AR-1 N923DJ Texas 15-12-18

Below is AutoGyro's Height-Velocity Diagram for the MTOsport, with a recommended blue dashed line describing precisely
the flat and low roundout/flare that I portrayed in my green line.

One takes risks at 20 feet/30mph IAS and 4 feet/10 mph IAS.

Safe flying,
Kolibri


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AutoGyro H-V diagram.png
 
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The above photos (15 April 2015 Michael Schreier) show ground effect on the right (Druckverteilung um einen Flügel in Bodennähe).

I'm adding to my post #99 and post #101 regarding the increased lift (air pressure underneath the rotor wing) during ground effect.



_____________
Posts by Doug Riley:
. . . yes, a gyro rotor experiences a very distinct ground cushion.
A gyro certainly has ground effect. You can feel it more distinctly by coming in with a little extra airspeed and no power.
Stan, that dovetails with what I tried to impress upon students about the Zen of gyro landings.

Many of them will over-rotate as they come through 10 feet, and then either zoom or pancake in (or both). I tried to get them to feel the effect you are mentioning -- let the ground cushion do some of the work of arresting your descent. Use less back stick than you think you need, and don't bring the nose any higher than level as you pass through ten feet. Work with the machine's natural tendency to level out.

Save the rest of your back-stick until you're a foot off and suitably slowed down.
My own experiences over nearly 40 years of gyo flying is that there certainly is a ground effect -- most apparent in lightly-loaded machines. This effect helps the pilot "round out" at the bottom of a descent. In fact, it will fool a student into popping back up again if he has adequate airspeed on approach and does not anticipate the cushion. My advice to students to overcome this problem has been to use less back stick than you think you'll need to level out.


_____________
I never completely understood the explanations that I've seen for ground effect. Some say the ground is reducing wing-tip vortices, thus improving wing efficiency. Some say the ground is creating a "high pressure cushion", again improving the wing efficiency. In any case, my own experience is that ground effect is noticeable in gyros. I feel it mostly during roundout and flare in a lightly loaded gyro (both single and tandem) using a Dragon Wings rotor. I feel a noticeable change in the descent attitude in the last 10-15 feet - more than what I would have expected for the amount of back stick used for the roundout. To me it feels as if I am hitting a cushion of air, and I have always attributed that feel to ground effect.
Udi 12-13-2012, 09:41 PM

Two phenomena are involved when a wing approaches the ground. Ground effect is one name for both effects which is sometimes confusing. These two phenomena are sometimes referred to as span dominated and chord dominated ground effect. The former results in a reduction of induced drag (D) and the latter in an increase of lift (L). The designations span dominated and chord dominated are related to the the fact that the main parameter in span dominated ground effect is h/b (height/span), whereas in chord dominated ground effect it is h/c (height/chord).

Chord dominated ground effect
As described above, ground effect increases lift. The air cushion is created by high pressure that builds up under the wing when the ground is approached. This is sometimes refered to as ram effect or ram pressure. When the ground distance becomes very small the air can even stagnate under the wing, giving the highest possible pressure, pressure coefficient unity.

The high pressure air cushion can clearly be seen in the illustrations. The pressure around an airfoil has been calculated with and without ground effect, both at a five degree angle of attack. In free air the (2D) lift coefficient was 0.8 and at a ground clearance of 0.05 times the chord it was 1.1. [Kolibri note: This is an increase of 37.5%.] The high pressure at the bottom of the airfoil in ground effect is caused by the ram effect.

The combined result of the two phenomena described above is an overall increase of the ratio between the lift and the drag (L/D). The lift increases when the ground is approached and because of the increasing lift the induced drag may not even decrease in absolute numbers, but even a slight increase still leads to an increased L/D ratio.

Figure 3. shows a computer simulation of a conventional wing profile both in ground effect and free flight.
By comparing the total Cl (Co-efficient of lift) of both, it can be seen that the same wing in Ground Effect has an increase in lift of approximately 75%.
You can also see that the pressure below the wing has increased dramatically, this is called the dynamic air cushion.

