Pitch vs. power - causes of excessive sink and hard landings

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DaveInPA

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Dave
Hi All - I am well aware I am overthinking it but I’m quiet obsessed with absolutely understanding what is happening during different landing situations. I’m a PPL working on my commercial and I’ve got a great CFI who comes with me on gusty days to the smallest and most challenging spots to practice landings.

I am getting good but There are still times When I’m solo when my airspeed on short final is fine and maybe a tad fast (65kts in a 172). I try to keep my descent to 500 ft/min as well. However when I think I can round out and get into ground effect It’s like I drop right through and land hard without a chance to flare. When I detect it I give it a little power and that helps.

I’ve been studying the lift equation, re-reading stick and rudder, and anything else I can find. In the short short final phase to slow the descent i pull back to slow the descent. That seems to make it worse or just not help at all. I really don’t think I’m slow enough to be stalling and my AOA certainly doesn’t feel like it’s in stall territory.

Shouldn’t increasing the AOA slow my airspeed but also give me a little lift to reduce the descent? AlSo why is this happening in the first place? What causes some roundouts to be smooth and others for me to drop like a rock unless I punch in some power?

Would appreciate any thoughts on causes of hard landings when airspeed and descent rates seem on target...
 
You want to look at the total drag curve, not the lift equation. But I'm not sure what you're doing. You pull back on final before flare time? Not sure I can figure out what's happening without flying with you, or at the very least seeing a video.
 
Are you on final already stabilized at idle power, or are you changing power to idle “at the fence” or thereabouts?
 
What is your sight picture? It should be mostly ahead, but turn your eyes to the left about 20% of the time to help judge your height above the runway. That is especially important if the runway is wider or narrower than you are used to.
 
I’m usually on final with a little power and I go to idle at the fence and right before I’m over the runway. The other day everything g looked fine and when I started to flare it just dropped and I landed with a hard thud. Once I felt the drop I started to pull back but I seem to drop faster when I pull back. This seems to happen more when I’m solo then with a heavy (240lbs?) instructor next to me. Maybe with less weight I’m flatter and I don’t realize it. Maybe I need more pitch...
 
Look at figure 12-3 in the Airplane Flying Handbook to see how steeply the drag curve rises as blade pitch is reduced. (I'm guessing that you are flying behind a fixed-pitch prop and have no control over blade angle, but I want you to see the drag curve). A windmilling prop, which is what you have when you pull the throttle to idle, is effectively a flat plate bolted onto the end of the crankshaft, and what you are dealing with is called flat plate drag. Next time you go out to do pattern work, try leaving just a tad of throttle applied....not enough to cause a pitch change. As you reduce throttle approaching (not in) the flare, it is pulling the airplane forward...when you pull it back to the stops it windmills and creates drag. Between those extremes there is a "sweet spot" where the prop is neither pulling nor causing drag...sorta like putting your car in neutral with a stick shift.

Bob Gardner
 
65kts, short final,power off, especially with full flaps in a 172 is challenging to make consistent good touch downs. You are so low energy that there is very little energy (read time) to round out and flare for a nice landing. I would try either approaching a bit faster, or with a bit of power, or with less flaps, so you are not dissipating energy as quickly during the round out and flare. I typically approach power off, about 70kts and 10-20 degrees fo flaps. If you touch down in a nose high attitude your actual touch down speed will be the same (maybe a couple knots faster with less flaps). You might need to move your aim point a bit closer to the end of the runway to touch down in the same place you usually do.

Brian
CFIIG
 
bobmrg said:
Look at figure 12-3 in the Airplane Flying Handbook to see how steeply the drag curve rises as blade pitch is reduced. (I'm guessing that you are flying behind a fixed-pitch prop and have no control over blade angle, but I want you to see the drag curve). A windmilling prop, which is what you have when you pull the throttle to idle, is effectively a flat plate bolted onto the end of the crankshaft, and what you are dealing with is called flat plate drag. Next time you go out to do pattern work, try leaving just a tad of throttle applied....not enough to cause a pitch change. As you reduce throttle approaching (not in) the flare, it is pulling the airplane forward...when you pull it back to the stops it windmills and creates drag. Between those extremes there is a "sweet spot" where the prop is neither pulling nor causing drag...sorta like putting your car in neutral with a stick shift.

