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Steve Brown

Well Known Member
I have noticed an uncomfortable tendency to pitch up during a slip, especially with full flaps. So, I've tended to avoid full flap slips. I am somewhat afraid of losing elevator authority due to air stream blanking.

Anyone else experience this?

Are my concerns real or imagined?

My airplane is O-320 with Catto 3 blade prop, When traveling with baggage, it tends to be near aft CG limit during landing. Seems to be worse then, but the effect is noticeable solo with no baggage.

I tend to add some airspeed while slipping. I do this because I'm cross controlled and I don't really know what the stall speed is in that situation.

Is this aerodynamically valid?

I'm wondering if the pitot not being pointed directly into the air stream during the slip makes the airspeed read lower than it is. If so, increasing indicated airspeed may be completely unnecessary.

I could go up to altitude and test the indicated stall speed with full cross control. Since this airplane is not meant to be spun, and I haven't spun an airplane since 1990, that seems like a very bad idea.

Opinions / theories on all this welcomed. Hard facts based on 9A flying experience greatly appreciated.

Also, related, anyone spun their 9? Even partial turn? Flaps, no flaps, etc?

After looking at the RV12 test videos, I'm having a hard time believing that Vans didn't successfully spin test this airplane.
 
I flew a 9A for 400+ hours. I did not notice any bad habits during slip. I think you are over thinking it.
 
First, we all know that all airplanes are individuals; no two are alike. That being said, I have slipped many times in my 9A, although I can't remember a hard slip with near aft CG. I have never noticed anything unusual. Yes, I did practice at altitude and no, I didn't attempt any stall (even to determine stall speed) with the controls crossed. I do have AOA and it has always seemed to remain accurate in a slip. I don't attempt to add speed during slips (normally.)

If you are uncomfortable slipping, you might try just reducing your approach speed slightly. Pulling the nose up a bit will increase your descent angle considerably, or at least it does on mine. This of course works best early on in final and requires caution unless you are carrying excess airspeed. I also have a Catto 3 blade on a 320. I have my idle rpm set very low, which helps also. You might try watching your altitude on base--if it appears high, I overshoot the turn to final a little, causing extra turning which of course costs altitude. It isn't pretty, but it does work. Practice at altitude and see what happens.

Bob Kelly
 
Going inverted with a cross controlled -9

Here is an old post by Andy Karmy: Yes full flap slips work just fine with the 9a, I have heard from others that some of the other RV?s don?t slip well, but my 9a seems to nicely come down if you slow it a bit and throw it into a slip.Now, cross controlled stalls, that?s another matter. There I was at 6000ft doing my flight test cards like a good little test pilot... Hum ok, time for the cross controlled stall series, slow down, feed in rudder and aileron until fully crossed, keep pulling... holding.... bam I?m inverted looking up/down at the ground! Yikes, continue the roll through to right side up. WOW what what that all about? Well, climb back to 6000 and try it again, same thing... Hum, note to self, never feed in full rudder and aileron while in the traffic pattern.Now don?t get me wrong, you have to have the nose way up, everything is looking very strange and feeling totally out of any sense of normal before you even get close to this happening, but it was fun! Andy
 
I just did a slip in my 9A this morning. I did not notice the nose up tendency. I took a video of my flight (6 minutes long) and the slip is at the end (5:11 min) of the video.

http://3limafoxtrot.com/video/1-18-2008flight.wmv

The video include two low approaches and a final landing. On the final landing (3:58 min), you may notice that I pulled the power idle (I set my idel at 550 rpm). I usually fly my pattern wider than today and do my 90-80-65 kts with full flap. It usually works well. This morning my downwind was close to the runway and had to slip on my final.
 
Leland said:
...ok, time for the cross controlled stall series, slow down, feed in rudder and aileron until fully crossed, keep pulling... holding.... bam I?m inverted looking up/down at the ground!...
Click to expand...
Guess you can't answer as you are not the original poster, but I would be curious if this was a slip or a skid. Skids tend to be much nastier.
 
