"Upthrust" please

[megawatt]

Could someone please clarify for me the term upthrust, specifically as it relates to motor orientation in a pusher configuration? I am building the Teltech BD-5, and have read within threads here that the setting of motor "upthrust" is the single most important building step. Is upthrust on a pusher opposite of downthrust on a tractor model? If one were to view the model from the side, would the top of the firewall be tilted back? Also, why does a pusher tend to force the models nose down at high throttle settings with the motor set at 0 degrees?
I want to make sure I orient the firewall in this model correctly, as remounting it would not be fun. Just like to double check what I think is correct. Thanks

Jeff

[Megowcoupe]

Jeff

As I understand it, upthrust means that adding power will be equivalent to adding up elevator. In a BD-5, it's short coupled with the thrustline well above the wing. I have a hunch that what's going on is thrustlines above or below a wing affect trim changes under power- not tractor or pusher which should largely be irrelevant.

When thrustlines are either higher or lower than the centerline of the wing, there is a rotational force applied to the wing which results in the leading edge pitching upward or downward. High thrustlines pitch down, low thrustlines pitch up. Some designers have tried to use an altered thrustline to compensate. By angling the motor to shove down on the tail in a high thrustline pusher configuration, some of the pitch changes under power may be ameliorated. An alternative suggestion is to be cautious on your throttle and don't make sudden changes.

HTH- and take the above with a grain of salt- this is just how I'm puzzling it through.

Sam

[megawatt]

Hi Sam. Your explanation certainly brought to light an obvious point that I had missed altogether, that being the rotational forces applied when the thrustline is seriously offset. Seems to me I remember fighting power on vs. off with my seaplane, which had the motor mounted on a pod high above the wing. I have a hard time getting past the "downthrust required" mindset normally asscociated with most model designs.
The picture in the assembly manual for the BD-5 looked almost like the firewall was favoring a downthrust attitude, but it is not clear enough to bank on. The manual states"it's proper inclination should be 3 degrees up, and 2 degrees left when viewed from the rear".
The verbage COULD be intepreted one of two way's.
Thanks for your help Sam.

Jeff

[jimsky]

I built the pusher in the attached photo. The motor is quite high off the wing surface. Initially with the motor level the plane really wanted to "nose down" when the throttle was applied. I tipped the front of the motor up several degress and the nose diving on throttle application dissapeared.

Jimsky

[Sparky Paul]

It depends on where the motor thrust line is relative to the vertical c.g. how you tilt it to change the thrust angle.
"Downthrust" with the motor in front is exactly that.
"Downthrust" for a pusher is the opposite..
like this>>>>
for a pushe motor mounted above the c.g. the uptilt at the rear of the motor is "downthrust".
For a pusher motor mounted below the c.g., the uptilt acts as "upthrust".

[jrb]

If Jimsky’s motor pylon was placed @ the leading, would he need down thrust?

More up thrust @ the trailing edge?

@ the CG?

@ the same 3 locations on the bottom?


How about the nacelles of a DC-9?

[Sparky Paul]

Originally posted by jrb
If Jimsky’s motor pylon was placed @ the leading, would he need down thrust?

More up thrust @ the trailing edge?

@ the CG?

@ the same 3 locations on the bottom?


How about the nacelles of a DC-9?
.
The principle has been illustrated.

[Mikerjf]

Most if it's just the vertical cg and the thrustline.

Mike

[megawatt]

[QUOTE]Originally posted by Sparky Paul
.
The principle has been illustrated. [/QUOTE/]

And a nice illustration it is. Thanks Sparky. The concept is more complicated than it appears.T
To reduce the power on "down pitch" tendency on the BD-5, I will be positioning the motor mount at 3 degrees "up". ie: the top of the mount when viewed from the side will lean forward?
"Upthrust" This will "lever" the nose up on power.
Is this correct?
Sorry I'm being so "thick" with this.:rolleyes: It must be the grey hair......
Jeff

[Quacker]

Hmmm... I'd have thought that the thrust line had more to do with the center of drag than of gravity.

