Best margin above a take-off obstacle

WaspAir

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In a glider, with just a little practice, you easily can tell airspeed accurately by sound alone, but gyros have too much extra noise for that to work as well.

Glider pilots often use an audio variometer (VSI) so that they can fly continuous low speed turns with lots of bank, close to stall, to rise in turbulent thermals, avoiding other gliders circling just above and/or just below them, without looking at the panel. It makes a tone that varies with your rate of climb or descent. After a while you don't exactly hear it -- it becomes more of a subliminal thing so that you're aware of what's happening but not consciously checking or actively listening, while adjusting your turn shape and position to optimize climb rate.
 

ventana7

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In a glider, with just a little practice, you easily can tell airspeed accurately by sound alone, but gyros have too much extra noise for that to work as well.

Glider pilots often use an audio variometer (VSI) so that they can fly continuous low speed turns with lots of bank, close to stall, to rise in turbulent thermals, avoiding other gliders circling just above and/or just below them, without looking at the panel. It makes a tone that varies with your rate of climb or descent. After a while you don't exactly hear it -- it becomes more of a subliminal thing so that you're aware of what's happening but not consciously checking or actively listening, while adjusting your turn shape and position to optimize climb rate.
Audio variometers are also nearly instantaneous unlike a vsi in an airplane or gyro which has considerable lag. in an earlier post I mentioned using a dot on the windshield lined up with the horizon to maintain precise altitude during steep turns. The dot will have far less lag than your VSI, altimeter or the G feeling in the seat of your pants.
Rob
 

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Behind the power curve is defined by a drag greater than the propeller thrust due to a too low forward speed. So it's a downhill flight despite full power.
JC, please do not perpetuate this false definition.

Your gyro has a minimum power-required speed, the speed at which it can fly straight and level with the lowest power and thrust. It is the point at which increasing parasitic drag and declining induced drag from the rotor meet on a graph. If you are flying at this speed, flying any slower will require more power. It is then you are on the "backside of the power curve," or "behind the power curve."

You can be behind the power curve and still be climbing. Getting so slow that full power will not maintain altitude is an extreme case.
 

Jean Claude

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I agree, your definition is correct. In France, aerodynamicists say for that: "second fligt mode" because "first flight mode" is the normal mode of airplanes.
But the gyroplanes is often flying in this second mode, even when cruising. It is not a dangerous criterion for them. The danger comes only when the maximum power can no longer give an ascending flight.
Hence their different definition .
 
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Philbennett

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This is an interesting topic and one I’ve debated here in the UK and I’m not entirely sure that its something widely taught or even talked about over here - to the degree that should a pilot elect to take off using Vx or Vy as their datum I don’t think it would correlate that well with the POH.

First of all the referenced airspeed is quite a bit lower than what is widely taught in the UK - where typically 65-70mph is used certainly to the first 300ft as a guard against issues should the engine fail. Of course the immediacy in the need to climb away and the deck angle would be a surprise (certainly if Vx is referenced) to many in the UK.

Next is the process in getting airborne. We have talked many times about the wheel balance phase and how long that should be and that will have a large influence on the ability to achieve POH Vx/Vy take off distances. The other huge factor is the time taken to achieve 100% throttle and lately in the UK some advice is that 100% throttle isn’t even required.

The point being if your usual take off process is <100% throttle (at any point), a focus on wheel balance and a higher than usual climb out airspeed then trying to reference Vy and certainly Vx will be a big variation and I doubt the ability of many to achieve POH data.
 

Jean Claude

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My purpose is not to change the good takeoff practices on a clean deck.
I'm just saying that to pass a near obstacle with the best margin, it's not by waiting Vx in the ground effect before climbing at this speed. It is better to do it as soon as a little slower speed is reached. And of course, with full throttle!
 

Philbennett

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Sorry JC i maybe causing some thread drift and I was really thinking aloud.

I guess the point I was making JC is that at some point operation of the aircraft with reference to Vx or Vy and the POH may well be because of potential limitations found during flight planning - I.e my runway is 500m grass and it’s 30C at MTOW.

Sure you can always say no but that’s not really proper flight planning - that’s just giving yourself a big margin to the actual limitations of the aircraft. For some sensible but not all that intelligent nor really a great reflection upon pilot training.

