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CPL written exam question confusion


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Need a little help with an exam question...

 

I've already asked a bunch of CFIs about this question in the ASA test book for the FAA CPL written. So far no one agrees with the answer the book indicates as correct. Here's the question and the answer choices. The "correct" answer is suppose to be C. I don't see how RPM would increase? If this is true, please explain.

 

RTC Question #5715 (ASA page 2-23)

 

Using left pedal to assist a left turn during an autorotation descent will probably cause the rotor RPM to...

 

A. increase and the airspeed to decrease.

B. decrease and the aircraft nose to pitch down.

C. increase and the aircraft nose to pitch down.

 

(From FAA-H-8083-21, "use of antitorque pedals to assist or speed the turn causes loss of airspeed and downward pitching of the nose, especially when the left pedal is used.)

 

Thanks!

Susie

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Because when using left pedal in a left turn during an auto when the torque has been removed from the airframe, now you're assisting the aircraft in "wanting" to rotate counter clockwise (rather than working against the rotor system rotating counter clockwise.) Therefore you should see a slight increase in your RPM's going into that left turn. If you were to utilize right pedal in a right turn during an auto, you'd be using tail rotor thrust against the airframe which is naturally already wanting to rotate left with the rotation of the rotor system, instead of right. Make sense?

 

That probably isn't the best to way to describe it, at least I think that is right. Somebody chime in if I am incorrect.

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  • 2 weeks later...

If I remember correctly there is also a questions about the negative pitch on the TR. It has the same background.

In powered flight:

Engine produces power, MR spins counterclockwise, torque makes the fuselage spin clockwise, TR counteracts that torque. More MR pitch, more torque, more left pedal and if no power is added the RPM goes down.

 

In Autorotation:

MR produces "power", torque makes the fuselage spin counterclockwise, negative pitch on the TR counteracts that torque.

If the pilot increases left pedal in a turn he/she decreases the negative pitch on the TR and therefore the drag. More power is left to increase the MR RPM.

 

The available power is always distributed between MR and TR. The more pitch is in the TR the less power is available for the MR.

 

Hope that helps

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If the pilot increases left pedal in a turn he/she decreases the negative pitch on the TR and therefore the drag. More power is left to increase the MR RPM.

 

The available power is always distributed between MR and TR. The more pitch is in the TR the less power is available for the MR.

Pretty sure it won't have anything to do with the TR thrust.

 

TR Thrust in autorotation is pretty damn close to 0, the reverse torque effect you describe is not very strong. The tiny changes in TR thrust won't be enough to cause an appreciable change in RPM, unless maybe if full left pedal is used (in which case you'd get a decrease in RPM, but even that would be masked by all the other horrible things that would happen to the helicopter)

 

 

Here's my take on it:

Adding left pedal in a left turn will cause the nose to pitch down. This will cause an increase in rate of descent, which, after a short delay, leads to an increase in RPM. It should say "nose to pitch down and RPM to increase" in that order.

 

It could also simply be a typo, given the quote from FAA-H-8083-21.

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The good news is that this question wasn't on the CPL test. I took the test last week, and thanks to the ASA Test Prep materials I only missed one question. Those of you studying for the CPL written need to know the differences between a Turn Coordinator and a Turn & Slip Indicator. There were lots of questions about weather and weather services, and be sure to read the METARs carefully. If the METAR says OVC005 then don't bother with the temp/dewpoint convergence computations for determining cloud bases. There was a mean weight and balance question regarding the repositioning of cargo, so study up on weight shift. There was also an "airplane" question regarding parasite drag and induced drag, but just look at Figure 1.

 

With respect to left pedal assist in the auto, I think my instructor and I will try it and write down the answer next time. My experience in the 22 is that lowering the nose in the auto increases airspeed followed by a reduction in RPM. Works every time. I think I have done every form of slip and pedal combination in my autorotation training, with an increase in RPM only occurring in hard banking turns and the flare to landing. When I grow up I'd like to do some autos in 206, or maybe even an Enstrom 280FX.

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I took the test last week, and thanks to the ASA Test Prep materials I only missed one question.

 

I'd like to do some autos in 206, or maybe even an Enstrom 280FX.

 

First, congratulations Susie! Missing one is really quite an accomplishment. I remember a similar weight shift question as well!

 

206? What no R66 in your future? I've done autos in both and while I think the R66 has a longer glide, you could describe them both as..E A S Y !!!

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My experience in the 22 is that lowering the nose in the auto increases airspeed followed by a reduction in RPM. Works every time.

 

Other way around - lowering the nose leads to a momentarily loss of RPM due to the reduced load factor, followed by an increase in airspeed. Which is then followed by an increase in RPM due to the increased rate of descent.