Example: Take a typical high wing plane like a Cub or Cessna. Load the belly with something heavy like a camera--anything to bring the CG down good and low. Apply power and without some downthrust, the nose is going to come up. What am I missing?

Q

[Mikerjf]

I wasn't to sure about whether the cg or center of drag was the major player so I did some experimenting with my Nieces Teddy, the stock setup has the batteries in the bellypan and the plane has a fairly large nose down pitch moment under full power. Moving the battery pack up into the canopy and under the wing reduced the nose down pitch moment to almost nothing. Flies much better now and doesn't go nose up when you pull power from straight and level.

Mike

[jrb]

My experience shows that most models fly on/near their thrust line (my Gallery: http://rcgroups.com/gallery/showgallery.php?cat=500&ppuser=497 ).

That is hold the thrust line level (prop disk vertical) and look at wing incidence, etc.

The wing should have a slight positive incidence; allows for climb with added power.

Too much incidence lead to a stall! Repeating to a climb stall combo called ballooning. Especially true if the model glides very well but balloons under power.

I’m in the process of over powering a Sonic Liner with Kyosho Learjet nacelles (500+ watts) and have provisions to adjust the thrust angle ( http://www.rcgroups.com/forums/showthread.php?postid=1950149#post1950149 ). It will initially be set so that the wing has a bit of positive incidence when the thrust line is level.

[Sparky Paul]

Originally posted by megawatt
[QUOTE]Originally posted by Sparky Paul
.
The principle has been illustrated. [/QUOTE/]

And a nice illustration it is. Thanks Sparky. The concept is more complicated than it appears.T
To reduce the power on "down pitch" tendency on the BD-5, I will be positioning the motor mount at 3 degrees "up". ie: the top of the mount when viewed from the side will lean forward?
"Upthrust" This will "lever" the nose up on power.
Is this correct?
Sorry I'm being so "thick" with this.:rolleyes: It must be the grey hair......
Jeff
.
The fix for the nose-down is what is shown in the illustration.
There will be less of a nose-down moment by tilting the motor as shown.

[Sparky Paul]

Originally posted by Mikerjf
I wasn't to sure about whether the cg or center of drag was the major player so I did some experimenting with my Nieces Teddy, the stock setup has the batteries in the bellypan and the plane has a fairly large nose down pitch moment under full power. Moving the battery pack up into the canopy and under the wing reduced the nose down pitch moment to almost nothing. Flies much better now and doesn't go nose up when you pull power from straight and level.

Mike .
The c.g. IS where the "center of drag" is!
All forces on an airplane are related to the c.g. whereever it is.
Moving the battery pack up moves the vertical position of the c.g. up.

[Quacker]

"The c.g. IS where the "center of drag" is!"

If the center of drag was where the center of gravity was, rudderless aircraft would never fly.

As an excercise, imagine a plane with a very small parachute attached to the tail. The plane will still fly, the CG will hardly be affected, but the center of drag will have been moved aft substantially.

Another example: the center of drag on high wing aircraft with a low drag fuselage will likely be up near the wing because drag is created when lift is created. If you make the fuselage twice as heavy as before, the vertical CG will move down, but the drag components will remain almost the same. I say almost because the increased weight will require more lift and thus more drag to fly. The center of drag will therefore move up while the CG will move down. So, this example shows that the relationship of the center of drag to the center of gravity is easily changed.

Q

[Mikerjf]

Moving the battery pack up changes only the vertical location of the cg. The airframe wasn't changed, the center of drag is the same.

Mike

[Quacker]

Right Mike. Only a small change in what you said: The center of drag is not a static value like the CG. Rather, it changes dynamically in flight, depending on the sum all factors creating drag that moment in time.

