Model Aircraft Aerodynamics

In another thread (www.modelflying.org.uk/forum/model-build...-frog?start=30#48810) I posed the idea that a slow flying model such as the BMFA's Frog, flew better with a thin single covered wing rather than a fatter double covered one. Alan Cantwell posted the following:

"Andy, i can see where you are coming from with the wing being thicker, but would the air not break up in the normal way on contact with the leading edge, and the crossbraces? therefore automatically being a thick wing?

i can remember, as an aside, a friend of mine flying on the nats display line, he was flying a glens Cap, with a zenoha 62 petrol engine, so, quite a size, his tow surface covering came off at the leading edge, and blew back words, the entire structure was on show, after the initial shock, and after a few test manouveres, he flew it round--carefully, and landed as normal, would the leading edge, in this case, be breaking the air up as normal? enabling it to fly on the underside covering, or was he just a lucky git"

I don't have the specific answer to your questions Alan, but my understanding of low speed aerodynamics is that the airflow doesn't have the energy to stay attached to the flying surface. As the wing gets thicker, the breakaway point moves towards the leading edge, the limit being the point where the aircraft won't fly. There is also a significant reduction in stalling angle as the airspeed reduces with very low speed wings stalling at around 5 - 8 degrees incidence rather than the more normal 15 degrees of a classic wing. With indoor models, a roughened surface (screw the tissue into a ball before using, for example) causes turbulence which energises the airflow and helps it to stay attached. The drag increases, but so does the lift giving a better lift/drag ration which provides an advantage. In the case of the Frog, I suspect that the leading edge will cause turbulence that will help the airflow stay attached giving a better lift drag than if the surfaces were faired in or double covered. In respect of your colleagues aeroplane, the drag would increase and the lift reduce. There was obviously enough lift for the aircraft to fly and enough engine power to overcome the extra drag but where the lift was coming from, upper or lower surface is a moot point and one I can't answer.

I'll now open the debate.......your views please?

well, i bow to the master, youve done more of this than a simple engineer, so, over to others

My source volume was Martin Simons book "Model Aircraft Aerodynamics". I highly recommend it as a good introduction to the aerodynamics of slow flying models. It certainly helped me to understand what was going on in the model world, which, in many ways is quite different to full size craft.

having said wrinkles in the tissue on the wing does this not create any pockets of larger molecules of air that may disturb the balance of the air flow?

The wrinkles create turbulence Sarah which makes drag. But it also energises the airflow which keeps it atached to the wing. I'll draw some diagrams when I get time.

Sarah asked:" Where is the point of start to measure for 4 degrees? Ie are you talking each convex rib has to be 4 degrees or the angle of the wing to the motor stick has to be 4 degrees?

The CHORD LINE of the wing is the line directly between the LE and the TE. The angle the CHORD LINe makes with another line is the incidence. LE up is normally positive, LE down is normally negative.

The RIGGING INCIDENCE is the angle the CHORD LINE makes with the FUSELAGE DATUM (a line joining the front to the rear of the fus).

The AERODYNAMIC INCIDENCE is the angle between the CHORD LINE and the relative airflow over the wing.

FF models tend to fly at about 3 - 5 degrees AERODYNAMIC INCIDENCE so the wings are normally fixed at about 3 degrees to the FUSELAGE DATUM. If the wing is mounted flat on the fuselage, the model will fly nose up. If the wing is mounted at excessive RIGGING INCIDENCE, the model will fly nose down.

A classic undercambered model section will create lift from an AERODYNAMIC INCIDENCE of about -3 degrees to about +10 degrees.

When ever i have designed a plane, i used my mentors maxim, 000 on engine thrustline, wing& tail incidence, one of these models, a tailed delta, was from flat sheet, it was all 0, and it flew superb, both ways up, with swpet wings, my mentor used semi root, and sym tips or was it the other way round?? Hmm the Zlin Trener, high swept wing eading edge high performance, but not that fast flying aerobatic machine, used a sym root, with a highly tapered leading edge, straight trailing edge, with a foil change, from sym at root, to undercambered at tip (bet that was a bitch to build in jig) airfoils, dont you just luv um!!
Even a passenger jet nearly becomes undercambered when its slats and flaps move to there full, i always try to get a wing seat, just to watch the wing do its work, facinating!!

