Slips

[PT20J]

2 hours ago, A64Pilot said:

The other day at the Fl Mooney lunch fly in I followed a Mooney out for departure, during taxi his elevator was full down, what was that about? can two aircraft have that much difference in trim ?

All Mooneys up through the M20J had trim bungees which are centering springs with the centering point variable as the stabilizer incidence is adjusted by the trim system. This is why the elevator is in trail with the stabilizer when the trim is in the takeoff position. According to Bob Kromer, the M20K, with it's heavier engine/prop moved forward had some undesirable flight characteristics and the easiest fix was to change the gadgets in the elevator control system to a bob weight and a down spring that has variable tension depending on the stabilizer setting. This is why all Mooney's from the K on sit on the ground with the elevator deflected fully downward. Bob didn't recall exactly what the undesirable characteristics were as this happened before his time at Mooney. But, a bob weight is generally incorporated to increase stick force per g, so it's a good guess that the controls were too light during turns and pull ups.

Skip

[PT20J]

The only Cessnas I recall having any limitation on slips with flaps were some of the 40-deg flap C-172s. It's arguable that it is actually a limitation since the placard reads AVOID SLIPS WITH THE FLAPS EXTENDED.

[A64Pilot]

5 minutes ago, PT20J said:

All Mooneys up through the M20J had trim bungees which are centering springs with the centering point variable as the stabilizer incidence is adjusted by the trim system. This is why the elevator is in trail with the stabilizer when the trim is in the takeoff position. According to Bob Kromer, the M20K, with it's heavier engine/prop moved forward had some undesirable flight characteristics and the easiest fix was to change the gadgets in the elevator control system to a bob weight and a down spring that has variable tension depending on the stabilizer setting. This is why all Mooney's from the K on sit on the ground with the elevator deflected fully downward. Bob didn't recall exactly what the undesirable characteristics were as this happened before his time at Mooney. But, a bob weight is generally incorporated to increase stick force per g, so it's a good guess that the controls were too light during turns and pull ups.

Skip

Pitch instability in that there wasn’t enough stick force increase with displacement when aft CG. You must have increasing stick force as you displace it. One way of dealing with it is anti servo tabs on the tail, we already had those. I dislike the bob weight myself as G forces influence it, so I increased spring force on the S2R-H80 to get it to pass. Average crop duster being of course an Aeronautical Engineer removes the down force spring as it makes the stitch force lighter, so I used two springs hoping they would only remove one.

Mooney must not have been bad if they didn’t go to anti servo tabs etc., they can be VERY effective in increasing stick force.

If you ever fly a loaded airplane that requires very little stick force to climb or descend, less than normal anyway, be very careful, you may be outside of aft CG.

I didn’t know that about long bodies, but it makes sense, I extended the nose 11 inches and lost some pitch stability, Mooney extended both ends?

[A64Pilot]

11 minutes ago, PT20J said:

The only Cessnas I recall having any limitation on slips with flaps were some of the 40-deg flap C-172s. It's arguable that it is actually a limitation since the placard reads AVOID SLIPS WITH THE FLAPS EXTENDED.

I thought all the Fowler flapped one did, (old Cessna’s don’t have Fowler flaps) but with a little reading it seems only the older models with 40 degrees are prohibited and only with full flaps, newer models with only 30 degrees are no longer prohibited

https://www.aopa.org/news-and-media/all-news/2009/january/flight-training-magazine/slip-solution

So I guess those that I know that do aren’t crazy like I thought.

[jaylw314]

43 minutes ago, A64Pilot said:

Pitch instability in that there wasn’t enough stick force increase with displacement when aft CG. You must have increasing stick force as you displace it. One way of dealing with it is anti servo tabs on the tail, we already had those. I dislike the bob weight myself as G forces influence it, so I increased spring force on the S2R-H80 to get it to pass. Average crop duster being of course an Aeronautical Engineer removes the down force spring as it makes the stitch force lighter, so I used two springs hoping they would only remove one.

Mooney must not have been bad if they didn’t go to anti servo tabs etc., they can be VERY effective in increasing stick force.