So what I’m saying is if you get taught to climb out at 70mph having pre-rotated to 200rrpm, wheel balanced for an indeterminant length of time and on part throttle then the performance data in your POH will be night and day away from what you’re achieving.

Indeed if you use our virus self isolation to look at some YouTube clips of take offs for Gyroplanes and time things then use fairly basic acceleration/ time / distance maths to work out the take off distance you’ll see that pretty much 100% of pilots you see are using far more distance relatively lightly loaded than the POH gives at MAUW.
 

ferranrosello

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Vx is an objective parameter. Vy too. But take-off techniques are not. What is good for a grass runway at 30ºC is not good for another situation.
To take off as shorter as possible JC technique may be good (I don't know) to achieve the shortest possible ground roll. But not the shortest take off distance (climbing to 50 feet). For that you need to get full advantage of ground effect until achieving Vx, an then climb.

But there are still other questions:
1.- Balancing to the mains... If you put the the front wheel at 50 mm high yo will use much more raunway that if you use only 10 mm.
2.- If yo make the take off roll with the front wheel on the ground it will be much better still but... no presure of this wheel against the ground: is dangerous.
3.- The stick management is crucial. You need keep its movement to the minimum. A small additional movement will cost at list 50 feet in your take off roll.

Taking all these questions into consideration, the take off distances depends much more on the pilots ability that in any other thing.
 

ferranrosello

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ELA 07 does not recommend leaving the pre-rotator engaged with the cyclic back so in my opinion if I don’t come full back and commence moving forward immediately after pre-rotation there will be significant rotor rpm loss.

In my opinion accelerating the rotor back up to 300 rotor rpm wastes energy.

The POH I have for the ELA 07 has Vx at 50kts (55mph, 90kph)
Leaving the prerotator engaged and ease the stick back is forbidden in Ela, because the mechanical prerotation system will be destroyed. This is the only reason.
 

Jean Claude

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Vx is an objective parameter. Vy too. But take-off techniques are not. What is good for a grass runway at 30ºC is not good for another situation.
Are you saying that the best choice of forward speed for climb depends on the nature of the ground you leaved?

To take off as shorter as possible JC technique may be good (I don't know) to achieve the shortest possible ground roll. But not the shortest take off distance (climbing to 50 feet). For that you need to get full advantage of ground effect until achieving Vx, an then climb.
The rolling distance before the wheels leave the ground is not my subject. This is the best speed forward, during the climb, which I am talking about. And it's not Vx, as you think.
 

ferranrosello

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The normal climbing speed after take off is Vy. The best forward speed for obstacle clearance after take off is Vx.
The wind has an impact in Vx. The air density has an impact in both. Vx is higher (not lower) at high density altitudes than at sea level. All these are facts, not a question that depends on everyone opinion.
Now my opinion:
The stick handling in take off has a direct impact in the take off roll. If we finish the take off roll at Vx and the we start climbing, then the take off roll has a direct impact in the obstacle clearance too.

Additionally, if you get airborne slower than Vx, in spite of the shorter take off roll you will have less obstacle clearance (what is to say, take off distance will be longer). In no wind conditions the best performance in obstacle clearance will be obtained by a combination of:
1.- Prerotate as much as possible prior to take off.
2.- Very nice, or better, perfect control handling. (Includes no side slip and full power in take off and climb until obstacle clearance).
3.- Get airborne exactly at Vx.
4.- Maintain Vx in the climb with perfect handling.

However, we are humans and we are not perfect. Every deviation from perfection will create less performance...
 

Tyger

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Additionally, if you get airborne slower than Vx, in spite of the shorter take off roll you will have less obstacle clearance (what is to say, take off distance will be longer). In no wind conditions the best performance in obstacle clearance will be obtained by a combination of:
1.- Prerotate as much as possible prior to take off.
2.- Very nice, or better, perfect control handling. (Includes no side slip and full power in take off and climb until obstacle clearance).
3.- Get airborne exactly at Vx.
4.- Maintain Vx in the climb with perfect handling.
However, we are humans and we are not perfect. Every deviation from perfection will create less performance...
This comports with what I was taught for "short-field" takeoff – basically keeping it on the ground till Vx, then keeping it at Vx till over all obstacles.
The opposite was true for "soft-field" takeoff, which is to get it off the turf, into ground effect, as soon as possible.
And the combined advice was... Don't attempt to take off from a field that is both short and soft!
 

