Height vs. Velocity diagram explained

Vance

Gyroplane CFI
Staff member
Gyroplane height vs. velocity curve.

Before a client begins flight instruction with me we cover the forty pages in the Rotorcraft Flying Handbook that involve rotor aerodynamics, the nomenclature of a gyroplane, basic maneuvers and emergency procedures. https://www.ronsgyros.com/Gyro_Handbook.pdf

It is not unusual for a client to ask me to show them how to do some gyroplane maneuver they have read about on the internet.

A vertical descent is one of those and often the particulars of what is practical and safe are exaggerated on the internet.

I carefully explain that a vertical descent is safe as long as you respect the Height vs. Velocity curve that we saw in the Rotorcraft Flying Handbook. It is a visual representation of how much altitude we need for a successful landing when the engine goes quiet.

At zero indicated air speed even at full power I will descend and without power I will likely be descending at something over 1,000 feet per minute or around twelve miles per hour. Most gyroplanes will require repairs if I hit the ground with a vertical speed of twelve miles per hour so to avoid repairs I need to use my approach speed to arrest the descent with the goal being to run out of altitude just before I run out of airspeed.

The FAA calls this a flare and it is begun fifteen to twenty feet above the ground and involves bringing the cyclic gently rearward to slow down and arrest the descent. Because of the number of gyroplane landing accidents the FAA is very focused on this approach speed and has a practical test standard of plus or minus five miles per hour of what is listed for approach speed in the Pilot’s Operating Handbook for that particular gyroplane.

Most people underestimate the altitude lost accelerating back up to the approach speed so we practice this at a safe altitude.

According to the chart at zero indicated air speed if my engine goes quiet I had better have three hundred feet of altitude to build back up to my approach speed before I hit the ground hard.

The chart also represents that if I have thirty five miles per hour indicated air speed I would like to have one hundred feet of altitude if my engine goes quiet in order to build back up to my approach speed.

The chart also represents what is an appropriate takeoff curve.

The chart is drawn for pilots with average skills in a typical gyroplane and some people feel they are superior pilots flying an exceptional gyroplane.

There is likely a Height vs. Velocity chart in the pilots operating handbook for your particular gyroplane.

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WaspAir

Supreme Allied Gyro CFI
The chart also represents what is an appropriate takeoff curve.
This aspect requires careful interpretation, because the dimensions on the chart are unusual. The horizontal axis represents speed, not distance, so plotting a safe climb-out on it does NOT show you a path over the ground.

If both axes were in feet, you would see a climb profile as expected when watching a take-off. This chart, however, shows something very different, determined not by flight path, but by the way you accelerate as you climb.

Likewise, it does not show climb rate or height vs. time. If you were, for example, to hold ten feet and thirty knots for an hour or two, it would appear as just a single dot on this graph, not a line or a curve, even though you might have made it into the next county by then.

I find it is one of the most easily misread things in a flight student's studies.

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All_In

Gold Supporter
Very interesting we created our own HV curve before we actually practiced our first vertical descent.
Henry's procedure was to flight test at altitude only one data point of AG915 Centurion HV curve = 20MPH.

That is the only airspeed I've been flying and I have doubled the tested data point = height above the ground so I'm at 100 feet AGL when I pull up to 65MPH.
Even if I was at 10 feet above the runway at 65MPH on it is a very sedate landing with 100 AGL it's almost like you did not do a vertical descent when you are over a runway or taxiway.

What is the danger in practicing vertical descents at twice the aircraft's demonstrable HV curve height?
Is the only danger hitting the ground hard? Or is there some unknown danger, to me, that I'm not taking into account?

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Vance

Gyroplane CFI
Staff member
Very interesting we created our own HV curve before we actually practiced our first vertical descent.
Henry's procedure was to flight test at altitude only one data point of AG915 Centurion HV curve = 20MPH.

That is the only airspeed I've been flying and I have doubled the tested data point = height above the ground so I'm at 100 feet AGL when I pull up to 65MPH.
Even if I was at 10 feet above the runway at 65MPH on it is a very sedate landing with 100 AGL it's almost like you did not do a vertical descent when you are over a runway or taxiway.

What is the danger in practicing vertical descents at twice the aircraft's demonstrable HV curve height?
Is the only danger hitting the ground hard? Or is there some unknown danger, to me, that I'm not taking into account?
I feel as usual much of our communication challenge is semantics John.

The definition of a vertical descent I am using is a vertical descent at zero airspeed.

In my lexicon a power off descent with 20 miles per hour of indicated airspeed is low airspeed with a high rate of descent.

The value I find in the height vs. velocity chart is to show what effect airspeed and altitude have on my chances of a successful landing with various combinations of airspeed and altitude.

The slower I am going the more altitude I need to have a successful power off landing.

I need to accelerate from whatever low airspeed I am going to my approach speed before it is time to flare. The chart shows that I need about 185 feet of altitude to accelerate from 20 miles per hour to approach speed and flare for a normal power off landing.

