Kitfox abrupt departure sensitivity to low speed stalls

[Ramos]

No doubt, this site is awesome. I am following this thread and have followed many others here with great interest. The honest, friendly sharing of knowledge gained through experience that takes place here is invaluable to someone such as myself. I have pursued Kitfox information since 1985. First in magazines and now the internet. Actually got to see a few in person for the first time this last Summer at the WAAAM fly-in. Have yet to sit or fly in one. None the less, I DO possess the necessary passion and am close to having the means to finally turn the dream into reality. What you folks provide to me is both knowledge and inspiration. Thank you all for being so welcoming and patient.

I apologize for the extreme 'thread drift', here. We now return to the regularly scheduled program...

[DesertFox4]

Kind words Ramos for the kind and generous folks on this website. Glad they keep you inspired to reach that goal of owning your own Kitfox soon. My first Kitfox took 15 years to obtain. Best decision aviation wise I made in 34 years of flying. Changed my life in so many positive ways.

[jamesbdunn]

... I used aluminum and with some tweaks in the the installation hoped to eliminate the oil canning. ... Being aluminum, what I did was heavier than the LLE, but it was not available yet.

I visited Lowell and his Kitfox; a beautiful plane. Obviously a passion for the details.

[jamesbdunn]

I understand that Vx depends upon the Kitfox model and modifications.

However, I see videos and stories of Kitfox planes and their pilot "hanging from the propeller" [exaggeration]. But it brings into question [for me] what happens when an engine fails [gas starvation scenarios, mechanical, propeller...]?

Can an airplane hanging from the propeller have enough airspeed to throw the stick forward in-time to enter a glide? Or, does the airplane immediately enter a severe stall and not have sufficient altitude to enter a glide before hitting the ground?

What goes on in my mind, at times is only a sampling of a broader understanding of related physics and experience. So please do not read this as questioning pilot experience. I pose this as a question to learn about potentially other influences in play.

Seems like hanging on the propeller and having an engine failure potentially can instantly transition from high angle of attack to severely stalled.

Is this the purpose of Vx?

• To insure sufficient inertia into relative wind,
• gravity not being a super major factor in reversing the direction of acceleration in relation to relative wind,
• sufficient margin to a stalled condition in terms of providing the pilot with sufficient time to respond,
• not having to make large control movements and causing stall/spin

Is the purpose of Vx to have sufficient margin for engine out to ensure entering into a stable glide?

[jtpitkin06]

Wow.., a lot to answer.

No kitfox has ever been powered to be able to "hang on the pro" as some of the aerobatic overpowered wonders like the Extra. Helicopters hang on the prop. Kitfoxes do not.
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Q.
Can an airplane hanging from the propeller have enough airspeed to throw the stick forward in-time to enter a glide? Or, does the airplane immediately enter a severe stall and not have sufficient altitude to enter a glide before hitting the ground?

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Ans.

At Vx there is sufficient energy in a Kitfox to lower the nose and enter a glide. However, there may not be sufficient altitude. There is also a delay in the pilot reaction when an engine quits of about three seconds. The danger comes when the nose pitches down and the pilot over-reacts by pulling the nose up, creating an accelerated or secondary stall.

There is never any reason outside of aerobatics to climb at a speed less than Vx.

For normal and short field take offs the obstacle is assumed to be 50 feet. So unless you have giant sequoias off the end of your runway you clear the obstacle and then accelerate to Vy without losing altitude. The Vx climb is very short in time. If your aircraft climbs at only 600 rpm or 10 feet per second, then the obstacle climb is only 5 seconds. Probably half that in most Kitfoxes.


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Q. Seems like hanging on the propeller and having an engine failure potentially can instantly transition from high angle of attack to severely stalled.

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Ans. At speeds below Vx, possibly. But this is assuming the engine fails and the pilot tries to hold the nose up during the reaction period before establishing a glide. All the more reason to transition to normal climb as soon as the obstacle is cleared.

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Q. Is this the purpose of Vx?