Did the models you talk of Alan have radio control.....0-0-0 set-up don't work for free flight unfortunately.

yes, they all where, the free flighters all had positve incidence, and clark y foils, motors had downthrust, dont know about the tails

Well here is my twopenny'oth.
At very slow speeds air behaves more like treacle and tends to stick so does not completely 'break away' and lose lift as in the conventional manner. Irregular surfaces (like a built up wing but with only one side covered) will increase the drag but it will have less impact on the lift.

For rubber free flight the target is duration not distance. To fly using minimum power the key elements are weight and sink rate rather than the lift to drag ratio. For a given amount of energy (i.e. a rubber motor! ) a light apparently inefficient airframe with a poor lift to drag ratio but low sink rate can actually stay up longer than a more efficient but heavier one.

To parody a Colin Chapman saying :
"For duration add lightness"

Quorneng wrote: Well here is my twopenny'oth.
At very slow speeds air behaves more like treacle and tends to stick so does not completely 'break away' and lose lift as in the conventional manner. Irregular surfaces (like a built up wing but with only one side covered) will increase the drag but it will have less impact on the lift.

For rubber free flight the target is duration not distance. To fly using minimum power the key elements are weight and sink rate rather than the lift to drag ratio. For a given amount of energy (i.e. a rubber motor! ) a light apparently inefficient airframe with a poor lift to drag ratio but low sink rate can actually stay up longer than a more efficient but heavier one.

To parody a Colin Chapman saying :
"For duration add lightness"

That's an interesting concept Q, but I don't think you've gone far enough with the analysis before making the conclusion.

There are four forces on an aircraft in flight: thrust, drag, lift and weight. A glider is a special case of zero thrust, so in that case it's just lift weight and drag. In level flight, thrust equals drag, lift equals weight.

If we lighten the model, we need less lift. The amount of lift we get depends on the surface area of the wing, the lift coefficient of the wing and the square of the airspeed. i.e.if we double the speed we get four times the lift. We can't do anything about the surface area of the wing or the lift coefficient on a given model, but if we reduce weight, we reduce the lift requirement and therefore the model will fly slower.

In the same way as lift, drag depends on surface area, the drag coefficient and the square of the speed. Again, if we double the speed, we increase the drag by a factor of four. But by reducing weight, we reduce speed, and therefore reduce drag. The drag reduction requires less thrust, so we need less rubber, which further reduces weight and we are on a winning line.

Putting the two together, the better the lift drag ratio, the less drag we will get for a given amount of lift and therefore the less thrust we need which leads to even less rubber.....etc

Looking now at a glider, or a duration model in the glide. The three forces must balance in a stable glide. Drag acts along the line of flight, lift acts perpendicular to the line of flight, and weight acts vertically downwards. If you draw a vector diagram, you'll see what I mean. Increasing weight means increasing lift which means increasing speed. Increasing speed means increasing drag and again, to stabilise the aircraft, the forces must be in balance. Because the aircraft is pointing downwards, any increase in speed means an increase in sink rate, so more weight equals less time in the air. But again, if we improve the lift drag ratio of the wing, we reduce the glide angle and therefore the sink rate.

The lift drag ratio is a fundamental of the airframe. If we improve it we'll improve the flight performance proportionally.

There's a good diagram of an aircraft in the glide showing the forces here: www.pilotfriend.com/training/flight_training/aero/gliding.htm

....and here's one for level flight: www.pilotfriend.com/training/flight_training/aero/forces.htm

This is normally such a "heavy "subject, but the way this is being explained on here is fantastic!!

I feel we are very honoured to have someone like Andy to explain .

Thanks Andy

Garry