There are two aerodynamic changes associated with the ground effect: (i) a reduction of induced drag and (ii) the presence of an effective air cushion. When an aircraft is flying close to the ground surface within a distance of one wingspan, the induced drag experienced by the aircraft is reduced because the vertical component of the airflow around the wing tip is limited, and the trailing wing tip vortices are disrupted by the ground (see Figure 3b). The downwash intensity is therefore reduced leading to a beneficial effect on lift and drag. If the aircraft is flying extremely close to the ground, within roughly 1/4 of the wingspan, the air flow between the wing and the ground is compressed to form an air cushion. The pressure on the lower surface of the wing is increased creating additional lift. Both of these effects lead to an increase in the lift-to-drag ratio (Figure 4).
 
"There are two aerodynamic changes associated with the ground effect: (i) a reduction of induced drag and (ii) the presence of an effective air cushion. When an aircraft is flying close to the ground surface within a distance of one wingspan, the induced drag experienced by the aircraft is reduced because the vertical component of the airflow around the wing tip is limited, and the trailing wing tip vortices are disrupted by the ground (see Figure 3b). The downwash intensity is therefore reduced leading to a beneficial effect on lift and drag. If the aircraft is flying extremely close to the ground, within roughly 1/4 of the wingspan, the air flow between the wing and the ground is compressed to form an air cushion. The pressure on the lower surface of the wing is increased creating additional lift. Both of these effects lead to an increase in the lift-to-drag ratio (Figure 4)."

So - two distinct factors in play: first, a reduction in induced drag, which happens within one wing(rotor) span. Since induced drag is drag created by lift, that seems to mean the drag is reduced FOR THE SAME LIFT, correct? So, no change in LIFT due to this factor (but decreased drag) - correct?

Second factor is the additional lift created by compressing air under the wing - WITHIN 1/4 WING(ROTOR) SPAN. My Magni's rotor is 8.535m in diameter (span). 1/4 of that would be 2.134m. The hub is 2.700m off the ground. That seems to mean, at best, I could get ground effect if my wheels are less than 0.57m (22in) off the ground. But since the rotor is stalling at the hub and the tips are coning, it's even less than that where the rotor is generating lift (i.e. I'd need to be considerably less than 22' off the ground). Is that correct?

From what you've quoted here, it seems like we do indeed get reduced drag and therefore increased lift-to-drag RATIO but the actual increase in lift per se is negligible at best for most gyros - and not present above, say, 18 inches at absolute best for the Magni? Do you read this the same way?

Also not mentioned here, and I'd probably need to do some homework to substantiate this, but I believe whatever lift is actually generated within 1/4 wingspan is inversely proportional to the wing loading. Our rotors have very high wing loading, making the effect on lift even less, I suspect.

No question I feel the benefit of ground effect (and use it) on departure and when "landing long" (I no longer call it "air taxiing") but it appears to be, for all intents and purposes, it comes from decreased drag and not (also) increased lift. By contrast, a low-wing aircraft gets significant benefit from both.

/Ed.
 
Nicely assembled and articulated Ed.

There are some gyroplane pilots that feel there is no ground effect in a gyroplane because the air is going up through the rotor.

There have been some spirited arguments about this.
 
but the actual increase in lift per se is negligible at best for most gyros - and not present above, say, 18 inches at absolute best for the Magni?
Do you read this the same way?
No, EdL, I don't read it that way because the paper didn't claim that increased lift is zero between 25-100% of wing span altitude.
In fact, they confirm a <100% of wing span cushion later on in Section 5.2 for rotary wing aircraft.

Also, the paper is dealing with WIG vehicles which are supported nearly fully by the air cushion, thus their focus on within 25% of wing span.

My personal experience echoes that of Doug's and Udi's and many others, that a discernible "air cushion" exists for gyros very low over the runway.
Try flaring very low and you'll not only feel it, you'll have to reduce AofA in order not to balloon.

I remain baffled at why WaspAir and others deny the existence of IGE increased lift.

Regards,
Kolibri
 
OK, Kolibri - you’re on your own.

