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Okay Gurus - Question on engine placement - 2/20/2008 3:34:10 AM   
happypappy


 

Posts: 477
Joined: 4/9/2005
From: Eustis, FL, USA
Status: offline 		Am thinking of getting into pylon racing in a stock class. The aircraft would be a low wing design. Am wondering what the effect would be if I lowered the thrust centerline and made up for it by adding weight forward. I know I will gain a little inherant stability by a more forward C/G but what other gains or drawbacks would I see? Only talking about 1/8"or so. Think about the flight path - only straight lines and left turns at full throttle. Can go to high rates after the race is over to counteract the forward c/g. This oughta' give you guys something to think about!

< Message edited by happypappy -- 2/20/2008 3:35:01 AM >
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RE: Okay Gurus - Question on engine placement - 2/20/2008 5:36:45 AM   
highhorse


 

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From: Memphis, TN, USA
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quote:

Am wondering what the effect would be if I lowered the thrust centerline and made up for it by adding weight forward


Well, I'm not sure, but it seems to me that the point of building a racer is to go fast ? (yeah, I know, smart-elic) Working from there, the problems that come to my (admitedly questionable) mind are:

1) lowering the centerline, as you've already alluded to, MIGHT cause the nose to naturally pitch up. I say MIGHT because I don't know where the CG (in a vertical sense as opposed to a "balance point" or the center of drag are. Maybe there'll be a pitching moment, maybe not?

2) If there IS an upward pitching moment, why not put the thrust line in a location that doesn't call for some sort of speed-robbing compromise?


I LOVE your question tho, and it's making me think. I'm sure someone much smarter than me will cime in soon and enlighten both of us. Or at least ME, seeing as how I'm the lower threshold

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RE: Okay Gurus - Question on engine placement - 2/20/2008 5:50:14 AM   
Mike Connor



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Joined: 1/10/2002
From: Tulsa, OK, USA
Status: offline 		A forward CG causes more drag. Not what I would want on a race plane

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RE: Okay Gurus - Question on engine placement - 2/20/2008 7:00:40 AM   
highhorse


 

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quote:

ORIGINAL: Mike Connor

A forward CG causes more drag. Not what I would want on a race plane

Yup, my thought too... EXCEPT when / if the thrust line produces pitch up moment which needs to be compensated for, then the fwd balance is a wash except when power is reduced. At which point the extra drag from the up elev doesn't matter 'cus he's not racing around at rreduced power.

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RE: Okay Gurus - Question on engine placement - 2/20/2008 2:08:36 PM   
crasherboy


 

Posts: 112
Joined: 10/21/2006
From: Bryant Pond, ME, USA
Status: offline 		One sure way to find out,is to try it! We don't learn by just thinking about it!

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RE: Okay Gurus - Question on engine placement - 2/20/2008 2:22:15 PM   
HighPlains


 

Posts: 2666
Joined: 3/2/2003
From: Over da rainbow, KS, USA
Status: online 		Is this a quickie design or something else?

< Message edited by HighPlains -- 2/20/2008 3:15:48 PM >

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RE: Okay Gurus - Question on engine placement - 2/20/2008 3:08:39 PM   
da Rock



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From: western, NC, USA
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quote:

ORIGINAL: highhorse


quote:

ORIGINAL: Mike Connor

A forward CG causes more drag. Not what I would want on a race plane

Yup, my thought too... EXCEPT when / if the thrust line produces pitch up moment which needs to be compensated for, then the fwd balance is a wash except when power is reduced. At which point the extra drag from the up elev doesn't matter 'cus he's not racing around at rreduced power.



Problem is, nobody knows if changing the thrust line will need any compensation. OK, the airplane knows, but it won't tell us until we fly it.

Lots of aerodynamic actions don't always result in any kind of compensation. And certainly don't automatically require specific compensations. BTW, moving a CG so much that you have to make up for the loss of elevator effectiveness with high rates is going to turn that airplane into a heavy flying thing. Moving a CG forward reduces the elevator's effectiveness. What happens then? The elevator has to deflect more to get the same pitch change it used to get with less deflection. What does that give the airplane? More drag. You really want to place the CG wherever the elevator is most efficient and effective, and going forward is the wrong way to get there. When you move a CG, you should always expect that the elevator's effectiveness is going to change. Because it does. Sometimes we notice, sometimes not. But the airplane always does. With racing planes, the easiest to read symptom is it's speed in the turns.