Bob Gardner
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Ok that makes a lot of sense...full idle and I’m adding a lot of drag. What I’m stuck on is why does more drag cause the plane to drop through ground effect even when adding more pitch? I know excess airspeed causes float and I can detect that and deal with it no problem. When I nail a landing I have time to round out and pitch to stall warning and know the feel for that as I can sense the ground effect. However when i land hard it’s like the plane flies right through the ground effect and no amount of pitch will prevent it. Does drag overcome the pitch and have more an impact on my lift - therefore the plane drops?
 
DaveInPA said:
Ok that makes a lot of sense...full idle and I’m adding a lot of drag. What I’m stuck on is why does more drag cause the plane to drop through ground effect even when adding more pitch? I know excess airspeed causes float and I can detect that and deal with it no problem. When I nail a landing I have time to round out and pitch to stall warning and know the feel for that as I can sense the ground effect. However when i land hard it’s like the plane flies right through the ground effect and no amount of pitch will prevent it. Does drag overcome the pitch and have more an impact on my lift - therefore the plane drops?
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Region of reverse command. Though at 65 knots it shouldn't be a problem. Perhaps the airspeed indicator in your aircraft is out of calibration, and reads too high. Other than that my guess would be just you're flaring too high.
 
DaveInPA said:
Ok that makes a lot of sense...full idle and I’m adding a lot of drag. What I’m stuck on is why does more drag cause the plane to drop through ground effect even when adding more pitch? I know excess airspeed causes float and I can detect that and deal with it no problem. When I nail a landing I have time to round out and pitch to stall warning and know the feel for that as I can sense the ground effect. However when i land hard it’s like the plane flies right through the ground effect and no amount of pitch will prevent it. Does drag overcome the pitch and have more an impact on my lift - therefore the plane drops?
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As you get further on the back side of the curve, which is what you are doing in the round out and flare, it takes more pitch change to accomplish the same increase in lift, and the drag increases much more quickly, exacerbating the problem.

obviously I’m not watching you in the airplane, but you’ve made a couple of comments that make me think you’re starting the round out a little early.
 
Some good thoughts here which I'll explore more in my own plane next time I'm out (when I get my plane's annual done!!!). Just a shot in the dark to the original poster... does this happen at every airport, or just a specific one or two you usually practice at? My home field is KNOWN for having a spot near the threshold at one end where the wind is obstructed and the sink rate increases unexpectedly... well.. unless you've flown there enough to expect it.
 
Pitch adds drag as well as lift. When you are slow, it's more drag than lift, so you start to sink faster and slow down. This is what gives you your plop. Try this at altitude and watch your sink rate as you slow and pitch up. You aren't "stalled" but you are mushing. Glider pilots know this well. If you are slower than your minimum sink speed, you start to sink faster than min sink. In the mush state, when you try to flare, there is little to no energy left. Instead of pitching up, try pitching down just a touch until you are very close to the runway, then shift your sight picture to the end of the runway and gradually pitch up, but not fast enough to start to climb. The plane will settle on the runway.
 
We used to approach at 55 kts in 172s with full flaps. Most bad landings start with too much speed. I'd suspect pitot-static problems here.
 
Dan Thomas said:
We used to approach at 55 kts in 172s with full flaps. Most bad landings start with too much speed. I'd suspect pitot-static problems here.
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That works fine as long as you are carrying power to slow the descent.

N model says 55-65 flaps down or and 60-70 flaps up, but says nothing about power setting, except for short field which says 59kts power off, full flaps, but then you trying for a steep descent and touching down short, with low energy is more important than touching down nicely.

Brian
 
An abnormally gradual flare, started abnormally high (~50 feet) might produce the described phenomenon. OP should really fly with a CFI to figure out the real problem and how to solve it.
 
Dan Thomas said:
We used to approach at 55 kts in 172s with full flaps. Most bad landings start with too much speed. I'd suspect pitot-static problems here.
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Yep, I cannot understand how 65KTS is too low on energy unless the pitot/static system is way off. Hell they will float power off full flaps from 60KTS two up. 55KTS is good.
 