As your article noted, there is indeed a ground effect but as it also effectively notes, for gyros it is, for all intents and purposes, because of reduced drag causing an improved RATIO of lift to drag (factor 1) and not increased lift.

“When an honest but mistaken man learns of his error he either ceases to be mistaken - or he ceases to be honest”. I saw that somewhere...
 
As your article noted, there is indeed a ground effect but as it also effectively notes, for gyros it is, for all intents and purposes,
because of reduced drag causing an improved RATIO of lift to drag (factor 1) and not increased lift.
Where does it exclude gyros?
Your summary inaccurately implies that increased lift is an all but negligible portion of the ratio. The paper certainly does not claim such.
Finally, you're ignoring the other papers I quoted.

For gyros in ground effect, increased lift exists from the chord-dominated ground effect, and it's discernible.
I'm hardly alone on this.
Rather, what has been creditably challenged is the below assertion:


1. Ground effect does not provide a "cushion" to a gyroplane. It provides a reduction in induced drag which enhances performance when close to the surface. It is not an upward force to overcome downdrafts, and in no way acts like a spring to resist descent. The benefit you seek from it isn't real. It will not absorb any kinetic energy from dropping. Those who talk about a "cushion" of air don't understand the physics; ignore any sources that put it in such terms. There will simply be no advantage in your procedure from this phenomenon.
 
Did you actually read your post or mine? Yours clearly said the LIFT part from the air cushioning happens within 1/4 of the wingspan and I showed how, on a Magni at least, that’s at best within 22 inches of the ground and considerably less with coning in the area of the rotor making lift.

I also said there is indeed a ground EFFECT and I use it. It’s from decreased drag causing the ratio and therefore benefit of (an unchanged) lift increasing.

So, how does this change life, anyway? That’s rhetorical - don’t answer. It doesn’t.
 
OK - perhaps technically I was wrong in an earlier post; probably more "incomplete" than wrong.

I mused that wing loading was involved in the issue. Your post on this essentially proves this is not completely true; it’s chord, not wing loading.

Chord dominated ground effect
As described above, ground effect increases lift.
The air cushion is created by high pressure that builds up under the wing when the ground is approached. This is sometimes refered to as ram effect or ram pressure. When the ground distance becomes very small the air can even stagnate under the wing, giving the highest possible pressure, pressure coefficient unity.

The high pressure air cushion can clearly be seen in the illustrations. The pressure around an airfoil has been calculated with and without ground effect, both at a five degree angle of attack. In free air the (2D) lift coefficient was 0.8 and at a ground clearance of 0.05 times the chord it was 1.1. [Kolibri note: This is an increase of 37.5%.] The high pressure at the bottom of the airfoil in ground effect is caused by the ram effect.


As noted, the lift-generating second piece of the ground effect is Chord dominated. The chord (not diameter) of my rotor is about 8 inches. Per the above, "...at a ground clearance of 0.05 times the chord..." means we're talking about 0.05 times 8 inches - 0.4 inches!!! The rotor needs to be 0.4 inches from the ground to get the 37.5% increase in lift you noted. At 8 feet - 12 rotor chords - I bet the impact is mathematically at the same impact as "free air". I'd say that most definitely substantiates the "creditably (sic) challenged" assertion you re-quote in #147 above. Also, I suspect the 1/4 of wing diameter cited in the later quote is for wings of far more than 8" chord.

So, not "wing loading" per se but chord length. I stand "corrected" - or "more complete".

Would you now agree your citations support the idea that Ground Effect, while present, is indeed due to reduced drag and not to increased lift for gyros?

/Ed
 
Yours clearly said the LIFT part from the air cushioning happens within 1/4 of the wingspan and I showed how, on a Magni...
I didn't say, and that paper didn't claim, that lift occurs only within 25% of wingspan.
It begins to occur within 100% of wingspan according to the same paper.


Would you now agree your citations support the idea that Ground Effect, while present, is indeed due to reduced drag and not to increased lift for gyros?
No, because multiple aeronautical papers clearly state that when there is ground effect, it's due to two concurrent factors: span-dominated (less induced drag) and chord-dominated (increased lift).
Lessened induced drag doesn't explain all the physics, nor the anecdotal "cushion" evidence of many gyro pilots.

