MaxP or Towering
Joined: Sep 2001
Posts: 1,051
Likes: 0
From: CA
The MP "constant angle" departure from a hole gives me the creeps. It really does. Lifting into a hover and then pulling all she has while launching off your comfortable energy conserving ground cushion is scary.
My greatest desire is to get the rotor above the trees. Once in the clean air the aircraft will either want to fly or it will want to settle.
If it settles....then it is an easy lowering into a nice known quantity.
If it settles during the constant angle departure then you have issues with the planet.
I have seen plenty of guys launch off helipads around here with the intent of doing the constant angle departure and EVERY time I can honestly attest that I had a moment of asking myself if we were going to make it (I usually do the "jump on the imaginary brake pedal thing that guys do when their wife drives them somewhere......
)
Just the slightest drop in the wind or a swirl to the rear and the landing gear was in the trees with no room or power to solve the problem.
A vertical climb to 30-40 ft, FEELING what the machine wants to do up there and then a transition into fwd flight with an emphasis on airspeed over altitude is much more comfortable.
Five cents worth from a NZ/OZ/CA perspective???
My greatest desire is to get the rotor above the trees. Once in the clean air the aircraft will either want to fly or it will want to settle.
If it settles....then it is an easy lowering into a nice known quantity.
If it settles during the constant angle departure then you have issues with the planet.
I have seen plenty of guys launch off helipads around here with the intent of doing the constant angle departure and EVERY time I can honestly attest that I had a moment of asking myself if we were going to make it (I usually do the "jump on the imaginary brake pedal thing that guys do when their wife drives them somewhere......
)Just the slightest drop in the wind or a swirl to the rear and the landing gear was in the trees with no room or power to solve the problem.
A vertical climb to 30-40 ft, FEELING what the machine wants to do up there and then a transition into fwd flight with an emphasis on airspeed over altitude is much more comfortable.
Five cents worth from a NZ/OZ/CA perspective???
Last edited by Steve76; 17th July 2003 at 09:57.
Joined: Jan 2003
Posts: 365
Likes: 15
From: CYPG
Advanced Take-off's
From behind the curtain in Canada, we have to teach what is called the "vertical take-off" for situations with no space for acceleration, and the "Towering" when you have some space. That is put forth in the Flight test standards, so that is it. However, I agree that the Vertical route is more comfy, since you can always go back into the hole you left, even if it may be a somewhat hard landing...
In the states we thaught the Max performance route, which I found slightly different than the towering. Here you don't have to use all power to get out, sufficient to get you going is enough, as long as you got enough. I personally like that better, but still better is the vertical departure, since you don't have to do a quick stop or rapid deceleration in case of those pesky warning lights.
Don't know if that is of any help but...
In the states we thaught the Max performance route, which I found slightly different than the towering. Here you don't have to use all power to get out, sufficient to get you going is enough, as long as you got enough. I personally like that better, but still better is the vertical departure, since you don't have to do a quick stop or rapid deceleration in case of those pesky warning lights.
Don't know if that is of any help but...
Joined: Jan 2001
Posts: 420
Likes: 0
From: AB, Canada
Very interesting post. I just spent the last week doing multiple vertical departures (straight up, clear the barrier, move forward). Experienced loss of wind a couple times and decided I'd prefer to get bigger engines or less cargo.
A couple points to add:
1. Climbing vertically can actually use less power than hovering OGE. Induced flow changing blade angle of attack is the biggest culprit, if I recall correctly. Rather than thinking about it, try it. Hover OGE, note power. Increase collective slightly to initiate a climb. Once climb established, set power to same as recorded hover OGE power. You should still notice a climb. Lower power slightly to see if you can maintain a climb. I tried it in a B412 and it worked.
2. Max angle of climb cannot be computed for helicopters as easily as it can for f/w since our thrust vector changes. Too much to go through here, but just get a basic f/w aerodynamics book for the analysis. Something could be done for helicopters, but many more factors would have to come into play (especially AUW and DA). Point is, it's not as easy to obtain.
Matthew.
A couple points to add:
1. Climbing vertically can actually use less power than hovering OGE. Induced flow changing blade angle of attack is the biggest culprit, if I recall correctly. Rather than thinking about it, try it. Hover OGE, note power. Increase collective slightly to initiate a climb. Once climb established, set power to same as recorded hover OGE power. You should still notice a climb. Lower power slightly to see if you can maintain a climb. I tried it in a B412 and it worked.
2. Max angle of climb cannot be computed for helicopters as easily as it can for f/w since our thrust vector changes. Too much to go through here, but just get a basic f/w aerodynamics book for the analysis. Something could be done for helicopters, but many more factors would have to come into play (especially AUW and DA). Point is, it's not as easy to obtain.
