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Stalling The Fokker D.21  
User currently onlineptrjong From Netherlands, joined Mar 2005, 3906 posts, RR: 19
Posted (2 years 10 months 2 weeks 22 hours ago) and read 3363 times:

Hello dear Tech forum,

I'm writing a book on the WW II Fokker D.21 fighter, and since I'm not a pilot, I'm coming here to ask your help regarding the aircraft's flying characteristics.

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The D.21 had a fairly nasty tendency to stall into a spin, both at high speeds, whenever the stick was pulled too hard, and at slow speeds, for example when landing a little slow.

Dutch pilots say the unintended spins were always towards the left. I'm fairly certain this is related to engine torque. The Bristol Mercury engine was right-hand drive (turning clockwise when seen from the pilot, I suppose). Does this make sense?

The Finns produced a Pratt & Whitney Twin Wasp Junior-engined version, which was left-hand drive. So can I safely assume this version had a habit of spinning towards the right, the aircraft being very much unchanged apart from the engine installation?

The famous aerobatics pilot Michel Détroyat flew the D.21 prototype, and insisted that lateral stability was unaccepable and that wing dihedral should be increased from 1.5 deg to 7-10 deg. A redesigned wing was built and tested, but the great Finnish fighter leader Eka Magnusson rejected it and evidently thought the flying characteristics with the original wing were quite acceptable, and that the D.21 was reasonable bang for Finland's buck (having good climb rates and pretty good diving characteristics).

So my conclusion is that the high anhedral version was excessively stable, preventing the fighter from snapping into a roll quickly. And that Magnusson's view was the warrior's view, while Détroyat's view was the aerobatic pilot's view, demanding perfect flying characteristics for things like sustained turns and loopings. When Détroyat recommended the P-36 to France, this was apparently largely based on that aicraft's perfect flying characteristics.

Again, does this make sense?

Thanks a lot in advance,

Peter 


The only difference between me and a madman is that I am not mad (Salvador Dali)
23 replies: All unread, jump to last
 
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17001 posts, RR: 67
Reply 1, posted (2 years 10 months 2 weeks 21 hours ago) and read 3343 times:

Quoting ptrjong (Thread starter):

The famous aerobatics pilot Michel Détroyat flew the D.21 prototype, and insisted that lateral stability was unaccepable and that wing dihedral should be increased from 1.5 deg to 7-10 deg. A redesigned wing was built and tested, but the great Finnish fighter leader Eka Magnusson rejected it and evidently thought the flying characteristics with the original wing were quite acceptable, and that the D.21 was reasonable bang for Finland's buck (having good climb rates and pretty good diving characteristics).

Classic design compromise issue. What is more important, maneuverability or stability? And where is the sweet spot?

Quoting ptrjong (Thread starter):
So my conclusion is that the high anhedral version was excessively stable, preventing the fighter from snapping into a roll quickly

I think you mean dihedral.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently onlineptrjong From Netherlands, joined Mar 2005, 3906 posts, RR: 19
Reply 2, posted (2 years 10 months 2 weeks 21 hours ago) and read 3341 times:

Dihedral, yes, sorry. Positive V angle.


The only difference between me and a madman is that I am not mad (Salvador Dali)
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17001 posts, RR: 67
Reply 3, posted (2 years 10 months 2 weeks 19 hours ago) and read 3310 times:

In ye olde days before CFD or even really good slide rule aerodynamics designs were way less optimized. So you would get these things.

Test pilot input was also much important because there really were a lot of unknowns even after the first aircraft had been assembled. Détroyat wanted to increase dihedral from 1.5 to 7-10. That's a huge change in the flying characteristics, and mostly based on rather unquantifiable seat of the pants factors.

For that matter, the flying style and preferences of the individual test pilots probably also contributed in a way they don't today.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineKELPkid From United States of America, joined Nov 2005, 6346 posts, RR: 3
Reply 4, posted (2 years 10 months 2 weeks 13 hours ago) and read 3235 times:

Quoting ptrjong (Thread starter):
Dutch pilots say the unintended spins were always towards the left. I'm fairly certain this is related to engine torque. The Bristol Mercury engine was right-hand drive (turning clockwise when seen from the pilot, I suppose). Does this make sense?

