V speeds for a RV-6

[rv6ejguy]

Originally Posted by tin man

A lot of good baseline speeds, how about BEST GLIDE?
Tom

80-85 knots on my combo.

[hevansrv7a]

Best Glide and perhaps some other speeds can only be correct at one point on the lines for density altitude and aircraft weight. I'm not sure that this applies to all speeds, but it does apply to many of them. Angle of attack is a more consistent indicator. I know that POH's use speed, but that doesn't change this.

If you can measure AOA, you can observe the speed and note it. AOA will be more correct over more varieties of situations.

Example (debated in this forum before, but I'm still convinced): Best Glide is about the same as Best L/D in no-wind conditions and giving away some errors from prop pitch, idle vs. stopped, etc. If you can determine the AOA at which you are A. maintaining level flight and B. using the least power then that's IMHO the AOA for best L/D. Why? because total drag is at a minumum as measured by power required for level flight and because the lift is a constant equal to the weight at that moment.

OK, now find the (different) airspeed at which the same AOA happens while engine is not producing power (you choose how realistic to make this). That's your best glide speed under those conditions.

Just a suggestion.

[Kevin Horton]

Originally Posted by hevansrv7a

Example (debated in this forum before, but I'm still convinced): Best Glide is about the same as Best L/D in no-wind conditions and giving away some errors from prop pitch, idle vs. stopped, etc. If you can determine the AOA at which you are A. maintaining level flight and B. using the least power then that's IMHO the AOA for best L/D. Why? because total drag is at a minumum as measured by power required for level flight and because the lift is a constant equal to the weight at that moment.

Not quite. The power required for level flight is equal to the drag times the speed (with the appropriate conversion factors to sort out the units). So, the speed for minimum power required is less than the speed for minimum drag.

For more info, look at the two plots of power vs speed, and drag vs speed in the Maximum Endurance, Maximum Range, and Optimum Cruise Speeds article on the EAA Chapter 1000 (Edwards Air Force Base)page.

[hevansrv7a]

Originally Posted by Kevin Horton

Not quite. The power required for level flight is equal to the drag times the speed (with the appropriate conversion factors to sort out the units). So, the speed for minimum power required is less than the speed for minimum drag.

For more info, look at the two plots of power vs speed, and drag vs speed in the Maximum Endurance, Maximum Range, and Optimum Cruise Speeds article on the EAA Chapter 1000 (Edwards Air Force Base)page.

Well, Kevin, I am finally convinced! I only had to read the article about a dozen times. Thank you. It's great to have your expertise on the forum and this is the second time you've helped me understand an aspect of that drag curve.

Now we have a way, starting again with minimum power for level flight (MPLF). It just takes a little more work. and some graph paper. Those of us with PctPwr or FF readouts will have an easier time of this. When you've done the extra work described in the article, you have the speed at which least drag can be found and that's the AOA you want, with or without power. I don't think that the speed for MPLF is necessarily the same as best glide speed. Is it?

With the formula in the article and a little algebra, can we be lazy and just multiply the MPLF speed by 1.316 to get best L/D speed and take that AOA?

[Kevin Horton]

Originally Posted by hevansrv7a

Now we have a way, starting again with minimum power for level flight (MPLF). It just takes a little more work. and some graph paper. Those of us with PctPwr or FF readouts will have an easier time of this. When you've done the extra work described in the article, you have the speed at which least drag can be found and that's the AOA you want, with or without power. I don't think that the speed for MPLF is necessarily the same as best glide speed. Is it?

With the formula in the article and a little algebra, can we be lazy and just multiply the MPLF speed by 1.316 to get best L/D speed and take that AOA?

The theory is good, so in principle you should be able to take MPLF times 1.316, and call that best glide speed. But, this only works if you use CAS. If you use IAS, the result could be affected by differing airspeed errors at the two conditions. The speed for MPLF will be fairly slow, so it is possible that the airspeed accuracy could be affected by significant static source position error. If you want a reasonably accurate result, you also need to do testing to establish the relationship between IAS and CAS.

The best L/D speed that you get from this method would be valid with prop removed, I think. It should be very, very close to the best glide speed with prop stopped. The best glide speed with prop windmilling will be a bit slower, I think, due to the extra drag from the windmilling prop.

If you measure glide performance with engine at idle you'll get a bit different result, as the effect of idle thrust or drag will vary with speed, and this will pollute the results slightly. But, given the risks of testing with engine stopped, this is probably a good compromise, unless you can minimize the risk by testing right overhead an airfield.

[Bad Sheila]

1) I have just installed an AOA on my RV6 (Dynon Skyview) This is new to me. How do I find my best glide speed using the AOA color bars?

2) If the engine quits in flight and the prop has stopped, Is it wise to increase the airspeed from best glide (80 kts?) to get the prop turning for a restart. (Only at a high altitude of course)

[Kevin Horton]

Originally Posted by Bad Sheila

1) I have just installed an AOA on my RV6 (Dynon Skyview) This is new to me. How do I find my best glide speed using the AOA color bars?

2) If the engine quits in flight and the prop has stopped, Is it wise to increase the airspeed from best glide (80 kts?) to get the prop turning for a restart. (Only at a high altitude of course)

You do the same flight testing that you would do without an AOA system - i.e. record altitude vs time during descents at a range of speeds. Also record the AOA reading at each of these speeds. Do each speed twice - once with the wind off the left wing tip, and once heading the other way with the wind off the right wing tip.

After landing, calculate the rate of descent (ROD) for each run, and average the ROD for the two runs at each speed. Plot ROD vs speed and draw a smooth curve through the data. Draw a line from 0 speed and 0 ROD and see where it is tangent to the curve - that is the best glide speed.

Look at your flight data, and see what the AOA reading would have been at the best glide speed. That is the AOA for best glide.

[fl-mike]

Thread drifter alert:
Kevin,
How do you re-calculate max range speed and Carson's speed to account for winds aloft? If max range speed is 100mph, and you have a 100mph headwind, you may plow through a lot of air, but you don't get very far. If you have a tailwind, it would seem max range speed would shift toward max endurance speed.

[Kevin Horton]

Originally Posted by fl-mike

Thread drifter alert:
Kevin,
How do you re-calculate max range speed and Carson's speed to account for winds aloft? If max range speed is 100mph, and you have a 100mph headwind, you may plow through a lot of air, but you don't get very far. If you have a tailwind, it would seem max range speed would shift toward max endurance speed.

The easiest way to understand this problem is to consider the graphical method to determine these speeds. To find the speed for max range in the zero wind condition, you plot fuel flow vs TAS (or power required vs TAS, as fuel flow should be proportional to power). Draw a line from zero speed and zero fuel flow that is tangent to the curve. The point where this line touches the curve is the speed for max range. You can see an example in the Performance Charts section on this page on the EAA 1000 site.

If we want to see the effect of wind, we plot fuel flow (or power) against ground speed instead of TAS. If we have a head wind this shifts the curve to the left, and the tangent line hits it at a point which represents a higher TAS than in the zero wind case.

For Carson's speed, we do the same thing, except we plot drag against TAS (or against ground speed if we have wind to worry about).