Matthew.
Joined: Jun 2001
Posts: 251
Likes: 0
From: Australia
isn't the whole idea of a constant angle departure, to get out when you CAN'T tower out (go strait up and clear every thing by 10 feet or so)?
strait up has to be the safest way, the chances of losing lift at a critical moment or just mis judging the power required for a max p T.O, are much greater. if there's not enough power to go strait up then looking at another way is the go. i shall experiment!
STEVE, if just the landing gear was in the trees, you've done well!
strait up has to be the safest way, the chances of losing lift at a critical moment or just mis judging the power required for a max p T.O, are much greater. if there's not enough power to go strait up then looking at another way is the go. i shall experiment!
STEVE, if just the landing gear was in the trees, you've done well!
Joined: May 2002
Aviation Qualifications: ATP+Mil
Posts: 18,633
Likes: 1,072
From: Downeast
Much chatter about profiles....now how about some discussion on power checks...power available checks...wind checks....determining available takeoff distance to barriers....determining height of barriers....determining direction, velocity of wind, and gust strength and duration....consideration of mechanical turbulence effects....sun direction...??? All things that go into making an informed decision as to when, how, and what way to takeoff.
Let's define a confined area.....not all confined areas are the same....would you always take off downhill for instance? Have you ever operated from an area that a slight uphill departure could lead to 4,000' altitude gain after 100 meters whereas downhill put you into a narrow steeply descending rocky floored canyon?
This is not a black and white issue....nor does one set of rules always fit the circumstances. Those of our numbers that have done some serious bush flying understand that concept. It all comes down to Risk Management.
Let's define a confined area.....not all confined areas are the same....would you always take off downhill for instance? Have you ever operated from an area that a slight uphill departure could lead to 4,000' altitude gain after 100 meters whereas downhill put you into a narrow steeply descending rocky floored canyon?
This is not a black and white issue....nor does one set of rules always fit the circumstances. Those of our numbers that have done some serious bush flying understand that concept. It all comes down to Risk Management.
Joined: Apr 2003
Posts: 3,012
Likes: 1
From: USA
heedm,
You are correct that once stabilized in an appreciable vertical climb, the power is less than that you would need for the same climb done at Vy, but all climbs need more power than steady level flight. I have done some careful work to see what it takes to make a vertical take off as compared to a steady HOGE, and I have never successfully risen above about 2 rotor diameters without using 1.5% more power than the power needed to HOGE. Even with that 1.5% more, the climb rate was a paltry 50 feet per minute.
PPRUNERS, Give it a try. Take a very still day, and just HOGE at 1.5 times the rotor diameter (65 feet for an S-76, for example) and carefully note the power. Try to have no climb or descent, it might take a little averaging of the torque swings. A good way to do it is to release the collective, and just see if you average a nice HOGE, rather than get involved in a nice pilot-in-the-loop oscillation. Radar altitude really helps here. Set back down to a low hover and then pull exactly that power. In my attempts, I could only climb slowly to a few feet above the 1.5 diameter, then watch as the aircraft bobs up and down in neat altitude oscillations around the hover height.
I could only avoid this oscillation by starting all over again and using about 1.5% more indicated torque (actually 2% of the power I was pulling, ie 2% of 75% total torque). If I pulled the extra power, I slowly and steadily climbed right up to several hundred feet.
You are correct that once stabilized in an appreciable vertical climb, the power is less than that you would need for the same climb done at Vy, but all climbs need more power than steady level flight. I have done some careful work to see what it takes to make a vertical take off as compared to a steady HOGE, and I have never successfully risen above about 2 rotor diameters without using 1.5% more power than the power needed to HOGE. Even with that 1.5% more, the climb rate was a paltry 50 feet per minute.
PPRUNERS, Give it a try. Take a very still day, and just HOGE at 1.5 times the rotor diameter (65 feet for an S-76, for example) and carefully note the power. Try to have no climb or descent, it might take a little averaging of the torque swings. A good way to do it is to release the collective, and just see if you average a nice HOGE, rather than get involved in a nice pilot-in-the-loop oscillation. Radar altitude really helps here. Set back down to a low hover and then pull exactly that power. In my attempts, I could only climb slowly to a few feet above the 1.5 diameter, then watch as the aircraft bobs up and down in neat altitude oscillations around the hover height.
I could only avoid this oscillation by starting all over again and using about 1.5% more indicated torque (actually 2% of the power I was pulling, ie 2% of 75% total torque). If I pulled the extra power, I slowly and steadily climbed right up to several hundred feet.
