Flying on the back of the curve?

steve5248

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I am in the process of reading Abbots' book on flying gyros and talking with the local PRA guys and there is a concept I'm having a hard time visualizing. Apparently, flying "on the back of the power curve" is somewhat difficult and potentially dangerous, but I can't picture it in my mind. Can someone describe it to me in a way that I can see it?

Steve
 
Yeah, I'll give it a shot. There is a speed -- without going into a lot of math or aerodynamics -- at which an aircraft flies most efficiently. If you fly faster than that, it takes more power to overcome parasite drag. If you fly slower than that, it takes more power to overcome induced drag. Slower -- behind the power curve -- takes more and more power, because the wing (or rotor) becomes less and less efficient at producing lift, and requires more and more power to maintain altitude. (A fixed wing aircraft, if slowed down just a little more, will stall. A rotary wing aircraft, will just settle faster and faster.) You fix this problem by increasing airspeed, but since you're already at full throttle, you can't do it by adding power. The only thing left is to nose over and trade altitude for airspeed. This is no problem unless it occurs with little or no altitude to trade. This is known as running out of altitude, airspeed, ideas, and experience at the same time, and has been known to smart just a little.

Dr. Rob
 
It's actually not that difficult to understand. First, you need to understand that for a given speed you require a certain amount of power to maintain altitude and fly at that speed straight and level. The power you need to maintain altitude is not the same for all speeds. You'll probably agree with me that going very fast requires more power than flying a bit slower.

Turning to the other extreme, you'll probably agree with me that going very slow also requires more power maintaining that altitude than if you went a bit faster.

So we now know that going very slow and very fast requires a lot of power. From this we can deduce that somewhere in the middle there exists a certain speed which requires the least amount of power to fly straight and level. Going either faster or slower will use more power and you have to advance the throttle some to keep the altitude.

When you fly "behind the power curve" you are flying slower than that particular most efficient speed.

What's different between flying the "front" or the "back" of the power curve? Well, let's assume that you're already at max throttle and you want to gain some altitude. If you're flying faster than this most efficient speed, i.e., you're in front of the power curce, then you simple pull back on the stick and up you go.

If you're flying real slow at max throttle and you want to gain some altitude, you can't simply pull back on the stick. That would slow you down even more and require more power to just maintain the altitude. Since you're already maxed out on the throttle, instead of climbing you will descend!

So the real important difference between flying "in front of" or "behind" the power curve is that you can gain altitude by pulling back on the stick in the former case but not in the latter. If you want to climb from a situation far behind the power curve you will have to increase your speed first. Since you cant add more throttle (you're already at max power, remember) you will first have to drop the nose to gain speed and cross over to the front side of the power curve. Then you can gently pull back on the stick and climb away.

-- Chris.
 
From the pilot's viewpoint, flying behind the power curve feels like lugging a low-powered car uphill while stuck in 5th gear. The pedal is to the metal, but you aren't speeding up and you're climbing poorly to boot.


The causes of the two conditons aren't the same, but the limited options to the operator feel similar. In most gyros, you will not encounter this problem as long as you hold airspeed to at least 40-45 mph. In some heavy gyros, this speed might be a little higher.

Recognizing, entering and recovering from "backside" flight is part of a normal flight-training syllabus.
 
It's much easier to understand this if you actually look at the graphic representation of the "power curve." I can't seem to find one that's not copyrighted, or I'd post it here. (If you scroll about 2/3 down this page you'll find a good one. It's labeled "Power vs Speed.")

If you talk to gyro people, many will say that "behind the power curve" means you're at full power and still descending. That's a misunderstanding.

"Behind the power curve" simply means you're flying more slowly than the speed at which minimum power is required to stay straight and level. That's sometimes called "Minimum Required Power Speed," and is often about 45 MPH for a draggy single-place gyro. In such a machine, if you're climbing steeply at 40 MPH, you're behind the power curve...and climbing!

When you see a skilled gyro pilot flying that same draggy single-place the length of the runway with his main gear 6 inches off the surface at 20 MPH, he's flying way behind the power curve, but he's not descending. He would simply need more power to fly any slower without descending. He may or may not have that reserve power. If he does, he could add power and fly at 19 MPH.
 
So, does this mean that a gyro can't safely cruise below 40 MPH? I've always heard that the great strength of the gyro is it's low-speed / low-altitude performance. The ability to cruise along at 20 MPH without worrying about stalling is what has drawn me to gyros. Can a gyro be configured with sufficient reserve power, and a pilot be trained for it, such that flying behind the curve is not "an accident looking for a place to happen"?
 
Ask yourself what happens if you're cruising along at 20 mph & the engine quits - if you have sufficient altitude it's not a problem, if you're at 50' it's not going to be pretty.
 
One of the best descriptions I have encountered was an article Greg Gremminger wrote for Rotorcraft in 2007

Here is a quote from it.

