behind the power curve!

[Redbaron]

at what airspeed are you behind the power curve? my gyrobee will not fly any faster than 40-45 mph and then slightly above that the whole airframe starts hoppin! :wacko: I've done a few engine idle descents from altitude holdin 40-45 mph and its one hell of a descent rate! I get it on the ground nice but I'm worried something ain't right! my buddy said my blades are coned up a good bit I dunno if its too much! I also seem to have a very slow shake in my stick, can anyone help, doug R?:violin:

ok I realize I'm above the power curve, shut up grant! :) I'm just worried about the airframe hoppin and conning angle!

[birdy]

at what airspeed are you behind the power curve?
The lowest AS at which the machine starts to sink for wotever power settn youv got it on.
The frame hoppn is probably resionance caused by off center blades, and the rrpm at anythn over your 45mph is where the resionance starts.

Wot aint rite bout getn it on the ground nice?

If you blades are out of cone, itll shake all the time.

[Redbaron]

hey bird above 45mph it starts to feel like a slight bucking from a horse! :suspicious: I guess you could call it airframe hoppin!

at what airspeed are you behind the power curve?
The lowest AS at which the machine starts to sink for wotever power settn youv got it on.
The frame hoppn is probably resionance caused by off center blades, and the rrpm at anythn over your 45mph is where the resionance starts.

Wot aint rite bout getn it on the ground nice?

If you blades are out of cone, itll shake all the time.

[Passin' Thru]

Jeff, for most gyros of your size and weight the peak of the power curve is between 45 and 50 mph. This is the airspeed at which the gyro will fly straight and level with the LEAST power. Its the point where the rotor has the least drag. Any slower than that requires more power due to the increased rotor drag, any faster than that also requires more power. ( I have a plot somewhere that denonstrates the curve but I can't find it right now)

The term "behind the power curve" generaly means airspeed so slow there is not enough power available to maintain altitude, usually around 20mph, depending on disc loading and power available.
The real problem with that is the more you try to hold it up with the stick, the faster you sink, even with wide open throttle! To regain altitude you must lower the nose (stick forward) to regain airspeed, even if it requires a slight dive.

Are you sure your AS indicator reasonable accurate?
Do you have a rotor tach? What is your rotor speed?

I'm not sure what you mean by a "slow shake". There is a condition where mechanicaly stiff blades are not properly pre coned and bending loads are imposed on the blades, positive or negative, and you get a wierd kind of wobble, sort of like toy spinning top slowing down. (There is a name for it but I can't remember it right now)

Blades out of track can shake your airframe like a bucking horse if sever enough. Usualy out of track is felt as "butt thimping".
I would reccomend you have someone with more experience check over your machine and perhaps give it a test flight. You may be surprised at what you learn that could improve your flying experience and safety. :yo:

[Redbaron]

hey thanks pete, believe it or not but I understand flying above the power curve! :p 40-45mph seems to be my top speed pete, 9-10 mph is the slowest! :p If gyroron flew my bee he would probally say this is the easiest bee I have ever flew! :)


Jeff, for most gyros of your size and weight the peak of the power curve is between 45 and 50 mph. This is the airspeed at which the gyro will fly straight and level with the LEAST power. Its the point where the rotor has the least drag. Any slower than that requires more power due to the increased rotor drag, any faster than that also requires more power. ( I have a plot somewhere that denonstrates the curve but I can't find it right now)

The term "behind the power curve" generaly means airspeed so slow there is not enough power available to maintain altitude, usually around 20mph, depending on disc loading and power available.
The real problem with that is the more you try to hold it up with the stick, the faster you sink, even with wide open throttle! To regain altitude you must lower the nose (stick forward) to regain airspeed, even if it requires a slight dive.

Are you sure your AS indicator reasonable accurate?
Do you have a rotor tach? What is your rotor speed?

