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Maximum distance/gal

swjohnsey

Well Known Member
Stupid question: I'm flying my RV-4 equipped with IO360 with CS prop. I'm headed for Hawaii and have past my no turn back point and realize I might not have enough fuel to make it. What speed and power setting would I use to stretch what I have left?

I read through some articles that seem to indicate best glide is probably the speed or close to it. I played with this a little on the transition to Oshkosh trying to fly 90 knot approach. 90 knot/105 mph requires a very small power setting like 15 in/1500 rpm. I tried leaning at this setting and couldn't ever get an EGT rise. I got down to around 3 GPH on the flow meter but got a little nervous being only 1,000 AGL.

Are there any gotchas running at these tiny power settings? Is there an optimum altitude? Will a little bit of flaps help?
 
Flaps, no.

Altitude is always better, reduces the pumping losses across the throttle. Ideally up in the oxygen altitudes where you can use wide-open throttle and still make 40%-50% power for an indicated airspeed near Vy - this will likely be at the upper teens or low 20's depending on your airplane. Drag is based on indicated airspeed, but thin air gives you a large boost on true airspeed.

At those low power settings you cannot harm your engine with lean mixture - just keep going lean until it stumbles, and richen it until you're happy. You may not see much EGT change with very low throttle settings.

Best glide speed, or slightly above it, would be your maximum range - at expense of the clock. "Carson Speed" is something you might want to study up on.
 
+1 on Greg’s post. A few details:
1. In theory you want best glide speed, but in practice a little faster- because the cruise props most of us use are more efficient at speeds higher than best glide.
2. That’s ‘best glide speed’ with the wind considered. Tailwind, slower (but never less than max endurance); headwind faster than no-wind best glide.
3. Up high is better, for reasons Greg stated, but you then have to calculate the extra gas it takes to climb. So the optimum altitude depends on the distance to be covered. And don’t forget #2- you usually don’t want to climb into a stronger headwind.
 
That is the object of this exercise. I'm going to experiment some more flying 90 knots/105 mph at very low power setting (15/15) and leaning agressively . . . but at a little higher altitude so that I won't be embarrassed it the engine quits.
 
Stupid question: I'm flying my RV-4 equipped with IO360 with CS prop. I'm headed for Hawaii and have past my no turn back point and realize I might not have enough fuel to make it. What speed and power setting would I use to stretch what I have left?

I read through some articles that seem to indicate best glide is probably the speed or close to it. I played with this a little on the transition to Oshkosh trying to fly 90 knot approach. 90 knot/105 mph requires a very small power setting like 15 in/1500 rpm. I tried leaning at this setting and couldn't ever get an EGT rise. I got down to around 3 GPH on the flow meter but got a little nervous being only 1,000 AGL.

Are there any gotchas running at these tiny power settings? Is there an optimum altitude? Will a little bit of flaps help?

In this scenario, best GPH (maximum endurance) is irrelevant. What you should be trying to determine is best gallons per mile (maximum range). They are two different animals.

https://en.wikipedia.org/wiki/Range_(aeronautics)

https://en.wikipedia.org/wiki/Endurance_(aeronautics)
 
Maximum range is what I'm after. Gallon per hour is just used to get an idea about fuel consumption.

On another note, what would be the most fuel efficient climb, speed and power setting. I generally climb 25 x 25 full rich until I hit WOT. I thought about leaning a bit.
 
I'm just glad you finally used the word "exercise." Last night I envisioned you using an Inmarsat connection to get real-time advice. :eek:
 
Maximum range is what I'm after. Gallon per hour is just used to get an idea about fuel consumption.

On another note, what would be the most fuel efficient climb, speed and power setting. I generally climb 25 x 25 full rich until I hit WOT. I thought about leaning a bit.

What altitude would you be climbing to initially?
It may make sense to do step climbs as you burn fuel and reduce the load.

As a point of reference, the Cessna 182T has placarded full power fuel flow rates vs altitude. After take off if the fuel flow is above that rate you are allowed to lean it down to that rate. So even at 4000' you can do some leaning per Cessna in that plane. Upshot? Leaning can be done in climbs. If you haven't looked at all Mike Busch's webinars on lean of peak ops... do so. I recall he talks about LOP climbs.
 
