Which would produce the slowest rotor RPM?

[Jay Bunning]

Question: Which would produce the slowest rotor RPM?

A: A vertical descent with power.

B: A vertical descent without power.

C: Pushing over after a steep climb.

Edited October 22, 2013 by Jay Bunning

[Astro]

Blowback,...maybe?

[WolftalonID]

In a climb you have high pitch settings. This is the cause for the low rpm. Its called high drag profile. I may be mistaken and would need to verify with my test book, but this question I believe was related to gyroplanes. They dont have engine driven rotors except during initial climb..I think....

 

Even if that last part is wrong, in a decent, we are using autorotational forces and low pitch settings which reduce our drag profile. Engines have limited power, the higher the pitch setting the more they work. The easier to get low rotor rpm.

 

Ok edited input here. In a climb your wind is coming from above the rotor disk and the relative wind to the pitch angle of the rotor blade is positive but not at an angle to exceed lift. If at the top of the climb you push over instead of lower collective, you move upward movement to forward movement. This changes relative wind from above in the climb to in front. Pitch angle has not changed and results in relative wind striking a high pitch angle, exceeding the angle capable of producing lift, and or an angle that exceeds available power and you get rotor droop, or low rotor rpm.

 

The same principle when we recover from low rotor rpm, we lower collective, AFT cyclic, roll on power. Reducing pitch attitude of the airfoil and increasing our rpm with power and using wind power under the disk.

 

The same principle in an engine out drill. Lower collective and AFT cyclic. Never push over. We fly rotor craft, not fixed airfoil craft. Rotor energy is controlled by collective and power. Thats why they are on the same stick.

 

Final edit: after review of my FAA test book, the question found on page 2-18 of ASA commercial test prep had no direct indication it was for gyroplanes. Therefore I apologize for taking up reader time in my initial guess work that it may have been. Basic aerodynamics of the rotor do reflect why the answer is what it is, and i agree the FAA explanation is rather copoutish.

 

One might want to review relative wind, angle of attack, induced drag to understand what is being implied by this Government eval test.

Edited October 9, 2013 by WolftalonID

[WolftalonID]

Just an observation of the OP. I noticed your making free training videos for helicopter pilots. Then post questions on basic aerodynamics such as this.

 

Are you a certified CFI with a rotorcraft rating? Curious as to your credentials to instruct students.

[apiaguy]

The OP question was valid... it is difficult to understand the intent of some FAA questions... depending on how you set the question up the answer could vary and the FAA usually doesn't give any set up for a standard question. As to the answer... I don't know what they are getting at or what they want you to know... I could postulate but won't. IChris to the rescue... he is so good with words.

As to Wolfboy... your post is riddled with inacuracies and guess work and then you go off questioning the OP's credentials as though one must have a higher knowledge, understanding and know the answer before being the one to instruct others. That is called rotorcraft douchebaggery. A common problem that is repairable with FAA field approval.

[WolftalonID]

Yes I did guess on the gyroplane association. Many questions on the commercial test are geared toward them and often confuse those of us who are not trained in them....hence the direct reflection on the guess.

 

The rest...is not inaccurate. It may not reflect your higher learning and vocabulary choices that I noticed you failed to inform us all about. However were fast to throw names like an internet bully.

 

I am commercial instrument rated with a CFI all in helicopters. My knowledge is not god, however after reviewing FAA-H-8083-21 I find no direct answer to the stated question ASA #5672 in the book. Both under helicopter maneuvers and gyroplane. Basic effect of aerodynamics on the rotor do answer the question completely.

 

I can answer the Op's question accurately, albiet minus iChris's profound indepth knowledge.

 

I can with my credentials ask as I did why the Op would not be the one providing the answer. He is a CFI. It was a question to him hoping he would fish.

 

You have no right to sit there and belittle, name call, and bully, and not provide anything towards an answer to assist the OP or any reader for that matter.

[apiaguy]

My point was not to diss your explanation of the question. (although I didn't like it)

It was my intent to belittle as you attempted to do to the OP....

Edited October 10, 2013 by AdminLB
Out of line name calling.