I'm not sure what you mean by a "slow shake". There is a condition where mechanicaly stiff blades are not properly pre coned and bending loads are imposed on the blades, positive or negative, and you get a wierd kind of wobble, sort of like toy spinning top slowing down. (There is a name for it but I can't remember it right now)

Blades out of track can shake your airframe like a bucking horse if sever enough. Usualy out of track is felt as "butt thimping".
I would reccomend you have someone with more experience check over your machine and perhaps give it a test flight. You may be surprised at what you learn that could improve your flying experience and safety. :yo:

[Gyro28866]

Redbaron
On my first gyro, I pitched the blades to 2 degrees (I thought) and tried to fly, and above 40 - 45 the bucking, as you call it, was so severe that my but was coming completely off the seat, I was told to re-pitch using shims, out of beer cans, double the thickness, and get 6 thousands, when I did this anything above 35mph would create a hop severe enough to bring me completely off the seat. I do not know what the pitch really was, but it was excessive. Oh yeah, this was on a set of Benson blades.
What I found out was I had to make a set of tracking bars, and I pitched the blades at 1 1/2 degrees, some fine tuning to track the blades, and it is smooth as silk.
What was happening, was the Rotor RPM was toooooooooo slow, and the coning angle become excessive due to a lack of centrifical force. As I understand this now, the danger of a slow turning rotor and large coning angle can cause a blade to fail in flight. needless to say the end results would not be pretty.
I did not use a rotor tach back then, so I do not know what the rrpm was, but visually it was a lot slower.
I created a tach out of a schwinn speedometer for less than 20 bucks and the rotor is turning at + or - 400 in straight and level at 45mph for me now, and is very smooth.

I think I would look at pitch first and then tracking.
Maybe, some of the experts here can chime in.
Thanks

[Doug Riley]

Behind-the-power-curve means flying slowly enough that the aircraft will not climb (or will barely climb) when power is applied. It does not mean flying so slowly that the aircraft will not maintain level flight -- that's something different. On most small gyros, a behind-the-power-curve state can occur anywhere between 40 mph and 30 mph, depending on prop pitch and engine power. Generally, the lower the engine power, the higher the speed at which behind-the-curve will happen.

A light gyro should have almost zero rotor vibe. Tuning a rotor is a methodical procedure that's been outlined countless times in the literature. I won't re-type any of those articles.

Briefly, "slow" rotor vibes (or hops, if you like) are one-per-rev. Most rotor out-of-tune conditions are one/rev. It can result from out-of pattern ("string"), out-of-balance (chordwise or spanwise) or out-of-track. These adjustments spill over into each other, so you may have to run through the series a couple of times to get it dialed in.

It is extremly unlikely that a small-gyro rotor would turn so slowly that it would "not have enough centrifugal force" in the structural sense. We all used to run Bensen blades at 2-plus degrees. It improved their efficiency a bit, compared to the stock 1.5-deg. setting.

If a rotor cones more than 3-4 degrees, it probably will be bending more than the designer intended, though. Coning angle is relatively easy to calculate using simple high-school trig and the formula for centrifugal force. You have to know your RRPM, though.

Unusually high coning angles will require a higher teeter bolt location. If the teeter bolt isn't relocated, the rotor will experience excess 2/rev vibes. All 2-blade rotors have some, but the resulting stick shake should be a half inch in each direction or less. 2/rev doesn't create the "hopping" sensation.

All this stuff can be looked up pretty easily.

[Redbaron]

hey doug, my stick moves like I'm churning butter in slow motion if that makes sence, in maybe a 1 inch circle, but only at faster speeds about 45-50mph! at real slow speeds behind the power curve there is no stick motion, but I do have some slop in the controls! thanks for the help


Behind-the-power-curve means flying slowly enough that the aircraft will not climb (or will barely climb) when power is applied. It does not mean flying so slowly that the aircraft will not maintain level flight -- that's something different. On most small gyros, a behind-the-power-curve state can occur anywhere between 40 mph and 30 mph, depending on prop pitch and engine power. Generally, the lower the engine power, the higher the speed at which behind-the-curve will happen.

A light gyro should have almost zero rotor vibe. Tuning a rotor is a methodical procedure that's been outlined countless times in the literature. I won't re-type any of those articles.