Thanks guys

Seems like the responses ranged from "thinking to much", which I've been known to do and "inverted", which I have not yet done in an RV9

I think one thing I want to do is ground speed comparison between cross-controlled and coordinated, using the same indicated airspeed.

If I can safely maintain my same indicated airspeeds during a slip, that takes one variable out.

My experience in stalling this airplane is that it is very benign with no flaps, but quite abrupt with full flaps. I'm guessing you'd have to be very light on your feet to keep it right side up with any significant cross control during a full flap stall.
 
I had a -9a though sold it some time ago.

My own experience was that because of the very large fin, to do serious slipping was quite difficult, because the rudder loads became quite heavy. To loose height I found it much better to slow right down as a previous poster mentioned. It was the opposite of my Supercub which I slip a lot.

I also think the poster who asked if the experience of it flipping over was talking about a slip or a skid was asking the right question. In a steep slip it is very hard to get enough AoA to stall. If that is not obvious think about a 90 deg bank. How would you stall the wing then?

It always seemed very well behaved to me. I think the pitch up is because the controls are very light. Anyway, that aside go and play with it at height until you are comfortable. If you are nervous of spinning I would take an hour with someone who can get you up to date.

Have fun!
 
Steve Sampson said:
I also think the poster who asked if the experience of it flipping over was talking about a slip or a skid was asking the right question. In a steep slip it is very hard to get enough AoA to stall. If that is not obvious think about a 90 deg bank. How would you stall the wing then?
Click to expand...

???

I don't follow your logic - the wing will stall when it exceeds it's critical angle of attack - it has no idea and does not care what the bank angle is. Greasy side down, 90 degrees, flat inverted, anywhere in between - does not matter - the airfoil only cares about angle of attack for stalling. You can easily roll into a 90-degree bank and pull enough elevator to exceed the critical AoA, and you're stalled.
 
I have never had any problems doing full flaps slips at 70 MPH. I have done them at 65 MPH but that is getting close to the edge. Slowing it down to 60 MPH really sets up a good sink rate but my preference is the slip.

BTW, Andy's description of the -9 rolling on it's back is basically a low engergy snap roll. He did the correct thing by keeping the aileron in it to roll it right side up. Don't try this unless you are ready to take the correct action by using the aileron and not pulling on the stick to split-S out.
 
airguy said:
???

I don't follow your logic - the wing will stall when it exceeds it's critical angle of attack - it has no idea and does not care what the bank angle is. Greasy side down, 90 degrees, flat inverted, anywhere in between - does not matter - the airfoil only cares about angle of attack for stalling. You can easily roll into a 90-degree bank and pull enough elevator to exceed the critical AoA, and you're stalled.
Click to expand...

Airguy, think about it in the limiting case. If you put in 90 deg of bank (which you wont get to because you will run out or rudder) the fuselage is doing ALL the lifting. The requirement of lift from the wing has dropped to zero, which is lucky because the airflow is now moving spanwise and the elevator authority will also be dropping to zero.

I am not saying you cant stall in a slip but if a straight sideslip is what you want to do, or even a practical turn to final, as the bank gets steeper, it gets harder to stall. A skidding turn is a different thing.

It is well covered in Sammy Mason's book, something like "Stalls and Spins".
 
Aaahhh - clear - you're talking about a severe slip scenario. I'm on board with you now.
 
Cross-control stalls part 1

Steve, I put together some answers to some of your questions. I will then follow with another post to more fully explain my thoughts.

From your questions….

Anyone else experience this? ((Not me.))

Are my concerns real or imagined? ((Some are real. You need to know the cross control stall speed or stay well above it. It is difficult to determine the cross control stall speed for all the variables. So stay well above it in the pattern. Pitot is probably not an issue.))

My airplane is O-320 with Catto 3 blade prop, When traveling with baggage, it tends to be near aft CG limit during landing. Seems to be worse then, but the effect is noticeable solo with no baggage. ((Aft CG makes the stick forces lighter, so your perception that the event is worse makes sense. I am just not sure what the event is.))

