Downthrust/rightthrust revisited.

[1705493]

A little while back, the subject of engine offset thrust was discussed and kind of beat to death. My contribution was that you can't be dogmatic about it and I illustrated my point with two Norvel .074 powered aircraft. The mid-wing sport of my own design had down and right thrust built in, as is my custom. The Tsunami is a .40 sized pattern ship shrunk down to suit the .074 and the engine is set dead straight. Both aircraft, I claimed, flew about the same, quite neutral in all axis, no bad habits etc.

Not quite, as it turns out. The discussion prompted me to pay more attention the next time I flew both of them at the same session. One thing I noticed with the sport design was that in a tight loop, I was automatically compensating for a slight roll drift as the loop was completed. This didn't happen with the Tsunami. As I recall now, I noticed this when the plane was first flown but I attributed this to perhaps a heavy wing panel and made a mental note to balance the wing next time I thought of it. Well, It never occured to me again and for two years now, I've been subtley correcting for a design flaw I didn't know I had,,, or chose to ignore.

While doing an overhaul of the engine, I decided to take out all of the right thrust with a wedge and I also made a wedge that took out both the right thrust AND downthrust.

Eliminating the right thrust produced a plane that completed perfect loops with no additional input other than just up elevator. Outside loops were cleaner too. In fact, the plane had a whole new feel to it that really, in retrospect, was much more like the Tsunami which has no offset thrust in either direction.

Eliminating the downthrust was not so rewarding, however. The plane became quite unmanageable due to the gross difference in lift between full throttle and idle. With the downthrust, trimming elevator for full bore, required just the smallest bit of up pressure when reverting to full idle. Quite manageable, even when hotdogging at low altitude. Without the downthrust, way too much down trim was required at full bore and way too much up elevator was required to maintain flight at idle.

It seems my old habit of automatically installing lots of right thrust misled me in my need for same. On the other hand, downthrust,,,, at least on THIS plane was still required.

Just to set the record straight. No math, no theory, just imperical evidence.

Pics to follow if I can get them to work.

[1705493]

Tsunami and mid-wing sport of my own design.



Shoot, can't do it. Limited to 100KB and one is 137 and the other 300.

Moderator,,, is there any way to make this work?

[hul]

don't know about thrust, but you can resize your pictures with Irfan View:
http://www.irfanview.com/
or JLB Image:
http://www.bivio.com/jon_bondy/files/HomePage/software.html
both are free

Hans

[raptor22]

A little while back, the subject of engine offset thrust was discussed and kind of beat to death. My contribution was that you can't be dogmatic about it and I illustrated my point with two Norvel .074 powered aircraft. The mid-wing sport of my own design had down and right thrust built in, as is my custom. The Tsunami is a .40 sized pattern ship shrunk down to suit the .074 and the engine is set dead straight. Both aircraft, I claimed, flew about the same, quite neutral in all axis, no bad habits etc.

Not quite, as it turns out. The discussion prompted me to pay more attention the next time I flew both of them at the same session. One thing I noticed with the sport design was that in a tight loop, I was automatically compensating for a slight roll drift as the loop was completed. This didn't happen with the Tsunami. As I recall now, I noticed this when the plane was first flown but I attributed this to perhaps a heavy wing panel and made a mental note to balance the wing next time I thought of it. Well, It never occured to me again and for two years now, I've been subtley correcting for a design flaw I didn't know I had,,, or chose to ignore.

While doing an overhaul of the engine, I decided to take out all of the right thrust with a wedge and I also made a wedge that took out both the right thrust AND downthrust.

Eliminating the right thrust produced a plane that completed perfect loops with no additional input other than just up elevator. Outside loops were cleaner too. In fact, the plane had a whole new feel to it that really, in retrospect, was much more like the Tsunami which has no offset thrust in either direction.

Eliminating the downthrust was not so rewarding, however. The plane became quite unmanageable due to the gross difference in lift between full throttle and idle. With the downthrust, trimming elevator for full bore, required just the smallest bit of up pressure when reverting to full idle. Quite manageable, even when hotdogging at low altitude. Without the downthrust, way too much down trim was required at full bore and way too much up elevator was required to maintain flight at idle.

It seems my old habit of automatically installing lots of right thrust misled me in my need for same. On the other hand, downthrust,,,, at least on THIS plane was still required.

Just to set the record straight. No math, no theory, just imperical evidence.