 

The initial loss of RPM happens because the coning angle is decreased as the disc is unloaded. Due to the conservation of angular momentum, the rotor has to slow down.

Then, rate of descent increases, which introduces more induced flow, which in turn creates a greater autorotative force and increases rotor speed.

 

A constant loss of RPM happens again when the nose is lowered excessively and airspeed is increased above the limits for autorotative flight. A steep nose down / diving attitude creates airflow that is more parallel to the rotor disc, in other words a loss of induced flow.

 

 

You can try the above in your R22, just take an instructor with you.

From, say, 60kts 102% RRPM in autorotation, apply forward cyclic without moving the collective. After an initial loss of RPM to somewhere in the mid-90%s, depending on how much cyclic was used, RRPM will start to recover and come up fast. I would suggest to recover before RRPM starts to decrease again, because you will be close to Vne by then. Be careful when trying this and avoid large control inputs.

Edited by lelebebbel
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  • 4 weeks later...

Other way around - lowering the nose leads to a momentarily loss of RPM due to the reduced load factor, followed by an increase in airspeed. Which is then followed by an increase in RPM due to the increased rate of descent.

 

The initial loss of RPM happens because the coning angle is decreased as the disc is unloaded. Due to the conservation of angular momentum, the rotor has to slow down.

Then, rate of descent increases, which introduces more induced flow, which in turn creates a greater autorotative force and increases rotor speed.

 

A constant loss of RPM happens again when the nose is lowered excessively and airspeed is increased above the limits for autorotative flight. A steep nose down / diving attitude creates airflow that is more parallel to the rotor disc, in other words a loss of induced flow.

 

 

You can try the above in your R22, just take an instructor with you.

From, say, 60kts 102% RRPM in autorotation, apply forward cyclic without moving the collective. After an initial loss of RPM to somewhere in the mid-90%s, depending on how much cyclic was used, RRPM will start to recover and come up fast. I would suggest to recover before RRPM starts to decrease again, because you will be close to Vne by then. Be careful when trying this and avoid large control inputs.

 

You say the increased rate of descent causes a build in RPM, but from my understanding the way that air is going through the rotor disc plays a large factor also. With the increase in nose down pitch the air is moving through the disc at more of an angle, so regardless of the increase in airflow it is still at an angle that doesn't help build RPM. I don't think the reduced load factor has anything to do with the decrease in RPMs initially, its all about how that air is hitting the rotors. Think about when you start the flare, if you want a real good build in RPMs, a bit more aft cyclic will give you that. Reason for this is the entire rotor disc is exposed to that relative wind.

 

I can honestly say I have never experienced an increase in RRPM during an auto while pitching the nose down. Not saying you are wrong, I am by no means an expert. I'll give it a try tomorrow but this is quite opposite of what I have experienced and learned. I only have experience in the 22 and 44 but still.

 

As for the TR producing little to no thrust, Tim Tucker made it extremely clear in the safety course that if the TR is attached, it IS producing thrust, regardless of the amount of right pedal that is being used. He was quite adamant about it actually.

Edited by CaptainDune
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I don't think the reduced load factor has anything to do with the decrease in RPMs initially,

 

It does. Shove the cyclic forward and the RPMs drop instantaneously, long before any change of airflow could possibly occur or take effect. This is purely coriolis effect, or conservation of angular momentum.

 

 

With the increase in nose down pitch the air is moving through the disc at more of an angle, so regardless of the increase in airflow it is still at an angle that doesn't help build RPM.

 

As I said, an overly steep pitch down attitude will lead to a loss of RPM for several reasons, but we are not usually going anywhere near those pitch angles in autorotation. Dropping the nose from, say, a 0 degree attitude to -5 or -10 degrees does not cause a significant reduction of airflow through the disc (geometry tells us that the change would be minimal, a few % at best)

On the contrary, the resulting increased rate of descent causes an increased airflow and increased RPMs.

 

I'm not making this stuff up, just go and try it yourself. Watch out though, this can easily lead to a rotor overspeed. You need quite a bit of collective to control the RPM as speed builds up, and then you'll need even more, because you have to pull the cyclic back to control the airspeed. Do this at sufficient height.

 

The increase of RPM during the flare is a combination of increased load factor and increased airflow.

Edited by lelebebbel
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  • 2 weeks later...

My two cents. I believe the answer is wrong on the exam.

 

Left pedal should decrease RPM because it is going to try to produce anti torque and increase angle of attack. Increase in angle of attack should lead to more drag on your tail rotor, thus lowering the RPM.

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