Briefly, "slow" rotor vibes (or hops, if you like) are one-per-rev. Most rotor out-of-tune conditions are one/rev. It can result from out-of pattern ("string"), out-of-balance (chordwise or spanwise) or out-of-track. These adjustments spill over into each other, so you may have to run through the series a couple of times to get it dialed in.

It is extremly unlikely that a small-gyro rotor would turn so slowly that it would "not have enough centrifugal force" in the structural sense. We all used to run Bensen blades at 2-plus degrees. It improved their efficiency a bit, compared to the stock 1.5-deg. setting.

If a rotor cones more than 3-4 degrees, it probably will be bending more than the designer intended, though. Coning angle is relatively easy to calculate using simple high-school trig and the formula for centrifugal force. You have to know your RRPM, though.

Unusually high coning angles will require a higher teeter bolt location. If the teeter bolt isn't relocated, the rotor will experience excess 2/rev vibes. All 2-blade rotors have some, but the resulting stick shake should be a half inch in each direction or less. 2/rev doesn't create the "hopping" sensation.

All this stuff can be looked up pretty easily.

[Doug Riley]

Sounds like one/rev. These vibes aren't normally airspeed-sensitive, but slop in the head or stick assembly might allow you to feel them more at one speed or another.

Check spanwise (seesaw) balance, string and tracking. If you adjust any one of them, fly before changing anything else, to see if there's an improvement.

Do chordwise balance last, because it's trial-and-error and can be a pain. McCutchen blades have a handy theaded adjuster; most others rely on beercan shim washers placed under the lips of the teeter "tophat" bushings. It's very possible to go the wrong way on this and make things worse, so proceed in small increments.

There should not be a lot of end play on the teeter bolt-bushing assembly -- just a few thousandths to make room for the grease. A lot of play allows the rotor to slip back and forth chordwise along the teeter bolt and can create an intermittent vibe. If there's too much play, add beercan shims in equal numbers to each side. Then move one or more from one side to another as needed to adjust chordwise balance.

Two/rev is airspeed sensitive, but it's a true vibration, not a stirring-the-cake stick motion. Fixing it requires moving the teeter bolt hole up or down in both the teeter block and the towers by the same amount (don't move the hole in only one part, or you'll mess up the teeter stops). This adjustment requires a machine shop to rework the parts accurately.

BTW, if your gyro truly won't go any faster than 45-50, you probably have too much pitch in your blades. That can create excess flapping that could contribute to 2/rev.

[Trez]

Behind-the-power-curve means flying slowly enough that the aircraft will not climb (or will barely climb) when power is applied. It does not mean flying so slowly that the aircraft will not maintain level flight -- that's something different.

***************

Actually Doug, the gyro community seems to use the term "behind the power curve" differently than all the other types of aviation out there.

BTPC is a term that describes a regime of flight in which the addition of engine power (straight and level and climb) not only will not increase airspeed or allow climb, but in severe enough conditions will cause further decrease in airspeed.

IOW, it is a condition in which power alone will not allow an increase in airspeed (straight and level) and only lowering the nose (losing altitude) will result in increased airspeed.

As I have always understood the concept, trading altitude for airspeed is the ONLY recovery! Dramatic, HUH/!!

It's not an easy concept to digest unless you look at a graph (the curve) that depicts this phenomenon. Seems to be clearer in that form.

Not to stir the pot too much, but another term that seems to be erroneously applied to gyros is "ground effect". .....:drama:

OK come at me!! .....:eek:

I'm sure there are some who would disagree with me.

Trez

******************

[Doug Riley]

We probably do use the term differently. We have a vast airspeed range below our best rate/angle-of-climb speeds. In that range, controlled flight is possible (right down to zero mph, in fact), but climb is not available without increasing airspeed. In contrast, a simple FW plane with no flaps has a much smaller range of speeds below best rate/angle-of-climb, before it stalls.

Aerodynamically, ground effect kicks in when the ground begins to block the circulation of wingtip vortices. Gyro rotors do have tip (OK, edge-of-disk) vortices. I imagine (though I haven't seen it made visible with smoke/dust) that the distinct ground cushion we feel is caused by roughly the same phenomenon as in a FW.

