CFI Pop Quiz #304 - Speed/Lift

[Chris Burgess]

Why does increasing speed also increase lift?

A) The increased velocity of the relative wind overcomes the increased drag.
B) The increased impact of the relative wind on an airfoil's lower surface creates a greater amount of air being deflected downward.
C) The increased speed of the air passing over an airfoil's upper surface increases the pressure, thus creating a greater pressure differential between the upper and lower surface.

[Paul_Zurawski]

Gonna go with (B)

[JByrd]

Respectfully, none of the above as written.

In my opinion "C" comes the closest with the inverse of the phrase "increases the pressure" to "decreases the pressure".

Jim

[Doug Riley]

Uh-oh, Jim's resurrected the old (and supposed) conflict between Bernoulli and Newton. This old chestnut has resulted in more hangar-flying hot air than any other one except downwind turns. We've gone around with it here on the Forum a few times.

The conflict is an illusion. A wing will, we know, fly without camber. In fact, if you've ever tossed a dime-store balsa glider, you know that a wing that is merely a thin flat board will make lift. IOW, bouncing air off the bottom works all by itself.

A wing works by accelerating air molecules. An acceleration can be either a change of speed, a change of direction, or both. The acceleration of air done by the balsa glider is mostly the "change-of-direction" type -- bouncing air off the bottom of the wing. Airfoil-shaped and cambered wings do more of the change-of-speed type of acceleration. In either case, however, the wing's activity obeys Newton's law of action-reaction. Bernoulli speaks to a specific sub-type of this activity (the speed-up), but there's no conflict and no need to "choose" between them.

[pwendell]

C is another one of those deceptive FAA answers. The increased speed of airflow over the top (curved) part of the airfoil DECREASES pressure, producing a very modest amount of extra lift.

B is the correct answer. Newton wins again :)

[ventana7]

I thought the lift got greater because when CFI's draw the picture of lift they always make the arrows BIGGER. in the drawing-- aren't the arrows what keeps the plane up in the air?
Rob

Oh I forgot... money is what keeps the plane aloft.

[EI-GYRO]

I'm with JBYRD on this one.

[JByrd]

Yes, here we go again.

Stall the wing and drastically increase that volume of downward deflected air from the increased impact on the lower surface. It shall keep you aloft...... at least from the inference from the FAA question.

Think about that while your field of view inverts and the view above your forehead becomes green or brown.

I can pull out a lot of discrete pressure port data that shows only about 2-6% of all lift (F) from a typical wing shape comes from redirection (M*A) of air from impact with the lower surface at any positive angle of attack from 0-22 degrees with respect to the chord.

The exception to the typical 2-6% contribution is in ground effect - The surface pressure underside of the wing facing the ground is slightly raised more because of the 3-D limitation on flow movement downward by the ground. The contribution of lift from the lower surface due to ground effect becomes more like 5-12%.

Bernoulli and Newton are shaking hands and smiling above or below the wing.

In wind tunnel work we generally do not even discuss the significance of the lower surface contribution of lift or design for optimization of it until about Mach 1.8 or greater. At Mach 3.4 and above the contribution of body lift generally makes subsonic wings almost entirely unnecessary. At that speed the contribution of upper surface lift is much reduced and the lower surface redirection of flow takes over.

Answer B makes sense at about Mach 3.4 or better.

Jim

[ben]

i think it's c

[Chris Burgess]

"B" is most correct. Why does increasing speed also increase lift? (B) The increased impact of the relative wind on an airfoil's lower surface creates a greater amount of air being deflected downward.

Jim pointed out a reason people pick the wrong response, they fail to see the one "incorrect" word in an otherwise correct answer. Frankly, I wouldn't want to fly an airplane with only Newton's help.

[Ga6riel]

Yes, here we go again.

The exception to the typical 2-6% contribution is in ground effect - The surface pressure underside of the wing facing the ground is slightly raised more because of the 3-D limitation on flow movement downward by the ground. The contribution of lift from the lower surface due to ground effect becomes more like 5-12%.

Jim

Thats not entirely the case
The contribution from ground effect isnt so easy to calculate, and this would be because the distance between the base of the foil and the surface can of course vary considerably. Limiting the contribution to between 5-12% greatly understates the art of the possible.

In tight circumstances the CL Max can be near doubled, this in an environment where drag is reduced and where, the drag vector visually described is perpendicular to the downwash angle, which is ofcourse considerably less.