Pics to follow if I can get them to work.

Cool. hey, what is the incidence and type of airfoil? Try taking some incidence out and you may still find no need for downthrust; just make sure the thrustline goes through the CG.

--Alex

[1705493]

Thanks hul, will have a look.

Raptor,

Yes, incidence is something to consider but I have done that and incidence is zero to the stab. The airfoil is very semisymmetrical and thicker than the norm. This gives it a lot of lift even at zero incidence. I'd be willing to bet that if it was fully symmetrical, the need for downthrust would be less but in that case, to fly, the incidence would need to be positive and we'd be back to square one.

From here on, depending on the aircraft, I won't be so quick to add right thrust but I'm convinced that down thrust has value,,, depending on the airplane, of course.

About the thrustline. I'm not too sure that through the CG is the deal. It would seem to me that through the center of drag is more appropriate. Maybe that's what you meant and that certainly is a factor. That is, a high wing is more likely to need downthrust than a low wing,,, just because the thrust line is below the main drag component, the wing. Notable is that the Tsunami's thrust line is uncommonly low. The nose has a very shark-like appearance and this puts the thrust line closer to the center of drag. All part of the mix and illustrates again, that it all depends on the plane.

[macr0t0r]

Right thrust is a funny thing; it's only required for planes that occasionally "hang" on the prop at a decent angle of attack (straight up does not apply). Light birds that are simply yanked through the air by their prop tend not to need it, since they don't develop much of an angle of attack versus the direction of travel and prop-blast.

Down-thrust is compensation for a plane that has positive incidence. However, since there must be SOME positive incidence (or at least positive angle of attack using elevator) in order to maintain flight, the down-thrust is rather nice for those that don't want to hang on the stick at lesser throttle.

[1705493]

Mac,

Both my Tsunami and the midwing are quite light and both will go vertical for a considerable distance from take off. Both are zero incidence realative to the stab. The Sport has a semi wing, the Tsunami is fully symmetrical. Even though the Tsunami is zero and it has a fully symmetrical wing, it flies. This implies that somewhere there's a positive angle of attack component. It appears that the low thrust line and the symmetrical wing contribute to its lack of need for downthrust. The midwing definitely NEEDS downthrust as its quite a handful without when jockeying the throttle. Which I do a lot with both.

BTW, if you've ever been afraid of small engines because they were finicky and didn't throttle well, the Norvel .074 is the exception. With just a high speed needle, (no idle adjustment) it throttles as well as any of your best .40 or .60 engines. The trick is that the main needle has a very broad range and can be used to get that perfect idle. Just dial it in at full bore and then dial it in further at idle. Recheck to see that your not too lean at full throttle and your done. Final tuning is done in the air. Go up high, throttle back and cruise down, punch the throttle and see what you get. If it smokes, land, tweak the needle in a click or two and repeat. At one point the needle will be perfect for both high and low throttle. Its quite amazing. One of the most powerful engines, ounce for ounce, I've ever seen.

Still no luck with the pics. Computers not my forte.

[macr0t0r]

@1705....that's an odd name...AMA number? :p
All I'm saying is that the wing has to somehow provide lift to counteract gravity. You can either have a non-symmetrical airfoil that lifts at 0deg angle of attack, or you can have a fully symmetrical wing with a slightly positive angle of attack. Even if you have the thrust/wing/stab all set to 0deg, your plane must still travel with a slight (though imperceptible) angle of attack to maintain level flight. The lighter the plane, the less angle of attack that is required. If everything is truly zero, then you will have to hold a bit of back-stick at lower throttle (unless you have the CG at or behind the center of lift).

The semi-symmetrical definitely will need downthrust since the change in lift at different speeds will be more dramatic.

[vintage1]

Right thrust is a funny thing; it's only required for planes that occasionally "hang" on the prop at a decent angle of attack (straight up does not apply). Light birds that are simply yanked through the air by their prop tend not to need it, since they don't develop much of an angle of attack versus the direction of travel and prop-blast.

Down-thrust is compensation for a plane that has positive incidence. However, since there must be SOME positive incidence (or at least positive angle of attack using elevator) in order to maintain flight, the down-thrust is rather nice for those that don't want to hang on the stick at lesser throttle.


I think rather downthrust is a compensation for either a fairly forward C of G, that requires progressively more up elevator to balance out at low speeds, or its a balance against offset thrust and drag centers - if thrust is below drag center (e.g. high wing plane) then there is a nose up moment under power.

