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Old 04-14-2012, 08:16 PM
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Default Helicopter Homework

This is for the helicopter pilots.

I have been given a simple reading assignment by my instructor over two topics. Translational lift (ETL) and the Height Velocity curve.

The HV Curve reading seems simple. Each airframe/manufacture will publish this for the airframe to let the pilot know about unsuitable airspeed and altitude configurations in respect to emergency operations. Example autorotations.

The other is Translational lift.

If I understand this correctly.....

When you move from a hover through translational lift to full forward flight that the aircraft requires control input to correct for performance changes of the tail rotor and main rotor to maintain aircraft attitude and azimuth.

does this sound right?
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Old 04-15-2012, 02:43 AM
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Jeb- I wish you the best as you learn to fly helicopters. It is fascinating to me to experience what I read about. Translational lift basically is the helicopter becoming more efficient and using less power as the airspeed increases as you take off from a hover and progress into forward flight. You can be in translational lift even while hovering, if there is a nice to moderate wind. .................................................. .................................................. ..... Let's just consider you are hovering when there is no wind. Your rotor is nothing but a huge air pump and it is pumping air from above the rotor , down through the rotor, and the air then otflows in all directions radially away from the helicopter. A lot of this outflow gets drawn back into a recirculating pattern that has it reintroduced back through the top of the rotor for another trip through the rotor. The outer tips of the rotorblades are creating vortices and there is a strong recirculation at the tips as the higher pressure air rushes outward and upward to fill in the low pressure area above the rotor. There is a strong doughnut shape circulation at the tips that cause a high inflow velocity down through the rotor at the outer disc area. .................................................. ..................... The faster the inflow is, the lower the angle of attack is on the rotorblades. To maintain a hover, this loss of angle of attack has to be compensated for by adding more collective. The drag increases as the AOA of the rotorblades is increased, and that's why it takes more power. This extra power going into the rotor causes the tail roror to work harder also as it has to counter the extra torque............................................ .......Now let's start moving forward out of this hover. You give a little forward cyclic and this tilts the rotor forward a little and the helicopter starts moving forward. As you initially start moving forward , the rotor starts having its incoming air start hitting the rotorblades slightly more horizontally instead of it all coming in vertically. This causes tha AOA of the rotorblades to increase. To maintain the same skid height, the collective can be reduced slightly as this slightly more horizontal bite of the relative airflow is causing more molecules per second to be deflected downward. This slightly smaller AOA also causes more right pedal as less torque needs to be countered by the tail rotor. The tail rotor also is becoming more efficient as it is operating in cleaner air as the helicopter transitions away from a hover............................................. ..................Now we are going faster and I see clues many times of starting to run into some debris from the recirculation at the front of my rotor. A little faster and the helicopter outruns this recirculating debris. The rotor is starting to get happier as the relative airflow is becoming increasingly more horizontal as the helicopter goes faster. Even less collective is needed , and more right pedal as less torque is being consumed by the rotor. .................................................. ...........As you go a little faster, your rotor blades start catching up with the tip recirculation and the blades start running into these vortices that create havoc with rapidly varying relative airspeeds hitting the rotorblades. This is the famous shudder you will feel as your rotor outruns these vortices. As your airspeed increases, your rotor is seeing even more horizontal air, and is really slicing into undisturbed air as the vortices are now trailing behind. The rotor is becoming more efficient as it doesn't start with as much inflow velocity, and can create the required induced flow with less AOA. That means less drag, less horsepower, less power needed by the tail rotor. This efficiency keeps increasing until you meet MPRS, minimum power required speed. You have gone through translational lift and any faster speed requires more collective and more left pedal from there on. I abreviated this translational explanation, as there are several other nuances that go on that would take much longer to type here. One such topic is transverse flow effect which causes the helicopter to roll slightly to the right. But thsts another topic I would be happy to use my vocabularly challenged brain on and put it into simple words. Hope this helps! Its fascinating to me to feel and see the clues that translational lift causes. If you want me to elaborate further, just ask. There are others here that can explain it better, and everyone can add their own words that can add to the explanation. Stan
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Last edited by StanFoster; 04-15-2012 at 03:00 AM.
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Old 04-15-2012, 03:14 AM
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LOL Stan. You had summed it up pretty well!!! The Effective Translational Lift occurs when the rotors reach undisturbed air and the effectiveness increases due to either directional movement of sufficient amount of wind.You can identify if by the shake of the frame and the right ward input on you cyclic and pedal input due to changes of the torque as Stan has explained so well. This will become so automatic you won't even recognize that you are doing it!