SNIP..
1. In theory you want best glide speed, but in practice a little faster- because the cruise props most of us use are more efficient at speeds higher than best glide.

+1

Several long cross country trips provided opportunity to do data runs at 8K’ to 12K’ to investigate the correlation with prop setting and MPG. I assumed the lower the RPM (reduction in engine pumping losses) the better. Going below 2400 RPM however I found a slight drop in MPG. I ended up at 2450 RPM. I concluded specific prop characteristics played a role in total system performance, perhaps more so than pumping losses in some areas of the operation curve.

Averaging through the data points, I found a 2% increase in MPG at 2450 RPM as compared to 2400 RPM and 2500 RPM. Engine setting typically 20 degrees LOP.

All these runs were at a good cruise speed (170-174 kts TAS). I suspect there is another sweet spot if you want to go slower but never investigated.

RV-8 with a stock IO-360-M1B and 74” Hartzell BA CS prop.

Bottom line - you will need to build your own data base to figure out the settings you are looking for.

Carl
 
On another note, what would be the most fuel efficient climb, speed and power setting. I generally climb 25 x 25 full rich until I hit WOT. I thought about leaning a bit.

I've done some of this, in my research for max range for RTW flights. I've loaded up on fuel and done some slower shallower climbs exploring exactly what you are talking about. My best results were climbing out (after initial takeoff WOT rich) by reducing to 2400 rpm, pulling manifold pressure back to 22" (constant speed prop in my case), and going lean of peak with advanced ignition. The engine will operate perfectly fine in this case with reduced power, and you climb slower at 500-600 fpm instead of 1200-1500, but you are doing it at much reduced fuel flow versus full power and rich. As I climbed up I would constantly adjust the throttle to keep 22" on the engine until I was WOT, then continued in the climb to final altitude.

I'm not totally convinced that this method is better than WOTROP on a normal climb and then aggressively WOTLOP, to get to the same point over the ground each way, but it's an interesting exercise.
 
I'm going to give that a try. I have the Lightspeed Plasma III ignition on the top plugs that advances the ignition timing but I don't have any control. What kind of speed and fuel consumption were you planning for?
 
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I'm going to give that a try. I have the Lightspeed Plasma III ignition on the top plugs that advances the ignition timing but I don't have any control. What kind of speed and fuel consumption were you planning for?

Just playing with various ways to achieve max range. I'm planning to take my 9A around the world one of these days, that requires some serious legs over water.
 
In 1993 Embry-Riddle did a study specifically on this subject for piston powered aircraft. They found that the type aircraft or how it was designed had no effect. Basically it found that it solely depended on aircraft weight and engine power setting. They found that the important thing was to find the minimum power speed (VmPwr) required to maintain straight and level flight with no flaps at specific weights. Then every other speed is a multiple of that speed.

Max range speed (Vmr) = VmPwr/.76 (Go furthest distance per gallon)
Max endurance speed (Vme) = Vmr*.83 (Stay airborne longest but not go anywhere)
Long Range Cruise Speed (Vlrc) = Vmr*1.07 (Get somewhere faster with min fuel)

These numbers are predicated on calm winds. A head/tail wind changes the actual speed required. Figuring out the change in required speed is also part of the research paper. How to find these speeds for your airplane can be found starting on Pg 22 of the provided link.

Since I fly very long distances, I made a speed chart specifically for my airplane. I have attached a copy of my last updated speed chart and I have found it is fairly accurate.

KIS4 Speeds.jpg
 
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Testing and tweeking

I’m amazed at what these RV’s can do, and also checking max performance for my RV-7a with O-360, CS prop. Take off, ROP, 21-22 MP, prop back to 2350, and climbing, adjusting to keep it constant at 21-22, then WOT, at 2350. Lean as I go, but watching those CHT’s to keep the temps down.

My standard 7a has 42 gallon, and did 950nm with 8 gallons to spare on landing. That was with a 17k tailwind @ 17,500’. I’ve done similar flights, and looking forward to trying different altitudes and prop settings.

Besides Greg, I’m very content to be in the plane for endless hours, non-stop. Of course, max range is one element of the long range experiment. O2 level checking at higher elevations and thorough fuel management are just as important. No one wants to be a glider.