Or both.

Mid wing aerobats with fairly rearward CG and symmetrical airfoilss ought to be zero zero, on everything, and any power on ballooning tamed by moving the CG back and rolling off any up trim.

However even then there will stillprobably be a need to add a little up for the best glide.

[feihu]

Eliminating the downthrust was not so rewarding, however. The plane became quite unmanageable due to the gross difference in lift between full throttle and idle. With the downthrust, trimming elevator for full bore, required just the smallest bit of up pressure when reverting to full idle. Quite manageable, even when hotdogging at low altitude. Without the downthrust, way too much down trim was required at full bore and way too much up elevator was required to maintain flight at idle.

It seems my old habit of automatically installing lots of right thrust misled me in my need for same. On the other hand, downthrust,,,, at least on THIS plane was still required.

Just to set the record straight. No math, no theory, just imperical evidence.

Pics to follow if I can get them to work.

It seems that the location of the vertical CG is higher/above that of the Tsunami resulting in a higher pitching moment due to power.

feihu

[1705493]

Both aircraft have the CG at a conservative location,,, neither too far aft or too far forward. But yes, a forwrd CG exacerbates the need for downthrust. Yes, a high wing, with its higher center of drag requires more downthrust. Yes, a semi or flat wing needs more downthrust than a fully symetrical wing. And in the latter case, this is because a flat or semi airfoil's lift is more sensitive to how fast it travels through the air.

I suppose my point was that while sidethrust can be useful (particularly with free-flight and control line ) it shouldn't be assumed, as I did, that it's always necessary. Downthust however, as proved here to my satisfaction on this plane, works well to moderate the effects of high and low throttle.

The folks that designed the Tsunami get a lot of credit for getting it all exactly right. My sport design is an original and doesn't have a cowl so I was able to alter the thrust as required. The second will have a cowl and with the engine thrust established, I can fair in a neat spinner, just like the Tsunami.

[Adrian Page]

I would be carefull assuming down thrust is good for all planes. Down thrust on a low wing plane with a high thrust line may be counter productive. Up thrust is rarely mentioned and so it is rarely used.....even when it needs to be. Vintage is right on the money bringing up the CG location. No other single adjustment will affect flight performance more. Don't believe it? Move the CG back 4" on your best trimmed plane..... :D
I don't add any thrust to my designs until I have flown them. This way I can counter whatever it is that needs countering. It saves wrestling with a plane that has built in down thrust that does not need it. It's kind of hard to tune out that sort of thing without cutting something.

Adrian

[feihu]

[QUOTE=1705493]
Downthust however, as proved here to my satisfaction on this plane, works well to moderate the effects of high and low throttle.
QUOTE]

Without downthrust, you are experiencing a severe pitch up with full power, which you compensate for by putting in a lot of down trim; so when you cut the throttle, your plane takes a nose dive and you have to retrim. And shifting the CG fwd or aft does not help.

This indicates that downthrust puts the thrustline very close or near the "vertical CG". which is the centroid of the weight distribution in your particular airplane; hence a minimum of pitching moments at high and low throttle.

I'm not surprised, many modelers have never heard of "vertical CG".

feihu

[1705493]

Adrian,

I agree that if the CG is grossly out of place, you'll have trimming problems but in this case, the CG is where it should be. I HAVE experimented with the CG and you and Vintage are both right in that regard.

Feihu,

Vertical CG IS new to me. Interesting concept. I suspect though, that it would only be a factor if you're hovering the plane (which I do). When hovering, just as in a helicopter, the vertical CG is important. But I'm not sure its a factor in the horizontal configuration where most aircraft spend most of their time. I'm inclined to think that the center of drag is much more important although if you visualize it, the center of drag and the vertical CG might be very close to each other. A high wing would have a higher center of drag AND a vertical CG closer to the top of the airplane.

At any rate, as any engineer will tell you, looks good on paper, but will it fly?? Can't guarantee it till you try and that's the value of not being dogamtic about the need or not for offset thrust.

PS,

Yes, upthrust isn't discussed much because few aircraft need it. The exception is on something like a float plane with the engine mounted on a pod, high above the wing and fuse. In this case you almost always see upthrust and the reasons for this are the same as downthrust on conventional aircraft. And, in this case, it seems reasonable to assume that the VERY low center of drag is the predominant factor.

