RCU Forums - View Single Post

12-14-2011 | 01:21 AM

#13

Aerobeez

Joined: Dec 2011

Posts: 22

Likes: 0

Received 0 Likes on 0 Posts

From: Baldwin Park, CA

RE: Thrust to weight

Quote:

ORIGINAL: foutsjoe

The short answer is that someone might have very general rule of thumb they could give you if they knew the plane (specifically the wing) type, but the long answer is no...not really.

All the thrust does for you is pull the wing through the air (which is what actually makes the plane fly). Different wings need different speeds in order to make enough lift to get the plane airborne. The wing ends up being the single most important factor in determining lift, so thrust alone is not nearly enough information. Any wing has a "coefficient of lift" which can get you to a number of how much the wing will lift at a certain speed, but that is not an easy thing to do. Other things such as drag, layout, etc also come in to play. Thrust to weight is not really a driving number in aircraft design, unless it is well over 1, and the plane can basically hover.

As an example, some competition planes I've designed are at the 55lbs cap at takeoff, and only have a .61 engine (roughly 10 lbs thrust). They fly, but not well. The wings are solely designed to handle that much weight without much power.

<p style="margin-bottom: 0in; border-top-style: none; border-right-style: none; border-bottom-style: none; border-left-style: none; border-width: initial; border-color: initial; padding-top: 0in; padding-right: 0in; padding-bottom: 0in; padding-left: 0in; ">Just browsing through the forum and saw this thread. Just wanted to give some input to the question. It's very true about what foutsjoe said about knowing the type of wing design in order to calculate if your plane can fly. Here is why:

In order for a plane to fly, the lift force has to be greater than the weight of the plane. In your case the lift force has to be more than 22lb.

The formula for calculating lift force is L =1/2ρv^2A(CL)
where L is the lift force,ρis the air density, v is the air velocity, A is the wing area, CL is the lift coefficient at a certain angle of attack.

Air density is a function of air pressure and temperature. An air density table is easily accessible online for the correct density at your flying condition. But for estimation purposes, at sea level and 20 degree Celcius, the air density is about 1.22 kg/m^2.

v is the plane's speed relative to the speed of the air moving across it. Ideally, assuming it's a no wind situation, v would simply be the traveling speed of the plane. 
you mentioned your plane produces 11 Lbs of thrust. The velocity of the plane can be calculated using F=MA where F is force (Thrust-drag), M = mass of the plane (in Kg), which is 22lb divided by 2.2, A is the acceleration of the plane (A=v/t),where t is the time traveled at that froce. so F = Mv/t,
Therefore, v = (thrust-drag)t/M. To calculate the drag, you have to know the drag coefficient of the plane, which is a function of the plane's geometry and surface material. It is a very complex calculation and it involves experiements for any specific shape. So let's just assume D is 0 for the purpose of this estimation. As such, we know that the speed would be somewhat less than what you would get using this formula, which we "boldly" assuming drag is zero(and we know in reality it's not). If you want to get technical about it, you could measure the speed using a speed sensor. Once you measure the speed you could use the same calculation to get the Drag and drag coefficient of your plane at that particular air condition. For your reference D =1/2ρv^2(Cd), which in this case A is the surface area of your whole plane.

Lastly, what you need to know is the CL. CL is a function of the lift coefficient and the angle of attack of the wings. And this is determined by the airfoil design. Each airfoil design has its own llife coefficient and it is represented by a curve. There are plenty of websites you can look up for lift coefficient for a certain airfoil design. Once you find the curce, you can get the value of CL at that particular angle of attack when your plane is running on the ground. You can now use that value to calculate the lift force your plane generates. If it's greater than 20Lbs, or 10Kg, your plane will fly. If not, considering getting a more powerful engine to get a higher traveling speed or find ways to reduce the weight of your plane. Hope this helps.

Here is a site you can look up data on different airfoils:
<a target="_blank" href="http://www.ae.illinois.edu/m-selig/ads/coord_database.html#K">http://www.ae.illinois.edu/m-selig/a...atabase.html#K
</a>reference: wikipedia
<a target="_blank" style="line-height: 16px; " href="http://en.wikipedia.org/wiki/Lift_coefficient">http://en.wikipedia.org/wiki/Lift_coefficient
</a>http://en.wikipedia.org/wiki/Drag_coefficient

Reply Like