There is some ability to flare more aggressively to save a flare and landing from less than optimal airspeed. The risk of things not working out is increased and that is a part of why the FAA practical test standard for approach speed in a gyroplane is plus or minus five mph.

In my opinion that the effectiveness of the horizontal stabilizer has a squared relation to velocity. In this example at 20 mph the horizontal stabilizer has less than a tenth of the control effect that it does at 65 miles per hour.

In my opinion there is always a danger is lowering the nose aggressively at low indicated airspeeds because I am unloading the rotor and it may not work as expected when I call on it to arrest my descent.

WaspAir

Supreme Allied Gyro CFI
One data point cannot possibly establish a curve; it's a mathematical impossibility, as there are infinitely many curves that pass through that one point.

If you are at 20 mph you are by definition not in a vertical descent. That would require zero airspeed.

I have substantial scepticism that you can reliably kill the engine while descending at zero airspeed and 100 feet, accelerate to approach speed, arrest the vertical descent rate with a cyclic flare, and touch down comfortably.

What is your vertical speed in a zero airspeed descent as you pass through 100 feet? For many rotorcraft, it would be in excess of 1000 fpm, which is rather a lot to arrest in only 100 feet with no forward speed.

I do not understand how you would "pull up to 65 mph" at any part of this process.

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All_In

Gold Supporter
One data point cannot possibly establish a curve; it's a mathematical impossibility, as there are infinitely many curves that pass through that one point.

If you are at 20 mph you are by definition not in a vertical descent. That would require zero airspeed.

I have substantial scepticism that you can reliably kill the engine while descending at zero airspeed and 100 feet, accelerate to approach speed, arrest the vertical descent rate with a cyclic flare, and touch down comfortably.

What is your vertical speed in a zero airspeed descent as you pass through 100 feet? For many rotorcraft, it would be in excess of 1000 fpm, which is rather a lot to arrest in only 100 feet with no forward speed.

I do not understand how you would "pull up tp 65 mph" at any part of this process.
I have no idea what my vertical airspeed would be at zero forward airspeed.
I've never done one at zero.
Nor can I think of why I would ever need, in an emergency, to do a zero forward airspeed descent when I'm above the HV curve.

BEN S

Super Member
Your at El Mirage in a borrowed Gyro at 150 feet, winds are about 35 mph and you took off directly into the wind. The engine goes cold iron and in the 5 to 10 seconds of time you have you have to realize the issue, make a determination on a course of action and implement it as the ground is coming up at you pretty damn fast, at this point you realize with your upward attitude the wind is now actually pushing you backwards a bit and you must throw the nose down to pick up enough speed to counter the wind so you don't land rolling backwards. You will be in effect coming straight down on a point.

mark biddle

Newbie
Need to take extra care when doing low forward speed vertical descents with a say 10 knot or more head wind because as you nose over the wind can momentarily be on top of the rotor and slow it some thus you may need a bit more height to build the rotor speed. Never thought about it until my instructor pointed it out.

Tyger

Super Member
I have no idea what my vertical airspeed would be at zero forward airspeed.
I've never done one at zero.
I would think you'd want to know that data. Determining it probably ought to be part of your phase-one testing...

Abid

AR-1 gyro manufacturer
I have no idea what my vertical airspeed would be at zero forward airspeed.
I've never done one at zero.
Nor can I think of why I would ever need, in an emergency, to do a zero forward airspeed descent when I'm above the HV curve.

The purpose of HV curve is not to tell you to do zero airspeed descents in an emergency.
It’s a curve that tells you at what height are you safe given a certain speed. The speed goes all the way to zero in the curve and the height relates to the relationship with each speed from where the pilot can regain sufficient acceleration to get to proper approach speed to do a safe flare with typical technique.
It guards us from flying too slow too low in case engine quits. I can only think that you would do a true vertical descent (as opposed to high rate of descent that maintains a speed above minimum controllable airspeed with power off - Vmca (power off) ) at zero indicated airspeed in an emergency if you have put yourself in a situation where the best place to land is exactly below you and you are just above your HV curve highest height. You may want to loose those couple of hundred feet doing a vertical descent without giving up position and then nose down (n engine) and regain safe speed to flare and land.

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WaspAir

Supreme Allied Gyro CFI
Need to take extra care when doing low forward speed vertical descents with a say 10 knot or more head wind because as you nose over the wind can momentarily be on top of the rotor and slow it some thus you may need a bit more height to build the rotor speed. Never thought about it until my instructor pointed it out.
There is danger from a rapid forward pitch in that you can create a low g situation and lose some rpm, but the problem is plainly not reversing the airflow direction through the disc with relative wind "on top of the rotor". In a very low airspeed very steep descent the relative wind seen by the disc will be nearly vertically upward. If your instructor thinks you can get flow from above the disc in that flight regime by pointing the nose a bit lower, he has some serious misconceptions about physics and aerodynamics.

