# Some thoughts on cross wind takeoffs.

#### Burrengyro

##### Gold Supporter
Hello Jean Claude,
Using my ELA07S at 450kg MTOW:
If I use your formula and graphs, the distance to takeoff, straight and level for a 45 degree 20kn (10.3m/s) crosswind is 50m???
The distance to takeoff, straight and level for zero wind but with an 8% downslope + gyro acceleration is 76m. I am assuming that there is no friction in the calculation of the increased acceleration due to the downslope/gravity. This would seem to indicate that the 8% downslope results in a reduction in distance to takeoff straight and level of 29m on the 105m for the same zero wind conditions for a level runway or the equivalent of a 5m/s (10kn) headwind on a level runway (71m).

There seems to be a critical velocity that has to be reached during the takeoff run where the rotor rpm decay is arrested and rotor rpm starts to increase steadily from 200 to 300 rpm. Assuming that we have a smooth bump-free runway, is it possible to accelerate the gyro more quickly in the first 3 seconds of takeoff roll so that the equivalent of your 20kn takeoff is achieved in zero wind conditions?
Best regards, John H

#### Jean Claude

##### Junior Member
Using my ELA07S at 450kg MTOW:
If I use your formula and graphs, the distance to takeoff, straight and level for a 45 degree 20kn (10.3m/s) crosswind is 50m???
Yes, 105 -(6.8* 10.3*Cos(45)) = 56 m

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#### Burrengyro

##### Gold Supporter
Yes, 105 -(6.8* 10.3*Cos(45) =56 m
Hi Jean Claude,
Thank you! John H

#### Jean Claude

##### Junior Member
The distance to takeoff, straight and level for zero wind but with an 8% downslope + gyro acceleration is 76m.
Where do you get this distance from? Is this an actual measurement on your sloped runway?

I am assuming that there is no friction in the calculation of the increased acceleration due to the downslope/gravity. This would seem to indicate that the 8% downslope results in a reduction in distance to takeoff straight and level of 29m on the 105m for the same zero wind conditions for a level runway or the equivalent of a 5m/s (10kn) headwind on a level runway (71m).
The shortened take-off distances due to the downhill runway, or to the headwind, or to a more energetic pre-launch, or to a stronger prop. thrust may possibly be equal, but the margins for the flapping divergence are not the same.
A too large extrapolation of your reasoning can therefore be dangerous.

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#### Tyger

##### Super Member
JC, what do you mean by "dentice"?

#### Burrengyro

##### Gold Supporter
Where do you get this distance from? Is this an actual measurement on your sloped runway?

The shortened take-off distances due to the downhill runway, or to the headwind, or to a more energetic pre-launch, or to a stronger prop. thrust may possibly be equal, but the margins for the flapping divergence are not dentice.
A too large extrapolation of your reasoning can therefore be dangerous.
Hi Jean Claude,
I used your graph to identify the velocity at takeoff point 75 Kph (21m/s) and acceleration at zero wind speed conditions. Then I added that acceleration figure 2.1 m/ss to the extra acceleration derived from gravity from the 8% downslope to calculate the new distance taken to takeoff S & L. I calculated the shorter distance from this new acceleration figure of 2.9m/ss. I ignored any reduced downslope acceleration due to friction and runway drag just for the purposes of approximate calculations.

I agree that too much extrapolation without actual runway tests could be dangerous. However, your figures and graphs are a great help to me. For my runway circumstances, I would deliberately change your formula from 105 - (6.8 x headwind speed) to 200 - (6.8 x headwind speed) to arrive at a more cautious estimate for safety reasons. All these calculations and estimates are to see if a runway is viable on available land with a slope in zero and crosswind conditions. Headwinds are an advantage.

I want to see if a 250m runway would safely work for my ELA07S and a Magni M16 100hp before any serious preparation work and land excavation is planned. The idea of a quick and short takeoff is to have enough runway still available to land ahead in case of engine problems. The bonus of the 8% downslope to assist takeoff acceleration is mainly useful in zero wind or crosswind conditions. Takeoffs using the upslope runway direction could require a much longer roll in low wind conditions and could be a problem.

I do not wish to apply the knowledge you share in an incorrect or unsafe way. I am happy to make mistakes on paper as long as I can correct them before being applied in reality.

As always, thank you for any thoughts and advice shared by you and all the forum contributors, especially if it helps others and saves me from myself! John H.

#### Jean Claude

##### Junior Member
The bonus of the 8% downslope to assist takeoff acceleration is mainly useful in zero wind or crosswind conditions.
As I said, extrapolating too far can be dangerous.
Thus 115 hp already gives almost too much forward acceleration with the nominal pre-launch of 220 rpm
Increasing it further by a downhill slope of the runway, reduces the distance only a little, because the autorotative torque not improves in the same proportions due the more extended stall on the retreating blade
Here is my simulation of a 450 kg (8.5 m) ELA 07S pre-launched at 220 rpm when the slope is 8%

SO

#### Tyger

##### Super Member
I would think an 8% downslope would help get your wheels unstuck a lot faster, regardless of wind conditions.
But I think it would make it a lot harder to land ahead on takeoff if there be any engine problem, especially on a runway as short as 250m !

