wing / airfoil
05-03-2011, 04:03 PM
#1
Thread Starter
My Feedback: (58)
Join Date: Nov 2002
Location: Knoxville,
TN
Posts: 1,807
Likes: 0
Received 0 Likes
on
0 Posts
wing / airfoil
hi guys,
I'm working on a government project and looking at doing a flying wing....
I need a airfoil that will give good lift and duration.. the whole platform will be about 4-5 pounds, and we need a 1-1.5 hours of power and glide.. I'm planing on using electric motor and a 5000-6000 3 s pack.
ideas.... length 60-80 inches
sweep ?
wing-lets ?
thanks
scott
I'm working on a government project and looking at doing a flying wing....
I need a airfoil that will give good lift and duration.. the whole platform will be about 4-5 pounds, and we need a 1-1.5 hours of power and glide.. I'm planing on using electric motor and a 5000-6000 3 s pack.
ideas.... length 60-80 inches
sweep ?
wing-lets ?
thanks
scott
05-03-2011, 05:01 PM
#2
RE: wing / airfoil
Try this and see if there is any useable info.
http://www.aerodesign.de/english/profile/profile_s.htm
Your needs are not typical of what sport fliers would want. I would assume you want something with a wide speed range at a medium to heavy wing laoding and decent L/D. If it's for Uncle Sam, I would bet a dollar that the payload keeps getting bigger as the project/program evolves.
http://www.aerodesign.de/english/profile/profile_s.htm
Your needs are not typical of what sport fliers would want. I would assume you want something with a wide speed range at a medium to heavy wing laoding and decent L/D. If it's for Uncle Sam, I would bet a dollar that the payload keeps getting bigger as the project/program evolves.
05-04-2011, 08:48 AM
#4
Join Date: Oct 2002
Location: Chilliwack, BC, CANADA
Posts: 12,425
Likes: 0
Received 22 Likes
on
19 Posts
RE: wing / airfoil
You're a long way from shopping for an airfoil yet if that amount of model info is all you've got.
The choice of airfoil will be determined by the final desired flight cruise speed. I'm assuming this is basically a straight line out, loiter then straight line back sort of flight path where turns are all done with shallow bank angles and open turns. So the vast majority of the flight is basically level flying or nearly level flying. But you need to figure out what wind strengths will be tolerated since there's not much point in flying a 30 mph cruise speed model in a 30 mph wind. And it's still not all that great to fly such a model even in a 20 mph wind since it could result in the flight pack power being used up before you can return to the launch point. Or it'll reduce the allowable range to less than that needed for the mission.
To find out which airfoil will work best for this craft you will need to firm up the required flight cruise speed, total final craft weight and wing area. In additon if this is a swept wing design you will need to take into account that the outer portions, which are highly washed out to aid in the wing's stability, do not and can not operate at as high a lift coefficient as the inner portion. So you'll need to figure out how much washout you'll be using and what the average or mean (they are slightly different) lift coefficient of the wing will be during cruise flight so you can determine how much induced drag there will be. Only once you figure out all this stuff will you be ready to go airfoil shopping for root and tip options.
Ideally you would fly at the best L/D speed for the wing so you consume the least amount of power to remain in level flight. Realistically a UAV does not "glide" other than to descend to a new flight altitude or to descend to a landing. Unless you're trying to build a self thermaling craft you're looking at simple straight and level flight for the majority of the mission running from power from the motor(s). And if the requirements of the flight mission require flying speeds significantly above that of the best L/D then you'll need more power to achieve the required flight speed. So again you need to firm up the mission requirements before setting the airfoil or flight pack parameters. The best configuration for a 25 mph cruise speed platform is going to be radially different from the best configuration for a 40mph platform. And a big part of this is the airfoils and amount of washout you'll use.
For example at your stated maximums of 5 lbs for 1.5 hrs and based on the rule of thumb of 13 watts per lb for light sport models to fly level you're looking at 65 watts just to maintain level flight at around the best L/D speed. From a 3S pack that means around 6 amps of power for the whole flight time. 6 amps for 1.5 hours is 9 amp hours or 9000 mah. So your 5000 to 6000 3s pack isn't going to cut the mustard unless you double it up. But that doesn't allow for the far higher power needed to climb to cruise altitude or to fly at higher speeds to cover more range or deal with windy conditions. It's highly likely that to allow for climbing, wind, running the onboard gear and having enough in reserve to avoid failure at the end of the mission you'll likely need more like 12 to 15 amp hours of onboard power. So call it two 6000's in parallel at a minimum to keep 5 lbs in the air for 1.5 hrs. And it's not unlikely that you'll need more than that depending on the final cruise speed requirement.
And if the mission calls for covering enough ground in that time that you can't do it at the relatively slow best L/D speed then some amount of power more than 13'ish watts per lb will be needed to achieve the required cruise speed. How much more will depend on how fast it needs to fly to cover the distance. If there's a need for a higher cruise speed it will also affect the choice of the airfoil, the size of the wing and the motor(s) and props used for flying. Testing of the final craft under various conditions will likely find that there's a cross over point where the speed and flight duration max out to give a maximum ground distance. But fighting headwinds out or back will alter this expected range to a high degree. Again testing of the completed craft at various speeds and motor powers will be needed to determine the final operating parameters.