Joined: Jan 2001
Posts: 420
Likes: 0
From: AB, Canada
I'll give that a try, Nick. Sounds much more believable than what I posted. I guess in the steady state we're able to use less power than required for hover OGE, but overall we need to use excess power to initiate that vertical climb.
This brings me closer to a theory I have on vertical takeoffs. We check power at 4' and then lower to 1', smoothly but rapidly increase to a high power setting. I believe that at 1' we get the maximum benefit from ground effect, obtaining the most excess power to initiate a climb, and then maintaining a high power setting we obtain the maximum vertical rate of climb possible. It may be that this technique allows you to climb to an OGE hover even when you don't have enough power to maintain the hover (although the rates of climb would be incredibly slow and it would require exceptional skill and luck to see it).
I think the 1' makes more sense than starting from a 0' hover because there would be fewer control inputs and more efficient use of available power.
Matthew.
This brings me closer to a theory I have on vertical takeoffs. We check power at 4' and then lower to 1', smoothly but rapidly increase to a high power setting. I believe that at 1' we get the maximum benefit from ground effect, obtaining the most excess power to initiate a climb, and then maintaining a high power setting we obtain the maximum vertical rate of climb possible. It may be that this technique allows you to climb to an OGE hover even when you don't have enough power to maintain the hover (although the rates of climb would be incredibly slow and it would require exceptional skill and luck to see it).
I think the 1' makes more sense than starting from a 0' hover because there would be fewer control inputs and more efficient use of available power.
Matthew.
Joined: Apr 2003
Posts: 3,012
Likes: 1
From: USA
Matthew,
You are right on both counts.
The power to hover OGE is less than the power needed to climb vertically, but you get a speedier climb rate by starting the takeoff at a low hover. or gear ightly on the deck, using the extra power from the IGE hover (about 12% power available) to make the climb happen.
In a narrow sense, a vertical climb is actually more efficient than a Vy climb, due to rotor inflow effects - almost twice as much climb rate for the change in torque. In other words, if we add 100 horsepower to a vertical climb, we get almost twice the climb rate than if we add 100 horsepower to a helo in forward flight. But since the HOGE takes so very much more power than forward flight, Vy is always the best climb speed in terms of peak climb rate for the total power consumed.
As a matter of interest, the "elevator factor" is perhaps worth discussing. Basically, if you start in level flight and add some power, the exact climb rate can be calculated. A horsepower is 550 foot-pounds per second, the power needed to continuously lift 550 lbs at a vertical speed of foot per one second. Take the weight of your aircraft, and the horsepower that you add to the level flight, and the exact climb rate can be calculated.
For an S-76, at 10,000 lbs GW, if we add 10% torque (which is 160 Horsepower in a B or C), we add 550x160=88,000 fl-lbs. This is enough to cause a climb rate of 8.8 feet per second, or 528 feet per minute. Actually, the rotor is about 86% efficient, so we lose 14% of that climb, and end up with 454 feet per second.
The climb-descent chart in the US Army flight manuals is based on this effect. For the Black Hawk, 100% torque is 2828 horsepower, so use that calibration to check the chart.
You are right on both counts.
The power to hover OGE is less than the power needed to climb vertically, but you get a speedier climb rate by starting the takeoff at a low hover. or gear ightly on the deck, using the extra power from the IGE hover (about 12% power available) to make the climb happen.
In a narrow sense, a vertical climb is actually more efficient than a Vy climb, due to rotor inflow effects - almost twice as much climb rate for the change in torque. In other words, if we add 100 horsepower to a vertical climb, we get almost twice the climb rate than if we add 100 horsepower to a helo in forward flight. But since the HOGE takes so very much more power than forward flight, Vy is always the best climb speed in terms of peak climb rate for the total power consumed.
As a matter of interest, the "elevator factor" is perhaps worth discussing. Basically, if you start in level flight and add some power, the exact climb rate can be calculated. A horsepower is 550 foot-pounds per second, the power needed to continuously lift 550 lbs at a vertical speed of foot per one second. Take the weight of your aircraft, and the horsepower that you add to the level flight, and the exact climb rate can be calculated.
For an S-76, at 10,000 lbs GW, if we add 10% torque (which is 160 Horsepower in a B or C), we add 550x160=88,000 fl-lbs. This is enough to cause a climb rate of 8.8 feet per second, or 528 feet per minute. Actually, the rotor is about 86% efficient, so we lose 14% of that climb, and end up with 454 feet per second.
The climb-descent chart in the US Army flight manuals is based on this effect. For the Black Hawk, 100% torque is 2828 horsepower, so use that calibration to check the chart.