OP here and reading these thoughtful responses is getting me closer to understanding it a bit more. I am most likely starting my round out too high. I’m probably over the numbers but still too high over the runway. When I start to pitch up - even slightly to flat for the roundout - I increase drag much more than lift at 65kts. I immediately start to lose airspeed quickly and therefore lift. Then my 500 ft/min descent rate increases instead of decreases as it should when I round out closer to the runway. In other words I don’t want to change my pitch attitude until I’m no farther than 10 feet.

so at slow speeds pitching up creates more drag than lift and that causes airspeed to decrease which decreases lift. Closer to ground I’m in ground effect and have less induced drag which explains why more pitch (ie flare) keeps me aloft.

still looking for a magic graph that shows these factors at work.
 
Google “Polar curve glider”. Not exactly the same as you have an engine, but the concept holds.
 
You mention a few times this is more a problem when solo, lighter. Could that be doing something with CG and your sight picture too?
 
DaveInPA said:
OP here and reading these thoughtful responses is getting me closer to understanding it a bit more. I am most likely starting my round out too high. I’m probably over the numbers but still too high over the runway. When I start to pitch up - even slightly to flat for the roundout - I increase drag much more than lift at 65kts. I immediately start to lose airspeed quickly and therefore lift. Then my 500 ft/min descent rate increases instead of decreases as it should when I round out closer to the runway. In other words I don’t want to change my pitch attitude until I’m no farther than 10 feet.

so at slow speeds pitching up creates more drag than lift and that causes airspeed to decrease which decreases lift. Closer to ground I’m in ground effect and have less induced drag which explains why more pitch (ie flare) keeps me aloft.

still looking for a magic graph that shows these factors at work.
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Exactly. 172’s are relatively benign at allowing this. Planes with higher wing loading such as Cherokee’s will drop very fast and hit hard.

Faster, less flaps, or a bit power will give you more time to ease it closer to the runway And get the correct site picture before it stops flying.

Brian
 
DaveInPA said:
When I’m solo when my airspeed on short final is fine and maybe a tad fast (65kts in a 172). I try to keep my descent to 500 ft/min as well.
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When I am on short final, I have absoluetly no idea what is my vertical speed. I have much more important places to be looking during short final than at a laggy VSI.
 
DaveInPA said:
Then my 500 ft/min descent rate increases instead of decreases as it should when I round out closer to the runway. In other words I don’t want to change my pitch attitude until I’m no farther than 10 feet.
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Again with the VSI right over the runway. You are spending too much time looking at the instruments and not out the window where you can see what the airplane is doing now, not what it was doing a few seconds ago...
 
DaveInPA said:
still looking for a magic graph that shows these factors at work.
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I'd imagine you're an engineer. This does NOT help when it comes to learning to fly. I suggest stop trying to mentally crunch numbers and graphs and start feeling for how much "weight" you feel in the yoke when you start rounding out. Too soft? Add a squeeze of power. Too stiff and ballooning and/or floating down the runway? Reduce your approach speed. Feel it, look out the window. Fly the airplane.
 
I didn’t see any comment on how much flaps you are using. In our club 182 20deg flaps with allow you have have that nice power off over the numbers and flare to get those mains to touch down gently. 40 deg flaps a completely different beast. You need to be whipping it until right before touchdown and really stay on top of that flare. I typically only use them when I need to get it down on a spot.
I would imagine the 172 would be quite similar.
Trim is your friend.
 
It sounds like on some of your landings you think you're higher than you are. Not looking down the runway?
 
JonH said:
It sounds like on some of your landings you think you're higher than you are. Not looking down the runway?
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Geeze... Do not tell him that. Next thing he will be staring at is the altimeter! :p
 
RoscoeT said:
I'd imagine you're an engineer. This does NOT help when it comes to learning to fly. I suggest stop trying to mentally crunch numbers and graphs and start feeling for how much "weight" you feel in the yoke when you start rounding out. Too soft? Add a squeeze of power. Too stiff and ballooning and/or floating down the runway? Reduce your approach speed. Feel it, look out the window. Fly the airplane.
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I didn't pick up on it, but you're right. Thinking that studying the lift equation will help your landing is bizarre.
 