My research folder on Ground Effect is now at 164.5 megs. I've read completely through several technical papers and many articles and forum posts.
You're welcome to read them all for yourself.
I've only quoted excerpts that specifically mentioned an "
air cushion", but much more corroboration is contained within the documents.
I will concede this, however: the general understanding of the increased lift has lagged behind that of lessened induced drag.

Regards,
Kolibri

Kolibri's Ground Effect folder.png
 
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EdL, I don't watch TV and thus cannot decode your twice repeated and likely impolite signoff, nor do I care to.


_______________
If the aircraft is flying extremely close to the ground, within roughly 1/4 of the wingspan, the air flow between the wing and the ground is compressed to form an air cushion.
The pressure on the lower surface of the wing is increased creating additional lift.
Both of these effects lead to an increase in the lift-to-drag ratio (Figure 4).

My RAF with 30' Sport Rotors are 98" above the ground. In percentage terms of wingspan above the ground:


0' - 27.22%
1' - 30.55%
2' - 33.88%
3' - 37.22%

RAF 0-3 feet AGL - ground effect on induced drag.png

Within a wingspan of the surface, ground effect is neither "present" nor "not present" but a continuum.
Increased lift does not instantly manifest at AGL of roughly 25% wingspan, as EdL over-read into the paper,
rather it has become strong enough to be betokened an "
air cushion" by the authors.

Earlier in this thread I made the case for rounding out low during turbulence, at about 6 feet.
When I am below 6.8 feet, I am within 50% of wingspan, and 50% is where ground effect anecdotally seems to become discernible by pilots.
At 1-2 feet I undoubtedly feel the "cushioning" sensation of increased lift.

Although gyros don't see as much ground effect as a low-wing Cirrus or even a high-wing Cessna, they nevertheless do experience it.

Regards,
Kolibri
 
First, the elephant in the room: you chose to resurrect a topic that died over three weeks ago and you did so in two threads - this one and the one about you and Vance "disagreeing". You've apparently collected over 2,000 documents, over 164MEGABYTES worth, on this topic - to apparently try to prove you're right and NOT to actually add meaningfully to gyro flying. And despite all that, you STILL don't see that YOUR OWN DATA does not prove your point. That's not normal, dude.

Look CLOSELY at your graph above. It specifically says decrease in induced drag, NOT increase in lift. Nobody (certainly not I) has disagreed that ground effect is in play for gyros and that it's because of decreased induced drag. What has been disagreed with - and what you STILL have not been able to demonstrate, is that there's a measurable increase in lift FOR A GYRO. Your references talk about a CHORD-based effect on lift. You do know what wing chord is, right? And you do understand it's on the order of 8" for the typical gyro, right? And that 8" is held 8-9 FEET above the ground, right? Which means even when the gyro is fully on the ground that spinning wing is 12-13 TIMES ITS CHORD LENGTH from the ground. By contrast, a Warrior's wing chord is about 5 feet and is about 3 feet off the ground at rest - only 2/3 its chord length. DO YOU SEE THE DIFFERENCE? To say a gyro GETS LIFT from ground effect is EQUIVALENT to saying a Warrior GETS LIFT from ground effect when it's 5x12=SIXTY FEET IN THE AIR. Would you accept the possibility that, at 60 feet in the air, a Warrior is not experiencing meaningful lift from ground effect?

And you realize we're talking about a gyro, where the rotor is always in autorotation from air going UP through the rotor and not a helicopter with air going DOWN from the rotor (and therefore has its own, different contributing factors for ground effect), right?

I will hazard a guess that the reference that said the lift is present generally within 1/4 of wingspan was GENERALIZING about LOW-WING, FIXED WING aircraft for simplicity, since the general public reading such a generalized description would possibly not even know about gyros, let alone care about how ground effect influences them and thinks more of an airliner, whose wing chord would indeed generate lift when within 1/4 wingspan.

So, once again, YES, a gyro experiences ground effect - because of a decrease in induced drag and NOT because of increased lift; at least you have not shown any proof of the latter.