Mooney elevators have control horns and tapered rear edges, both which serve the same function as anti-servo tabs (and less complicated)

[A64Pilot]

52 minutes ago, jaylw314 said:

Mooney elevators have control horns and tapered rear edges, both which serve the same function as anti-servo tabs (and less complicated)

No, the horns actually reduce the force as they move opposite the flight control, often called a balanced flight control, the bigger the horn, the less force required to displace it. Wish I had a pic of a Thrush rudder horn, it’s probably 20% of the total rudder. An anti-servo tab moves with the flight control, so as you move the trailing edge of an elevator up, the anti-servo tab also moves up, pushing the elevator back down.

I’m not sure what if anything the tapered trailing edge does control force wise but I don’t think it plays into it. I think it’s to match the main wings appearance wise, a straight horizontal would look goofy with tapered wings

Horns on a Mooney judging from their size primary function is to hold the weights. They are pretty small, but it’s not a big airplane and Mooney control forces are sort of high for this size airplane, especially ailerons and you should harmonize controls, nobody likes an airplane who’s controls require different efforts. I’m not picking on Mooney, it’s an IFR traveling machine, not a crop duster, it should have controls on the heavy side.

 

[PT20J]

55 minutes ago, A64Pilot said:

I thought all the Fowler flapped one did, (old Cessna’s don’t have Fowler flaps) but with a little reading it seems only the older models with 40 degrees are prohibited and only with full flaps, newer models with only 30 degrees are no longer prohibited

https://www.aopa.org/news-and-media/all-news/2009/january/flight-training-magazine/slip-solution

So I guess those that I know that do aren’t crazy like I thought.

Two perpetual myths about Cessnas are that slips with flaps are dangerous and that they have Fowler flaps.

According to Cessna, the flaps are "single slotted". This is a common oral question on the practical test. If you say Fowler, you get to look it up in the POH.

The article cited notwithstanding, only the 40-deg flap C-172s had a note about slips with the flaps down. And it wasn't a prohibition: as I said earlier, the placard reads AVOID, not prohibited. There is also the following note in the Normal Landing section of the POH: "Steep slips should be avoided with flap settings greater than 20 deg due to a slight tendency for the elevator to oscillate under certain combinations of airspeed, sideslip angle, and center of gravity loadings." 

 

 

[A64Pilot]

Cessna’s definition non withstanding, I believe it’s a Fowler flap, just like GE engines don’t have coolers, they have heat exchangers, same thing different name

Fowler flap[edit]

A split flap that slides backwards, before hinging downward, thereby increasing first chord, then camber.^[14] The flap may form part of the upper surface of the wing, like a plain flap, or it may not, like a split flap, but it must slide rearward before lowering. As a defining feature – distinguishing it from the Gouge Flap – it always provides a slot effect. Invented by Harlan D. Fowler in 1924, and tested by Fred Weick at NACA in 1932. They were first used on the Martin 146 prototype in 1935, and in production on the 1937 Lockheed Super Electra,^[15] and are still in widespread use on modern aircraft, often with multiple slots.^[16]

[PT20J]

12 minutes ago, A64Pilot said:

A split flap that slides backwards, before hinging downward, thereby increasing first chord, then camber

And, that's why they are single slotted rather than Fowler, The motion of the flap opens up a slot. It does not extend aft to increase the wing area first and then rotate downward to increase camber.

[jaylw314]

53 minutes ago, A64Pilot said:

No, the horns actually reduce the force as they move opposite the flight control, often called a balanced flight control, the bigger the horn, the less force required to displace it. Wish I had a pic of a Thrush rudder horn, it’s probably 20% of the total rudder. An anti-servo tab moves with the flight control, so as you move the trailing edge of an elevator up, the anti-servo tab also moves up, pushing the elevator back down.

