# AR-1 N57AR - Texas - 20.8.23

This comes back once again to the confusion surrounding the very term, "blade flap".
It has been used to refer to
1) the normal vertical motion of rotorcraft blades to compensate for dissymmetry of lift;
2) actual retreating blade stall itself; AND
3) "the blades begin[ning] to flex and bend"

The problem keeps arising in these discussions because people are using a single term that can have very different – although related – meanings.

But as we've all seen, lots of folks are very imprecise with words here (loose v lose, break v brake, etc.). Why should this be different?

It wasn't about hand spinning. You have missed the point.

I know the point. I understand it well and I am proof that you can understand what is going on without hand spinning rotors. We all understand gravity works without jumping from a tall building. It is the responsibility of the teacher to do thorough ground school to make students understand what a rotor flap/sailing is, how it can happen and what circumstances it can happen in and how to prevent getting into those circumstances. Not just rote learning of how to pre-rotate and takeoff but understanding on why it is done that way and what is the significance of the order of operations, stick position, balancing (loading disc) and not accelerating too fast outrunning the rotors. Once you are beyond the rote memorization of the process and actually understanding why it is done that way, the chances of you having a blond moment significantly reduce.
How many instructors cover these topics in detailed ground briefings for takeoff lesson. In the US it is much less than it should be imo.
Secondly, you can speed up rotors without hand spinning in taxi practice with your instructor.

I explained in another thread that there is a simple relation between TAS and Rrpm like the HV curve that can be drawn for each model depending on rotor diameter/chord and rotor disc angle (seen here as stick position) that I call the RV curve which is more or less a straight line.

This graph is "typical" and shows IAS because it was for a specific density altitude.

Mike G

This comes back once again to the confusion surrounding the very term, "blade flap".
It has been used to refer to
1) the normal vertical motion of rotorcraft blades to compensate for dissymmetry of lift;
2) actual retreating blade stall itself; AND
3) "the blades begin[ning] to flex and bend"

The problem keeps arising in these discussions because people are using a single term that can have very different – although related – meanings.

But as we've all seen, lots of folks are very imprecise with words here (loose v lose, break v brake, etc.). Why should this be different?

I feel this picture from the FAA handbook leads to a general misunderstanding of the problem. I like to think of it as two distinct things with the same name: low and high speed blade flap. The diagram showing low speed. But high speed being what really wrecks gyros. And yes, I am sure someone will disagree.

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I explained in another thread that there is a simple relation between TAS and Rrpm like the HV curve that can be drawn for each model depending on rotor diameter/chord and rotor disc angle (seen here as stick position) that I call the RV curve which is more or less a straight line.
View attachment 1159575
This graph is "typical" and shows IAS because it was for a specific density altitude.

Mike G
That is a very interesting graph because if I am
Interpreting it correctly then should your average pilot pre rotate to c.200 rpm then pull the stick all the way back he seems to have a significant margin before flapping conditions ever occur - indeed one might argue could never occur given we need to see 80km/h (50mph) airspeed which is quite significant and with the drag of the rotors even a 914 powered aircraft doesn’t accel. very fast.

Hi Mike

I must admit I am quite surprised at the margins implied in this graph

If I am reading it correctly I can be at a little over 40 knots, with the stick aft, at only 200 RRPM.

That seems scary to me but I know you have done a lot of development work and maybe I am just very conservative

I pretotate to 120 RRPM, bring stick back and go to 150. I start my takeoff roll and slowly increase throttle as the nose wheel lifts off, about 180-220. Apply power and RRPM quickly picksup, at 50 Kts I lift off, fly along the runway at abt 55 then climb out. Not rocket science, am I missing something? Single place machine with 80 HP 912ul.

I pretotate to 120 RRPM, bring stick back and go to 150. I start my takeoff roll and slowly increase throttle as the nose wheel lifts off, about 180-220. Apply power and RRPM quickly picksup, at 50 Kts I lift off, fly along the runway at abt 55 then climb out. Not rocket science, am I missing something? Single place machine with 80 HP 912ul.

