 |
Matronics Email Lists
Web Forum Interface to the Matronics Email Lists
|
| View previous topic :: View next topic |
| Author |
Message |
dave.saylor.aircrafters(a
Guest
|
 Posted: Sun Mar 03, 2013 7:46 pm Post subject: Behind the curve, was RV-10 Stall Characteristics |
 |
|
Am I correct in relating minimum sink to best angle? And best glide
distance to best rate? Or is it the other way around, or just
nonsense?
FWIW, I use about 80 KIAS for best angle, and I see about 95 for best
rate. I usually try to use the AOA for best glide, though, at the
split bar. Are other people seeing very different numbers?
Then basically, at some point on the approach are we almost certainly
behind the power curve? My understanding has been that it meant you
were "dragging it in", using a lot of power and still sinking, and
that if you pulled the power off without lowering the nose, you'd
stall.
Maybe I'm stating the obvious...
Dave Saylor
831-750-0284 CL
On Sun, Mar 3, 2013 at 5:29 PM, Bob Turner <bobturner(at)alum.rpi.edu> wrote:
| Quote: |
Being "behind the power curve" means that you are flying slower than minimum sink speed. If you plot the power required to maintain level, unaccelerated flight, it is U shaped, sort of. The bottom, minimum point is the speed at which you will have minimum sink in a power out situation. (It is not best glide speed). In many light aircraft minimum sink speed is just a little faster than stall speed. Let's say it's 65 kias. Then to fly straight and level at 64 kias takes more, not less, power than at 65 kias. Flying straight and level at 63 kias requires even more power. For this reason this region is also called the "region of reverse command". In a power off glide, as already pointed out, once you are below minimum sink, a lower speed results in an increased vertical descent rate (since it requires more power which in this case is coming from gravity). In high density altitude takeoffs (limited engine power) pilots sometimes get into trouble by lifting off at too low an airsp!
eed. e.g., they lift off at 60 kias and climb in ground effect to 30 ft agl. They then find themselves in a situation where they are using every bit of power they have to just maintain level flight. If they had instead lifted off at 65 kias they would find they needed less power to maintain that same level flight at 30 ft agl, and could use the excess power to accelerate or climb.
--------
Bob Turner
RV-10 QB
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=395563#395563
|
| | - The Matronics RV10-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?RV10-List |
|
|
|
| Back to top |
|
 |
Bob Turner
Joined: 03 Jan 2009
Posts: 885
Location: Castro Valley, CA
|
| Posted: Sun Mar 03, 2013 8:41 pm Post subject: Re: Behind the curve, was RV-10 Stall Characteristics |
|
|
In an ideal airplane best rate of climb speed would be the same as minimum sink speed. But real props, even CS props, can only be optimized at one speed, and that is nearly always cruise speed. The result is that as airspeed increases from 60 to 70 to 80 to 90 .... knots the prop efficiency is increasing, and by quite a bit. The increase of prop efficiency pushes Vy to much larger values than minimum sink. Graphically you would plot power available (basically 260 HP times the prop efficiency) on top of power required. Vy is where the difference between the two curves is a maximum. Maximum level speed is where the curves cross, on the right.
It is unlikely you get behind the power curve on a typical approach prior to the flare. Minimum sink for an ideal airplane is 0.77 (actually the fourth root of one third) times best glide, typically just a bit higher than stall speed. But if you drag it in just a few knots above stall, then yes.
If you want to know exactly, just fly straight and level at 70, 69, 68, etc knots, and note the power setting needed. When the power required starts going up, you're on the back side.
| | - The Matronics RV10-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?RV10-List |
|
_________________
Bob Turner
RV-10 QB |
|
| Back to top |
|
|
jkreidler
Joined: 13 Feb 2008
Posts: 151
Location: Sheboygan Falls WI
|
| Posted: Mon Mar 04, 2013 6:02 am Post subject: Re: Behind the curve, was RV-10 Stall Characteristics |
|
|
All, one of the best books around to explain all of this is "Aerodynamic for Naval Aviators". The book is available for free download on the FAA's web site at the address below. Actually they have a number of free books with good info, check out: http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/
http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/00-80T-80.pdf
Since nearly every build eventually becomes a test pilot it is a good idea to understand the basics.
Please know that below is my interpretation of ‘behind the curve’, I do not have formal training in aerodynamics – but I did stay at a Holiday Inn Express.