I, too, am planning for RTW and appreciate this thread and feedback from others (thanks Greg)!
 
In 1993 Embry-Riddle did a study specifically on this subject for piston powered aircraft. They found that the type aircraft or how it was designed had no effect. Basically it found that it solely depended on aircraft weight and engine power setting. They found that the important thing was to find the minimum power speed (VmPwr) required to maintain straight and level flight with no flaps at specific weights. Then every other speed is a multiple of that speed.

Max range speed (Vmr) = VmPwr/.76 (Go furthest distance per gallon)
Max endurance speed (Vme) = Vmr*.83 (Stay airborne longest but not go anywhere)
Long Range Cruise Speed (Vlrc) = Vmr*1.07 (Get somewhere faster with min fuel)

]

I don’t understand. How can max endurance speed differ from minimum power required speed?
 
I don’t understand. How can max endurance speed differ from minimum power required speed?
It also took me a while to understand what it meant.

Max endurance takes into account pilot fatigue which min power doesn't. At min power the pilot will be working so hard to avoid a stall, he will quickly become exhausted and will have to land. That cuts flight endurance significantly.

For example; if you are put into a hold for an extended amount of time, you want to use the minimum fuel so go to max endurance speed. Is it a "bad" term"? Maybe. But that is what they came up with so please don't shoot the messenger.

;)
 
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It also took me a while to understand what it meant.

Max endurance takes into account pilot fatigue which min power doesn't. At min power the pilot will be working so hard to avoid a stall, he will quickly become exhausted and will have to land. That cuts flight endurance significantly.

For example; if you are put into a hold for an extended amount of time, you want to use the minimum fuel so go to max endurance speed. Is it a "bad" term"? Maybe. But that is what they came up with so please don't shoot the messenger.

;)

This is the craziest thing I have ever heard (sorry about shooting the messenger). How does one quantify pilot fatigue? Besides, there are many airplanes where max endurance is comfortably above Vs. And for our Hilo bound pilot, he could set the autopilot to hold max endurance, then take a nap to avoid fatigue!
 
How does one quantify pilot fatigue?
The same way you quantify pilot attitude. Take data (poll) from participants then statistically analyze the results. None of these numbers are absolute, just really good starting points and extremely close for our purposes.

And for our Hilo bound pilot, he could set the autopilot to hold max endurance, then take a nap to avoid fatigue!

Our Hilo pilot wants max range (Vmr) not max endurance (Vme) so apples vs oranges. At max endurance (Vme) he would stay in the air longer but would slowly go nowhere. With max range (Vmr) you trade fuel consumption for distance before the fuel runs out. Just like if you turn your car on and leave it in a parking lot with the engine running it might run for 36hrs but you went 0 miles on that one tank of gas. If you drive at 55mph the engine might only run for 6 hours but you will be 330 miles away on that same tank of gas.

Long range cruise (Vlrc) trades a little bit more fuel for a lot more speed. You have slightly less range but you get there much faster. On my long trips (8hr+) I normally fly slightly faster than Vlrc, but upon arrival I still have plenty of reserve.

IMHO the important thing about this research is that these speeds are proportional and the methodology can be used in any piston powered airplane. There are other techniques that a pilot can use that increases or decreases the actual numbers, but this paper did not delve in them.

:cool:
 
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Of course you’re correct, our Hilo guy wanted max range. I just got so distracted by this idea that a quantified method of evaluating ‘pilot fatigue’ would be part of an engineering assessment of max endurance that I cannot think straight. By this logic max endurance is set by how long I can go without a bathroom break. Real world, yes, but as an aircraft parameter?
 
It also took me a while to understand what it meant.

Max endurance takes into account pilot fatigue which min power doesn't. At min power the pilot will be working so hard to avoid a stall, he will quickly become exhausted and will have to land. That cuts flight endurance significantly.

For example; if you are put into a hold for an extended amount of time, you want to use the minimum fuel so go to max endurance speed. Is it a "bad" term"? Maybe. But that is what they came up with so please don't shoot the messenger.

;)

I don't see anything remotely resembling this "analysis" in the paper itself. There's no mention of fatigue at all, unless I missed it somewhere.
 
yes

Please reference the section on fatigue.