• To insure sufficient inertia into relative wind,
• gravity not being a super major factor in reversing the direction of acceleration in relation to relative wind,
• sufficient margin to a stalled condition in terms of providing the pilot with sufficient time to respond,
• not having to make large control movements and causing stall/spin

Is the purpose of Vx to have sufficient margin for engine out to ensure entering into a stable glide?
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Ans. None of the above. Vx is best angle of climb for the purpose of clearing an obstacle. Nothing more. It is a calculated number achieved by plotting speed (x axis ) vs rate (y axis) and drawing a line from 0/0 to tangent on the curve. Where the tangent contacts the curve is Vx. Best rate is the highest point (y value) on the speed/rate curve.

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Summary. If your runway has an obstacle, lift off so as to rotate into Vx without a speed overshoot. Clear the obstacle then you accelerate to best rate or cruise climb.
If the engine quits, establish a glide pitch and aim for your emergency landing point.


John Pitkin
CFII 47 years.

[jiott]

Just my understanding here:
The purpose of Vx is maximum sustainable climb angle at full power; it has nothing to do with engine out or stall recovery.

In my Kitfox flight training we did many power-on stalls where it felt like you were hanging on the prop. As long as you kept the ball reasonably in the center, the stall was a very straight ahead nose drop that was easily recovered by enough forward stick to regain flying speed. If done properly there was nearly no loss of altitude. Response time had to be immediate but not a panic situation. A stall in a Kitfox (I'm speaking of an SS7) is nothing to be feared, ball centered, unless you are very close to the ground. I know of a friend who actually purposely stalls occasionally on a short approach over trees to quickly lose altitude, then a quick forward stick to break the stall and land. I am not recommending this and have not done it myself, but apparently the Kitfox stall is so benign and controllable it can be done.

So an engine out at Vx may or may not cause a stall, depending on how quick you are on reacting to it, but in either case is a very controllable event.

[Esser]

Well I'm sure this will start an argument but I never ever use Vx in any plane I fly. There is really no need. If you are flying on such a short runway that you have to follow the POH to the letter to clear an obstacle or you won't make it, that runway is too short. There is no way you can guarantee your engine is making the same power as the day the poh was written. A fresh engine and a 2000h engine aren't the same.

Originally being a glider pilot, my flying style is all about energy management. My shortfield technique is get it off the ground as soon as possible, stay in ground effect and build up excess speed until I feel I have to pull up to clear the obstical and then bleed off the excess speed in the steep climb until I hit Vy then climb out as normal. You gain more more energy in less time thus way. Practice it yourself at a regular airport and you will be amazed how high you can get in the same distance as if you just pulled it off at Vx and climbed out.

Mileage may vary.

I also recommend reading Contact Flying by Jim Dulin. He talks about techniques like this and energy management at length. Best $10 you'll spend on flying ever.

[jiott]

I agree with you Josh. Nearly all Vx climbs are unnecessary and are for show off at fly-ins or when you are flying with your Super Cub buddy. I'll admit I do them often just to keep in practice and make sure I can hold a nice steady Vx airspeed in the rare case I would ever really need to do it. I fly a lot in the mountains, but would never deliberately use an airstrip where Vx is required. However, that said, I can see where a pilot thru his own stupidity or inattention could get trapped in a blind canyon with no turnaround room and maybe need to do a Vx climb to clear the ridge. Stuff happens, so we do need to have the skills to fly at the edge of the envelope. Just my 2 c.

[flflyer]

I just read this whole thread. Lots of points being brought up. I also think my model 6 wing works fantastic at slow speed as is. I also agree that some gap seal or VG's on the tail could help it flare better with an engine out.

I've had the displeasure of an engine failure on takeoff also, many years ago, and a partial power loss on takeoff just a few years ago. If I'm taking off, I'm planning which way I'm going to turn at which altitude. I even will takeoff slightly downwind to get a better scenario if I lose power at low altitude. The Kitfox handles normal airports so easily that you can easily find a place to put down, but on small strips, it really pays to think about what's in front, to the side, and to have a turnback strategy.

