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Author Topic: Aerodynamic Problem.  (Read 613 times)

Offline RogerGreene

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Aerodynamic Problem.
« on: September 17, 2019, 03:40:50 PM »
This thought has been bugging me for quite a while.

On a typical stunt plane when making an inside loop the flaps are down and the elevator is up. However, with a combat plane that has the elevator hinged on the trailing edge of the wing and the control surface points down, the plane goes down. Now back to the stunt plane: if the elevator push rod comes off the control horn so the elevator does not move, which way will the plane go when the flaps go down?


Thanks,
Roger
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Online Ken Culbertson

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Re: Aerodynamic Problem.
« Reply #1 on: September 17, 2019, 04:48:31 PM »
This thought has been bugging me for quite a while.

On a typical stunt plane when making an inside loop the flaps are down and the elevator is up. However, with a combat plane that has the elevator hinged on the trailing edge of the wing and the control surface points down, the plane goes down. Now back to the stunt plane: if the elevator push rod comes off the control horn so the elevator does not move, which way will the plane go when the flaps go down?


Thanks,
Roger
Down, had it happen three times over the years.  Saved the third one.  It really depends where the center of lift falls relative to the center of gravity but in most cases - down.

Ken

Ken
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Offline Dan McEntee

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Re: Aerodynamic Problem.
« Reply #2 on: September 17, 2019, 05:08:29 PM »
    This is why you do not want to give up elevator when trying to take off out of tallish grass if your landing gear isn't placed correctly and/or too short. The flaps get the prop wash first and drive the nose down. If anything, hold a touch down elevator.
   Type at you later,
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Offline Tim Wescott

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Re: Aerodynamic Problem.
« Reply #3 on: September 17, 2019, 05:43:11 PM »
This has been chewed over at some point on this forum, but I'm too lazy to do a search right now.

On full scale planes with flaps the elevator trim can go either way depending on the airframe.

Full scale or stunter, just don't break the elevator linkage!
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Online Ken Culbertson

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Re: Aerodynamic Problem.
« Reply #4 on: September 18, 2019, 07:00:07 AM »
Back at VSC 16 or so, my 52 Nobler kept going low, so I gave it some up and BLAM, into the ground. Seems the badly soldered  washer  came loose from the push rod to the elevators, thus the flaps were now elevators. Broke the nose off and sent it into the next circle.. Every one was so happy. LL~ H^^
Same airplane, same cause that I posted below.  Third time I was luckily at the top of an outside square loop when I hit down and it went up.  After saying WTF loud enough to be heard, I somehow remembered the first two and was able to keep it level up high and sort of land without breaking anything when the tank ran out.  My first two met the same fate as yours.

Does anybody last remember having to think down is up?  It has been muscle memory for me since the late 50's.  Those 4 minutes to run our the tank were the longest I can remember!

Ken
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Online Ken Culbertson

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Re: Aerodynamic Problem.
« Reply #5 on: September 18, 2019, 11:32:13 AM »
I have never consciously thought down was up. I just flew.

 As a side note, after that incident, I learned about ball joints and have never use a soldered connection in the control system since.  D>K  I have made other mistakes to take their place.  LL~ LL~ LL~ LL~ LL~
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Offline bob whitney

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Re: Aerodynamic Problem.
« Reply #6 on: September 18, 2019, 08:37:59 PM »
Yup  Mid 90;s Dayton cash bash  took a proto speed ship off with the controls reversed. flew it 10 lops just giving it down . as it got close to the ground then gave it full up and when it was high enough I swapped the handle over and finished out the flight.  needless to say I never made it into the pylon y1
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Offline Air Ministry .

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Re: Aerodynamic Problem.
« Reply #7 on: September 18, 2019, 10:12:56 PM »
 if the elevator push rod comes off the control horn so the elevator does not move, which way will the plane go when the flaps go down?

Into the ground .  :(

Inverted out of the wingover , first time down flap lifted the nose . The second time they thought they were elevators , flaps & elev. went DOWN

So the funny rattling noise wasnt the big end .

Offline Chuck_Smith

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Re: Aerodynamic Problem.
« Reply #8 on: September 20, 2019, 07:04:00 AM »
I love this question, because it highlights a key concept of subsonic aerodynamics.

It's common, but unfortunately incorrect, to think of the airflow as a linear flow from front to back. At subsonic speeds this just ain't the case.  What happens at the back of the plane affects what happens at the front. You need to visualize the flow field and how it warps and twists around the object moving through the air.

I've spent some time with aero on race cars and it blew some of the engineer's minds to understand that what they were doing with the rear of the car affected what happened at the front of the car, but it does. 

In it's simplest analysis the flap deflection changes the moment coefficient of the wing, and on a - let's say a flying wing config a downward flap deflection causes a downward  pitching moment. With no horizontal stabilizer/elevator to add another moment in the opposite direction the wing will pitch in the direction of the flap deflection. The total pitching moment on the airplane is the sum of all the moments due to the wing, tail, landing gear, thrust, underwing stores, etc.

But - and it's a BIG but, it also alters the flow field around the ENTIRE airplane. On a conventional aircraft this may manifest itself as either a nose up OR nose down total moment on the plane with a free stick response as would be the case here.

Some airplanes pitch up when the flaps are deployed and some pitch down. It all depends on the total geometry of the and how the tail sits in the downwash of the wing.

As a general, and I mean really general rule with many exceptions - low wing planes will pitch nose down when the flaps are deployed, and conversely on high wings. But T-tails maybe not so much.

So, if you whack the fuselage off an Evolution or SV11 just aft of the flaps, the controls will reverse. Pretty easy to evaluate the moments due to lift there. Leave the tail on but disconnect the elevator and it's anybody's guess.

Probably not the answer you want but remember, we're talking about invisible flow fields that you need to visualize.

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Chuck





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