Three things cause turning tendencies in single-engined propeller powered airplanes:

#1). Engine torque. Since every action has an equal and opposite reaction, the airframe will tend to turn (and roll, in the case of a really big engine  ) the opposite way from the engine. The F4U Corsair was notorious for rolling over at slow airspeeds if the pilot applied the throttle too quickly...

#2). Corkscrew effect. The wake of the propeller creates a corkscrew-shaped area of disturbed airflow, which in most planes hits the rudder.

#3). P-Factor. When climbing or descending, one of the propeller blades will be "flatter" than the other one relative to the airflow in front of the aircraft. The flatter blade creates more drag, causing the airplane to turn in that direction.

Yes, an all three of these contribute to turning tendencies in a single-engined proppeller powered aircraft. All I know is that with an engine that turns the "American" direction, they cause a left turning tendency   (IIRC, that is counterclockwise as you face the engine, clockwise if you're sitting behind it...).

P.S., I believe most Bristol engines turned the "European" way, which is clockwise while facing the engine, counterclockwise from behind...



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineGST From United Kingdom, joined Jun 2008, 930 posts, RR: 0
Reply 5, posted (2 years 10 months 2 weeks 12 hours ago) and read 3212 times:

Quoting KELPkid (Reply 4):

Three things cause turning tendencies in single-engined propeller powered airplanes:

In addition to these airborne turning characteristics, the propeller also tends to turn aircraft on the ground run. Spitfires with merlin engines used to turn left on the ground run, but those with griffin engines turned right due to opposite rotation. This might not be a problem in most cases, but in the case of carrier borne seafires, turning left was much preferred as it steered you away from the island. The griffin engined variants were sometimes disliked for the reason that they would try to bury themselves in the island.

Sometimes the ground run turning effect was quite severe, there was a joke that you could tell typhoon pilots because they always had one leg much more muscular than the other, the aircraft required significant rudder pedal pressure to keep the aircraft straight on the runway due to the naper sabre engine.


User currently onlineptrjong From Netherlands, joined Mar 2005, 3906 posts, RR: 19
Reply 6, posted (2 years 10 months 2 weeks 10 hours ago) and read 3178 times:

Thanks for all the replies.

[quote=KELPkid,reply=4] (IIRC, that is counterclockwise as you face the engine, clockwise if you're sitting behind it...).

Six years ago you were more positive, writing on this forum:

Quote:
American engineers have always been of the opinion that an aircraft engine should turn counterclockwise as viewed from the front, and clockwise as viewed from behind.

 

So that means the Twin Wasp turns clockwise seen from behind, and since the Finns needed to change propellers, the Mercury must have turned counter-clockwise seen from behind, as you suggested. Can the Brits confirm this? And would the expression 'right hand drive' thus refer to the turning tendency of the aircraft rather than to the engine?

This doesn't seem to explain how the Mercury D.21 stalled towards the left. But maybe I have this concept wholly wrong, because I am now informed from Finland that both D.21 variants stalled 'first to the left and immediately afterwards to the right', according to test reports. So isn't this related to the turning direction of the engine at all? If not, why whould the stall always be towards the left first?

The Finns ultimately reduced the stalling tendency by fitting 6-hole slots in both wing tips.

Peter 



The only difference between me and a madman is that I am not mad (Salvador Dali)
User currently onlineptrjong From Netherlands, joined Mar 2005, 3906 posts, RR: 19
Reply 7, posted (2 years 10 months 2 weeks 7 hours ago) and read 3117 times:

Quoting ptrjong (Reply 6):
I am now informed from Finland that both D.21 variants stalled 'first to the left and immediately afterwards to the right', according to test reports

This was about minimum speed stalls.



The only difference between me and a madman is that I am not mad (Salvador Dali)
User currently offlineKELPkid From United States of America, joined Nov 2005, 6346 posts, RR: 3
Reply 8, posted (2 years 10 months 1 week 6 days 16 hours ago) and read 2959 times:

Quoting ptrjong (Reply 6):

So that means the Twin Wasp turns clockwise seen from behind, and since the Finns needed to change propellers, the Mercury must have turned counter-clockwise seen from behind, as you suggested. Can the Brits confirm this?