Jean Claude

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Ferran, maybe you haven't read the first message.
Here is quantified example based on a typical curve Vf vs Vz.
You can easily place points C (best climb rate) and B (best climb angle), and It is clear to everyone that at point A the angle is less favorable than at point B.

However, you also know that to climb, you need the extra prop thrust corresponding to the "slope drag".
So, if you prefer to stay at full throttle in level flight, this supplement will produce an forward acceleration.

It is easy to calculate the slopes at A and B and the corresponding forward accelerations in level flight, since Acc. = g * Vz / Vf
At 58 km / h, in flight level, accéleration will 1.8 m / s2
At 75 km / h, in flight level, acceleration will 2.1 m / s2

So, to accelerate from 58 km/h to 75 km/h, you have to wait about 2.6 s, during which you will have traveled about 50 meters still at ground level
While climbing immediately at 58 km / h (< Vx), you would already be at a height of 2.6s x 3 = 8 meters.
A scale drawing will show you that it is only above 60 meters (180 ft) that the Vx choice becomes interesting.

Sans titre.png
PS: With headwind, choosing A is even more interesting.
 
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ferranrosello

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Than you for your explanation Jean C.
The assumption you are doing about getting airborne at 58 km/h seems reasonable. However my point is what if I don´t get airborne until being very closed to 75 km/h. Then the induced drag during the take off roll would be weaker and would use less than 50 meters to reach 75 km/h.

This is a variable that depends very much of the take technic used.
There are no discussions about this question. The safest Take off technique is to keep a moderate nose up attitude in the take off roll. In my experience, the lower the attitude the better the take off performance. To get airborne at 58 km/h implies a moderate nose up attitude...

PS: Totally agree about the head wind point. And if you are flying at high density altitude Vx will be faster that what is stated in the Manual... But this is another question, our machines flying manuals are very poor, consequently you need to solve the lack of information with knowledge.
 
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Philbennett

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Completely agree Ferran and it’s something that isn’t discussed enough in training. We have an absurd situation where we might test our student at the end of his training with knowledge of items in the POH but as far as it relates to take off performance literally have no idea how that data was achievedand therefore can’t have trained the pilot to achieve it....
 

Jean Claude

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However my point is what if I don´t get airborne until being very closed to 75 km/h. Then the induced drag during the take off roll would be weaker and would use less than 50 meters to reach 75 km/h.
Ferran,
The drag of the rotor during the takeoff roll is not parasitic: It mainly translates the energy absorbed to increase the kinetic energy of rotation of the rotor: Rotation energy = Drag * distance
Therefore, not taking the air before 75 km / h will give indeed less rotor drag, but will increase the distance of run. So, the 50 meters gained for reach 75 km/h after takeoff are precedently lossed during the rolling.
 

ferranrosello

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Jean Claude,. I've never said nothing about parasitic drag. I've talked about induced drag... And the induced drag for a flying gyrocopter is much bigger at 58 km/h than at 75. But if the gyro is rolling below 70 km/h is still much lower.

What you say about the energy transfer to the rotor would be correct if the efficiency in this transfer was the same at 58 km/h than at higher speeds. However the efficiency is better at faster speeds.
Ferràn
 

Jean Claude

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I misspoke, Ferràn. I meant that the induced drag you are talking about is a useful drag (not parasitic) even when you are rolling, because it is only the lift of the rotor which produces the increase of the rpm.
Thus, rolling faster, the energy required (induced drag * distance) will remain the same to reach the same rpm.
 

ferranrosello

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Ok, thank you, Jean. I agree that the energy required will be the same. However not all induced drag is transformed in rotor rpm... The efficiency depends on rotor blades angle of attack, that is to say forward air speed.

It would be interesting to know how interact airspeed, rotor rpm antirotative lift and autorotative lift in the take off roll, in order to find what is the best combination to get the best performance.
 
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