The HV curve is an actual graph (Figure 1) that depicts the minimum combination of height potential energy and speed potential energy required to make a safe landing IF the engine is not available – if the engine QUITS! Or more accurately, it depicts the area of flight, combination of height and velocity, that should be avoided. The HV curve mostly applies if the engine quits. You might not ever get in trouble flying within “Deadman’s Zone,” but should we really take a chance with the engines we fly?

If you want to read the entire article here is the link

http://www.magnigyro.com/features/features.html
 
So, does this mean that a gyro can't safely cruise below 40 MPH? I've always heard that the great strength of the gyro is it's low-speed / low-altitude performance. The ability to cruise along at 20 MPH without worrying about stalling is what has drawn me to gyros. Can a gyro be configured with sufficient reserve power, and a pilot be trained for it, such that flying behind the curve is not "an accident looking for a place to happen"?

Steve,

With sufficient power you could cruise along at 20 mph. It would not be efficient and you would still be flying way behind the power curve which means if the engine goes quiet you aren't going to have many options. You could increase the level of safety by having impact absorbing landing gear like the G-Force Landing Gear available on Butterfly Gyros. But you simply need to understand flying that slow and close to the ground would come with inherent risks. If you are doing this super slow flight from 500 ft altitude or higher, then the risks are greatly reduced. Most gyroplanes need to be able to dive and trade some altitude for additional airspeed so they can then flare and come to a short soft landing. Just dropping out of the sky with no flare will be enough to do major damage if not totally destroy most Gyros.

Even the Butterfly Gyros with G-Force Landing gear hold some forward speed when coming in for a no flare landing. (15 mph is the slowest continual descent with no flare I have seen Larry do) Larry has demonstartated a successful stop and drop landing (with no flare) from about 25 ft but he won't go higher than that, with zero airspeed and expect the ship to fly away untouched. Every kind of flying has it's limitations and Gyros are no exception. However Gyroplanes still can do things other aircraft would not attempt to do.
 
...The HV curve is an actual graph (Figure 1) that depicts the minimum combination of height potential energy and speed potential energy required to make a safe landing IF the engine is not available...

The height/velocity curve is a separate concept. It's important not to confuse it with the power curve.

The H/V curve is a graphical representation of combinations of height above the ground and airspeed which will allow a safe landing in an engine failure. The portion which indicates an insufficient combination of height and airpseed is often called the "dead man's zone."

You can be ahead of, or behind the power curve at any altitude.
 
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(Edit: it seems there are two other people that beat me to my post but since I already typed it, I'll let it stand as it is.)

Steve, I feel you are muddying the waters a bit by introducing yet another curve. The H-V curve, also called height-velocity diagram, tells you something entirely different from the power-velocity curve.

To answer steve5248's question regarding low speed capability of a gyro:

A gyro can fly slower than most fixed wing airplanes. Anything slower than about 40 mph and you'll be slower than the most efficient speed. All this means is that you are burning more fuel to go slower than you would if you just sped up a bit. The danger if you fly very slowly is that you can't make a flare and touch down safely should the engine quit. Since you require power to gain speed yet have none available once the engine turns quiet your only option to gain sufficient speed for landing is to trade some altitude for it. If you're also missing sufficient altitude you'll hit the ground hard.

The HV-curve Steve Greenwell refered to tells you how high you have to fly so that you still have sufficient altitude to exchange for speed in order to make a normal landing.

-- Chris.
 
So, does this mean that a gyro can't safely cruise below 40 MPH? Can a gyro be configured with sufficient reserve power, and a pilot be trained for it, such that flying behind the curve is not "an accident looking for a place to happen"?

Steve, there's nothing unsafe about flying behind the power curve, as long as you have sufficient altitude to allow reaching a safe landing area and flaring in an engine failure. You'd also need some combination of height and airspeed if the engine failed while flying in front of the power curve.

If you're too slow AND too low, that's when the problem happens. That's a height/velocity curve issue, nothing to do with the power curve.

If you want to fly at six feet AGL, you need enough airspeed to accommodate your reaction time and allow execution of a flare, and being in front of the power curve does not guarantee you enough speed. You're in what's called the "dead man's zone" of the H/V curve. There is no way to do that safely unless you have 100% engine reliability, and science isn't there yet.
 
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Thanks for the "High Velocity" article, it does a great job of explaining it. So, it seems that in all of these U-Tube videos I've been watching, where guys do incredibly short takeoffs and landings, they're really gambling. They make it look like it's so simple and safe your grandmother could do it, but in reality if their engine quits, they're dead. What about the videos of the Monarch demonstrating slow speed drop/landings with their G-Force landing gear?
 
If you want to fly at six feet AGL, you need enough airspeed to accommodate your reaction time and allow execution of a flare, and being in front of the power curve does not guarantee you enough speed. You're in what's called the "dead man's zone" of the H/V curve. There is no way to do that safely unless you have 100% engine reliability, and science isn't there yet.

I don't follow this bit; maybe I'm misreading it. It would seem to make a safe take-off profile impossible.

