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  #51  
Old 03-24-2021, 01:25 PM
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DanH DanH is offline
 
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Quote:
Originally Posted by terrye View Post
OK Dan, I'll bite . It's not either of Liston's books "Aircraft Engine Design" or "Powerplants for Aircraft". So what's the title and the author?
Terry, I don't know. It was a long time ago. As you probably know, the Air University Library is on Maxwell AFB, and civilian access requires an active duty sponsor. I wasn't sure if I could borrow any books, so I was was simply copying relevant pages with the library copying machine.
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  #52  
Old 03-24-2021, 07:22 PM
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What's F1?
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  #53  
Old 03-25-2021, 05:15 AM
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Originally Posted by emsvitil View Post
What's F1?
Fundamental frequency, first natural frequency, lowest vibratory mode. Different users may notate it otherwise, but here's the picture.

Aircraft propeller mass moment of inertia is almost always much higher than powerplant mass moment of inertia, so imagine a simple two element system as a large flywheel connected to a smaller flywheel with a shaft, hanging free in space....two intertias connected by a stiffness.

Using your giant magical hands, hold the big wheel stationary and turn the small wheel a few degrees, in a manner which twists the shaft, now a torsional spring. Release both wheels at the same time. The wheels will begin to oscillate in opposition. The angular displacement of the large inertia will be less than the angular displacement of the small inertia, but the period (rate, cycles per second, hertz) will be identical. The period at which they oscillate is the natural frequency (F1) of the simple two element system, a function of the inertias and the connecting spring rate. Change an inertia or the spring rate to change the frequency.

A real system can (and does) have multiple inertias and connecting stiffnesses. The number of available natural frequencies (F1, F2, Fx, etc) and vibratory modes is always total inertias less one. I've attached some examples below. Note the "mode shapes", a name which seems to stem from the 2D drawings depicting them in old texts (example below). At F2 and higher, the various inertias take up different rotation directions in relation to other inertias.

Note that natural frequencies are passive. They are the frequency at which the mode oscillates if excited, which is the key concept in application. When excited at a forcing frequency other than the system natural frequency, nothing much happens. When excited at the same frequency, the system resonates; the amplitude of the oscillation grows, and without damping, can theoretically reach infinity. It's theoretical because (1) there is always some damping, and (2) no connecting stiffness is infinitely strong...
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Last edited by DanH : 03-25-2021 at 05:18 AM.
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  #54  
Old 03-25-2021, 07:21 AM
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Bit 'o thread drift here, and I'm guilty as any. Torsional vibration is a subject more homebuilders should contemplate, in particular those who want to fly alternative engines. However, it is a big, complicated subject, and this thread is about Brian's RV-8...so let's just follow his progress with the new engine.

Hopefully Verner understands the torsional stuff!
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  #55  
Old 03-27-2021, 04:27 PM
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Quote:
Originally Posted by DanH View Post
Bit 'o thread drift here, and I'm guilty as any. Torsional vibration is a subject more homebuilders should contemplate, in particular those who want to fly alternative engines. However, it is a big, complicated subject, and this thread is about Brian's RV-8...so let's just follow his progress with the new engine.

Hopefully Verner understands the torsional stuff!
LOL, I figured that I would just sit back and let you guys hash this one out. Anyways......I posted some more pictures on the FB page yesterday. Progress is coming along smoothly. To those of you who don't have a FB account. Sorry, but I don't use any third party pages to store photos just for VAF. I might be wrong, but you might not need an a FB account in order to see the page.
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  #56  
Old 03-27-2021, 05:16 PM
ksauce ksauce is offline
 
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Quote:
Originally Posted by DanH View Post
Fundamental frequency, first natural frequency, lowest vibratory mode. Different users may notate it otherwise, but here's the picture...
OK, OK, back to the thread but, Dan, this is really interesting. Thanks for posting.
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  #57  
Old 03-27-2021, 06:33 PM
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Brian, it's a new day...photo hosting right here on the VAF servers. Just click the little bitty paperclip icon above the "Reply To Thread" screen and post photos, easy as pie.
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  #58  
Old 03-27-2021, 09:44 PM
Andy_RR Andy_RR is offline
 
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Bedtime reading...

http://www.enginehistory.org/NoShortDays/TV.pdf
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  #59  
Old 04-07-2021, 04:01 PM
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The Verner 9S is proving to be a strong engine. The one thing that I am changing about it is the fuel delivery system (carb). I am adapting an Airflow Performance FM-150 to the engine. In order to do this I designed an adaptor that fits the unit to the engine and also houses the flow divider and fuel injectors. Along with an SDS duel fuel pumps setup I think this will really make the engine a winner.
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