[Doug Riley]

The trouble with crediting lift mostly to airfoil camber (the speed-up) is that (balsa gliders aside) symmetrical wing sections work OK on real airplanes, and you can even fly a cambered wing upside down.

OTOH, I'm happy to believe that the downward redirection of air flowing over the TOP of the wing contributes mightily. That's the part that is lost rather abruptly when the wing stalls.

The bottom surface keeps on deflecting air downward post-stall, but even the resultant thrust from THIS activity points farther and farther aft. It ends up mostly as drag. The lift portion is a diminishing return.

Those pressure-port data would be interesting to see, but I suppose they are double-super-secret.

[Ga6riel]

I guess this is a really bad time to mention inverted flight

[Canadian Rhino]

If it is the bottom side, why dont fixed wingers fly with thier flaps down all the time? I think the loss of vacume on the top of the wing at slow speed is just another word for stall!:D
Ground effect on a fixed wing I can understand but on a rotary wing much higher of the ground with a much smaller wing surface area, I have to wonder.:rolleyes:

[Doug Riley]

A bullet that ricochets off a concrete wall at muzzle velocity will take some concrete with it. OTOH, you wouldn't expect a divot in the wall if you slow-pitched the same bullet at the wall by hand. The force applied to the wall by the change in direction of the impacting mass depends on the speed of the mass. Actually, it's a function of the square of its speed.

Planes don't fly with their flaps down at high speeds because they don't have to. At high speeds, enough lift is created without the need for flaps. Flaps add to the wing's camber (and sometimes to its area) and increase the Bernoulli effect as well as the "bounce-off" effect.

Airplanes optimized for low-speed flight sometimes use highly-cambered wings (wings with a drooped trailing edge; the same thing as flaps, only permanent).

Gyros experience a very pronounced ground effect. At high airspeeds, it's like plowing into a pillow. Gyro students frequently over-flare on landing because they don't make allowances for the automatic cushioning effect caused by their nearness to the ground.

[Ga6riel]

Ground effect on a fixed wing I can understand but on a rotary wing much higher of the ground with a much smaller wing surface area, I have to wonder.:rolleyes:

on a rotary wing you need to consider the swept area or disc area, not the actual rotor itself. Looking at it that way theres a lot more area for reaction to take place. About 1/2 the wingspan above the surface is considered the affected zone

[Chris Burgess]

I am of the "opinion" that ground effect is nil. The major factor of landing "float" is the loading and spin-up of the rotor as you flare.

[Doug Riley]

Hmm. My experience is that you can fly the length of the runway in ground effect at a significantly lower power setting than you'd need at the same airspeed out of ground effect.

I ran a 1500 VW engine on my first gyro. It would ONLY maintain altitude within ground effect. You could force it up to about 50 feet by overspeeding on the ground and then hauling the stick back, but it would only zoom up for a moment, then run out of steam and descend back to 5 feet. It would cruise the length of the 2800-foot runway nicely at 5 feet, though.

I must have spent 50 hours flying man-high. I finally bored the engine out to 1800 cc and got up into the pattern.

[Chris Burgess]

Hmm, we must have flown the same VW. I had one "act" the same way. High speed on the ground, then nudge it smartly back into the air, where it slowed and fell back to earth. That was what several mph did for efficiency and induced drag. We also bored the engine and fixed the problem, not enough thrust. I get the picture, just still don't give the ground any credit. Again Doug, just IMHO.

[mceagle]

It would cruise the length of the 2800-foot runway nicely at 5 feet, though.
I must have spent 50 hours flying man-high.
It was such that taught many "old" gyronauts not to pio. There was simply not enough room.
Even today I believe that a student has no place being in the circuit pattern if they cannot fly the length of the strip at 5 ft.

[Friendly]

Tim ,
What is so hard about flying the runway at 5 feet??

[birdy]

Nuthn hard bout it Mark, thats why students should do it, coz its easy. Besides, there is't the 'fear facter' [ first taste of alt] to make him over correct and start a POI at 5'.

[Friendly]

Ok, I guess I did not understand the post. I thought Tim was saying some people fly the pattern and they have difficulty flying the runway at 5 feet.

[Doug Riley]

Mark: In machines that are prone to PIO, it's easy to detect the onset of oscillation if you're close to the ground. A little oscillation of a couple feet will catch your attention if you're low. You probably won't notice it at 1000 feet until after it's developed into something nasty.

