Cambo

My Feedback: (2)

Hi guys,

Was thinking about this in my head and can't seem to figure the answer. Lets say we have 2 planes of the same model. Both have the same engine turning the same prop at the same rpm. One weighs 10 pounds and the other weighs 15. Assuming the wing could lift both would the planes travel the same speed in straight and level flight.

Jimmbbo

The 15 lb airplane will require a higher angle of attack (AOA) to create the additional 5 lb of lift to maintain level flight. Higher AOA causes an increase in induced drag, slowing the airplane's airspeed...

HTH!

Jim

MajorTomski

Or the heavier plane would have to fly at a higher speed at the same AOA to stay level.

Because of the weight there's a trade off in your presumption. The heavier plane could fly at the same speed, but it would take more power and a higher angle of attack.

LouW

The heavier plane could indeed fly at the same AOA at a higher speed, and the induced drag would be the same, but the form drag would be higher. A heavier airplane simply requires more power to fly. Interestling if the airplanes are identical except for weight, the glide angle would be esentially the same but the heavier one would glide faster. The best glide angle is dependent on lift/drag ratio which remains basically the same.

Cambo

My Feedback: (2)

Still doesn't make sence to me. Wouldn't the angle of attack be the same with both planes in level flight?

Maybe someone needs to explain what AOA exactly is. I have an idea but am not sure.

buzzard bait

AOA is the angle at which the wing meets the air. As someone said above, everything else equal, the heavier plane needs a higher AOA to fly level than the lighter plane.

Conceptually, drag has two components, induced drag and parasitic drag. The parasitic drag will be the same for both planes because they are the same design. But induced drag is the drag that results from creating lift. A heavier plane needs more lift so it will have more induced drag. That will happen through a higher angle of attack. You'll need a bit of 'up' trim on the heavy plane compared to the light one. That will slow it down.

As you reduce the throttle and try to maintain level flight, the heavier plane will quit flying at a higher speed. So the lighter plane can not only fly faster in level flight, it can also fly slower.

Obviously the heavy plane will be slower when climbing too. But when it starts heading down it's a different story. The heavy plane will be faster when descending. Suppose you are upwind and out of gas. The heavy plane will be the most likely to make it back to the field. It's faster going downhill.

Cambo

My Feedback: (2)

Interesting, this is macking sence now.

HighPlains

My Feedback: (1)

Actually, the AOA for minimum drag depends on the airfoil. Some airfoils have a higher drag when lightly loaded, than when carrying some additional weight.

Jimmbbo

Attaching two drawings - one of a generic lift curve, one showing AOA...


Lift (blue line in drawing) increases with AOA until the critical AOA is reached and the wing stalls.... Higher AOA, more lift, but also more drag (red line in drawing).

With constant thrust, drag slows the airplane down, so you either live with the slower speed, or increase power and fuel consumption to make up for the loss...

crasherboy

Lets keep things simple. thrust versus drag,,, lift versus gravity. The heavier it is the more lift required to maintain height,the more drag ,the more power required to maintain speed. So you can see from that ,the heavier the more power required to maintain the same height and speed. Lets imagine an example. Lets say that I go to Alaska with a Piper super cub[say the plane weighs 1200 # and has 125 hp]. Now say that I shoot a moose that dresses out to 1200 #. Tell me ,after I load in the moose and myself,how much power would it take to even take off,that is if the plane could carry that much cargo? Don't worry about my figures .I am just trying to help you see the connection of my first sentence. The last I knew Piper cubs both regular and super are used for this purpose,cause they are so good at slow and easy landings.etc.

Cambo

My Feedback: (2)

This is great stuff guys. I think i understand it.

Before i was thinking that the wing could hold any amount of weight (in straight flight) up to a point were it simply couldn't hold it any more. But know i see that the more weight the higher angle of attack you need. To flip that around, if you had a plane that was insanley lite for the size of the wing, would you actually need negative AOA to maintain altitude without rising?

MajorTomski

Yep, that is why you occasionally see a B-52 doing a takeoff with flaps and the plane is actually nose down and climbing.

B.L.E.

Adding weight increases the speed at which the best glide ratio happens, it does not nessesaily give the plane a better glide ratio. A lot of gliders, both models and full scale sometimes carry ballast to help penetrate headwinds. Full scale gliders usually carry water ballast that can be dumped during flight so the glider can land light.

BMatthews

Wellllll.... technically deploing the flaps alters the centerline of the wing since that line is from the leading to the trailing edges. So while the B52 may be climbing with a nose down attitude it's actually got a highly positive angle of attack for most of the wing.

As for the insanely light weight vs angle of attack. Yes it is possible and happens on occasion. Cambered airfoils get lift from both the camber of the shape as well as the angle of attack. There's a spec for airfoils called "zero lift angle" which is the angle of attack where no lift is produced. This would be the case if you were in a vertical dive or climb and held elevator to hold the model on the vertical line. For a symetrical airfoil this angle is always 0degrees. For any cambered airfoil it's some negative number.

Cambo, you can see all this and play with it yourself. Google for foilsim and go play with size, weight, camber, angle of attack and flying speed. It's very educational to see the lift vs speed and angles and to learn the lift coefficients we operate at. It's geared to full size but it works for model sizes and speeds as well as long as you know that "our" size wings stall at around 7 to 9 degrees positive. So don't look for lift above that angle that means anything for us.

BillS

I have a 5 1/2 pound Kougar and a 6 1/2 pound Kougar. The glide slope at idle is as different as night and day. At high throttle I don't notice any difference in the attitude of either airplane. At low speed the heavy one requires more elevator.

Bill

Cambo

My Feedback: (2)

This is great stuff guys, thanks for the explanation

JimCasey

My Feedback: (1)

The lift force at a given AOA is proportional to the square of the airspeed. If you double the weight of the plane it has to go 41.4% faster to maintain altitude at the same AOA. Power required goes up as the cube of speed, so it will take 2.83 times as much power if you double the weight.

rmh

I wondered when someone would state the obvious-
thanks Jim-
Why---should a plane that is heavier go faster (discounting gliders )
Why the ----do you think engines were added in the first place?
When all else fails in researching answers for "aerodynamic rule" questions -- just look at logic.
Way back when- we were avid newcomers to the new sport of drag racing.
It quickly became apparant that that changes we suspected which would make the car go faster - DID make it go faster -but in tiny increments .
then when I got into powered models - same thing all over again
it took huge increases in power and in weight reduction (both) to go a little bit faster .
My favorite gag -that still persists- is that a heavier pattern plane will do better in the wind than a light one