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I had this problem recently. Did my private with 80 hours in a 172, then started instrument in a school that runs Beech Sundowners and my landings were terrible. Used to the shallower descent and higher pitch attitude the 172s liked, i was rounding out way too high and bleeding airspeed way too fast. The best advice I got from my CFII and chief instructor was this: Try making your descent a little steeper by forcing yourself to come in a little high and going to idle(typically a dot or two high on the PAPI). That way you're not messing with power and it's one less variable. Chuck in your flaps and stabilize at your final approach airspeed(airspeed is KING), and nail that speed right down to your aiming point. Gently round out a couple inches over the pavement, moving your eyes down the runway. Chief's words were "take it down as close to the runway as you *dare*, without hitting the nosewheel". My landings got way better/smoother after that.

Remember you're not *trying* to land, you're simply flying/gliding the plane as close to the runway as possible, as long as possible, holding it off until it's done flying and gently settles down on the wheels. Don't overthink it and get tangled up in proper procedure. You'll get a feel for what the plane likes. Planes don't like being forced to land, they'll kick and scream until you give them what they like. It's all about energy management and dissipating all that potential(height) and kinetic(speed) energy just in time for a smooth touchdown.
 
I’m definitely in the camp that 65kts is fast in a 172. 1.3 VSo is 52kts at gross.

What helped me get consistently better landings was to stop thinking of the flare as a distinct action I needed to take. Instead, I level off just above the runway and don’t let the wheels touch. Eventually you’ll run out of energy and you’ll sink the final few inches to the runway. But unless I’m approaching really steep, it isn’t as much a distinct flare as it is a gradual level off.
 
DaveInPA said:
Still looking for a magic graph that shows these factors at work.
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You're not gonna find it......the physics change and are non linear. Those charts are shown in the POH are linear and are created from test pilot data.
 
Have you ever practiced slow flight over the runway. I found it to be a great exercise to help in landing problems... approach normally. When you get to the flare add power to keep the airplane from touching down. Fly as far as you can down the runway a couple of feet (barely) off the runway. Try to get Very slow and in a landing attitude with the stall horn screaming. If you chop power you’ll squeak a landing. Or you can add power and go around for another pass... Fun, and very helpful. Also Skyhawks will float and make a precarious touchdown if your approach speed is much over 1.3xstall. I truly believe the “ground effect” you speak of is a myth in high wing (Skyhawk) airplanes...
 
I am not a CFI, but it sounds like more of a sight picture problem combined with you wanting to know the physics (likely over analyzing as a result).

For site picture, here is what I was taught and have seen it repeated many times by respected CFI.

Go to a big runway and fly 2ft off the runway at 65 knots with your CFI. Get used to the sight picture. Get some weights, as close as practical to CFI weight. Repeat. Then reduce the weight, move it to the back seat, or remove it completely. The sight picture changes based on weight and CG. You need to learn to adjust based on current load.

Good luck,

Tim

Sent from my HD1907 using Tapatalk
 
Flying behind the power curve is much easier if you establish it early on the final leg, and extending the final leg to give yourself more time isn't a bad idea. Stabilized approach sound familiar? Pitch for airspeed, power controls rate of descent. Pin the view of the runway and maintain it all the way down. Everything takes care of itself.
 
dmspilot said:
I didn't pick up on it, but you're right. Thinking that studying the lift equation will help your landing is bizarre.
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Wanting to link flying experience to theory is good! This particular approach may be misguided for reasons already provided, but not bizarre in my mind.
 