This airport (https://www.airnav.com/airport/K30) has something above just a 2% grade (paved); I have preferred to takeoff downhill, and land there uphill, even with a bit of tailwind.
I imagine an 8% grade would look like a ski slope in comparison!

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#### Mike G

##### Junior Member
John (Burrengyro) and Jean Claude

If (as I suspect) JC's calculations are somewhat based on the results of my testing, don't forget that I go to WOT as soon as I release the pre-rotator and bring the stick back.

If John's TO procedure is "I often advance the throttle to WOT within 25m of the start of the takeoff roll" that's half of his TO roll before he's at full throttle. This will help avoid blade flap or sailing but will extend his TO run to beyond the calculation.

Also JC's calculations are based on 2200N (500 lbf) thrust, you'd better make sure that you really have that. One of the discrepancies we found with my measurements was that I obviously wasn’t getting the thrust out of my old 1,000 hour 912S that JC assumed I was.

Plus check the stick position (into/outof wind) discussions for cross wind take offs. This will have an impact on your take off run and flapping risk.

As I said before JC's calculations are remarkably accurate but, I've learned the hard way, that if you want to fly to his limits you'd better fly to the same degree of accuracy.

Here is a graph I'm preparing for the presentation of my GWS to explain the flapping alarm to the uninitiated. I’ve modified it for your 8.5 m rotor.

For the stick back (@20°) acceleration, It translates to

Rrpm<2.5 x TAS (kph) = danger,

Rrpm>2.5 x TAS (kph) = safe

Here is the same graph with the basic alarm limits for the GWS. The actual alarms lines are not simply straight lines and are more complicated but it gives you an idea.

Here is an actual recording of a 150 Rrpm pre rotator take off with an 8.7 M rotor overlayed on the graph for that rotor. You can see that it flirted with the red alarm. You can also see that I needed to be at about 40 kph before I recovered my initial pre rotation Rrpm and about 60 kph before the rotor really started to accelerate. If this had been a downhill or a stick into Xwind TO the flat part of the curve would have been much longer and the risk of flapping much greater.

Mike G

#### Burrengyro

##### Gold Supporter
As I said, extrapolating too far can be dangerous.
Thus 115 hp already gives almost too much forward acceleration with the nominal pre-launch of 220 rpm
Increasing it further by a downhill slope of the runway, reduces the distance only a little, because the autorotative torque not improves in the same proportions due the more extended stall on the retreating blade
Here is my simulation of a 450 kg (8.5 m) ELA 07S pre-launched at 220 rpm when the slope is 8%
View attachment 1153625
SO
I would think an 8% downslope would help get your wheels unstuck a lot faster, regardless of wind conditions.
But I think it would make it a lot harder to land ahead on takeoff if there be any engine problem, especially on a runway as short as 250m !

This airport (https://www.airnav.com/airport/K30) has something above just a 2% grade (paved); I have preferred to takeoff downhill, and land there uphill, even with a bit of tailwind.
I imagine an 8% grade would look like a ski slope in comparison!
Hi Tyger,
It must get interesting using the downsloping runway 06 for take-off and landing on the upsloping runway 24 if you have a lot of traffic flying in and out? I think an 8% downslope works if it rounds out to about 100m of upslope at the end of the runway to help the landing. John H

#### Burrengyro

##### Gold Supporter
John (Burrengyro) and Jean Claude

If (as I suspect) JC's calculations are somewhat based on the results of my testing, don't forget that I go to WOT as soon as I release the pre-rotator and bring the stick back.

If John's TO procedure is "I often advance the throttle to WOT within 25m of the start of the takeoff roll" that's half of his TO roll before he's at full throttle. This will help avoid blade flap or sailing but will extend his TO run to beyond the calculation.

Also JC's calculations are based on 2200N (500 lbf) thrust, you'd better make sure that you really have that. One of the discrepancies we found with my measurements was that I obviously wasn’t getting the thrust out of my old 1,000 hour 912S that JC assumed I was.

Plus check the stick position (into/outof wind) discussions for cross wind take offs. This will have an impact on your take off run and flapping risk.

As I said before JC's calculations are remarkably accurate but, I've learned the hard way, that if you want to fly to his limits you'd better fly to the same degree of accuracy.