It would be interesting since fatigue is basically subjective, and will be vastly different between every pilot and every situation...
 
Interesting threads ladies & gentlemen :)

They’ve been quite a few RTWs flying RVs already, and I don’t recall any of them flew at speeds this low. In any case the probably longest crossing, Westbound CA to HI could justify flying max range if one wants to maximize reserves.

I did some testing whilst preparing for my little visit to North America last Summer, and an important factor, for me only, was to not spend too much time over open waters during the crossings. I therefore flew these crossings at higher a speed than over land.
And another consideration was the frequency of technical inspections, and things such as oil changes, etc. Flying faster also saves on those…
 
Trying to remember back to my ERAU days and where I got this from. It isn't in this paper, obviously, but I remember vividly the discussion in the class about why you don't fly at VmPwr for prolonged periods of time. FWIW, this paper came as part of an even older (1960's) research paper by NASA on the same subject.

Let me keep looking through some old papers and get back when/if I find something.

They’ve been quite a few RTWs flying RVs already, and I don’t recall any of them flew at speeds this low. In any case the probably longest crossing, Westbound CA to HI could justify flying max range if one wants to maximize reserves.
The speeds in my chart are NOT for any RV. These speeds (VmPwr, Vme, Vmr and Vlrc) are different for different airplanes but it shows how to find them for your airplane.

Remember:
None of these numbers are absolute, just really good starting points and extremely close for our purposes.
 
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And another consideration was the frequency of technical inspections, and things such as oil changes, etc. Flying faster also saves on those…

This seems like a fairly simple calculation. We know fixed hourly cost like 50hr oil changes. Fuel consumption varies quite dramatically with power setting, but also affects speed. Be interesting to put this together in a spreadsheet, factor in headwind/tailwind and see what numbers get spit out for most economical speed. Gets even more interesting if one is attempting to skip an enroute fuel stop. Takes a lot of fuel (and time) to simply stop and get more fuel.
 
This seems like a fairly simple calculation. We know fixed hourly cost like 50hr oil changes. Fuel consumption varies quite dramatically with power setting, but also affects speed. Be interesting to put this together in a spreadsheet, factor in headwind/tailwind and see what numbers get spit out for most economical speed. Gets even more interesting if one is attempting to skip an enroute fuel stop. Takes a lot of fuel (and time) to simply stop and get more fuel.

It takes even more time, fuel, and cash if you run out...
 
IMHO, two interesting real life practical applications of this research are:

HEADWIND RULES OF THUMB
1. If cruising at or above Vlrc do not adjust speed unless headwind component exceeds 25% of Vlrc (TAS). Since Vlrc= 1.07Vmr minor headwind conditions receive automatic compensation.
2. For each 5 knots that headwind exceeds the .25 Vlrc threshold, increase cruise TAS 2 knots above no-wind Vlrc.
3. Example: VlrcC=120 KTAS and headwind component is 60 knots. Excess headwind is 6O-(120x.25)=30 knots. Therefore, cruise speed should be increased to 120+((30/5)x2)=132 KTAS. Ground speed is increased from 60 to 72 knots, reducing enroute time by 17%, while fuel burned (per ground mile) is reduced 3.7%.

TAILWIND RULE OF THUMB
Decrease TAS 1 knot for every 2 knots of tailwind component, but not below 0.8Vlrc
Example: Vlrc=120KTAS and tailwind component is 36 knots. To maximize range, decrease speed to 120-(36x.5)=102 KTAS=.85Vlrc. Specific range increases 3.5%, although enroute time is increased 13%.

IOW, increase TAS with strong headwinds, decrease TAS with strong tailwinds.
 
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I'm going to do a little testing tomorrow or the next day, 3500 AGL, 15 in hg, 1500 rpm at around 100 mph indicated leaned to around 3 gph and see if I encounter anything unexpected. I suspect the oil temp and cylinder head temps will be very low. I'm going to fly for at least an hour in that configuration fine adjusting power setting
 
The speeds in my chart are NOT for any RV
Yes, I would have guessed that ;)

It still is a pretty nifty chart, and thanks for that ER ref, one never stops learning (the more so when the forgetting accelerates…).
 