FlySince9 said:
Have you ever practiced slow flight over the runway. I found it to be a great exercise to help in landing problems... approach normally. When you get to the flare add power to keep the airplane from touching down. Fly as far as you can down the runway a couple of feet (barely) off the runway. Try to get Very slow and in a landing attitude with the stall horn screaming. If you chop power you’ll squeak a landing.
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Doing that in a 172 is a prime way to get a tailstrike and a bunch of hidden damage.
 
dmspilot said:
I didn't pick up on it, but you're right. Thinking that studying the lift equation will help your landing is bizarre.
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I appreciate all the help but I also get that perfecting landing is 100pct about going out and doing a lot of landings. Not sure why taking a quick peek at my airspeed and descent rate on short final is a bad idea. It’s a quick glance and tells me if I’m gonna be fast and float or if I’m getting to slow or descending too fast. Kinda like one last look at the 3 greens of the landing gear on short final.

really my question is more about aerodynamics and why does pulling back for more pitch cause more sink when I’m over the numbers. True there is no graph that will teach me to land better....but knowing more about what is happening with the plane I always thought was part of being a good pilot. But I do appreciate the different responses and the reminder to just go and fly and get a feel for the plane as the most important objective.
 
DaveInPA said:
I appreciate all the help but I also get that perfecting landing is 100pct about going out and doing a lot of landings. Not sure why taking a quick peek at my airspeed and descent rate on short final is a bad idea. It’s a quick glance and tells me if I’m gonna be fast and float or if I’m getting to slow or descending too fast. Kinda like one last look at the 3 greens of the landing gear on short final.
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I was trained to go back and forth between looking at the aim point (out the windshield) and the airspeed. Normally you shouldn't be looking at your VSI because you can get the same information by looking out the windshield. Looking at the VSI on one or two landings while training just to help you get an idea of roughly what that descent rate looks like out the window is probably okay, but it should not normally be part of your scan at all on final.
 
DaveInPA said:
I appreciate all the help but I also get that perfecting landing is 100pct about going out and doing a lot of landings. Not sure why taking a quick peek at my airspeed and descent rate on short final is a bad idea. It’s a quick glance and tells me if I’m gonna be fast and float or if I’m getting to slow or descending too fast. Kinda like one last look at the 3 greens of the landing gear on short final.

really my question is more about aerodynamics and why does pulling back for more pitch cause more sink when I’m over the numbers. True there is no graph that will teach me to land better....but knowing more about what is happening with the plane I always thought was part of being a good pilot. But I do appreciate the different responses and the reminder to just go and fly and get a feel for the plane as the most important objective.
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Not sure you mean when you say you check the VSI to make sure you're not descending too fast. You should be descending in a straight line to your aiming point on the runway, not any particular rate.
 
Have you thought about putting a camera on one of your tiedown hoops pointing at your landing gear? You can get an adapter on Amazon for $60 (search for 'camera airplane mount'), and there are action cameras for about the same price. They're not nearly as good as the GoPro ones, but they're good enough to be able to see what's happening. Getting an external view might help you give you a better idea of what you're actually doing.
 
DaveInPA said:
I appreciate all the help but I also get that perfecting landing is 100pct about going out and doing a lot of landings. Not sure why taking a quick peek at my airspeed and descent rate on short final is a bad idea. It’s a quick glance and tells me if I’m gonna be fast and float or if I’m getting to slow or descending too fast. Kinda like one last look at the 3 greens of the landing gear on short final.

really my question is more about aerodynamics and why does pulling back for more pitch cause more sink when I’m over the numbers. True there is no graph that will teach me to land better....but knowing more about what is happening with the plane I always thought was part of being a good pilot. But I do appreciate the different responses and the reminder to just go and fly and get a feel for the plane as the most important objective.
Click to expand...

Here is an exercise I use to show VFR students just how inacurate and laggy the VSI can be. I set up a 1000 fpm climb using the VSI as a reference. Then with them looking at the VSI and me looking outside, I very quickly pitch down to level flight while they count out loud how long it takes the VSI to get back to 0. Typically it takes >5 sec for the VSI to "catch up" to what the airplane is really doing. Looking at the VSI when that close to the ground not only gives you "old" information, it takes your eyes away from where they need to be... Out on the runway making sure you are still aimed at the right spot and that nothing has wandered out on the runway.

If you have the airplane configured (and trimmed) correctly your rate of descent should be fairly consistent and if you are not aimed at the right spot, a small power change will fix it, all without looking inside the airplane.

This might help:


Note: He does not talk about how to fix being long or short. That is the small power changes I mentioned earlier.
 
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