I tend to add some airspeed while slipping. I do this because I'm cross controlled and I don't really know what the stall speed is in that situation. ((See my following comments))

I'm wondering if the pitot not being pointed directly into the air stream during the slip makes the airspeed read lower than it is. If so, increasing indicated airspeed may be completely unnecessary. ((Pitot is likely ok. See comments on static source. Keep the speed up until you know for sure.))
 
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part 2

During a typical 1-G stall, stick position sets the elevator, elevator sets the AOA, AOA determines stall. Thus stick position determines stall. It makes no difference if you are wings level or banked. Speed does not matter. You can stall at any speed when the stick sets the stall AOA. CG, power, and sideslip do cause the stick position/AOA relationship to vary. There are a number of other effects, such as pitch rate and AOA rate that can affect the pitching moment and stall AOA but I think that those are minor in this type of scenario.

Sideslip has two main effects on stall that I can think of. One, it affects the flow across the wing, including blanking from the fuselage and flow from the prop. The required aileron to maintain a steady-heading sideslip also increases stall speed. At a steady-heading sideslip, the stall is asymmetric, thus the high roll rate.

Two, the static sources (static pressure) can cause an increase or decrease in indicated airspeed with sideslip. An increase in static pressure will result in a decrease in both indicated airspeed and indicated altitude. A decrease in static pressure will results in an increase in both indicated airspeed and indicated altitude. Sideslips should not affect pitot (total) pressure unless the sideslip angles are 10 degrees or more. That is why the tips of the manufactured pitot probes are beveled on the outside.
At altitude, check it out by feeding in left rudder and the right rudder at lower and lower airspeeds to see if the indicated altitude moves up or down (don?t do it too rapidly and don?t do large rudder doublets). If indicated altitude jumps up with pedal, typically it will jump up with left pedal AND right pedal (also true for both pedals if the indicated altitude jumps down). Changes in indicated altitude due to sideslip are more apparent than changes in indicated airspeed. Fifty feet of altitude jump will result in about a 4 knot jump in indicated airspeed.
If indicated altitude jumps up, your airspeed indicates a false high. That is not good as the true airspeed is closer to stall than indicated. If the altitude jumps down, then airspeed indicates a false low.

Crossed controlled stalls can put you inverted in many airplanes (see Andy Karmy?s experience). I remember a slow-flight training session in Bonanza. I let the ball get out (crossed controlled) and as I touched the pedal, the airplane stalled. It was so abrupt that my head banged off the side window. But, as my head was banging off the window, the control column was moving forward. Thus, there was little rolling excursion.

Slips in my 9A seem normal. As noted above, it is hard to get a lot of rudder until near stall. At normal approach speeds, I get about 200-300 fpm with moderate force on the pedals. The odd part to me is that the airplane yaws quickly, but the roll is slow to develop.

Trying to fly indicated airspeed based on ground speed indications may be risky. I would bet that you will not get the resolution you need for staying out of stall.
 
Clarification

John C said:
....... As noted above, it is hard to get a lot of rudder until near stall. At normal approach speeds, I get about 200-300 fpm with moderate force on the pedals. ..........
Trying to fly indicated airspeed based on ground speed indications may be risky. I would bet that you will not get the resolution you need for staying out of stall.
Click to expand...

Typically, I slip rudder to the firewall. Yes, rudder forces are high, but I just push hard enough to overcome them. I'm pretty sure the angle is more than 10 degrees.
I've used this technique on every airplane I've flown. Have gotten tail buffeting on some, which causes me to back off. I read a warning about some Mooney's (not the flavor that I owned) going flat in full slip below a certain airspeed.
The test with the ground speed is only intended to "calibrate" the effect of slip on indicated airspeed, not to be used as an operational technique.

As far as the techniques mentioned for getting down on a regular basis, slowing down well in advance is much easier on passengers.

On the other hand, aggressive slips are useful for emergency short approach, for staying within glide distance of the field, for late letdowns by ATC, for "popping out" too high and too hot, etc
 
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