The two effects interact near the ground. A gyro will commonly lift off at an airspeed that will not allow any climb at all. Ground effect gives us enough of an efficiency boost that we will pick up airspeed with time, however. Once we hit 40, 45 or whatever, the gyro will begin to climb. Many gyros will require gradual FORWARD movement of the joystick during this interval; in that sense, this BTPC speed range is a region of control reversal (you must push forward to climb). OTOH, if you think of the stick as an airspeed control instead of a climb-dive control, these stick movements will seem less odd.

If flying straight and level at a BTPC airspeed far from the ground, airspeed might not increase even at WOT, and instead a dive might be required to get "over the hump" and climb. Initiating this dive will, again require forward stick.

[barnstorm2]

Barron, what did your instructor say about this question?

Perhaps he/she should take a look at your gyro.

Typically, instructors can and will verify your aircraft is properly setup and not dangerously out of trim before giving you a solo sign-off.

.

[dragonflyerthom]

Red

An Instructor will also expose you to the many different problems you might encounter including behind the power curve before you will solo.

He will check your gyro out pointing out little things that can become big things. This includes the correct angle of your blades.

good Luck

[barnstorm2]

Red

An Instructor will also expose you to the many different problems you might encounter including behind the power curve before you will solo.



Quite correct.

No instructor that I know of would let a student solo without first educating on and also demonstrating being behind the power curve, it's dangers and recovery.


.

[Gyro28866]

Red
what blades are you using???

[GyroDoug]

***************
BTPC is a term that describes a regime of flight in which the addition of engine power (straight and level and climb) not only will not increase airspeed or allow climb, but in severe enough conditions will cause further decrease in airspeed. Trez
******************

Trez,

I am certainly no expert in this matter and am simply making an observation here. I have often seen a Gyroplane flying very low and very slow with a very nose high attitude. (often in the 5 to 10 mph range) I have always thought that was flying BTPC. However, they have always been able to lower the nose, which increases the speed and eventually allows them to start climbing. Also when someone is trying to get off the ground in as short a distance as possible and they force their machine off before it is really ready to fly, they are described as taking off BTPC. However in both of these situations, they are still able to gain speed by lowering the nose, but not trading any altitude (because they have none).

So my question is: are these situations I am describing not really flying BTPC? because they don't fit the definition above.

[Fl90]

Well, there's the back side of the power curve and behind the power curve.

My opinion only; behind is where you trade altitude for speed to climb again and the back side is where you need more power than MPRS to maintain altitude.

I'm on the back side often and behind once in a while.

[Fl90]

I should have completed the back side definition with:

able to add power to climb, or forward stick for speed (without loss of altitude).

[Redbaron]

rotor hawks, the riveted kind! I always fly high and hot, hehe, just like the fixed wingers! :)


Red
what blades are you using???

[Doug Riley]

Terry, remember that BTPC speeds normally are quoted for flight out of ground effect. On takeoff, you may be behind, but, thanks to the added efficiency in ground effect, you can power your way to a faster speed and eventual climb.

In a low-powered machine like a 447, you may have a long wait, though. My 447 Gyrobee will wallow off at about 17 mph, but you'll be too old to be flying by the time it gets up to climb speed (35+). It's more efficient to put the nosewheel down and pick up some speed, then give it gentle back stick to lift off in the high 20's. Still behind, but not by so much.

Many high-powered gyros (which means a lot of them these days) have decent climb potential even way below best rate/angle of climb speeds. These gyros can be flown out of ground effect at speeds that traditionally would have been considered BTPC, and still chew out of it. This kind of flying can be risky, though, no matter how much power you have, as you're depending absolutely on the continued noise. If the engine quits and you are at, say, 50 feet at 20 mph, you may not be able to regain enough airspeed during your glide to avoid a no-flare smackdown.

There's some discussion of this, with pics and graphs, in the FAA Rotorcraft Flying handbook.

[Resasi]

Doug I was inquiring some time ago about the downside of having lots of power, and have come to understand that that was certainly one of them. The ability to power off using massive thrust, but then being in a situation that depended upon there being no engine failure.