With racing planes, move the CG forward and the tail has to produce more downforce just like with any other plane. The forward move of the CG increased the wing loading. The downforce increase comes from the better leverage the CG just got. Now the tail has to produce an equal downforce. Put that airplane in a turn and the centrifugal amplification of the forces really mounts up. And with racing planes, it's something you really don't want. With others, who cares.

With most models, there isn't enough room to move a motor mount enough to change much at all. And most people who race fast planes made them fast by adjusting everything in speed testing. The actual environment gives the best answers.

Also, moving 1/8" really isn't going to change anything radically anyway. So it makes more sense to wait and see if anything needs to be changed to compensate for whatever results.

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RE: Okay Gurus - Question on engine placement - 2/20/2008 3:36:37 PM   
dick Hanson



Posts: 10061
Joined: 12/12/2001
From: slc, UT, USA
Status: online 		I wondered if anybody would pipe up with that answer- I havent raced in a long time but just like anything else there are parameters which seem to improve the performance -
The CG- needs to be as far aft as PRACTICAL
This means that you experiment (no formulas for this --) until you tune in the model and control surface settings to still retain control -easily but that's it .
Basically a well tuned racer is almost at a neutral setup - (almost unstable) so that only minimal forces are needed to change directions or make corrections
This can be dangerous for a newcomer or ham fisted flyer.
As for the thrustline -
The typical high thrustline requires some UP EL to counter the low wing position lift/drag force
This really does helpmake things more stable as you need to hold a wee bit of pressure on the stick at all times . I flew the fastest when I had complete confidence in being able to hold the "line" I wanted .
It is all a compromise. On the old quicky 500 racer I had - I knew I could slot it exactly where I wanted it to be . This tends to drive off others who are afraid of a collision. Just like car racing- there is some bluffing needed
Anyway the thrustline unless it is below the wing -is no big deal --- for most purposes
if you know how to setup the model with a high thrustline - - they work too - I had a semi scale Spirit of St Louis with a K&B 20 that went like stink - and was easy to control (back in about 1974)
The latest Formula models are too fast for me - These things really have developed into trained rats--.

< Message edited by dick Hanson -- 2/20/2008 3:40:59 PM >

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RE: Okay Gurus - Question on engine placement - 2/20/2008 3:37:42 PM   
happypappy


 

Posts: 477
Joined: 4/9/2005
From: Eustis, FL, USA
Status: offline 		I agree with your replies so far. To answer some of the questions and statements - the airplane is not a Quickie design, it is a simple low wing class that is currently on the upswing.
Da Rock I also agree with your answers so far about deflection of control surfaces when the c/g is changed however the upthrust I am talking about would be compensated by moving weight forward and the control surfaces would again be neutral. What result in handling through the course would this produce? No change? Better? Why?
Okay - here is another question. This one is on overall airflow. The aircraft will have no cowl over the engine, only cheeks on the sides of the engine, and the engine must be mounted upright. I am allowed up to a 2.5" spinner...do I mount the engine close to the cheeks to try and keep the spinner close in for airflow or mount the engine farther forward thinking that the spinner may have a better chance to deflect airflow outward and around the cheeks a little more? Another question on airflow coming soon!

< Message edited by happypappy -- 2/20/2008 3:39:17 PM >

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RE: Okay Gurus - Question on engine placement - 2/20/2008 4:46:15 PM   
dick Hanson



Posts: 10061
Joined: 12/12/2001
From: slc, UT, USA
Status: online
quote:

ORIGINAL: happypappy

I agree with your replies so far. To answer some of the questions and statements - the airplane is not a Quickie design, it is a simple low wing class that is currently on the upswing.
Da Rock I also agree with your answers so far about deflection of control surfaces when the c/g is changed however the upthrust I am talking about would be compensated by moving weight forward and the control surfaces would again be neutral. What result in handling through the course would this produce? No change? Better? Why?
Okay - here is another question. This one is on overall airflow. The aircraft will have no cowl over the engine, only cheeks on the sides of the engine, and the engine must be mounted upright. I am allowed up to a 2.5" spinner...do I mount the engine close to the cheeks to try and keep the spinner close in for airflow or mount the engine farther forward thinking that the spinner may have a better chance to deflect airflow outward and around the cheeks a little more? Another question on airflow coming soon!

