I grew up in an environment where a man's word was his bond and Superman was for truth. justice, and the American way. It was troubling, for a young instructor to find out he was teaching sanctioned techniques that were inefficient at best and dangerous when not paired with extra avoidance of natural power.

Now put that young instructor in Monte Vista, Colorado teaching in a C-140 and short wing Grumman trainers. FAA seminars were giving the soft field takeoff bad press in the mountains while he was finding the basic low ground effect takeoff (think soft field) the only safe way to get off in the hot afternoon.

There can be problems with soft field technique, mainly too high ground effect. We were taught book airspeed for Vx, Vy, Vso, and slow speed at altitude. What was not emphasized was that Vx is Vy at high density altitude and that the airplane descends in a mush before stall. But mainly nothing was taught about the tremendous energy difference in six inches ground effect verses six or even three feet ground effect. He learned that flying overloaded Cobras in Vietnam. They wouldn't hover higher than six inches at the most. Also the FAA didn't teach lowering the nose to stay as close to the runway as possible.

At least now, after decades of less efficient normal, short, and soft field techniques, sanctioned training and testing materials and techniques teach acceleration in ground effect on all takeoffs. Unfortunately they don't mention the key safety elements : "level in low" ground effect. At least now students are less "hung out to dry."

Contact,

I've been working on my soft field takeoffs lately.

Typical takeoff configuration is a pretty neutral trim setting so the plane pretty much wants to find a Vy attitude right after rotation. Do you recommend starting with some nose down trim or do you just push the nose down to maintain level flight in ground effect?

I'd guess I am five feet above the runway. Truth is, I'm scared to experiment with going lower for fear of touching back down. Would an inadvertent touch down be that bad? Assuming my rate forward along the runway is much greater than my rate of descent down to the runway, how big a deal is touching down at 75mph?

Allan

Allen,

Cary recommends starting with nose down trim, which would be best for those who change the trim. I always stayed low and trim is same for cruise and full flaps, which I always used. So, I never changed from cruise trim.

Your airplane is more powerful and heavier than most trainers. The control wheel is heavy. Most pilots could haul it off sooner, requiring greater pitch attitude followed by a significant push forward to level in low ground effect.

Try flying low at cruise speed, going lower and lower with each pass. Do not slow down or land. The airspeed will make the controls sensitive. Use only rudder and elevator on final and over the runway. Throttle and prop set for takeoff. Use elevator to just tick the grass. Aileron ONLY to keep wing level or down to prevent drift. Best done without crosswind The taller the grass the better. We are too busy during landing and a bit worried on takeoff to fine tune accurate height. Fine tune at cruise.

After the above drill, you will find touching down briefly on takeoff to be no problem at any speed. There is no substitute for finding the ground by touching down. Until we do, we really don't know/believe.

Have fun and report results. I know you're not going to bang it down.

Jim

Quote
" But mainly nothing was taught about the tremendous energy difference in six inches ground effect verses six or even three feet ground effect. He learned that flying overloaded Cobras in Vietnam. They wouldn't hover higher than six inches at the most. Also the FAA didn't teach lowering the nose to stay as close to the runway as possible."

Hovering in ground affect reminds me of demonstration of that. We were in Navy CH-46 on the USS Sacramento vertreping mail an supplies to a mine sweeper. Antenna from boat got into rotor disk lost about 3 feet of one blade. We were about 10 miles from Sac. and about 2 miles from land (South Vietnam) Pilot elected for closets' spot to land. Turned out we were inside Chu Lai air base. We were able to get marines to help us change rotor blade. Captain of the Sac wanted to show appreciation for their help and sent a few cases of beer and some ice cream for the guys who helped. CO of the base contacted Sac skipper and wanted to know if he had any more beer. Of course he agreed to give them a couple hundred cases. Base CO said they would send Huey,s out to pick it up. First one landed on our flight deck and loaded up a lot of cases then tried to hover, no way had to unload some. Tried again same result. He finally got airborne. Next one landed and loaded up....was able to barley hover, to our surprise disappeared over the fan tail. We all ran to fan tail just in time to see him get into translational lift just above the water. Whew close one. Next one paid more attention to weight, guess the first 2 were thirstier then the last one. Lesson learned about ground affect over deck disappears when not over deck















c

DonC wrote:" But mainly nothing was taught about the tremendous energy difference in six inches ground effect verses six or even three feet ground effect. He learned that flying overloaded Cobras in Vietnam. They wouldn't hover higher than six inches at the most. Also the FAA didn't teach lowering the nose to stay as close to the runway as possible."