If you look at page 42 of the PDF (book page 24) you see a plot of the coefficient of lift (CL) relative to the angle of attack (AOA). At the top of the curve the AOA increases while the CL decreases up until the wing stalls. This is one aspect of 'behind the curve'. The second aspect of 'behind the curve' is found on pages 46 & 47 of the PDF (book pages 28 & 29), where they talk about the drag coefficient (CD) relative to lift (L/D). This says that as the AOA is increased the CD will increase. The AOA which yields the best L/D is called L/D Max (the most lift for the least drag), at AOA’s greater than what produces L/D Max you are using more AOA to produce less lift with more drag until stall. The powerplant (gravity or Lycoming) needs to provide extra power to overcome the extra drag to produce lift. You will notice that everything refers to angle of attack and not airspeed as a wing will always act the same at a certain angle of attack, not true with airspeed. Which is one reason why AOA indicators are such neat tools; they help you know where the edge is rather than trying to predict the edge based on airspeed combined with other factors. One last point from page 46 of the PDF (28 of the book) – notice the differences in shapes of the curves for different airfoils. Some of the curves have fairly flat backsides (NACA 63-006), while some have dramatic drop offs (NACA 63-009). The flat backs are typical of training aircraft where the ‘back side of the curve’ wasn’t as bad. Our RV’s have steeper backsides as a result of the tradeoffs made to get a good high speed wing with low stall speeds. This is something that gets new RV pilots sometimes when they get on the backside, they are not as used to how much power it can take to correct a fast sink, remember that power can come from Lycoming or gravity, push the throttle or the stick, but it has got to happen quicker and more aggressively than that trainer we all got used to. I do not have a plot for the RV-10 – sorry.
Feel free to correct me on this, as I said I do not have formal training in this area of engineering. Beyond that please take some time to investigate this for yourselves, you may learn something, or at very least find a cure to insomnia. Above all please continue to share with the group things that surprised you, I have not experienced this one first hand, but I remember when a slip nearly got me (see the archives).
Thanks – Jason
| | - The Matronics RV10-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?RV10-List |
|
_________________
Jason Kreidler
4 Partner Build - Sheboygan Falls, WI
Tony Kolar, Kyle Hokel, Wayne Elser, Jason Kreidler
N44YH - Flying - #40617 |
|
| Back to top |
|
|
Bob Turner
Joined: 03 Jan 2009
Posts: 885
Location: Castro Valley, CA
|
| Posted: Mon Mar 04, 2013 11:04 am Post subject: Re: Behind the curve, was RV-10 Stall Characteristics |
|
|
Jason, since you said it's okay to offer some corrections...
One of the hardest concepts for physics students to grasp is that when they're holding a big rock in front of them, they are in fact doing no work (in the physics sense), since work (also called energy) requires a force acting through a distance.
Power (which is energy per second) cannot be directly compared to lift or drag, which are forces. So L/D max (I find it easiest to think of the lift fixed at the airplane's weight, and the drag varying) is where the least force is required to keep the airplane in unaccelerated flight. But it is not where the least power is required, because at slightly lower speeds the force is slightly greater, but the distance per time (speed) is lower, and the product of the two (power) is less. So you end up with minimum sink (minimum power required) speed being less than best glide speed. Remember best glide is not an attempt to minimize power, but rather to maximize the ratio of forward distance traveled over vertical distance lost. In some ways comparable to the fact that maximum range speed is higher than maximum endurance speed.
So you don't get behind the power curve (the region where it takes more power to maintain level flight as you go slower) until you are slower than minimum sink speed, which, as I said, is 0.76 (fourth root of 1/3) slower than best L/D speed for an ideal plane.
| | - The Matronics RV10-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?RV10-List |
|
_________________
Bob Turner
RV-10 QB |
|
| Back to top |
|
|
jkreidler
Joined: 13 Feb 2008
Posts: 151
Location: Sheboygan Falls WI
|
| Posted: Mon Mar 04, 2013 1:02 pm Post subject: Re: Behind the curve, was RV-10 Stall Characteristics |
|
|
Thanks Bob - let me say this a bit differently, maybe we are saying the same thing.
As you reduce power to slow down and increase AOA to maintain level flight you will continue to reduce drag, at some point (L/D Max) the drag will again start to increase, which is the point you have moved to the backside of the curve. You will need further increase AOA and increase power to maintain level flight, until you slow to a speed where maximum AOA is reached and the wing stalls regardless of power setting. Important to note the distinction, before reaching L/D Max you can decrease power, increase AOA and maintain altitude, after passing L/D Max you will need to increase power and AOA to hold altitude. The higher power setting is required to offset the increased drag. In order to get out of the stall we need to reduce the AOA either by adding enough power to overcome the drag plus the power needed to alter the relative wind (AOA), or by pushing the nose over to alter the relative wind (AOA) – ideally both.
