that delivers about 160 watts and pulls 21 amps WOT. The plane will go vertical for quite a ways (almost hovers) and is very nicely aerobatic. At 1/2 throttle it flies very nicely, climbs well and will do loops and easy patterns.
Is this what you are looking for?
Tom
I use watts for calculating thrust. As a very loose rule (and you will learn to tweak this with experience) 35-50 watts to the prop for every pound of plane is good for a glider assist, 50-75 watts/pound is OK for easy sport/trainer flying, 75-100+ watts/pound will give you a decent aerobatic plane, and 150 watts/pound will give you 3D. I have a 32 oz Stick with an AXI 2808, 3S pack, and a 9X6 prop that delivers about 160 watts and pulls 21 amps WOT. The plane will go vertical for quite a ways (almost hovers) and is very nicely aerobatic. At 1/2 throttle it flies very nicely, climbs well and will do loops and easy patterns.
Is this what you are looking for?
Tom
09-07-2005, 10:34 PM
#6
Member
My Feedback: (3)
Join Date: Jun 2005
Location: Kansas City,
MO
Posts: 49
Likes: 0
Received 0 Likes
on
0 Posts
RE: How are you guys measuring thrust?
Xray:
Thanks! No, it is called an Stik-E from Global models. They don't make it anymore; I found it on eBay for about $20. On the box it says "A kit that won't amp out your wallet." It came with a 550 motor. The kit was horrible (at least mine was). The ribs were not consistent and the wood was really cheap. I changed the tail feathers from the typical round slabs of balsa to the built up ones you see Saved about an ounce at the tail which I am sure saved me about 4oz on the nose. I have the battery all the way forward to balance it since the AXI is quite light. But as it turns out it is a really sweet flying plane. It has a flat bottom airfoil and I now wish it was symmetrical. But a friend of mine took an ultra stick and reduced it for a .25 and that will be perfect for this set up I have! Anyways, for a second plane this one was just right!
Best of luck!
Tom
Thanks! No, it is called an Stik-E from Global models. They don't make it anymore; I found it on eBay for about $20. On the box it says "A kit that won't amp out your wallet." It came with a 550 motor. The kit was horrible (at least mine was). The ribs were not consistent and the wood was really cheap. I changed the tail feathers from the typical round slabs of balsa to the built up ones you see Saved about an ounce at the tail which I am sure saved me about 4oz on the nose. I have the battery all the way forward to balance it since the AXI is quite light. But as it turns out it is a really sweet flying plane. It has a flat bottom airfoil and I now wish it was symmetrical. But a friend of mine took an ultra stick and reduced it for a .25 and that will be perfect for this set up I have! Anyways, for a second plane this one was just right!
Best of luck!
Tom
09-08-2005, 07:12 AM
#7
My Feedback: (21)
Join Date: Jul 2002
Location: Spencerport, NY
Posts: 7,350
Likes: 0
Received 0 Likes
on
0 Posts
RE: How are you guys measuring thrust?
Watts is not the same as thrust, though there is a general relationship there (more Watts == more thrust). There's just no direct numerical relationship. Thrust simply can't be calculated using only Watts.
If you're looking for how we measure actual thrust, the simple answer is, we don't. Thrust is only half the picture. An airplane's power is actually divided for two purposes, thrust and speed. For the same amount of input power, you can have high thrust and low speed, low thrust and high speed, or a combination of the two. A balance needs to be found between the two so that the plane does what you want.
How do you figure out what the right balance is? You fly the plane. If it's too slow, you reduce the prop diameter a bit and increase pitch a little. Not enough thrust? You increase the prop diameter a bit and reduce the pitch a little. As long as you start out with enough Watts, the plane will fly. It's all fine-tuning from there.
If you're looking for how we measure actual thrust, the simple answer is, we don't. Thrust is only half the picture. An airplane's power is actually divided for two purposes, thrust and speed. For the same amount of input power, you can have high thrust and low speed, low thrust and high speed, or a combination of the two. A balance needs to be found between the two so that the plane does what you want.
How do you figure out what the right balance is? You fly the plane. If it's too slow, you reduce the prop diameter a bit and increase pitch a little. Not enough thrust? You increase the prop diameter a bit and reduce the pitch a little. As long as you start out with enough Watts, the plane will fly. It's all fine-tuning from there.
09-08-2005, 06:47 PM
#8
Senior Member
Join Date: Mar 2002
Location: Kingston,
ON, CANADA
Posts: 4,925
Likes: 0
Received 3 Likes
on
3 Posts
RE: How are you guys measuring thrust?
Tried a low pitched prop on my 1/4 scale Corby Starlett. Got off the ground in about half the normal distance but nearly lost it going around the patch just above a stall.
09-08-2005, 10:35 PM
#10
Member
My Feedback: (3)
Join Date: Jun 2005
Location: Kansas City,
MO
Posts: 49
Likes: 0
Received 0 Likes
on
0 Posts
RE: How are you guys measuring thrust?
ORIGINAL: Matt Kirsch
Watts is not the same as thrust, though there is a general relationship there (more Watts == more thrust). There's just no direct numerical relationship. Thrust simply can't be calculated using only Watts.
If you're looking for how we measure actual thrust, the simple answer is, we don't. Thrust is only half the picture. An airplane's power is actually divided for two purposes, thrust and speed. For the same amount of input power, you can have high thrust and low speed, low thrust and high speed, or a combination of the two. A balance needs to be found between the two so that the plane does what you want.
How do you figure out what the right balance is? You fly the plane. If it's too slow, you reduce the prop diameter a bit and increase pitch a little. Not enough thrust? You increase the prop diameter a bit and reduce the pitch a little. As long as you start out with enough Watts, the plane will fly. It's all fine-tuning from there.
Watts is not the same as thrust, though there is a general relationship there (more Watts == more thrust). There's just no direct numerical relationship. Thrust simply can't be calculated using only Watts.
If you're looking for how we measure actual thrust, the simple answer is, we don't. Thrust is only half the picture. An airplane's power is actually divided for two purposes, thrust and speed. For the same amount of input power, you can have high thrust and low speed, low thrust and high speed, or a combination of the two. A balance needs to be found between the two so that the plane does what you want.
How do you figure out what the right balance is? You fly the plane. If it's too slow, you reduce the prop diameter a bit and increase pitch a little. Not enough thrust? You increase the prop diameter a bit and reduce the pitch a little. As long as you start out with enough Watts, the plane will fly. It's all fine-tuning from there.
So far I have found this hobby to be about "feel"; Getting a feel for power systems, feeling how the plane flies, etc. For me it is one of the fun parts of it. I think anyone looking for exact answers will always be disappointed.
Tom
09-09-2005, 02:31 AM
#11
Senior Member
Join Date: Dec 2004
Location: Naersnes, NORWAY
Posts: 912
Likes: 0
Received 0 Likes
on
0 Posts
RE: How are you guys measuring thrust?
There are some prop-constant tables available and these can give you approximate thrust on spesific props depending on RPM-readings. An example: http://aircraft-world.com/prod_datas...pconstants.htm