Interesting, according to Wikipedia, the Bristol Mercury uses epicyclic gearing. I don't know if this form of gear reduction changes the rotation of the prop shaft. However, the Wikipedia article states that the ultimate goal is "Left Hand Drive" (whatever that means!).  



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineN243NW From United States of America, joined Jul 2003, 1626 posts, RR: 20
Reply 9, posted (2 years 10 months 1 week 6 days 14 hours ago) and read 2948 times:

Quoting KELPkid (Reply 4):
#3). P-Factor. When climbing or descending, one of the propeller blades will be "flatter" than the other one relative to the airflow in front of the aircraft. The flatter blade creates more drag, causing the airplane to turn in that direction.

That may be true, but what I was taught is the downward blade (the one on the right-hand side of the aircraft looking forward) at any nose-up aircraft attitude has a greater angle of attack than the descending blade, creating more thrust on the right-hand side of the propeller disc and therefore a slight left turning tendency due to lateral thrust asymmetry.



B-52s don't take off. They scare the ground away.
User currently offlineKELPkid From United States of America, joined Nov 2005, 6346 posts, RR: 3
Reply 10, posted (2 years 10 months 1 week 6 days 6 hours ago) and read 2857 times:

Quoting N243NW (Reply 9):
That may be true, but what I was taught is the downward blade (the one on the right-hand side of the aircraft looking forward) at any nose-up aircraft attitude has a greater angle of attack than the descending blade, creating more thrust on the right-hand side of the propeller disc and therefore a slight left turning tendency due to lateral thrust asymmetry.

Hmmm....not confirming or denying, but I know that in planes that have relatively large props in relationship to the aircraft size (e.g. Cessna 172 with an O-360 conversion), I have experienced right hand turning tendencies on nose down descents (don't usually notice this in most other singles)   The engine isn't making much power during descent, but I do know the prop is creating lift as long as it spins...



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlinedw747400 From United States of America, joined Aug 2001, 1257 posts, RR: 1
Reply 11, posted (2 years 10 months 1 week 6 days 3 hours ago) and read 2814 times:

Quoting N243NW (Reply 9):
but what I was taught is the downward blade (the one on the right-hand side of the aircraft looking forward) at any nose-up aircraft attitude has a greater angle of attack than the descending blade, creating more thrust

That is correct.

Quoting KELPkid (Reply 4):
The flatter blade creates more drag, causing the airplane to turn in that direction.

This is possible, but it is not the "official" definition of P-factor. So... don't say that on an FAA written!

Quoting KELPkid (Reply 10):
I know that in planes that have relatively large props in relationship to the aircraft size (e.g. Cessna 172 with an O-360 conversion), I have experienced right hand turning tendencies on nose down descents

I can't say I've noticed the same. I've flown 172s with 75" ("basic" 160 hp), 76" (the O-360 180HP), and 78" (IO-360 210HP) prop and have never seen a consistent trend between them.

I'd guess that your A&P set the trim tab more aggressively on the 180HP aircraft to counteract the greater left turning tendency of the larger engine at cruise. When running at low power, that trim tab is going to push the aircraft to the right more than a less aggressively adjusted tab on a lower power plane.



CFI--Certfied Freakin Idiot
User currently offlineKELPkid From United States of America, joined Nov 2005, 6346 posts, RR: 3
Reply 12, posted (2 years 10 months 1 week 6 days 1 hour ago) and read 2787 times:

Quoting dw747400 (Reply 11):
I can't say I've noticed the same. I've flown 172s with 75" ("basic" 160 hp), 76" (the O-360 180HP), and 78" (IO-360 210HP) prop and have never seen a consistent trend between them.

Try a chop 'n drop sometime...   (i.e. 750-1000 FPM descent). I'd be curious to know if you experience the same...



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineN243NW From United States of America, joined Jul 2003, 1626 posts, RR: 20
Reply 13, posted (2 years 10 months 1 week 5 days 23 hours ago) and read 2770 times:

Quoting dw747400 (Reply 11):
I'd guess that your A&P set the trim tab more aggressively on the 180HP aircraft to counteract the greater left turning tendency of the larger engine at cruise. When running at low power, that trim tab is going to push the aircraft to the right more than a less aggressively adjusted tab on a lower power plane.