One thing that will help is to drift crosswind, or even turn 10 degrees crosswind on climbout. The turn should be away from the crosswind if there is an actual crosswind component (there usually is). Obviously don't do it over houses or hangars, etc, but in general, if you drift crosswind on climb, you will need less room to turn back. This is because you only need a 180 turn, instead of a 235, then a 45 to get lined back up. If you lose power, and you've hit your altitude that you've worked out as a reasonable one, then you just turn into the crosswind (if there is one of course) and by the time you are on the runway heading you will be generally lined up with the runway.

I even make one more lap around the airport while gaining altitude if the conditions aren't good for landing nearby. That way, if there is some overheating, low oil pressure, or something giving clues, you are ready to do an engine out pattern. And you have time to double check the engine temps are stable.

I love flying sailplanes, and every landing is engine out. Everybody practices a rope break on takeoff, which is just like an engine out; the instructor intentionally releases the tow rope at 200' AGL, and you turn around and land. Easy in a sailplane, although they take a long time to turn compared to a Kitfox. In that exercise, you have to get the nose down right away, which is very much like a powerplane losing an engine on climbout.

BTW, I had the Sportsman cuff on my C180, it was fine. No matter how slow your plane will fly, you can always exceed the angle of attack that keeps the wing flying. I would get an angle of attack gauge before modifying the Kitfox. They are a lot of fun in slow speed scenarios, and will always tell you how much margin you have before stall.

Last point, I do believe that STOL planes that are 'hung on the prop' are in a risky situation, even though I've seen plenty of guys flying that way for super short landing demos. And yes, if you lost the engine, the critical angle of attack does change somewhat, and you need speed that you don't have. No reason to fly a Kitfox that way, on the backside of the power curve, where the wing is producing lots of drag. The reason the demo pilots do it is because the instant you chop the power, the plane will go down, which hopefully, is on the X during the landing contest, 1 foot below. And, it's about as slow as you can get a plane, at the cost of exposing it to big problems with an engine out.

[jamesbdunn]

Is the purpose of Vx to have sufficient margin for engine out to ensure entering into a stable glide?
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Ans. None of the above. Vx is best angle of climb for the purpose of clearing an obstacle. Nothing more. It is a calculated number achieved by plotting speed (x axis ) vs rate (y axis) and drawing a line from 0/0 to tangent on the curve. Where the tangent contacts the curve is Vx. Best rate is the highest point (y value) on the speed/rate curve.

John, and others

Thank you for clearing up my over-thought misconception of Vx. A helicopter as someone alluded to is flying Vx straight up (hanging from the propeller), and when power dies the aircraft drops (unless auto rotating). Vx has nothing to do with margin to stall during engine out.

I appreciate everyone's voiced experience.

My take-away: is that when taking off, and there are no realistic obstacles on long runways with reasonable density altitude, to lift off well above Vx and climb out at Vy to transition to a safer gliding altitude as soon as practical.

To get to an altitude where a non-urgent power off landing can be initiated.

To have an engine out plan of action for each phase from lift off to cruising climb.

I'm not well experienced. So what is obvious for the experienced pilot, I have yet to experience and pay attention to the details.

From the comments provided, there seems to be regions of the transition from wheels-off to cruising climb.

• Wheels-off,
• transitioning towards Vx while in ground effect,
• since at full throttle the pulling back to change AoA,
• sustained climb at Vx,
• relaxing the elevator to transition to Vy (or lift off near Vy and avoid Vx)
• climbing Vy,
• Vy extended to clear mountains or other tall obstacles (circling)
• attaining a safety margin altitude AGL,
• transitioning from Vy to climbing cruise.

So it seems that depending upon in which phase the engine out occurs, how much runway is left, and the airport configuration, the pre-planning is significantly different. Your options for engine out landing sites and what the plane needs to accomplish are different.

On a long strip in a 100 hp Kitfox, the altitude for climbing cruise might occur before leaving the end of the runway.

I'll have to think about the gliding altitude lost in a banked turn, and what height AGL is needed for each phase toward climbing cruise.

I have seen posted 1000' lost to glide 1.5 miles for a Kitfox 4, straight and coordinated.

How much altitude is lost in a gliding turn (plane effectively heavier)?

Effect of density altitude on gliding turn?

Other engine out influences to consider for flight planning?