I have never seen an HV curve for any rotorcraft for which minimum power required airspeed at 6 feet AGL is in the tall fat / left-side avoid region. (On helicopters, there is also a long thin flat low altitude / high speed / right-side avoid region, to accommodate reaction and transition to autorotative flight, that is not present for gyros and not pertinent here). If you're on the "front side" at 6 feet, won't you have at least 35 or 40 mph IAS for most designs, which is plenty to flare with, and below the "knee" of the curve?
 
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I'm beginning to lose sight of the benefit of flying a gyro. A Taylor Craft has a stall speed arround 20 MPH, I saw a guy fly one backwards once at an airshow. There was a 25 MPH wind blowing down the runway. He simply nosed into the wind and reduced throttle until he went backwards with the wind. The T-Craft has about an 8 to 1 glide ratio, a gyro has 4 to 1. If both are flying at 50 ft AGL and 30 MPH, what happens when their engines quit? Given a flat patch of ground straight ahead, the T-Craft has a good chance of setting it down safely. The gyro has no chance at all, he just dies. Am I understanding this correctly?

Steve
 
I don't follow this bit; maybe I'm misreading it. It would seem to make a safe take-off profile impossible...If you're on the "front side" at 6 feet, won't you have at least 35 or 40 mph IAS for most designs, which is plenty to flare with, and below the "knee" of the curve?

JR, I'll concede your point, at least as far as the capability of the machine is concerned. The helicopter H/V diagram needs to accommodate time for a transition in collective pitch setting, which the gyro does not, but in both cases the diagrams are about the machine, and do not account for pilot reaction time.

There are lots of accounts of gyro pilots making hard landings and rolling machines when engines quit on takeoff. It may be that none of them were on the front side of the power curve, but I doubt it.

I suspect that most takeoffs in gyroplanes are made behind the power curve, so while a safe takeoff may be possible, it may also be uncommon.
 
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If both are flying at 50 ft AGL and 30 MPH, what happens when their engines quit? Given a flat patch of ground straight ahead, the T-Craft has a good chance of setting it down safely. The gyro has no chance at all, he just dies. Am I understanding this correctly?

In the very limited scenario of an ideal landing area ahead, but outside the gyroplane's glide range, yes - the airplane wins.

But if the landing area is at all rutted, has brush or stumps, etc, the Taylorcraft still has to land at 20 MPH, which will be ugly. Almost any gyro can be plopped down vertically with no forward speed without serious injury to the pilot.

The realistic answer here is that there is no aircraft safe to fly at 50' AGL over stuff you couldn't land on. Helicopter pilots do it because it's their job, but they sometimes also pay the price.
 
I'm beginning to lose sight of the benefit of flying a gyro. A Taylor Craft has a stall speed arround 20 MPH, I saw a guy fly one backwards once at an airshow. There was a 25 MPH wind blowing down the runway. He simply nosed into the wind and reduced throttle until he went backwards with the wind. The T-Craft has about an 8 to 1 glide ratio, a gyro has 4 to 1. If both are flying at 50 ft AGL and 30 MPH, what happens when their engines quit? Given a flat patch of ground straight ahead, the T-Craft has a good chance of setting it down safely. The gyro has no chance at all, he just dies. Am I understanding this correctly?

Steve
I think you'll find the actual IAS stall at 1-g load for a Taylorcraft is more like 38 mph, and certainly not 20. Speeds like that or a little higher are typical for light taildraggers (around 33 knots or 38 mph).
The Taylorcraft simply cannot fly at 50 feet and 30 mph airspeed as used in your example. The gyro can.
The gyro can also go to zero airspeed so long as you have a few hundred feet below you (try that in a Taylorcraft!), and can hold altitude in the 20mph range where the Taylorcraft cannot fly at all.
A Taylorcraft can stall and spin in the pattern and kill you. A gyro cannot stall or spin. A gyro can land in its own length. A Taylorcraft takes quite a bit of room by comparison.

You have to find more creative ways to kill yourself in a gyro!
 
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Quote: "If both are flying at 50 ft AGL and 30 MPH, what happens when their engines quit? Given a flat patch of ground straight ahead, the T-Craft has a good chance of setting it down safely. The gyro has no chance at all, he just dies. Am I understanding this correctly?"

Steve, if you are flying either a T-Craft or a gyro at 50 feet AGL and 30 MPH, you are flying (either one) where you have no business being. With skill and a tad of luck, either of them should be able to land safely. But if you are there, you are either (1)there by choice (probably showing off) or (2) because you don't know any better. If you are there by choice, then you have accepted the risk involved, and have chosen to hang your ole' butt over the edge and hope nothing goes wrong that will keep you from pulling it back. If you are there because of ignorance (poor, or no, training,) this can be fixed by a good instructor. Remember, exceptional pilots exercise exceptional judgement so they don't have to demonstrate exceptional skill.

Dr. Rob
 
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