This sounds like the most likely culprit.

Quoting KELPkid (Reply 12):
I'd be curious to know if you experience the same...

Shock cooling?   



B-52s don't take off. They scare the ground away.
User currently offlineKELPkid From United States of America, joined Nov 2005, 6346 posts, RR: 3
Reply 14, posted (2 years 10 months 1 week 5 days 17 hours ago) and read 2722 times:

Quoting N243NW (Reply 13):

Shock cooling?

Step down fixes on a non-precision approach?   700 FPM is doable, but yeah, 1000 FPM is starting to get into unstable approach territory...my instrument instructor taught me to get down as quickly as possible when you reach a step down fix...



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineN243NW From United States of America, joined Jul 2003, 1626 posts, RR: 20
Reply 15, posted (2 years 10 months 1 week 5 days 16 hours ago) and read 2716 times:

Quoting KELPkid (Reply 14):
Step down fixes on a non-precision approach?

Ahh, touché  
Quoting KELPkid (Reply 14):
my instrument instructor taught me to get down as quickly as possible when you reach a step down fix

Mine too. No use dilly-dallying around and having to go missed because you weren't at MDA in time. IIRC, the entire distance from the FAF to the MAP is a protected area all the way down to the MDA. Ugh, a lot of acronyms in that sentence.  



B-52s don't take off. They scare the ground away.
User currently offlinedw747400 From United States of America, joined Aug 2001, 1257 posts, RR: 1
Reply 16, posted (2 years 10 months 1 week 5 days 16 hours ago) and read 2714 times:

Quoting KELPkid (Reply 12):

Try a chop 'n drop sometime..

Done that many times, never noticed any consistent difference between the planes.

Quoting N243NW (Reply 13):
Shock cooling?

Of course, shock cooling opinions range from "it is a non issue" to "you should keep your engine running 24/7 so it never cools down". The middle ground seems to agree that it is most prevalent on larger engines operated at low power settings and high rates of descent for extended periods. A thousand foot drop here or there isn't as much of an issue, particularly on small engines like the O-360. That said, you still want to treat the engine nicely--smooth power adjustments and avoid high speed idle descents when possible.

Quoting KELPkid (Reply 14):
Step down fixes on a non-precision approach? 700 FPM is doable, but yeah, 1000 FPM is starting to get into unstable approach territory..

During my IPC the other day my CFII cut us in a little to close, requiring a 1500+ FPM descent on a GPS approach. Had it been actual I'd have gone missed, but it made for a fun exercise. Drop the "speed brakes", power a couple inches above idle, and an Arrow will just come right down in a nice steady descent.



CFI--Certfied Freakin Idiot
User currently offlineN243NW From United States of America, joined Jul 2003, 1626 posts, RR: 20
Reply 17, posted (2 years 10 months 1 week 5 days 16 hours ago) and read 2710 times:

Quoting dw747400 (Reply 16):
Drop the "speed brakes", power a couple inches above idle, and an Arrow will just come right down in a nice steady descent.

You're not kidding. The Arrow has slightly better gliding qualities than a cinder block.  



B-52s don't take off. They scare the ground away.
User currently onlineptrjong From Netherlands, joined Mar 2005, 3906 posts, RR: 19
Reply 18, posted (2 years 10 months 1 week 5 days 13 hours ago) and read 2684 times:

Thanks for the replies. It's a fairly complex field and I obviously shouldn't attempt to understand everything about the Fokker D.21's stalling and spinning characteristics, let alone to explain it to others.

It boils down to this question for me - does it strike you as very odd that this aircraft type would tend do drop the left wing first, while having a prop that turns anti-clockwise, and would still tend to drop the left wing first with a prop that turns clockwise? Or are you saying: no, I can see a couple of possible reasons for that.

If the latter is the case, I am fine with not knowing the exact reasons. I just don't want to get my book shot down with 'Hey, it's utterly impossible that the version with engine x would tend to drop the left wing first' or 'it's utterly impossible that both the left-turning and right-turning engine versions would tend do drop the left wing first'.