Last edited by choppergabor; 04-15-2012 at 03:25 AM.
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Old 04-15-2012, 03:22 AM
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Gabor- You explained it excellent with brevity, and no callouses on your fingertips from typing. Stan
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Old 04-15-2012, 03:24 AM
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Quote:
Originally Posted by StanFoster View Post
Gabor- You explained it excellent with brevity, and no callouses on your fingertips from typing. Stan
Well yours is the full and through explanation. I used to have a great illustration somewhere I used to make copies to my students that was so nicely depicted......if I could just find it I would post it..... lemme look
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Old 04-15-2012, 03:38 AM
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Gabor- I like to explain in detail to another newbie what I as a newbie am experiencing. I love these topics and I find trying to explain them reinforces this stuff into my own head! Vance and I have had several conversations about how typing out our experiences helps our ownselves. I will honestly say I will be a fledgling helicopter pilot for many years to come. It never is boring learning this stuff! My explanations are geared for newbies, as they are written by a newbie! Stan
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Last edited by StanFoster; 04-15-2012 at 03:53 AM.
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Old 04-15-2012, 07:47 AM
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Default My take on your homework

Hello Jeb,

At the risk of seeming like an interloper I feel these two may be leaving out how the two assignments may be related.

In my training in a helicopter I soon learned that helicopters should not take off like they do in the movies.

The height-velocity diagram seemed to me to indicate that I should accelerate near the ground and get up to around 50kts before climbing above 5 feet.

As I began to accelerate there came a time at around 16kts to 24kts where translational lift was apparent and I had to make adjustments to keep from climbing into the dark side of the height-velocity diagram or introducing yaw.

Stan has done a nice job of explaining why so I am more focused on what to do about it.

I trained in 2002 so things may have changed and helicopter is not on my private pilot, rotorcraft, gyroplane license.

Thank you, Vance
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Last edited by Vance; 04-15-2012 at 09:13 AM.
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Old 04-15-2012, 08:58 AM
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cool thanks guys. the rotorcraft hand book does not some it up in laymans terms. i understood the dry explination but couldnt exactly picture it in my head about what was actually happening to the rotor. now i get it much better thanks stan. it make sense the way you explained it.

vance-my instructor explained that we would be performing most of our take offs near the graound and avoid the "hollywood" version for a while.

thanks again stan and gab!!
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Old 04-15-2012, 12:44 PM
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Jeb_ Glad to be of some help. Hey...we are both newbies......I have read a lot of helicopter books....and kind of have boiled down my version that I can understand. I dont mind going into more detail about translational lift, transverse flow, vortex ring state, HV curve, max performance takeoffs...etc...etc.... I am even more enthused about helicopters than ever.....and I have been obsessed since my first lesson.....

Looking forward to seeing your progress here if you have the time to share all the details.

Stan
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Old 04-26-2012, 08:06 PM
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Default I got my FAA Rotorcraft handbook

I got my rotorcraft handbook today.

yeah yeah i can download it for free.

i have a digital copy and have been looking through it.

i am an analog kind of guy.

stan and gabor.

thanks for the ETL discussion.

reading the book in physical form helps much more. i get the whole thing about the transition from hover to forward flight now.

i also have the 2012 FAR/AIM. good reading.

there will be more questions about VFR and controlled airspace to come.

Question: are you required a night cross country flight for your private pilots rating?
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Old 04-27-2012, 05:34 AM
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Default 61.109 Aeronautical experience.

(c) For a helicopter rating. Except as provided in paragraph (k) of this section, a person who applies for a private pilot certificate with rotorcraft category and helicopter class rating must log at least 40 hours of flight time that includes at least 20 hours of flight training from an authorized instructor and 10 hours of solo flight training in the areas of operation listed in §61.107(b)(3) of this part, and the training must include at least—

(1) 3 hours of cross-country flight training in a helicopter;

(2) Except as provided in §61.110 of this part, 3 hours of night flight training in a helicopter that includes—

(i) One cross-country flight of over 50 nautical miles total distance; and

(ii) 10 takeoffs and 10 landings to a full stop (with each landing involving a flight in the traffic pattern) at an airport.