Hovering in ground affect reminds me of demonstration of that. We were in Navy CH-46 on the USS Sacramento vertreping mail an supplies to a mine sweeper. Antenna from boat got into rotor disk lost about 3 feet of one blade. We were about 10 miles from Sac. and about 2 miles from land (South Vietnam) Pilot elected for closets' spot to land. Turned out we were inside Chu Lai air base. We were able to get marines to help us change rotor blade. Captain of the Sac wanted to show appreciation for their help and sent a few cases of beer and some ice cream for the guys who helped. CO of the base contacted Sac skipper and wanted to know if he had any more beer. Of course he agreed to give them a couple hundred cases. Base CO said they would send Huey,s out to pick it up. First one landed on our flight deck and loaded up a lot of cases then tried to hover, no way had to unload some. Tried again same result. He finally got airborne. Next one landed and loaded up....was able to barley hover, to our surprise disappeared over the fan tail. We all ran to fan tail just in time to see him get into translational lift just above the water. Whew close one. Next one paid more attention to weight, guess the first 2 were thirstier then the last one. Lesson learned about ground affect over deck disappears when not over deck















c

That one going in the book?[emoji41]

Don's story is a good reminder that for one to be flying in ground effect, it has to be close to the ground! That means if the terrain drops after the runway, the airplane better be well into its climb out. The closest I've experienced that was departing from Leadville's 16 about 7 years ago, with a DA of 12,100'. I was at about 2' AGL as the end came up and the terrain dropped away, but by then I had accelerated to about 95 mph and could start climbing.

On whether to trim nose down for a low ground effect departure, that's a technique I learned that is helpful to me. I find it easier to hold back pressure on the yoke, to keep the airplane from descending back to the ground, rather than push on the yoke to keep the airplane in ground effect. Assume that when the airplane is trimmed for take-off, it would fly straight and level at 65 mph. But as the speed builds past 65 mph, it will naturally tend to climb, which would require pushing on the yoke harder and harder to maintain that 2' AGL as speed builds to whatever climb-out speed (Vy, presumably) the pilot wants to use. I just have more control pulling than pushing--but each to his own. If a pilot can easily maintain low ground effect as the airplane's speed increases, by whatever technique he/she uses, I'm not going to argue about it. But if the pilot finds the airplane tends to climb out of low ground effect too soon, it's worth trying a little nose down trim to see if that works for that pilot--it works for me.

As for touching down again accidentally when already very close to the ground, it's unlikely that it will be problematic. I did that inadvertently 2 years ago, coming out of Alamosa after a weekend at La Garita, when I was distracted just after lifting off and was up to perhaps 75-80 mph. I had refueled, so the airplane was heavy, and the temp was pretty hot--don't recall the DA at the time, but Alamosa is at 7539', so I'd guess it was around 10,500' or so. I don't recall what it was, but something fell off the passenger seat and I instinctively reached for it. Dumb, I know, but that split second distraction was just enough that the airplane touched down again, and I took off again, hoping that nobody saw it. No issues, except a bit embarrassing.

Cary

Hey Allan,

I have F'ed up several times trying to fly in low ground effect and banged off the runway as a result. Most were just touches but a couple were pretty good thumps with no bad outcome. All on grass, none on pavement, which may be less forgiving, don't know. I also use a bit of nose down trim, which seems to make it easier for me. The main thing is to make sure your friends aren't watching, or you will be buying the beer.

Pete

You can touch down safely most of the time. During my duster days I touched several times at 120 mph or so. A few touches pulled me down to a 100 but most just woke me up. Not a recommended practice though. Every year a duster drives it in.

Don't get your feet down in corn. Piston airplanes won't come back out. You can go to sleep on cotton or alfalfa, however. Some plants stick up a bit taller. Just allow the wheels to hit those and keep the tick tick tick even.

Dynamic proactive elevator for/aft movement helps maintain desired low ground effect altitude. As with longitudinal alignment with rudder, bracketing makes it much easier than reacting to the nose going the wrong way.

When teaching swath runs in the Cub, we instructors watch for both L/R tail/nose wiggle and up/down nose wiggle. The pilot with a static nose in either plane is behind the aircraft. Going to sleep or getting behind is unacceptable in very low work.

That one going in the book?

Sent u a PM GB

Helpful responses, thanks all.

I'm re-reading Jim's book these days. Finding that I understand stuff that I didn't understand before. I wonder how many times that will happen in my flying days;-)

If the weather ever clears around here I'll be more experimental.

albravo wrote:Helpful responses, thanks all.

I'm re-reading Jim's book these days. Finding that I understand stuff that I didn't understand before. I wonder how many times that will happen in my flying days;-)

If the weather ever clears around here I'll be more experimental.

Guaranteed, it will happen every time you fly, unless you don't pay attention.

Cary

All this talk about ground affect ....leads me to a question about why that helo crashed in the Bin Laden raid . I would think the helo would be getting into ground affect especially since it was inside a walled compound which I think should increase the ground affect lift. Maybe some other problem....but official response loss of lift . What's your best guess Jim (contactflyer)

Probably shot down by supposed allies. Pakistan military and intelligence had a lot of irons in the fire.

I know zip about helicopters, other than that they're so ugly that the earth repels them, but what I've read about that incident was that it was vortex ring state that caused it to crash.

Cary

Cary wrote:I know zip about helicopters, other than that they're so ugly that the earth repels them, but what I've read about that incident was that it was vortex ring state that caused it to crash.

Cary

It's actually pretty hard to get into that state.

It's highly likely what happened was loss of performance due to high GW and high DA. Even with the best performance planning, unless you have sensors on the ground at your destination you're really just making an educated guess to what power you'll have available.

That state was too complicated for me, Cary, so I didn't go there. Before sensors, Cam, we old guys just stayed in low ground effect and pulled pitch until turns started to bleed and then put the collective back down a bit. Those young guys did " the best they could with what they had where they were at. " Good job, Teddy would have said.