If the aircraft were to hold the same airspeed as before the weight was added, additional lift at that speed would be needed. The additional lift would cause additional drag at the wing. since no other drag components were changed in the example, the center of drag would be changed. In this case upward.

Q

[Sparky Paul]

Read message 11.

[Quacker]

Well, I guess we disagree, Sparky. A single anecdote is not enough for me to suspend my understanding. I don't know anyone that has the expertise to resolve this difference of opinion, do you? I'm really interested in learning something here.

Q

[tim hooper]

Moved to Modellng Science.........

tim hooper

[Ollie]

An aircraft in unaccelerated flight (constant velocity) is in a state of dynamic equilibrium. That means that the sum of all the force vectors is zero and the sum of all the moments is also zero.

If the total lift and weight vectors are in line with each other and in opposite directions and also the thrust and total drag vectors are in line with each other and in opposite directions then the sum of the force vectors and the sum of the moments will be zero. The the thrust, total lift and total drag forces to produdce dynamic equilibrium do not necessarily have to pass through the CG but the weight force must.

For example, the paramotor configuration flying from right to left has the total drag vector above the thrust vector producing a clockwise moment. The weight vector ahead of the lift vector produces a counter clockwise moment which added to the pitching moment produces a total counter clockwise moment that is equal in magnitude to the clockwise moment so that the sum of the forces and the sum of the moments are each zero.

[Mikerjf]

Q;

Only a small change in what you said: The center of drag is not a static value like the CG. Rather, it changes dynamically in flight, depending on the sum all factors creating drag that moment in time.

Yep.

Ollie;

One question:

What's the paramotor configuration you're referring to?

Mike

[Ollie]

A paramotor is like a paraglider with the addition of a motor and pusher prop behind the flier's back. In other words it is a partachute with a canopy that is shaped like an airfoil in the direction of flight and the span is much greater than the chord. The canopy has curved anhedral. A very light weight chair hangs from shrouds for the pilot to sit in. A small engine and pusher prop are mounted on the back of the chair. It is foot launched. I used it as an example because the four major forces are clearly not colinear with each other and the direction of the resulting moments are easy to see.

http://www.paramotor.com/

[Quacker]

It's a perfect example.

Q

[Sparky Paul]

ALL forces on a plane react at the c.g.
Moving the c.g. changes the moments any particular contributor to the total forces will have..
Compensation for a large change must come from somwhere.
The "change in pitching moment" in message 11 for example.
Considering specific forces in isolation is erroneous.

[Quacker]

"ALL forces on a plane react at the c.g."

Au contraire.

[Quacker]

Sparky:

Since we disagree, I suggest that we recuse ourselves from offering additional expert opinion.

I think it's time to seek further information from an outside expert source. Do you know one? How about Dan Kreigh (IFO designer)? He has a great resume.

Q

[Sparky Paul]

Most modellers are unaware of the inertial axes of the airplane.
These are the pitch, roll and yaw axes about which the plane actually manuvers.
They need not and most frequently do not coincide with the usual fore-aft-lateral axes of the shape, and need not be orthogonal with each other.
In the situation where the motor battery is moved a substantial amount vertically, and the corresponding airplane response change notee as "pitching moment", the actual response is more sensitivity in pitch because the stabliziing pendulum effect of the battery in its original location has been diminished, there by reducing the amount of control deflection required to manuver the plane.
There will a be a corresponding diminution in the roll stability with the mass moved closer to the wing.

[Quacker]

As above...

Here is a link to Dan's resume, especially see paragraph #3:

http://www.wildrc.com/htmlpages/designer.html

Best,
Q

[Dan Kreigh]

Okay here we go.

I've been mostly an aircraft structural guy for the last 15 years. But here's my take:

Sparky is right in saying:
"ALL forces on a plane react at the c.g."

Quacker is also correct in saying:
"If the aircraft were to hold the same airspeed as before the weight was added, additional lift at that speed would be needed. The additional lift would cause additional drag at the wing. since no other drag components were changed in the example, the center of drag would be changed. In this case upward."

