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Jul 03, 2001, 04:07 PM
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rtideas's Avatar
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Why set downthrust on a motor? And can you have too much?


I mounted a MAS drive system in a Hangar-9 Aspire EP. I made the new motor mounts from scratch, but it has gobs of downthrust. I wanted to fly. It was perfect outside and was itching to fly. So left it.

It flies well, but some at the field asked why I had so much down thrust.

What the disadvantages/advantages of downthrust?

rt
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Jul 03, 2001, 04:36 PM
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maciek's Avatar
Downthrust is needed mostly in high-wing models to keep the plane from pitching up and stalling with the motor on. The dis-advantage is that some of the motor power is used to pull the plane down, instead of forward.
Jul 03, 2001, 06:46 PM
Another reason for down thrust is to counter the left yawing moment created at higher angle of attack because the downward moving half of the prop on the right side takes a bigger bite of air. This is called P-factor, and is the reason you usually need some right rudder on takeoff. The downward tilt will reduce the effect.

Too much down thrust would give you a right yawing moment during takeoff roll, as well as causing a nose up pitching moment when power is cut.
Jul 03, 2001, 06:52 PM
Registered User
Quote:
Originally posted by maciek:
Downthrust is needed mostly in high-wing models to keep the plane from pitching up and stalling with the motor on....
Naieve question - is that because of the gyroscopic effect of the motor and prop spinning, otehrwise couldn't you compensate for this by moving the CG forward????

Confused of San Franciso

Jul 03, 2001, 06:54 PM
Registered User
On the same subject, is there any good way to determine the thrustline angle relative to the wing , stab & c.g. as a starting point before flight testing? For instance, if the thrusline is above the C.G., would one change it to upthrust?
How about pushers?
Jul 03, 2001, 08:41 PM
RB
RB
Down thrust { and right thrust } is used to minimize trim changes due to throttle changes. Ideally you should never need to adjust the trims during a flight. The plane should fly straight and level at WOT and fly straight and level at low speed without having to adjust the trims.

If you have too much down thrust the plane will require up elevator trim to fly level at WOT. When you pull the throttle back the plane will balloon up.

Ray
Jul 03, 2001, 10:59 PM
Registered User
What is WOT?

Anybody know of any speed 400 motor mounts that are adjustable for downthrust?

Why would someone need right or left thrust?
Jul 03, 2001, 11:45 PM
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rtideas's Avatar
Thread OP
WOT is Wide Open Throttle, I believe.

IPE - In Plain English opposed to
NIPE - Nearly Incomprehensible Plane English

rt
Jul 04, 2001, 09:54 AM
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Mike C's Avatar
This is something I have been wondering about lately. Is there any rule of use for downthrust on high wing planes like 5 degrees or 3 degrees for best performance? Obviously you don't want anymore than needed because it is a waste of power. I have been setting up for downthrust and right thrust using the BR method. ('bout right method)
Jul 04, 2001, 09:58 AM
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Rotten Robbie's Avatar
If the motor is installed without any thrust offset the plane try ti pitch up and pull left under throttle.

To compensate it is customary to use an equal amount of down thrust and right thrust.

Some thing between 2 and 5 degrees.

Robbie
Jul 04, 2001, 10:20 AM
RB
RB
Hobby Lobby sells a Aeronaut bulkhead mount that works great and is easily adjusted for down or right thrust by using washers under the mounting screws. They also sell this mount with a gearbox but it is way over priced.

Ray
Jul 04, 2001, 11:00 AM
Registered User
Bill Glover's Avatar
Quote:
Originally posted by Mike C:
I have been setting up for downthrust and right thrust using the BR method. ('bout right method)
That's the only method I know of

For the average high-winger so long as you have a bit of downthrust and a bit of right thrust it will go OK. It's not critical, and many (most?) full-size aircraft have to be re-trimmed depending on throttle position!

Where you have a high thrustline and low centre of gravity/drag then you do need upthrust rather than downthrust. Pylon-mounted motors on gliders or flying boats are the usual examples.

Someone I know designed a model for aerial photography with the camera in the nose (pointing forwards) and the motor on a pylon above the wing. But he didn't have any upthrust - big mistake! Maiden flight, taking off with full throttle, model hurtling along the strip nose-down and showing no sign of lifting off. So he chopped the throttle, at which point the model abruptly pitched up, leapt into the air, stalled, and ... you guessed it ... went in nose-first, smashing the camera!
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Jul 05, 2001, 02:54 PM
aka: A.Roger Wilfong
gnofliwr's Avatar
One of Frank Zaic's books (Circular Airflow, I believe) goes into detail on this. He started out to investigate one thing and found that his test model kept looping under power, so he had to take a side trip to figure out why. He was dealing with free flight models, but the aerodynamics is the same. Also the Aspire is a sailplane so the wing/stab/thrust relationship is even more like a freeflight than your typical R/C sport model. I don't have the book in front of me so I'll have to go from memeory (always dangerous).

Basically, the problem is that if the model is trimmed for a glide the wing has about 6 degrees incidence with respect to the stab. If you add power a 0 degrees, what happens is the plane speeds up and the wing generates more lift. Since the stab is trimmed for a slower speed glide, the added lift will cause the plane to pitch up - given sufficient power, it will fly right over the top and do a loop. As maciek notes above, down thrust is used to counteract the pitch up tendency by essentially pulling the nose down.

There's a correct amount of down thrust for every airfoil/plane configuration. Too much down thrust and you'll power into a dive, too little and you'll nose up. As has been noted in a couple of other threads, a power nose up tendency is not entirely bad and is in fact a characteristic of some full size planes. On some planes, adding additional down thrust may exacerbate a dutch roll tendency or make the plane seen squirrelly at high throttle. Also, on aerobatic models, down thrust becomes up thrust when you're inverted - not all bad on a flat bottomed or semi-symetrical wing, but not what you're looking for on a pattern ship.

WRT the Aspire. If it's flying fine, leave the thrust line alone.

- Roger


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