I’m not sure what if anything the tapered trailing edge does control force wise but I don’t think it plays into it. I think it’s to match the main wings appearance wise, a straight horizontal would look goofy with tapered wings

Horns on a Mooney judging from their size primary function is to hold the weights. They are pretty small, but it’s not a big airplane and Mooney control forces are sort of high for this size airplane, especially ailerons and you should harmonize controls, nobody likes an airplane who’s controls require different efforts. I’m not picking on Mooney, it’s an IFR traveling machine, not a crop duster, it should have controls on the heavy side.

 

You're right, I got the anti-servo and balance tabs swapped.  I imagine most of the time they'd only be used on stabilator type controls

The tapered rear edge of the control surfaces serves to lighten forces.  The increase angle at the trailing edge creates extra lift and pulls the airflow in the other direction, helping decrease the control force needed.  I have no idea how it compares in magnitude to the control horns, though

[EricJ]

1 hour ago, PT20J said:

Two perpetual myths about Cessnas are that slips with flaps are dangerous and that they have Fowler flaps.

According to Cessna, the flaps are "single slotted". This is a common oral question on the practical test. If you say Fowler, you get to look it up in the POH.

The article cited notwithstanding, only the 40-deg flap C-172s had a note about slips with the flaps down. And it wasn't a prohibition: as I said earlier, the placard reads AVOID, not prohibited. There is also the following note in the Normal Landing section of the POH: "Steep slips should be avoided with flap settings greater than 20 deg due to a slight tendency for the elevator to oscillate under certain combinations of airspeed, sideslip angle, and center of gravity loadings." 

Back when I was young and bored and would just go try things with the Cessna 150s, we'd slip as much as it would let us, like fully crossed controls, with all angles of flaps.   Somebody said it was designed to not be able to stall out of a slip with full flaps, so of course I had to go try it.    Basically it just had so much turbulent air over the tail, especially with 40 degrees of flaps, that there was not enough elevator authority left to pull the nose up much, but if you put the nose down and got a little airspeed and then yanked it really hard it'd spin out of the slip rather than stall, which made sense.   You had to work really hard to get it to do it, though.   It wasn't anything I'd consider a dangerous characteristic at all.

One thing I did find in all that playing around was that it wasn't too hard to make it stall coming out of a slip.   If you had the nose kinda high-ish, or pulled it up a little while coming out of the slip, you could stall it pretty easily.  The slip is very draggy, so it's not too hard to be slow in the slip and the increasing elevator authority coming out of the slip, if you were trying to keep the nose up a bit, anyway, could get you in trouble if you weren't paying attention, particularly because the ASI isn't going to be accurate during a slip, anyway.   For this reason I just got in the habit of putting the nose down a little coming out of a slip.

The 172 we had at the time had 40 degrees of (manual) flaps, but I never tried the same stall-out-of-a-slip experiment with the 172s.   We did slip them a lot, though, even with full flaps.    Our field was only 2200 ft long, so if you were even a little high you were often motivated to get rid of the extra altitude somehow.    The nice thing about the 40 degrees of flaps on the old Cessnas with slips was that when you straightened it out, even if you had the nose down a little in the slip, it would stay slow with all that drag.   I'm always disappointed flying a 172 with only 30 degrees of flaps, you just really want that extra notch if you're used to it.

Slipping the Mooney seems pretty straightforward to me, and I've not noticed any undesirable characteristics, but I've not gone out and pushed the limits like we used to with the 150s.   Just doing typical slips, even with full rudder deflection, seems pretty easy, though.

[PT20J]

Mooney only uses beveled trailing edges on the ailerons. Mooney lateral control forces are fairly high. This is partly because there is less mechanical advantage in the system than many other airplanes of similar size as noted by fact that the yoke does not rotate through as large an angle stop-to-stop, and partly because so much of the wing is given to the flaps that the ailerons are necessarily short in span, requiring greater chord -- which increases the hinge moment -- to achieve the required area .

Originally the ailerons were not beveled. This feature was added about the time that the PC system came out and it may have been that the vacuum actuators required a reduction in the forces.