Max Conrad

Back in the day, Max Conrad would cross the oceans squeezing as much mpg as he could out of those comanches. I heard he would fly on the deck and pull the prop all the way back on the lycomings. He said you couldn’t do that with a continental and it survive.

50yrs later we have more knowledge like prop efficiency, mpg calculations via the EFIS, Carlson numbers, and so on. I’ve pulled the prop back on my -9 and it will turn an amazing low rpm and still fly. Sounds like a time to test again. I found that 2450 rpm was the sweet spot on the planes I’ve owned.
 
Hi All, I have a couple of questions for Stan. First, I admire the inititive to try a long over water flight. Greg Niehues built his plane to do just that.
So My first question is where in a RV4 are you putting the required fuel? When we helped to prepare a RV7 to do the same basic trip---then on to AUS, it was something like 135+ gallons. 4 leading edge tanks, a 15 gallon tank in the baggage area, and a 35 gallon turtle pak in the passenger seat. WAYYY over gross at take off. Pretty simple fuel system. I assume that Stan has something similar. Guessing he used Jon Johanson's system.

Its not just the fuel, but the survival gear----hopefully NOT needed, but its a long swim to Hawaii. I know that Greg tested his systems over a mileage course laid out over the US, and was successful on several occasions. Dont know about Stans testing plans. Something to think about---oil consumption in flight and replenshment.

Lots of little things to think about other than MPG fuel burn---Bill Harrelson would be a great person to consult with also.

Tom
 
This was just a thought exercise. I am planning no long over water flights. The RV-4 has stock tanks? I ran them dry individually in level flight and came out with 16 gallons usable in each tank to my surprise.
 
My flight calculations were for my worst case leg - California to Hawaii, westbound, against the likely headwinds to be found there. At normal fuel consumption and power rates, I can flight plan 155 ktas on 7 gph conservatively - a little more flow if lower altitude, and a little less if higher - but in that ballpark. With an average 20-knot headwind I would require 14:51 to cross the 2006 nm between Half Moon Bay and Hilo, with a fuel burn of 104 gallons actual for the crossing. I have 67 gallons in my wing tanks, plus 15 behind the seats in the "hidden smoke oil tank" area (a pair of 7.5 gal tanks), and I would still need a turtle-pac bladder tank in the right seat for another 50 gallons to give me a comfortable 4 hours of fuel margin. Takeoff weight would be right at 2200 pounds in that configuration with 800 pounds being fuel.

This was calculated on normal flight speed and fuel burns, without significant power reductions - just relying on WOTLOP at altitude for efficiency. It set my minimum equipment requirements for the fuel system. In my airplane I'm lucky with the oil consumption - I burn a quart in about 20 hours - but as Tom pointed out you will definitely need to think about that if you burn significantly more.

Definitely in the category of "thought exercise", but intriguing nonetheless, is this - with that same fuel configuration I could fly north to Prudhoe Bay, Alaska and tank up, then take off and fly north until I ran out of north, circle the pole a couple times for bragging rights, and come back to Prudhoe Bay. Absolutely no point to that flight other than bragging rights, but it has plenty of that to be had. That would top the charts, I think, in the area of inhospitable terrain for an emergency landing as well.
 
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RV4

Jon Johanson set 48 world records in his RV4, three times around the world. East, West and North South. Lots of articles about his flights, would probably tell you where to put fuel in RV4.
 
Jon Johanson set 48 world records in his RV4, three times around the world. East, West and North South. Lots of articles about his flights, would probably tell you where to put fuel in RV4.

Jon had:

117 litres standard RV4 capacity
43 litres in tank behind instrument panel
66 litres in wing tips (33 litres each side)
245 litres in ferry tank in back seat
471 litres total

his initial calculations here:

IMG_1130.jpg

After upgrading to electronic ignition on one side and a few other tweaks he concluded he needed 472 litres to make the Santa Barbara to Hilo leg. He ended with 471 litres instead of 472 because that's what the Australian authorities agreed to.

This thread made me pull out his book and read it once again.

IMG_1131.jpg
 
yep, a good read.
And nicely sits amongst some other distinguished aviators on my coffee table: inspiration and dreams :)
 

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Wing Efficiency?