Question on the formula you sight, is that good for all types of airplanes, or is the number different for say a Cessna versus a Cirrus?
Either way thanks for the feedback, and by all means let me (and the group know) if I still don’t get it, I am not sure I fully understand you distinction between power and force as it applies here, from a theoretical standpoint I understand perfectly. I guess when I say power I am referring to engine settings, maybe not the best form.
| | - The Matronics RV10-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?RV10-List |
|
_________________
Jason Kreidler
4 Partner Build - Sheboygan Falls, WI
Tony Kolar, Kyle Hokel, Wayne Elser, Jason Kreidler
N44YH - Flying - #40617 |
|
| Back to top |
|
|
Bob Turner
Joined: 03 Jan 2009
Posts: 885
Location: Castro Valley, CA
|
| Posted: Mon Mar 04, 2013 1:38 pm Post subject: Re: Behind the curve, was RV-10 Stall Characteristics |
|
|
Jason, not quite right yet.
Power is calculated by multiplying the force times the velocity. I know it is not intuitive but you can get away with less power by slowing down, even though the drag force goes up.
To make up numbers for the airplane, suppose at best L/D the force required to overcome the drag is 100 lbs, and the speed is 80 knots. The power required is 8000 (in some strange units). Now if you slow to 70 knots, the drag is up to, say 110 lbs. But the power required is lower, 7700 (same strange units), than it was at best L/D, because while the drag force went up, it went up more slowly than the speed went down. As you go still slower, the rate of increase of drag gets larger, and eventually it goes up faster than speed goes down. That's minimum sink speed, and slower than that you have the strange phenomenon that level flight requires more power if you want to go even slower. BTW, as speed goes to zero power required goes to infinity, not zero, because the drag goes to infinity "faster" than the speed goes to zero.
The usual ideal airplane model approximations are: parasitic drag proportional to v squared; wing lift proportional to angle of attack and v squared, and equal to weight, induced drag equal to angle of attack times weight which makes induced drag proportional to one divided by v squared. When you multiply the force (drag) by v to get power you end up with terms that are proportional to v cubed and 1/v. When you solve this equation you'll end up with v to the fourth, that's where the fourth root comes from. When you differentiate the v cubed term (to find the minimum) it becomes 3 v squared; that's where the 3 comes from. Minimum sink is fourth root of 1/3 times best glide. This is all a lot simpler if you can find graphs of "Power required" and "power available (including prop efficiency)".
| | - The Matronics RV10-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?RV10-List |
|
_________________
Bob Turner
RV-10 QB |
|
| Back to top |
|
|
jkreidler
Joined: 13 Feb 2008
Posts: 151
Location: Sheboygan Falls WI
|
| Posted: Mon Mar 04, 2013 6:50 pm Post subject: Re: Behind the curve, was RV-10 Stall Characteristics |
|
|
Have a look at the Naval Aviators Handbook starting on page 353 - they explain operating on the backside of the curve (region of reverse command). They reference power as thrust. You are going a bit further and talking about energy management. Thanks for your help in understanding this Bob - Jason
| | - The Matronics RV10-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?RV10-List |
|
_________________
Jason Kreidler
4 Partner Build - Sheboygan Falls, WI
Tony Kolar, Kyle Hokel, Wayne Elser, Jason Kreidler
N44YH - Flying - #40617 |
|
| Back to top |
|
|
Bob Turner
Joined: 03 Jan 2009
Posts: 885
Location: Castro Valley, CA
|
| Posted: Mon Mar 04, 2013 7:49 pm Post subject: Re: Behind the curve, was RV-10 Stall Characteristics |
|
|
Yes, that's a good reference. Look at the graph of power required at the bottom of page 98. It shows how minimum power occurs at a lower speed than max L/D, and in fact is about 0.8 less.
This reference frequently mixes turbojet engines with propeller driving engines. There are some important differences, explained around page 96 and following. Turbojets tend to put out constant thrust regardless of speed ( and so put out more power at higher speeds), while our internal combustion engines put out constant power, not changing with speed (actually we can get a small amount of extra power at higher speeds due to ram air pushing the manifold pressure up a bit).
| | - The Matronics RV10-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?RV10-List |
|
_________________
Bob Turner
RV-10 QB |
|
| Back to top |
|
|
|
|
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum
|
Powered by phpBB © 2001, 2005 phpBB Group
|
Get Email Distribution Too!
FAQ
Search
Memberlist
Usergroups
Register
Profile
Log in to check your private messages
Log in