Beveling was apparently discovered by accident. Here's a short history, and the best explanation I've found about how it works aerodynamically. From Abzug and Larrabee, Airplane Stability and Control: A History of the Technologies That Made Aviation Possible, Cambridge University Press, 1997:

[whiskytango]

28 minutes ago, EricJ said:

Back when I was young and bored and would just go try things with the Cessna 150s, we'd slip as much as it would let us, like fully crossed controls, with all angles of flaps. 

My first airplane was a '64 straight tail C-150 with 40 degrees of manual flaps available.  My home 'drome had a 7000 foot runway and got a lot of bizjet traffic.  Tower would tell us slow guys in the pattern to make a "short approach" when they had a bizjet on final.  The drill was to turn base when you got to the approach end of the runway, pull off the power, drop all 40 degrees of flaps, dive for the runway and slip as much as you dared.  Right before turning final you took out the slip.  The airframe was so draggy with 40 degrees of flaps out, floating was not a problem, and you could turn off at the first taxiway.  I don't remember what the airspeed was during that maneuver, but I assumed I was going too fast to stall.       

[0TreeLemur]

1 hour ago, PT20J said:

Originally the ailerons were not beveled. This feature was added about the time that the PC system came out and it may have been that the vacuum actuators required a reduction in the forces.

This is correct.  That's why the PC system won't work on the older M20's without the beveled ailerons.   Pretty clever bit of engineering to reduce aileron control forces.  Thanks for posting that article.

BTW- I loved flying a 172 with 40-degree flaps.  Felt like throwing an anvil out on a line from the nose and dragging a parachute from the tail.  Didn't need to do a slip with all that going on.

[PT20J]

19 minutes ago, 0TreeLemur said:

BTW- I loved flying a 172 with 40-degree flaps.  Felt like throwing an anvil out on a line from the nose and dragging a parachute from the tail.  Didn't need to do a slip with all that going on.

Me too!

[0TreeLemur]

Nerd alert!   There is a free pdf version of the book Skip cited above! 

https://assets.cambridge.org/97805218/09924/sample/9780521809924ws.pdf

Just read the introduction.   Seems very well written.  Recommended for those interested in the history of control.

[EricJ]

50 minutes ago, 0TreeLemur said:

This is correct.  That's why the PC system won't work on the older M20's without the beveled ailerons.   Pretty clever bit of engineering to reduce aileron control forces.  Thanks for posting that article.

BTW- I loved flying a 172 with 40-degree flaps.  Felt like throwing an anvil out on a line from the nose and dragging a parachute from the tail.  Didn't need to do a slip with all that going on.

I thought the 40 degrees was crazy good and we visited a Canadian flying club at the time that had an O-1 Bird Dog that had 60 degrees.    I really wanted to fly that just to play with the flaps and landing.   

[0TreeLemur]

This P-51 that they parked me next two last weekend had flaps down to ease egress from the wing.   Looks like 40- or 45-degrees to me.  

This a/c online: https://www.mustangsmustangs.com/p-51/survivors/serial/44-72826

[A64Pilot]

3 hours ago, PT20J said:

And, that's why they are single slotted rather than Fowler, The motion of the flap opens up a slot. It does not extend aft to increase the wing area first and then rotate downward to increase camber.

First notch of the 210 flap moved it afr along a track, the track is what differentiates a Fowler from a plain flap.

  What I call a poor man’s Fowler can be made by significantly dropping the hinge point, look at a Husky for an example, the dropped hinge point moves the flap aft opening up the slot. I wanted to do that with  the Thrush but wasn’t afforded the opportunity.

 

[A64Pilot]

6 minutes ago, 0TreeLemur said:

This P-51 that they parked me next two last weekend had flaps down to ease egress from the wing.   Looks like 40- or 45-degrees to me.  

This a/c online: https://www.mustangsmustangs.com/p-51/survivors/serial/44-72826

Yes, but Fowler due to the slot and airflow over the flap are hugely more effective, reason why airliners pretty much are all Fowler flaps. An airliner in landing configuration probably pretty much doubles the effective wing area with flaps and slats, they are incredibly efficient, look at their glide ratios. Absolute marvels in efficiency.