It does not matter - make cheeks as small as rules permit

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RE: Okay Gurus - Question on engine placement - 2/20/2008 6:45:24 PM   
BMatthews



Posts: 8949
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline 		Read and heed Dick's answers. Forget the upthrust, just point it straight ahead. Minimum frontal area and make it pointy as practical.

The height of the thrust line makes extremely little difference at our sizes and amounts of possible alteration. And especially if you're using a low wing. But the nose up effect from too far forward a CG combined with the amount of decalage needed to balace that will be there all the time. For max speed you want to put the CG back as Dick says.

< Message edited by BMatthews -- 2/20/2008 8:26:10 PM >


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RE: Okay Gurus - Question on engine placement - 2/20/2008 9:06:38 PM   
happypappy


 

Posts: 477
Joined: 4/9/2005
From: Eustis, FL, USA
Status: offline 		Okay Guys, have always been taught as a 30+ year GA pilot that a forward C/G has more inherant stability but it's easy to see how the aft C/G would work toward minimizing control movement and ,hey, I am not inside the plane. Will look at that and probably set it so there is zero deflection of the elevator at WOT in a straight line. Next question - back to engine placement again. We all know that air gaps cause drag. Two ways to look at it. Place the engine as far back as posible to get the cylinder head close to the firewall for minimum disruption of airflow and drag? Try and move the engine forward to allow the 2.5" spinner to push airflow away from the draggy frontal area of the plane? The forward placement may make the airplane think it's longer. Also have a little leeway in the placement of the cocpit canopy. According to the rules it has to "roughly" be where the advertising photo places it. It seems like if I can move it forward, closer to the engine, it should smooth the airflow coming off the cylinder head somewhat and reduce drag. Your thoughts? The engine, a Super Tigre, has the ability to move the muffler farther away from or closer to the engine. Am assuming that moving the cannister as close to the airframe as possible will reduce drag? This stuff is fun to me as the class is a bone stock class so paying attention to the little things can make small but positive differences. Thanks for your thoughts and input so far!

< Message edited by happypappy -- 2/20/2008 9:08:15 PM >

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RE: Okay Gurus - Question on engine placement - 2/21/2008 12:17:08 AM   
Mike Connor



Posts: 1384
Joined: 1/10/2002
From: Tulsa, OK, USA
Status: offline 		The sphere shape of the cylinder head is not good for drag as speed increases. I doubt you will get it close enough to the firewall or canopy to make much difference. The spinner may increase drag unless it blends into an engine fairing. A blunt trailing edge increases drag.

I predict the better pilot will win this race and not just the fastest airplane. Talking about drag reduction is fun but practice around the pylons will win the race.

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RE: Okay Gurus - Question on engine placement - 2/21/2008 3:02:19 AM   
BMatthews



Posts: 8949
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline 		Another voice of reason. Mike's right about the closest to the pylon course being the faster way around.

The canopy won't help unless it's right up against the firewall. So no help there. You may as well stick it where the pictures say. Placing the engine low on the firewall so that only the very head is sticking up will help a lot. And what I'd do is set up a 90 degree curved air baffle so that it scoops some of the air and deflects it around the back of the cyinder between the cylinder and the firewall. You don't need much but it's good to not have a dead zone. Helps the engine run a little cooler. Sort of like the helmet cowl on the control line speed models.

The GA stuff is right about a foward CG adding stability. But define "stability". For GA it's about the ability of the craft to quickly and positively return to the trimmed speed and attitude. But on a GA plane if you hold the stick or wheel and add power what happens? The nose comes up and you find yourself climbing at the trimmed airspeed. Cut power and hold the stick and you're suddenly gliding down at the trimmed airspeed. That's stability in the GA world.

For a biginning RC pilot this isn't a bad thing and it explains why so many trainers are set up with a forward CG. But as you've already stated and as so many other RC pilots have stated this trait is a pain when you move on to one of our usual grossly overpowered aerobatic or racing models that have more in common with an F18 than a Cessna 150. So to reduce that "stability" we move the CG back and that reduces this tendency to pitch up with adding power or diving when we take it away after altering the trim for high speed full throttle flying. For the need to monitor and alter the pitch yourself instead of letting the "stability" do it for you the model suffers a lot less from power induced pitching.


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Free Flighters go that extra mile........

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RE: Okay Gurus - Question on engine placement - 2/21/2008 4:07:37 AM