This is similar to when I drop the flaps on a high wing full size airplane and suddenly need lots of down trim to counter the upward pitching moment created by the additional wing drag about the lower center of gravity.

I know the full size BD5 has a lot of up thrust as depicted in Sparky's nice sketch in message 5. I know Burt Rutan's Vari Viggen had pitch control issues with such a high center of thust line and low center of gravity. More than one Vari Viggen pilot has tried to abort a takeoff only to find himself suddenly airborn after pulling off the power!

Quacker's comment here is interesting:
"If the center of drag was where the center of gravity was, rudderless aircraft would never fly."

The only way I can figure this statement might be true is if you are looking at a side view of a model and assume that the center of drag is considered the same as center of pressure. (center of pressure is where all the distributed pressures can be summed and applied to one point on the side profile of the vehicle without any moments that want to rotate the model.) So for a typical model to fly properly, you want the center of pressure aft of the center of gravity. An extreme example is an arrow with a heavy weight in the nose and tail feathers in the back that are used to shift the center of pressure well aft of the center of gravity. So Quacker would be correct in that the rudderless aircraft would have a tough time flying with the "center of drag" (pressure) located at the center of gravity (without an active control system).

But for most of this discusion, I was regarding a center of drag in the FRONT VIEW of the model and how thrust or changes in drag may create pitching moments about the center of gravity.

I had a little trouble understanding what the question was so I hoped I helped some.

P.S. Hi Sparky, I think we met at the heavy lift student competition?

[Quacker]

Thanks Dan.

My comment about the rudderless plane was a bit confusing in this context because I did not make it clear that I was referring to the yaw axis--which itself was not under discussion. I was hoping to illustrate the general case which applies to any axis of stability and unfortunatly picked an oblique example.

Q

[Sparky Paul]

Dan, we did meet there... Me and the other Paul (Susbauer)...he flew the pants off your IFO.
I understand the Davis plane will be at the Fort Worth SAE event this year... much lighter, but the same size! That should be interesting! :)
More on topic though, the "center of drag" idea is about as useful as "center of lift" today, with pitching moment explaining both concepts better. (I'd never encountered center of drag previously.)
The center of gravity change with flaps down is minor compared to the effect of the pitching moment on the plane.
I have a Kadet which cannot be controlled at full-flaps and full-power, the pitch-up is so severe, yet the c.g change would be minimal.
Shouldn't lowering the c.g. with flaps lower the "center of drag", and therefore lessen the effect of the entire wing on the dynamics of the plane around the c.g.?
On my spoiler gliders which use 1/8" sheet balsa for the surface, the c.g. change when up is probably so small as bo unmeasureable, yet the nose-down pitch is as severe at full deployment as the Kadet's nose-up.
The Kadet with the speed-brake down has no visible change in anything, unless I'm flying with another in formation, then the speed change can be seen.

[megawatt]

Very good disscusion. Lots of science. Thanks. From the information disscused in this thread, may I ask for one more clarification? On a normal tractor setup, right thrust is usually used to counter prop torque. Sooo, on this pusher config., the motor should offset to the right as viewed from above? Basically the same orientation as the firewall on a tractor? Anybody? I would love to see another sketch.(hint hint)

Jeff

[Quacker]

This is a good way to think of it: When your prop rotates, the torque tries to twist the plane in the other direction (roll axis). So look at your plane and prop rotation direction and imagine this. Now imagine that you only had a rudder to correct this. Which direction would you deflect the rudder? Now which direction would this send the nose? Give your motor some side thrust such that it sends the nose in this direction. For tractors, it's the same direction as you wnat the nose to go. For pushers, it's the opposite direction. (On a pusher, left thrust sends the nose right, etc.).

OK?
Q

[megawatt]

Thanks Q, I just needed another nudge! Crystal clear explanation.

Jeff