Let's assume - STRICTLY FOR DISCUSSION PURPOSES - there was increased lift. So what?!? Does that change whether or not one uses/accommodates ground effect - or how one does so? No. This seems all about something much more personal for you. That's not normal, dude.

Although you may not watch TV, it does appear you use Wikipedia. Go look it up.

Bye, Felicia

"When an honest but mistaken man learns of his error, he either ceases to be mistaken -- or he ceases to be honest."
 
over 164MEGABYTES worth, on this topic - to apparently try to prove you're right and NOT to actually add meaningfully to gyro flying.
This entire subtopic of increased lift is inherent to my discussion of rounding out low in turbulence to better handle energetic air.
So, yes, I am trying to add meaningfully to gyro flying.



Look CLOSELY at your graph above. It specifically says decrease in induced drag, NOT increase in lift.
EdL, naturally I'm aware of that. I used that graph to place my RAF's rotorspan at 0-3' AGL within a percentage context.
And, as the graph suggests, a reduction of induced drag by only 15-20% isn't enough all by itself to explain the sensation of gyros in ground effect.

Regarding the chord-dominated ground effect, you're applying a gyro's meager 8" chord to the formula as if it were a single fixed-wing.
It's not, as each blade creates lift 5-6 times per second (300-360 rrpm). Obviously, a different formula is required to measured a rotor's increased lift.



And you realize we're talking about a gyro, where the rotor is always in autorotation from air going UP through the rotor
and not a helicopter with air going DOWN from the rotor (and therefore has its own, different contributing factors for ground effect), right?
Uh, no:

If there is no down wash from a gyroplane rotor, what holds us up?
Vance 11-01-2008, 05:38 AM
Rotor downwash velocity is identical between gyro and helicopter if AUW, rotor diameter and airspeed are the same.
C.Beaty 08-12-2015, 08:37 AM
 
Kolibri;n1142597 said:
This entire subtopic of increased lift is inherent to my discussion of rounding out low in turbulence to better handle energetic air.
So, yes, I am trying to add meaningfully to gyro flying.


...but the elephant is still in the room...


EdL, naturally I'm aware of that. I used that graph to place my RAF's rotorspan at 0-3' AGL within a percentage context.
And, as the graph suggests, a reduction of induced drag by only 15-20% isn't enough all by itself to explain the sensation of gyros in ground effect.

Regarding the chord-dominated ground effect, you're applying a gyro's meager 8" chord to the formula as if it were a single fixed-wing.
It's not, as each blade creates lift 5-6 times per second (300-360 rrpm). Obviously, a different formula is required to measured a rotor's increased lift.


Actually, I entered this conversation with the simplistic idea that it did increase lift but all of the evidence you've provided says otherwise, and all of the effect I feel in the gyro (but not in a fixed-wing) is from reduced drag. I'm entirely open to the possibility, for example, that the rotor creates some kind of "virtual wing" with a "virtual chord" based on its diameter, or something, and that that creates lift in ground effect. But you say "Obviously, a different formula is required to measure(d) a rotor's increased lift." Possible but not "obvious" to a mere mortal like me. Please show me the formula from a reputable source and a description of the aerodynamic effect. And nothing I've seen you share suggests anything like my totally-made-up-but-to-me-plausible idea of a virtual wing is actually involved.

But even far more basic than that, the idea that "...each blade creates lift 5-6 times per second..." is, frankly, a stunning example of not understanding what's going on. It's creating lift continuously while spinning but that lift varies 5-6 times a second as the blades rotate, with one advancing and one retreating - unless, of course, one has zero airspeed, in which case it's generating lift more or less symmetrically and like a parachute generates lift. All of that net lift (summed from the stall, driving, and driven regions, in various parts of the rotor over the diameter and depending on the forward velocity) is applied to the rotor hub.


___________________________
"And you realize we're talking about a gyro, where the rotor is always in autorotation from air going UP through the rotor
and not a helicopter with air going DOWN from the rotor (and therefore has its own, different contributing factors for ground effect), right?"