Cessna nailed the flap “thing”

[201Steve]

12 hours ago, N201MKTurbo said:

In my old M20F I did them all the time. Now I have speed brakes, so I use them.

On my instrument check ride in ‘85, I forgot about a crossing altitude. The examiner asked “what altitude were you supposed cross XXX at”? I was about a mile from it and 1000 feet high. While under the hood, I pulled the power to idle and shoved the rudder to the floor. I crossed with the needle centered and about 50 feet high. He said “ well OK, that meets the requirement” and then “I can’t believe you did that”.

I did that during a practice ils approach with my cfii at the time. My glide slope indicator wasn’t working. I took the glasses off and slipped, trying to see if it would catch it somewhere along the way. He went biserk on me. “NEVER. EVER. EVER. SLIP ON INSTRUMENTS!!!” I was like, easy old man, it’s clear as glass out here. I took the glasses off. We are good Ike Turner!

[N201MKTurbo]

9 minutes ago, 201Steve said:

I did that during a practice ils approach with my cfii at the time. My glide slope indicator wasn’t working. I took the glasses off and slipped, trying to see if it would catch it somewhere along the way. He went biserk on me. “NEVER. EVER. EVER. SLIP ON INSTRUMENTS!!!” I was like, easy old man, it’s clear as glass out here. I took the glasses off. We are good Ike Turner!

Just because your instructor doesn’t know how to slip on instruments doesn’t mean you don’t. 
 

Just shove the rudder down and note the heading, then fly that heading. Not hard. 

[Scottknoll]

Semi-related question, I’ve always wondered why the elevator (Ovation) is slightly deflected upward in trimmed up cruise flight. You can see the horn pointed slightly down in this picture. I would have assumed it would be aligned with the horizontal stab.

First time in icing, seeing that made me wonder about ice accumulation on the horn. Never saw any, just made me think about it.

[EricJ]

1 hour ago, 0TreeLemur said:

This P-51 that they parked me next two last weekend had flaps down to ease egress from the wing.   Looks like 40- or 45-degrees to me.  

This a/c online: https://www.mustangsmustangs.com/p-51/survivors/serial/44-72826

P-51 flaps and inner gear doors come down on their own as the hydraulic pressure bleeds off after shutdown.    Usually the inner gear door close right away at startup as the hydraulic pressure comes up and then the flaps go to the selected position (usually up).    You can kinda tell how long one has been sitting by whether the gear doors are open and the flaps are down or not.   If they're closed it was likely shut down very recently.

Like this:
 

 

 

[PT20J]

56 minutes ago, Scottknoll said:

Semi-related question, I’ve always wondered why the elevator (Ovation) is slightly deflected upward in trimmed up cruise flight. You can see the horn pointed slightly down in this picture. I would have assumed it would be aligned with the horizontal stab.

First time in icing, seeing that made me wonder about ice accumulation on the horn. Never saw any, just made me think about it.

The original short bodies trimmed for cruise with the elevator in trail. The mid bodies trim with the elevator slightly trailing edge down. The long bodies frim with the elevator slightly trailing edge up. This is normal.

The airplane is in trim when the tail down force causes a moment that exactly balances pitch down moment of the rest of the airplane. If it trims with the elevator trailing edge down, that means that the stabilizer incidence angle  is set too much leading edge down and the elevator is reducing the tail down force created by the stabilizer. Conversely, when the airplane trims with the stabilizer trailing edge up, it means that the stabilizer angle is set too much leading edge up and the elevator is increasing the tail down force.

In order to fix this, Mooney would have had to redesign the empennage which would have been expensive. Since the trim system had enough range to compensate for the imperfection over the CG range they just left it alone. The trim range is pretty marginal in the landing configuration on some models which is why the trim ends up full nose up.

Some have fretted that the slight offset causes extra "trim" drag. But, it is probably minimal. Remember, Lopresti was looking for obvious drag producers on the 201 project and he would have certainly addressed it if it were significant. Actually, most trim drag is from the wing -- it is the increment of induced drag created by the extra lift required to offset the tail down force.

Skip