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JohnS

Member
I can think of a good use for vertical descent when flying a 2-stoke engine. They are sensitive to jetting and exhaust gas temperatures. Descending from altitude idling the engine with a strong headwind will wind-mill the prop. My gas temps went into alarm when I chopped the throttle so I either had to circle around and start my approach from further away or do a vertical descent. That solved the wind-milling problem.

Only trouble was, I carried the vertical too far into the HV curve, as I was focused on my landing spot. Had to add power and still only got about 30 mph for my landing. It was a firm landing, didn't hurt anything. Gotta train the eyes to judge what 150 feet AGL looks like in a vertical descent. (I'm remembering 150 ft for a light single gyro is minimum altitude from zero air speed, so a safe pattern altitude would be 300 ft.)

chrisk

Gyroplane CFI
I'd like to second what Abid said. The curve tells you the minimum safe speed, at a given height, if your engine quits. This might be useful knowledge should you be tempted to fly at 30 mph and 100 feet above the ground. Like wise, should you decide to climb out or approach to land at a lower speed.

Gyro28866

David McCutchen
In a larger heavier machine. It will require a bit more time to accelerate from "0" to "45" IAS. I would expect your machine at 45 ias with "engine out
" to be descending at +2000 feet per minute. That said, from 500' agl to stopped on the ground talking about it is less than 15 seconds.
Over the past 30 years in gyros, I have had 9 real world engine out dead stick landings. As an ole Mac driver, you get used to it.
get a copy of Chuck's spreadsheet and calculate your "wing loading"; it will shed a little light as to what you might expect for a rate of descent.
Start at 1000' agl 65kph, and pull the throttle. takes "0" altitude to get to a glide speed of 65..
Then at 55kph ias, and pull the throttle, see how much altitude loss to et back to the 65
then at 45 kph,
I think you will discover, at "0" IAS it will require 350-400' of altitude loss to recover the needed velocity to safely land on a runway.

DavePA11

Super Member
Hi Gyro28866 - after flying aircraft without starters I never liked shutting off the engine for dead stick landings. So only pulled to idle to practice. Is there significant more drag with engine out in the gyros from your experience you mentioned. What is different from idle vs engine out?

Aerofoam

Active Member
Typically a spinning, or windmilling prop creates more drag than a stationary prop.
The only exception to this is if the idle thrust speed of the prop matches the forward speed of the aircraft, then there
may be less drag idling, but usually you are looking at the drag from the size of the disk vs. he drag of the static blade.
I have actually had aero engineers argue this point, but it is very easy to demo by handing them a 10" RC prop on a shaft and having them hold it out the window of a moving car......

chrisk

Gyroplane CFI
Hi Gyro28866 - after flying aircraft without starters I never liked shutting off the engine for dead stick landings. So only pulled to idle to practice. Is there significant more drag with engine out in the gyros from your experience you mentioned. What is different from idle vs engine out?
The short answer is yes, the machine will have "less glide" with the engine shut off. The longer answer is it depends on the set up. If you assume a Rotax9xx series engine and an idle on the high side, you will notice a significant difference in glide distance by simply adjusting the idle speed to be a bit lower. I don't see a good reason to shut the engine off for practice landings, so I don't have experience with that.

WaspAir

Supreme Allied Gyro CFI
An actual shutdown makes a simulated emergency into a real one. Best to practice safely, plan conservatively, and don't expect luck to go your way on anything else after luck deserts you in the engine department.

Tyger

Super Member
after flying aircraft without starters I never liked shutting off the engine for dead stick landings. So only pulled to idle to practice. Is there significant more drag with engine out in the gyros from your experience you mentioned. What is different from idle vs engine out?
The CFI who gave me my checkride once had a real engine out in a Magni. He told me he needed to push the stick forward a lot more to regain airspeed than he did when pracising by bringing the engine to idle (Magnis do tend to pitch up when engine thrust is reduced).
You might be surprised by how much thrust an idling engine is actually giving you.

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Jean Claude

Junior Member
This depends mainly on the pitch of the chosen propeller, i.e. the cruising speed.

For a single-seater with a slow speed (small pitch) and Rotax 503/582
Let us assume a two-bladed propeller of 1.65 m diameter and 0.12 m effective chord.
The drag produced by this perpendicular surface at a relative wind of 22 m/s would be about 90 N (½ ρ S Cd V²)
Now, if these blades are rotating at 850 rpm ( with an engine idling at 2200 rpm) then a representative blade element (located at ¾ R) has a circumferential speed of 55 m/s . Its angle of attack would be zero for a pitch of : Atan (22/55) = 22 degrees. This is about 2 degrees more than the pitch angle of this propellers, whose pull would then be about -80 N (½ ρ S CLV²)
Thus, no noticeable change between stopped or idling propeller

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