Something doesn't sound right. How is your single seater lifting off at 50 knots. AR-1 lifts off at 40 knots and if I do soft field technique it can lift off at 38 knots. I climb out at 49 knots (Vx) or 54 knots (Vy)

That is a very interesting graph because if I am
Interpreting it correctly then should your average pilot pre rotate to c.200 rpm then pull the stick all the way back he seems to have a significant margin before flapping conditions ever occur - indeed one might argue could never occur given we need to see 80km/h (50mph) airspeed which is quite significant and with the drag of the rotors even a 914 powered aircraft doesn’t accel. very fast.
Please, note that Mike's diagram shows the Rrpm at forward speed, not the pre-launch Rrpm.
Don't forget that Rrpm starts to decrease during the run.
Here, a run simulation:

PJB
Yes I do see that JC but I guess the point I was making is that in your average 2 seater and the POH process there is some margin to disaster- I.e those that do get into trouble have had a serious and relatively long lapse.

Something doesn't sound right. How is your single seater lifting off at 50 knots. AR-1 lifts off at 40 knots and if I do soft field technique it can lift off at 38 knots. I climb out at 49 knots (Vx) or 54 knots (Vy)
I follow the POH, it's working well.

Yes I do see that JC but I guess the point I was making is that in your average 2 seater and the POH process there is some margin to disaster- I.e those that do get into trouble have had a serious and relatively long lapse.
Phil
During testing of the GWS with Abid in 2022 he ended up doing 140 rpm pre rotations and stick back full throttle. He got alarms but didn't flap the rotor. Some of it is discussed here https://www.rotaryforum.com/threads/gyro-warning-system.1145987/page-5

Considering that his POH says 180 Rrpm pre rotation minimum, yes you're right there is a lot of margin before you flap if you are considering a stick fully back take off.

That's why we developed the stick position alarm after testing with Abid because we both concluded that you had to really f.. it up to flap if the stick was fully back. Our conclusion was that it's more than probable that most flapping take-off accidents are due to the stick not being fully back during the initial take off role. Hence the need to develop the stick position alarm.

Mike

PJB
Hi Mike

I must admit I am quite surprised at the margins implied in this graph

If I am reading it correctly I can be at a little over 40 knots, with the stick aft, at only 200 RRPM.

That seems scary to me but I know you have done a lot of development work and maybe I am just very conservative
Mayfield

At about the 14 minute mark in this video
I explain how this graph works, it might help answer your question.

Mike

On the graph "kph" is kilometers per hour, correct? Not knots (which are already "per hour").

Writers should not assume their readers know what an acronym stands for. The proper form is to write out the entire title name, followed by the acronym in parenthesis the first time it is used. From then on it is acceptable to use the acronym alone.

What does GWS stand for? Great White Shark? Gyrocopters With Skirts? Gyroplanes Without Sisters? If I missed a proper reference, please kindly excuse my oversight, thank you.
Definately Great White Shark. They're flying now, so they are a significant threat to manned aircraft.

On the graph "kph" is kilometers per hour, correct? Not knots (which are already "per hour").
Tyger
Yes kph is kilometers per hour.
Mike

Different units are often the source of errors
Kt (knot) is for nautical mile per hour
mph is for US mile per hour
km/h is for 1000 meters per hour
m/s (meter per second) is used in scientific calculations
ft/mn ( foot per minute) tis used for aircraft ascent and descent

""Different units are often the source of errors
mph is for US mile per hour""

Indeed - Miles Per Hour is a British Imperial standards unit still used by the US and numerous smaller states often with a UK or US historical connection.

Variation in "standard" units is a potential source for misunderstanding and incidents - every day I listen to my local airband and hear the pressure settings given in hectopascals ( millibar ) to all and sundry - all good until KNIFE 71 or similar ( US CV-22 ) asks for it in Inches ( of Mercury ).

Standard standards and those standard to you but not standard to others standards !

The problem keeps arising in these discussions because people are using a single term that can have very different – although related – meanings.
In the UK most people refer to it as 'blade sailing', and why I refer to it as 'blade flap/blade sailing'.