Gyroplane vs. Altitude-Takeoff and landing

Vance

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The following is my opinion based on my experience flying and instructing at airports where the density altitude is over 5,000 feet to as high as 9,800 feet density altitude.

I don’t change approach or climb out indicated air speeds despite having a higher true air speed at 5,000 feet density altitude for a given indicated air speed.

I keep this ground speed difference in mind that is roughly 2% per 1,000 feet of density altitude because it changes the sight picture so at 5,000 feet density altitude the ground speed is ten percent faster at the same indicated air speed.

What this means to me is; if a client has the nose too high during the takeoff roll and we lift off early we may have trouble getting away from the ground. If they don’t have full power in on lift off it exacerbates the challenge.

Based on most gyroplane performance charts my takeoff distance to clear a 50 foot obstacle is typically increased by 60% at 5,000 feet density altitude in most of the gyroplanes I fly.

My rule of thumb from the Mountain Flying Bible is 12% increased takeoff distance per thousand feet of density altitude.

In other words to takeoff and climb over a fifty foot obstacle at 8,333 feet density altitude will require double the distance it does at sea level and standard temperatures.

My impression is my cyclic control inputs are larger and more aggressive. I have no data to quantify that and it may be my response to the sight picture of the faster ground speed.
 

XXavier

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I don't believe that there may be any change of the cyclic control. At altitude, with a lower density, the rotor spins faster, but the magnitude of the rotor thrust vector stays constant, and tilting it with the stick works the same way...
 

Vance

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My observation is I need a more aggressive final flare in The Predator when landing at altitude Javier.

I noticed it less in Cavalons but I still felt I was being more aggressive to get the same response.

As I wrote; it is just my impression and I have no data to back it up.

I don’t have enough experience with other gyroplanes to notice a difference between sea level and 5,000 foot density altitude.

I am not usually flying the same gyroplane at both altitudes.
 
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XXavier

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It sounds right to me. Landing with a lower air density means a higher ground speed, so that, in the flare, you need to tilt back the disk more strongly than when landing with a higher air density, i.e. at a lower ground speed...
 

WaspAir

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If you are following the same glide slope, the density altitude will increase your true rate of descent as well, which means a higher sink rate to arrest.
 

XXavier

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If you are following the same glide slope, the density altitude will increase your true rate of descent as well, which means a higher sink rate to arrest.

I feel confused now... It's true that, for the same glide slope, as you have a higher horizontal speed due to the lower air density, the sink speed will be higher too. But, given that higher horizontal speed, the same disk tilt (as when landing at a lower altitude) should be enough to arrest the higher sink speed...

Is it so...?
 

Vance

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If you are following the same glide slope, the density altitude will increase your true rate of descent as well, which means a higher sink rate to arrest.
This appears to me to be the seed of an epiphany.

It appears my thought process has been muddled about landings at high density altitude.

The Predator is so forgiving my landings were all satisfactory.

The Cavalons were less forgiving but I managed anyway.

I knew what to do to make the landing work; I just didn’t understand why.

I had some recent experience where the landing process seemed more difficult to teach than it should have been and part of why I wrote this was to address my confusion.

I have some thinking to do to align my experience with this elevated understanding.

I am beginning to feel confused on a higher level.

I feel this is time well spent.

Thank you J.R. Stark, you are the flight instructor I aspire to be and a friend I value.
 

Vance

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I feel confused now... It's true that, for the same glide slope, as you have a higher horizontal speed due to the lower air density, the sink speed will be higher too. But, given that higher horizontal speed, the same disk tilt (as when landing at a lower altitude) should be enough to arrest the higher sink speed...

Is it so...?
I am sorry, I missed your question. If I am understanding your question and J.R.'s post the higher rate of descent requires a more aggressive flare to arrest the descent. A lot like you need a more aggressive flare in a power off landing compared to a power on landing because you have a higher rate of descent.
 

ventana7

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The following is my opinion based on my experience flying and instructing at airports where the density altitude is over 5,000 feet to as high as 9,800 feet density altitude.

My impression is my cyclic control inputs are larger and more aggressive. I have no data to quantify that and it may be my response to the sight picture of the faster ground speed.
Vance
As you know the majority of my flying is at DAs from 8,000 to 12,000 feet. I don't really know how to describe what I feel in the cyclic but I describe it as the rotors not having as much "grip" on the air. Much like driving on snow or ice or even a wet road, you just don't have as much grip.

I don't know what the physics are.

Rob
 

DavePA11

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So Rob, does that mean the gyro will decend faster at the high altitudes and you have to take a less steep approach while landing since the flare "grips" less? I have only flown gyro at sea level, and never climbed higher than 4,000ft MSL since it was too cold up there flying bare foot in summer. Forget winter since way to cold to fly higher than 1000' AGL.
 

WaspAir

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Wings (whether rotary or fixed) fly according to the indicated airspeed they encounter (we could get picky and talk about calibrated speeds and so forth, but those subtleties aren't critical here). Rotor rpm is a true rotational speed (350 turns per minute is still 350 turns per minute whether at sea level or FL250 because of the way we define and measure it).

In the case of a spinning rotor in autorotation, a stable autorotation rate comes about from balancing of the contributions of driving and driven regions without pilot intervention, and those contributions depend upon the indicated airspeed encountered at those regions. You should expect to see higher rpm on your rotor tach when flying with lower air density, even if you do nothing different in your pilot technique, because greater true airspeed is necessary to get the same indicated airspeeds on the blades and to yield that stable autorotation rate. Fortunately, gyros can do this automatically without trouble with a free spinning rotor (unfortunately, helicopter rotor rpm in powered flight can be limited when they are up in thin air because of powerplant and transmission speed limit issues, so be glad you have a gyro.)

With respect to the ground for landing, you can get the same glide angles at the same indicated airspeeds at high altitude but you will follow that glide path at a quicker true speed, both vertically and horizontally. Think of it as rolling down a ramp; the angle of the ramp doesn't change at high altitude, but the true speed at which you move along the ramp goes up (while indicated airspeed is the same). You will both go forward faster and sink faster, but that doesn't mean you should change the angle of your "ramp". Choose the angle as you would at sea level, for terrain clearance, glide range in case of engine failure, fitting into the traffic pattern, and so on, flying at the indicated airspeeds to which you are already accustomed, but be prepared to adjust when and how you flare to accommodate the greater true sink rate you are carrying.

We are not talking about enormous differences here; a sink rate 10% higher is definitely noticeable but probably not catastrophically terrifying.
 

Vance

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The situation that prompted this thread was giving transition training to a couple who had trained in an MTO in Texas with one of the flight instructors I recommend, had purchased a Calidus and they were both having a challenge with taking off and landing from their field at 4,200 feet.

From the first flight it was evident the takeoff roll was longish but with 4,600 feet of runway available that wasn’t a problem. There were some bumps in the runway that were disquieting until take off was complete and I felt that required getting full power in sooner than in Texas.

Both had a tendency to land a little fast and this made some of the landings untidy.

A friend of theirs with a Calidus came up from Texas and had some less than elegant landings prompting a desire for transition training and someone to test fly the Calidus.

In my opinion Calidus flew very well and was well put together.

Because the Calidus is so slippery and the pilot is isolated from the wind we needed to get a little technical about the differences.

I was successful with the more experienced pilot and less successful with the other.

I felt like I wasn’t explaining the effect of the altitude well and thought about it at length on my drive home.

I find if I write things down it often clarifies things in my mind.

With the help of my friends on the Rotary Wing Forum I feel I may become a better flight instructor.

Thank you.
 
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