Uh, no:



If there is no down wash from a gyroplane rotor, what holds us up?
Vance 11-01-2008, 05:38 AM




Rotor downwash velocity is identical between gyro and helicopter if AUW, rotor diameter and airspeed are the same.
C.Beaty 08-12-2015, 08:37 AM



Sadly, again, a stunning example of not understanding what's going on. Air flowing up through the rotor drives a part of the rotor which generates lift - but that lift only partially, by itself, offsets the weight of the gyro. It takes forward movement, with a net angle of attack on the rotor, to generate enough lift to get - or even keep - the gyro up. Otherwise one would have a perpetual motion machine and you wouldn't need a pusher or tractor engine. I can't speak for what Vance was responding to (probably something similar to this thread, where I'm sure MY words will later be taken out of context too). So you're effectively saying gyros can truly hover? Please explain. But my point was a helicopter's rotor is POWERED, so there is indeed a net positive downflow of air, which is created by a different mechanism than a gyro's rotor's effect and is subject to ground effect but in ways which I suspect differ aerodynamically from a gyro's.

I can see why people get frustrated conversing with you. And although my "bye, Felicias" indicated I wasn't planning to continue the conversation, your comments here are so profoundly inaccurate regarding gyro aerodynamics that I wanted to make sure any student pilots here don't take them at face value. Heck - for anyone listening, don't take MINE at face value, either: chat with your instructor and make sure you understand what is and isn't going on. I'm very comfortable acknowledging when I'm wrong (as I did with the nosewheel issues in another thread), so if you find something credible along the "virtual chord" idea, then share it. But so far all I've seen you do is misunderstand what you've read in the over 2,000 documents you gathered up to try to make a point you still haven't been able to make.

But, seriously, I see no value in continuing this conversation when it's blatantly apparent you don't want to be "correct" or even improve gyro flying and safety - you just want to prevail. Not my cup of tea.
 
"...each blade creates lift 5-6 times per second..." is, frankly, a stunning example of not understanding what's going on.
It's creating lift continuously while spinning but that lift varies 5-6 times a second as the blades rotate
Goodness grief. Yes, the rotor disk is continuously creating lift . . . but because of blade rotation of 5-6x/second.
I only mentioned that to differentiate its chord vs. a fixed-wing chord.
Applying an 8" chord value to a fixed-wing ground effect formula would be sort of like trying to explain the effect of motion pictures from a single still frame.

A "virtual chord" is nice conversational term, and probably helpful to the discussion.
You ask for a formula of such; I haven't yet seen it mathematically represented.
My impression is that a gyro's ground effect hasn't been exhaustively studied. We are the red-headed step-child of aviation, you know . . .


But my point was a helicopter's rotor is POWERED, so there is indeed a net positive downflow of air,
which is created by a different mechanism than a gyro's rotor's effect and is subject to ground effect but in ways which I suspect differ aerodynamically from a gyro's.
You don't believe that a gyro's rotor exhibits a "net positive downflow of air" and only a helicopter's rotor does?

There are three old threads on the forum about gyro ground effect. I've read them all and saved them to HD.
I recommend them.

Regards,
Kolibri
 
Kolibri;n1142607 said:
You don't believe that a gyro's rotor exhibits a "net positive downflow of air" and only a helicopter's rotor does?

Of course I do - that’s why it flies. But it requires forward velocity to generate ENOUGH lift to do that. Your response implies you believe a gyro can generate enough lift without forward motion to enable it to hover like a helicopter. That would be a perpetual motion machine since the pusher/tractor engine would, by definition, be uninvolved.

Clearly you’re just plain argumentative. Fine. On the “fool vs. knave” spectrum your positions clearly suggest the former. I don’t want to make fun of you for something you apparently can’t help so I’ll stop here.
 
Your response implies you believe a gyro can generate enough lift without forward motion to enable it to hover like a helicopter.
Actually, that is solely your mistaken inference.

Because a gyro creates downwash in flight, it will create additional lift when flying close enough to the ground.
I will entertain any evidence that purports that a gyro rotor in ground effect is somehow immune from doing so.
That would require quite a remarkable aeronautical paper indeed.
 
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