Here is a graph I'm preparing for the presentation of my GWS to explain the flapping alarm to the uninitiated. I’ve modified it for your 8.5 m rotor.
View attachment 1153626
For the stick back (@20°) acceleration, It translates to

Rrpm<2.5 x TAS (kph) = danger,

Rrpm>2.5 x TAS (kph) = safe

Here is the same graph with the basic alarm limits for the GWS. The actual alarms lines are not simply straight lines and are more complicated but it gives you an idea.
View attachment 1153627

Here is an actual recording of a 150 Rrpm pre rotator take off with an 8.7 M rotor overlayed on the graph for that rotor. You can see that it flirted with the red alarm. You can also see that I needed to be at about 40 kph before I recovered my initial pre rotation Rrpm and about 60 kph before the rotor really started to accelerate. If this had been a downhill or a stick into Xwind TO the flat part of the curve would have been much longer and the risk of flapping much greater.
View attachment 1153628
Mike G
Hi Jean Claude, Mike and all,
This part of the thread may be a bit off the track from the original post re crosswind takeoffs. In a slightly obscure way, it feeds into both issues. I am learning from JC and Mike about the importance of making sure the rotor is always spinning at an appropriate flying speed for the ground speed of the gyro in takeoff mode. Rotor speed management for a successful takeoff, regardless of runway slope, condition, wind direction, is critical just as it is at the landing phase and up to the time the rotor stops.

In a downhill takeoff, pre-rotate to 220 rrpm (for my ELA07S), accelerate efficiently but not too quickly, to ensure that rotor rpm continues to build steadily until the wheels are off the ground, then apply WOT to maintain a safe and rapid climb.

For me, I can only do this by feel as my concentration is 100% on the initial nose lift because I am not observing any rpm gauges during the early takeoff phase. When the wheels are off the ground and WOT is applied, I have more time to scan the airspeed gauge and climb safely when it reaches 60mph. Mike's GWS looks like a really useful tool in these situations. Taking off from a short runway or a downsloping short runway at MTOW is not a problem if the wind is helping. In light winds or calm conditions, a short runway, at MTOW, may be a problem if the takeoff technique is not efficient for the conditions. The main problem with my proposed theoretical short downhill runway is there may be no room left to land ahead safely in event of an engine problem no matter how efficient I become with my takeoff technique.

The other issue is rolling wave-type crosswinds and turbulence generated by adjacent hills or trees. When the wind is directly crosswind from the NW, the air flows over a nearby hill and houses and rolls down onto our current grass runway. With a long runway, I have time to adjust and land ahead.

A short runway is better than no runway......maybe.
Jean Claude and Mike, thank you for helping me on the road to a better understanding of gyros. John H.

#### Tyger

##### Super Member
Hi Tyger,
It must get interesting using the downsloping runway 06 for take-off and landing on the upsloping runway 24 if you have a lot of traffic flying in and out? I think an 8% downslope works if it rounds out to about 100m of upslope at the end of the runway to help the landing. John H
It's not an airport I visit often, but I've never seen another plane (in motion) while I've been there. There is a skydiving operation, and the airfield is part of a golf course, so I imagine it gets a little busy on nicer days.

#### Jean Claude

##### Junior Member
It is a pity that the manufacturers do not specify in their flight manual the takeoff procedure in case of a downhill runway (just the new recommended pre-launch rrpm, depending on the slope)

#### Burrengyro

##### Gold Supporter
It's not an airport I visit often, but I've never seen another plane (in motion) while I've been there. There is a skydiving operation, and the airfield is part of a golf course, so I imagine it gets a little busy on nicer days.
Hi Tyger,
I got immersed in the crosswind thread as part of my own research into a suitable alternative runway in the event of our current runway becoming unavailable. I have some land which may lend itself to a 250m (830ft) runway, but with an 8% slope. There is a lot of earth moving required to make this project work. Hence the musings about downslopes and upslopes. Abid, Jean Claude, Mike G and others including yourself have added a lot to my understanding of runway issues and crosswind landings and rotor management with same. 250m runway is fairly tight for safety margins as you rightly highlighted. It may be time to look for a safer alternative. Two hours flying yesterday. Practice is always more fun than study. John H

#### Jean Claude

##### Junior Member
Also JC's calculations are based on 2200N (500 lbf) thrust, you'd better make sure that you really have that. One of the discrepancies we found with my measurements was that I obviously wasn’t getting the thrust out of my old 1,000 hour 912S that JC assumed I was.
Mike,
Don't worry. It's not the wear of your engine. It's just that the fit of propellers to engines is rarely perfect and I not know how it is unperfect.
So, a shorter chord will require a larger pitch setting, which will result in less static thrust.
Their thrusts, however, can be just as good in cruise flight.

An example, calculated by JavaProp for three bladed 1.73 m and 115 Hp

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#### Mike G

##### Junior Member
Jean Claude
Thanks, I know the engine isn't too bad I did a leak test and all cylinders are still remarkably tight.
I have the DUC Windspoon, I wonder if Javaprop could simulate that?

Mike G

#### Jean Claude

##### Junior Member
Mike,
The thrust curves obtained by JavaProp are accurate, even if the plane shape of the blades is a little different. But we must at least enter the aerodynamic pitch at 3/4R, which is difficult to exactly measure

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