I don't know enough to address this other than our RV wings are clearly not designed for high altitude efficiency but is it worthwhile to consider as a factor in choosing a cruising altitude for extreme range flight?

Glenn Wilkinson
 
I don't know enough to address this other than our RV wings are clearly not designed for high altitude efficiency but is it worthwhile to consider as a factor in choosing a cruising altitude for extreme range flight?

Glenn Wilkinson

The 9 wing is the best of them, it loves flying higher.
 
Flaps, no.


At those low power settings you cannot harm your engine with lean mixture - just keep going lean until it stumbles, and richen it until you're happy. You may not see much EGT change with very low throttle settings.

This is true at any altitude. When you are LOP, fuel flow is power. Just make sure all cylinders have peeked and you can ignore MAP and RPM. HP = 14.9 X Fuel Flow.
 
When I had my 'crazy' thoughts about building my 7A to try this, I came up with like 150 gallons. Mike Stewart gave me the idea of the twin leading edge tanks that gives you 84 gallons on 7A. So a baggage compartment tank, and right seat turtle pak.
Transfer pumps from the outboard tanks made sure to get all the fuel out of them using a modified Andair FS20X8T valve with an Imperial Brass valve, I built this for a client in Australia for another purpose, but it works. Facet transfer pumps on all the tanks to the selector valves, then to the AFP boost pump and forward. A bunch overkill, but, my backstroke isnt what it used to be.

Lots of pople have done it in RVs since with varying designs. The 7 I helped with that was going to AUS, had some neat stuff, but the concerning thing was oil consumption. So after a bunch of flights the comsumption per hour was calculated, and we devised a pump to transfer oil to engine to make up for the comsumption. YES this plane was overweight---and I was told they attempted an over gross landing. Didnt end well for the plane. I since have lost track of it.
 

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By my back of napkin calculations, going to best glide speed for cruise would cut fuel requirements just about in half.
 
By my back of napkin calculations, going to best glide speed for cruise would cut fuel requirements just about in half.

True enough - but that puts the length of flight time well out of reach. 15 hours is long enough, I'll add the fuel onboard and burn it to keep the time enroute reasonable. Ya'll 20-something kids might not mind a 24-30 hour flight across the ocean, but all these gray hairs I've got tell me I don't wanna do that.
 
Conrad

I think Max Conrad's Twin Comanche flight was 58 hours. The seat was a tank with a thin piece of foam. I actually got to sit in that TC many years ago.
I think Conrad started out at a fairly normal power setting and gradually reduced to around 45% power as he burned off fuel. He took advantage of the trade winds that were 20-25 knots on the tail even at low altitude. For anyone with an interest the Conrad single Comanche is in the museum in Liberal KS.
I believe Dick Rutan's world distance record still stands. Anchorage to Grand Turk in the Long Eze.
Extreme low speeds are counter productive, especially with any significant headwind.
 
Earthrounder

The first earthrounder in a homebuilt was Don Taylor in a Thorp T18. Oshkosh to Oshkosh in 1976.
One current distance record in a homebuilt is Bill Harrelson in a Lancair 4. Guam to Jacksonville Fl area. The Lancair holds 361 gallons for the record flights.
Conrads Twin Comanche held 800 gallons for the Capetown to St. Petersburg flight. 7878 miles 56.8 hours.
 
Got to fly a little today testing for best range. Went from KIKG to KTWX around 82 nautical miles. Indicating 100 mph CAS around 91 mph at 2,500. Power setting was 15 in hg and rpm was 1,500. Leaned it out to 3 gph. Engine ran smoothly with a hicup every once in a while. As you might expect cylinder head temp was low and I could get no egt temp. Oil temp was running around 160. I was concerned that I would foul plugs or that the engine would run rough when going back to more normal power setting. Turned out not to be a problem.

The RV-4 certainly flies differently at slow speed but not scary different, mushy. Brings back memories of my days in 150s and Citabrias. Shot a Bubba RNAV approach to 31 using Foreflight runway extention. Hit the box at 1500 AGL, turn to runway heading, 500'/min decent flying runway extention line. At 400' AGL decision altitude you are looking at the runway threshold set up to hit the 1000' marker.
 
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