OTOH, a machine with reasonable pitch damping is very unlikely to get into PIO in the first place, even in the hands of an inexperienced pilot. I've NEVER had anyone PIO my Dominator, and I've handed the controls over to complete novices dozens of times -- always at around 1000 feet.

Some of the "wisdom" about PIO and crow-hopping is left over from the days when all machines were more or less unstable and prone to PIO and PPO. Both tendencies can be designed out of a gyro, and should be. People should not have to tiptoe around a fundamentally vicious aircraft; they should insist that it NOT be vicious!

Gyro buyers have been too easy on the manufacturers in this regard, IMHO. We should kick more butt.

[Ga6riel]

I am of the "opinion" that ground effect is nil. The major factor of landing "float" is the loading and spin-up of the rotor as you flare.

i guess opinion can contravene fact, that said
if a gyro behaves like any other flying machine i can think of, and i include pelicans in that sample, and it demonstrates the tendency to float with minimal power at an altitude 50% of the span or less ~ then its in ground effect

[Doug Riley]

I'll try to remember to do some low-and-level runs next time I'm out and check the engine RPM needed to maintain level flight and steady airspeed. My recollection is that it's less for a given airspeed near the ground than at altitude, but why depend on memory?

[brett s]

Why would a gyro rotor not experience ground effect just like a helicopter rotor? Same principle, right?

[Ga6riel]

Why half the span?
seen in the figure attached, the maximum circulatory flow seen from front or rear elevation covers half the span. The presence of the ground greatly affects this flow.

The power of this effect can be seen in Zimmermans V-173 built by Vought's as seen here in the tunnel at Langley, and where the props countered the vortice flow thus greatly enhancing lift. This apparent 'boost' to Aspect Ratio was easily recalculated as the span + the downblade length of each prop.

[Ga6riel]

Why would a gyro rotor not experience ground effect just like a helicopter rotor? Same principle, right?
brett: as helo has an active rotor, its thrust equating to weight; then while in hover it is somewhat different. When in translative flight however, it is every part the same.

[Ga6riel]

Doug
be aware that you may 'feel' some tendency for the disc to flatten out, hence some manipulation to the stick

[Chris Burgess]

i guess opinion can contravene fact, that said
if a gyro behaves like any other flying machine i can think of, and i include pelicans in that sample, and it demonstrates the tendency to float with minimal power at an altitude 50% of the span or less ~ then its in ground effect

Never said "ground effect" did not exist for a gyroplane, I did however suggest I felt it's contribution to float was "nil" (not much). Remember also I was talking in the flare portion of a landing only, not cruise flight in ground effect. Rob, could you quantify it's contribution? I do not have your credentials nor will I pretend to know the answer. Chuck Beaty would surly have the numbers.

[birdy]

Less power is needed to maintain a given AS at ground level than at alt.
I don't know at wot alt the effect kicks in with FWs, but i know, ina gyro, theres conciderably less power needed at 12", than at 12'.

[Ga6riel]

Results vary according to the geometry of the type. The conundrum is that the lower you situate the wing, the more efficient the effect. So in a sense, the lower you are, the faster you travel. The reality however sees you rise with speed, untill eventually you climb out of GE. So the numbers are constantly changing based on the relationship with the height above the surface.

the little ESKA -1 posted elsewhere has a Cl Max of 2.25 IGE
this on a machine with a reverse delta wing, hence no flaps. So it would be fair to expect what, 1.2 in free space?

Power consumed would be around 40% of normal AC. Its 33 lb per HP means it can not fly in free space, but can make forays powered by momentum.

It is very dangerous for these machines in free space, as they have limited or dangerous stall performance, for instance, I beleive ESKA would tip stall, and therefore spin (having no aileron control). Usually machines optimised for GE have pretty terrible free space performance. They nearly all have very low aspect ratios, in order to achieve some usefull speed.

ESKA -1 specification
weight 992lbs
usefull load 485lb
weight efficiency 48.9%
wing area 148 sq ft
Power 30HP, M-63 motorcycle engine
Vmax IGE 75.8 mph
V cruise 68mph
take off speed 34mph
most effective flying altitude 1 to 5 ft

As this machine is a flying boat, there are variants that are land based machines. That have the advantage of not requiring much of the weight and regalia for landing on water. weight efficiencies for such machines are in the order of 60%.

seen below, an Orlyonok pilot dips a wing in the sea to assist turning