More power available simply increasing the area of the dead man's curve. OK if one realizes that and either stays within the parameters, or accepts the possible consequences.

[GyroDoug]

So having a Gyro like the Butterfly Aurora would give you the power (with a 100 HP Rotax 912) and with the G-Force landing gear that can take the no flare landing without any damage, the added safety to be able to cover a wider spectrum of the flight envelope. Sounds like an ultimate combination to me.

[Doug Riley]

And now for a brief commercial message...?

If the G-force gear has been improved from the model I've flown, then I'd answer a cautious "yes." That model, however, did not have a long-stroke nose gear leg. When you first touched down with the mains extended, you had to hold back the stick or the gyro would tip forward onto the (short) nosewheel. This could result in a capsize forward and to one side. Trouble is, holding the stick back after landing is risky in breezy conditions; standard procedure is to get it full forward immediately upon touchdown -- which you couldn't do on this particular Butterfly because of the short nose gear. You had to wait for the mains to settle in.

In the ol' days, the Cierva-style machines had stop-and-drop landing gear, but they were tail draggers. There was no need for a third, long-stroke gear leg.

Also, if the machine has some forward ground speed and lands on rough stuff, I imagine it will still trip-and-tip, even with long-stroke gear. To use this gear successfully, you must haul back on the stick and come in with very little foward ground speed.

So BSPC still has relevance.

[Resasi]

Along with training, knowledge, skill and airmanship.:)

[GyroDoug]

Also, if the machine has some forward ground speed and lands on rough stuff, I imagine it will still trip-and-tip, even with long-stroke gear. To use this gear successfully, you must haul back on the stick and come in with very little foward ground speed. So BSPC still has relevance.

Doug,

I would not presume to understand this better than you do as you have lots of experience with many different Gyroplanes and at least one Butterfly, but watching this video, it sure doesn't seem to me that there is any problem at all with bleeding off the airspeed and dropping in vertically with very little or no speed. even the engine out landings coming in at 25 mph with no flair don't seem to be a problem at all. I do see him landing on the mains and holding the nose up for about 1 second before he drops it. Are you saying that would be dangerous in a typical crosswind situation?

YouTube - Testing the G-Force Landing Gear. (http://www.youtube.com/watch?v=ajM3yOojPbg)

[Doug Riley]

Doug: Yes, holding the nose off once the mains touch is risky in a crosswind, in fact in any gusty wind. You should get the stick forward -- to the stop --ASAP in these conditions. You're vulnerable to being popped back up, or rolled by the crosswind, as long as you linger with the stick back. Yet, that's what's required if you must first wait for the gear to settle all the way in.

This "hold back 'til you settle" technique is necessary because there's no long-stroke nose leg on the gyros in the video, just as there isn't on my student's Butterfly that I've flown. I thought I heard that Larry was going to add one...?

Certainly, if you slow down in the air and drop in with close to zero airspeed, the gear will allow you to drop quite a ways onto unprepared (level!) surfaces and still not bend your gyro. However, my point was that the gear does not make low-level BTPC flight routinely safe. You may not have time, or the presence of mind, to slow down to zero. In that case, if the surface is unprepared, the wheels may catch and tip you, even though the gear won't bend from the landing itself.

All bets are also off if the surface has much slope. The long gear actually will make it somewhat more likely that you'll tip over.

Don't get me wrong -- stop-n-drop gear is a fine thing. It restores some of the utility that was left behind when Bensen revived the autogyro but used a rigid axle. It does not quite make low, BTPC a non-issue, though.

[birdy]

All bets are also off if the surface has much slope.
Ditto.
I near craped meself the time i landed a butterfly on uneven ground.
I didnt drop it, i landed like your sposed to. But coz the high wheel, on contact, started to tip the machine over, without enuff weight to activate the gear, the low side had most of the weight over it, and it sank quickly to the low side. Hard fast stick stoped it go'n assup, but not by much.

[EI-GYRO]

I always thought the G-force gear was a solution in search of a problem.

Just learn to land the damn thing. :)