[AdminLB]

My point was not to diss your explanation of the question. (although I didn't like it)

It was my intent to belittle as you attempted to do to the OP....

Apiaguy,

 

Although I do appreciate your participation as a long standing member of the forum, not sure you need to use such language to attack WolfTalonID. It is not uncommon to sincerely ask someone for their credentials to better understand their point of view. I understand how easy it is to take a word or simple sentence like "Then post questions on basic aerodynamics such as this" . . . in a context that appears to be condescending.

 

But I always like to give the benefit of the doubt (at least in this forum) and look at the totality of the posts from people. From my perspective, although the question could have been tightened just a smidge, I do believe it was sincere.

 

I also understand that we all get spirited and passionate and that often fires up the conversation. Honestly, I like that. I personally do not mind the disagreement in points of view, but no sense in going for the jugular right out of the gate. You pretty much changed the whole tone of the post when you launched "wolfboy", "douchebaggery", and then went for the kill with "Silvercock."

 

Let's please disagree with facts, not name calling.

[troll.detector]

Good feedback

[SBuzzkill]

... nevermind.

Edited October 10, 2013 by SBuzzkill

[apiaguy]

yeah... ok, maybe got a little crazy.... I'll give you that. But it was all true! haha.....

I'll try better to refrain from the name calling. Sorry Wolf....I'd like to extend the olive rotor blade and invite you to come up north and fly sometime.

[iChris]

 

Could someone help me understand WHY in a power-on situation, at the top of a climb a pushing forward of cyclic would lower RPM? Many thanks in advance and see question below:

 

Question: Which would produce the slowest rotor RPM?

A: A vertical descent with power.

B: A vertical descent without power.

C: Pushing over after a steep climb.

 

That test question #5672 is somewhat ambiguous for a helicopter question and the answer seems to imply that a cyclic pushover after a steep climb (power-on) would cause a lowering of the RPM. In general, it's a poor test question without much relevance to a students study. However, the question actually seems to be asking, which of the three choices would potentially produce the slowest rotor RPM prior to corrective action via governor or manual intervention. In that context it’s the one that is highest in power required and/or has the highest drag.

 

In most cases were a constant available power is being applied to the rotors system, any decay in rotor speed (RPM) is a function of the power required and of the rotor’s level of kinetic energy. Therefore, this type of question can be approached from the perspective of the power required vs. power available.

 

If the power available were fixed at a constant value, any increase in the power required would normally cause a decrease in RPM. If on the other hand the power required decreased, the excess in power available would increase RPM.

 

Induced drag works along the same line. Increased induced drag equals a decrease in RPM. Inversely, decreasing induced drag yields an increase in RPM.

 

In any case from the figure below, you can see, a vertical descent has the least amount of drag and requires the least amount of power vs. the cyclic pushover. Therefore, the cyclic pushover from a climb yields the best potential for the slowest rotor RPM out of the three choices.

 

Φ = inflow angle

θ = blade pitch

α = angle of attack

V_i = induced velocity

V_c = climb velocity

D_i = induced drag

D_o = profile drag

ΔL = lift vector

 

 

Edited October 19, 2013 by iChris

[WolftalonID]

Thanks for the feedback.

@Wolff: yes I am a CFI, please check out the videos, we welcome feedback/suggestions/corrections.

I am always learning, trying to be a better pilot and instructor. I was unable to give a definite expansion of this question to my student and neither could more experienced pilots i asked, so I came to the forum...

I hear ya there. Often I tend to come up with some questions that stump many at my school. Its always interesting to see the responses given though. It seems to amaze me the different perspectives other pilots have.

Great attitude about learning btw! We are always the student of the helicopter, and enjoy the adventure it brings along the way.

[pilot#476398]

I think this is just one of those trick questions put in there just to get you to think more, and to possibly try and prevent too many test scores of 100%! You look at the first two and think, "I can't see how either of those would do it,...it must be the third one,...or maybe...?" Dammit, I hate these kind of questions!"

 

Then it makes you think, "maybe I should have trained in someting without a governor?"

Edited October 18, 2013 by pilot#476398

[SBuzzkill]

You're going to see a spike in power and that would be your clue that if fuel wasn't added then you would see a drop in rotor RPM.