(3) 3 hours of flight training with an authorized instructor in a helicopter in preparation for the practical test, which must have been performed within the preceding 2 calendar months from the month of the test; and

(4) 10 hours of solo flight time in a helicopter, consisting of at least—

(i) 3 hours cross-country time;

(ii) One solo cross country flight of 100 nautical miles total distance, with landings at three points, and one segment of the flight being a straight-line distance of more than 25 nautical miles between the takeoff and landing locations; and

(iii) Three takeoffs and three landings to a full stop (with each landing involving a flight in the traffic pattern) at an airport with an operating control tower.
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Old 06-06-2012, 05:51 AM
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Default wrong pedals

i have one small problem when i am training.

if i am not directly focused on pedal work or my concentration is somewhere else i tend to push the wrong pedal in reaction to an yaw change.

for example....i start to yaw nose right i end up putting right pedal in. this really only happend when i am a little over loaded mentaly but its embaressing.

any tips or suggestions to work on this?
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Old 06-06-2012, 06:26 AM
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Think of which way you want to rotate, then push that pedal. It's like pulling the reins on a horse....pull to the left, it goes to the left.

Sounds like you simply need an hour so of pedal turn practice. Really cement that in your head because you really, really don't want to react that way when the ship yaws due to engine failure.

-John
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Old 06-06-2012, 03:20 PM
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Jeb .... you are a helicopter pilot in the making ..... I can tell by your posts .... thank you ..... I completely enjoy all of your experience .... I was glad to hear of your recent raise that helps keep you flying .... nearly every new helicopter pilot I met loves transit buses and macaroni .... and a raise once in a while .... and wants to quit by about hour 6 in training ... then we cross the line and the magic happens .... (smile) ....

Try your best to get a grasp of the theory ..... and then one day you will develop a translation lift and Height Velocity Curve devoted to your own personal safety as you fly your helicopter .... best wishes in your studies and training.

.... I want to send you a box of macaroni but I am too busy eating steak .....

One day I hope to stand around a steak pit with you sharing student flying experiences .... helicopter pilots have a reputation for only telling true stories .... we have a legend to uphold .... best wishes to you Jeb.

Arnie
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Old 06-06-2012, 03:35 PM
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Hey Jeb,

You remind me of ME!

Let me add my 2 cents to the H/V question you asked. I don't know anything about you so excuse me If I oversimplify.

1) In physics, there's 2 kinds of energy. A brick traveling 60 MPH horizontally can do a lot of damage. That is KINETIC ENERGY. A Brick sitting perfectly still at 5,000 above the ground can do a lot of damage IF DROPPED. That's POTENTIAL ENERGY.

2) A helicopter needs energy to spin the rotor and make lift. It usually gets that energy from the fuel, in the form of combustion.

3) If the engine were to quit in flight. You need energy to safely lower you to the ground by spinning the rotor.

4) That energy can be harvested from your KENETIC ENERGY (horizontal speed like the 60MPH brick), or from your POTENTIAL ENERGY (vertical height of the 5,000 ft brick), OR from a combination of the two.

5) The H/V curve is just a graphic plot of the altitude/airspeed combinations where, if the engine quit, you would have enough COMBINED energy to safely lower your butt to terra-firma.

To add even MORE understanding, A HEAVIER aircraft has more kinetic energy and more potential energy so an R-22 that has one pilot that weighs 150# and 1/4 tank of gas, does NOT give you as many options when the engine quits, as it would if you had your fat brother with you and a full tank.

Arguably, the thing that stands out most on the H/V chart....Go vertically up the ZERO airspeed line to the point where you exit the shaded area. That's the minimum hovering altitude you can get to the ground safely from, if the engine fails. (Except hovering autorotations of course)

The low altitude shaded area in the higher airspeeds, is the weird area where you have enough kinetic energy to get down, but there's not enough height to allow the feeble human brain the 1.5 - 2 seconds it needs to react, before hitting the ground.

Last edited by bryancobb; 06-06-2012 at 04:02 PM.
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