L/D max que
- Thread starter ppragman
- Start date
I am unsure if I am reading this right because you say BR exceeds BG. I take that to mean the Vbr speed is higher than Vbg, which is untrue. Best range correlates to the speed that requires the least power available, also known as min sink. It is typically 5 knots below that of best glide. I don't think the prop windmilling or not plays a role in a comparison of these two speeds.
Sure it changes drag and the speed of best glide occurs at min drag. So a windmilling prop would equate to a higher best glide speed than a feathered or stopped prop. But the power required curve is derived from the drag curve, so it would move as well. Conclusion, both speeds would change. If memory serves me the HP required curve is D*V/<some number constant...300 something>.
ppragman
No pasa nada.
What I'm getting at is this:
Sealevel best glide in the 207 is 80kts, published sealevel best range at 45% power is 106kts. True or calibrated doesn't give you 26 knots, so why is this?
I was understanding that best glide was at L/D max too, but I noticed this, and was alarmed.
I have two assumptions, but am not sure.
One, I know min sink is the best range because it requires the least amount of power to attain level flight. How many people would really cruise at 75 knots though?
Two, maybe it has to do with your engines efficiency at given power settings. To my knowledge, lower power settings are almost always more efficient. However, I have not flown many aircraft, maybe there is some equilibrium speed where the GPH provides X speed that equates to best range. :dunno:
ppragman
No pasa nada.
I thought L/D max was your best range, and thus best glide? Regardless:
Yeah, I dunno either, I'm reading some stuff about long range navigation, and reading a lot about Vbr and L/D max, here are the speeds for power settings at sea level for the 207:
75% power - 135KTAS
65% power - 127
55% power - 118
I was alarmed at the discrepancy so I pulled out an old 1976 172M POH,
Best Glide: 78KIAS
Best Range is at 55% power and 97KTAS, 45% power is closer (86KTAS) but it results in less range.
Ok, well, maybe something aerodynamic about the 172M is interferering.
So I am thinking there is an equilibrium point maybe, or perhaps since the best glide speed is published for "propeller windmiling" the L/D ratio at best glide speed isn't the L/Dmax for an engine generating power. Just some thoughts.
ppragman
No pasa nada.
Ok, I found this:
http://www.auf.asn.au/groundschool/umodule2.html
• Vbr — best range, or Vmd — minimum drag, is the speed that provides maximum L/D by producing minimum drag and thus the best power-to-speed ratio. This speed might utilise about 55% power and is usually flown at the lowest altitude where the throttle is fully open to obtain that speed. Vbr/Vmd decreases as the aircraft weight decreases from MTOW. It's rather boring to fly at that speed, wind conditions have to be taken into account, and the fuel saving may not be that significant compared to flying at a speed 10% faster. Also, the engine manufacturer's operating recommendations should be followed, but mixture is usually leaned, and minimum rpm set if a constant speed propeller is fitted. Vbr/Vmd has the same basic airspeed range as Vy and Vbg [below].
There is a difference in concept between Vbr and Vmd. Pilots of low-powered aircraft are generally not interested in the best power to airspeed ratio in cruise; ground speed achieved per litre is far more significant, so in some conditions Vbr equals Vmd but in headwind conditions Vbr is increased. Look at the diagram from section 1.7 at left and note the pink line that has been drawn from the junction of the vertical and horizontal axes tangential to the power required curve. That line just touches the curve at a position corresponding to the minimum drag airspeed Vmd. Now imagine a 30 knot headwind and start the tangential line from a point along the horizontal axis that is equivalent to 30 knots; that (the blue line) will be tangent to the power required curve at a position corresponding to a higher speed — Vbr for a 30 knot headwind. The rule of thumb is to add half the head wind to the basic Vbr, which, in this case, indicates a Vbr that is about 15 knots greater than Vmd. This is the same principle used by glider pilots to establish their best penetration speed — see the speed polar curves for optimum glide speed in the 'Coping with emergencies guide'.
http://www.auf.asn.au/groundschool/umodule2.html
I can't find a source right now, it is late and I am lazy. But I am certain of these:
Best glide: Longest glide distance, occurs at the tangent line on the power required curve, and equates to the speed of minimum drag.
Min sink: Longest time in the air, provides the longest range, and occurs at the speed of minimum power required.
You can see all of this from that picture you linked. The only thing you can't see is they talk about best glide as the longest range, and with engine power it isn't. I will source it for you sometime tomorrow if someone doesn't beat me to it.
Think of it this way, look at that curve. If you fly 5 knots slower you land at the bottom of that curve, min sink or minimum power required. So you take out approx 100 RPMs at a cost of 5 knots. In the mean time you probably save a half a gallon per hour.
This is an easy thing to figure out though. What is your fuel burn at the recommended setting and what airspeed do you get? What is the fuel burn and airspeed at best glide? At min sink? Do an airspeed/fuel burn ratio and you have your answer.
Grr I tried editing that out, but now the administration only lets you do that for 15 minutes after.
Hey doug, why?
Anyways, FWIW ignore the above post.
Flight Theory for Pilots agrees what what you posted. However, I am fairly certain I have two other books that contradict this. I may have misread them, but I will go back and look through them tomorrow and try to hunt that down.
Again, IGNORE THE PREVIOUS POST, IT MIGHT BE WRONG!
Hey doug, why?
Anyways, FWIW ignore the above post.
Flight Theory for Pilots agrees what what you posted. However, I am fairly certain I have two other books that contradict this. I may have misread them, but I will go back and look through them tomorrow and try to hunt that down.
Again, IGNORE THE PREVIOUS POST, IT MIGHT BE WRONG!
tgrayson
New Member
You have propeller/engine efficiency to consider, too, when you're talking about best range, but those issues don't matter when you're performing a glide. (But yes, it's true that the drag at a given velocity is greater power on than off, due to propeller slipstream scrubbing.)
The details can be found in Breguet's Range Equation. There are several forms of the equation, but for a piston, the variables are propeller efficiency, power specific fuel consumption, L/D, and quantity of fuel.
ppragman
No pasa nada.
Min sink is best endurance, L/D max is best glide when the prop is windmiling, and best range.
Thanks a bunch, I'll look into the equation!
ppragman
No pasa nada.
Oops , disregard what I wrote before.