Thanks in advance,

Peter 



The only difference between me and a madman is that I am not mad (Salvador Dali)
User currently offlinedlednicer From United States of America, joined May 2005, 543 posts, RR: 7
Reply 19, posted (2 years 10 months 1 week 5 days 4 hours ago) and read 2621 times:
AIRLINERS.NET CREW
DATABASE EDITOR

Quoting KELPkid (Reply 4):
Three things cause turning tendencies in single-engined propeller powered airplanes:

There is one more reason - manufacturing asymmetry. A lot of aircraft, especially in the pre-precision measurement days, had side-to-side asymmetry built in, usually because of slight errors in the manufacturing jigging. As a result, certain aircraft will always have a stall-break to the left or to the right.


User currently onlineptrjong From Netherlands, joined Mar 2005, 3906 posts, RR: 19
Reply 20, posted (2 years 10 months 1 week 5 days 3 hours ago) and read 2599 times:

Quoting dlednicer (Reply 19):

There is one more reason - manufacturing asymmetry. A lot of aircraft, especially in the pre-precision measurement days, had side-to-side asymmetry built in, usually because of slight errors in the manufacturing jigging

Hi David, thanks for a very interesting reply. I´m not sure why I didn't think of asking you...

Fokkers would be prime candiates for this, given the manufacturer's building methods. Fokker biplane upper wings would not even be exchangable between airframes. So this is an interesting point, although I wouldn't even suprise me if some D.21 were bent towards the left and some towards the right, rather than all towards one side.

Anyway, I assumed the turning direction of the propeller to be a major factor in the tendency to drop the wing on one particular side, but I´m gathering now that this is hardly the case, and that your answer to my question in reply 18 is ´no, I can see a couple of possible reasons´? I hope you can help me out!

Peter 



The only difference between me and a madman is that I am not mad (Salvador Dali)
User currently offlineKELPkid From United States of America, joined Nov 2005, 6346 posts, RR: 3
Reply 21, posted (2 years 10 months 1 week 5 days 3 hours ago) and read 2590 times:

Quoting dlednicer (Reply 19):

There is one more reason - manufacturing asymmetry. A lot of aircraft, especially in the pre-precision measurement days, had side-to-side asymmetry built in, usually because of slight errors in the manufacturing jigging. As a result, certain aircraft will always have a stall-break to the left or to the right.

Many World War II fighters (the Bf-109/Me-109 being a prime example) had some assymetry intentionally built in. The Bf-109 had a rudder which was canted a few degrees from vertical to compensate for single engine turning tendencies, and, IIRC, the engine mounts were also slightly offsett so that the engine didn't mount compeletely straight  



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineLarshjort From Denmark, joined Dec 2007, 1434 posts, RR: 0
Reply 22, posted (2 years 10 months 1 week 5 days ago) and read 2563 times:

Quoting KELPkid (Reply 21):
the engine mounts were also slightly offsett so that the engine didn't mount compeletely straight

Check the PC-21, it's roughly the same size and power as the Bf-109G. If you compare the spinner to the airintake in this picture you can see how offset it is.


PC-21/1616915/L/" target="_blank">View Large PC-21/1616915/M/" target="_blank">View Medium
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Photo © Javier Gonzalez - Iberian Spotters



/Lars



139, 306, 319, 320, 321, 332, 34A, AN2, AT4, AT5, AT7, 733, 735, 73G, 738, 739, 146, AR1, BH2, CN1, CR2, DH1, DH3, DH4,
User currently offlinedlednicer From United States of America, joined May 2005, 543 posts, RR: 7
Reply 23, posted (2 years 10 months 1 week 4 days 20 hours ago) and read 2524 times:
AIRLINERS.NET CREW
DATABASE EDITOR

Quoting KELPkid (Reply 21):
The Bf-109 had a rudder which was canted a few degrees from vertical to compensate for single engine turning tendencies, and, IIRC, the engine mounts were also slightly offsett so that the engine didn't mount compeletely straight

It goes beyond that - I have the Messerschmitt drawings and they show that the vertical tail is actually cambered to one side.

A lot of propeller-driven aircraft have thrust line inclination, both in pitch and yaw. I worked on the certification of a single-engine GA aircraft and can tell you that it has both.


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