So I hope you can see that there's a lot more to be done yet before you worry about which airfoil you should use.
One tool I can point you at which will be highly helpful is the Panknin wing twist formulas. A bit over half way down the page there are links to Excel spreadsheets that have all the formulas built in and make using them very easy. If you don't have Excel then download and install Open Office for free. The OOCalc application will run the Excel files.
http://www.b2streamlines.com/Panknin.html
These are used for determining the amount of washout twist needed with a swept flying wing to achieve stability. Note that this twist is speed sensitive. Try it with various weights and speeds and CG locations and you'll soon get a feel for how these parameters are important and why I'm saying that you're a long way from being ready to pick an airfoil if the information in your first post is all you have at the moment.
The choice of airfoil will be determined by the final desired flight cruise speed. I'm assuming this is basically a straight line out, loiter then straight line back sort of flight path where turns are all done with shallow bank angles and open turns. So the vast majority of the flight is basically level flying or nearly level flying. But you need to figure out what wind strengths will be tolerated since there's not much point in flying a 30 mph cruise speed model in a 30 mph wind. And it's still not all that great to fly such a model even in a 20 mph wind since it could result in the flight pack power being used up before you can return to the launch point. Or it'll reduce the allowable range to less than that needed for the mission.
To find out which airfoil will work best for this craft you will need to firm up the required flight cruise speed, total final craft weight and wing area. In additon if this is a swept wing design you will need to take into account that the outer portions, which are highly washed out to aid in the wing's stability, do not and can not operate at as high a lift coefficient as the inner portion. So you'll need to figure out how much washout you'll be using and what the average or mean (they are slightly different) lift coefficient of the wing will be during cruise flight so you can determine how much induced drag there will be. Only once you figure out all this stuff will you be ready to go airfoil shopping for root and tip options.
Ideally you would fly at the best L/D speed for the wing so you consume the least amount of power to remain in level flight. Realistically a UAV does not "glide" other than to descend to a new flight altitude or to descend to a landing. Unless you're trying to build a self thermaling craft you're looking at simple straight and level flight for the majority of the mission running from power from the motor(s). And if the requirements of the flight mission require flying speeds significantly above that of the best L/D then you'll need more power to achieve the required flight speed. So again you need to firm up the mission requirements before setting the airfoil or flight pack parameters. The best configuration for a 25 mph cruise speed platform is going to be radially different from the best configuration for a 40mph platform. And a big part of this is the airfoils and amount of washout you'll use.
For example at your stated maximums of 5 lbs for 1.5 hrs and based on the rule of thumb of 13 watts per lb for light sport models to fly level you're looking at 65 watts just to maintain level flight at around the best L/D speed. From a 3S pack that means around 6 amps of power for the whole flight time. 6 amps for 1.5 hours is 9 amp hours or 9000 mah. So your 5000 to 6000 3s pack isn't going to cut the mustard unless you double it up. But that doesn't allow for the far higher power needed to climb to cruise altitude or to fly at higher speeds to cover more range or deal with windy conditions. It's highly likely that to allow for climbing, wind, running the onboard gear and having enough in reserve to avoid failure at the end of the mission you'll likely need more like 12 to 15 amp hours of onboard power. So call it two 6000's in parallel at a minimum to keep 5 lbs in the air for 1.5 hrs. And it's not unlikely that you'll need more than that depending on the final cruise speed requirement.
And if the mission calls for covering enough ground in that time that you can't do it at the relatively slow best L/D speed then some amount of power more than 13'ish watts per lb will be needed to achieve the required cruise speed. How much more will depend on how fast it needs to fly to cover the distance. If there's a need for a higher cruise speed it will also affect the choice of the airfoil, the size of the wing and the motor(s) and props used for flying. Testing of the final craft under various conditions will likely find that there's a cross over point where the speed and flight duration max out to give a maximum ground distance. But fighting headwinds out or back will alter this expected range to a high degree. Again testing of the completed craft at various speeds and motor powers will be needed to determine the final operating parameters.
So I hope you can see that there's a lot more to be done yet before you worry about which airfoil you should use.
One tool I can point you at which will be highly helpful is the Panknin wing twist formulas. A bit over half way down the page there are links to Excel spreadsheets that have all the formulas built in and make using them very easy. If you don't have Excel then download and install Open Office for free. The OOCalc application will run the Excel files.
http://www.b2streamlines.com/Panknin.html
These are used for determining the amount of washout twist needed with a swept flying wing to achieve stability. Note that this twist is speed sensitive. Try it with various weights and speeds and CG locations and you'll soon get a feel for how these parameters are important and why I'm saying that you're a long way from being ready to pick an airfoil if the information in your first post is all you have at the moment.