[macr0t0r]

Vertical CG! Good call; I've been mounting motors on the centerline for so long I had almost forgotten about this. Yes, if your motor is mounted above the vertical CG, then this will have a pseudo down-thrust effect due to leverage of the thrust on the CG. As 1705493 stated, a motor that is mounted far above the vertical CG will require UP-thrust to counteract the pseudo down-thrust effect.

Basically, you want to mount your motor so that it applies a SLIGHTLY negative pitch-moment relative to the CG point (both vertical and horizontal) if you want to minimize change in pitch from throttle changes. As a cost, though, you will have to hold a bit of up-elevator during hover. No biggie.

[feihu]

1705493 -

Pitching moments of an airplane in the horizontal configuration are known to be caused by many things; and it is not my intention to discuss these things here.

Sufficient to say, frontal drag is just one of the many factors that affect pitching moment; another is the displacement of the thrust line from the CG. With the high power output of today's RC engines, it takes only a slight displacement to cause a noticable effect.

feihu

[hawaiianspork226]

hey sorry to throw a wrench in the works, but what about ducted fans/jets?

do they need down/up/lateral thrust to work well?

just wondering b/c i may be doing a conversion soon.... hehehe

-David

[1705493]

Thanks David,

Just when we had it all figured out YOU have to throw in ducted fans. :rolleyes: ;) :D :p

However, now that you mention it, I'd bet that torque reaction is not present because it's compensated for by flow straighteners in the duct.

[vintage1]

Also DF run at very high RPM and low diameter...=low torque anyway.


I haven;tseen many high wing big dihedral EDF models, so many of the reasons for thrust offset disappear....and mostly they only seem to fly WOTanyway :D

[Adrian Page]

When I fly my Gee Bee R2 or Z very slowly and then open the throttle very quickly, the nose is pulled down. This is after the planes have been trimmed to fly hands off at full throttle. The planes do not need any trim adjustments for different flight speeds. The planes have zero down or up thrust. Is this what we're talking about?

Adrian

[macr0t0r]

@Adrian: Pitch-down at throttle with 0deg down-thrust? That's different. You say that after the initial burst of throttle the plane returns to trim? That can be caused by either of the following:
1: Motor is mounted above the vertical CG (all your weight is below the centerline), so the initial "pull" before the rest of the plane comes to speed is causing a negative-pitch moment.
2: Your motor-mount may be "flexing" in relation to your wings and stabilizer when you hit the throttle, so that you momentarily have down-thrust.

That's the best I can think of unless I saw the plane myself.

[1705493]

I wonder if we need a definition of "Center of Gravity"

[vintage1]

No, its simple.

The geebee has a low wing. Under increase of power the thrustline is above CG and above drag center, so the nose pitches down, As speed increases, the downforce on the tailplane restores the attitude to neutral. You can tame that a little by adding upthrust and moving the CG back, but stability will be more marginal.

[1705493]

Vintage,

Good point. Many low wing pattern aircraft have sleek fuses with the low wing not all that low or much below the thrust line, like the Tsunami. The GeeBee has a fat fuse with the wing quite a bit below the thrust line much like you see on a seaplane with a pod. In the case of the seaplane, the effect is so exagerrated that not only do you not need downthrust, you actually need considerable upthrust. In the case of the GeeBee, it looks like it'd need just a bit and not as much as the seaplane.

I'm still not convinced that vertical CG has anything to do with this. Center of Gravity is supposed to be a single point that, if you had a way of suspending the model from that point, it would balance in all axis, yaw, roll and pitch. What we see commonly drawn on plans is the pitch BALANCE POINT, the designer normally not having any means, nor the need, to determine the true CENTER OF GRAVITY according to its proper definition.

Assuming that what is meant as the vertical CG is its placement (under its true definition) relative to the top or bottom of the fuse, I don't see how its location has any effect on the need for downthrust. I feel that the vertical CG is being mistaken for the center of drag.

If you visualize a helicopter as though it were a conventional aircraft in hover, its apparent that the vertical CG is relevant. I've flown choppers and the CG, if not exactly under the "prop" (or thrust line), will drift in hover and need constant correction. In the case of a fixed wing aircraft, the vertical CG would only be important when hovering and even then, its effect would be negligible,,, I think.

That would be an interesting experiment to do. Take my sport design and build a removeable, sleek pod, installed 4" above the wing. To distinguish between center of drag changes and actual vertical CG changes, the pod would first be flown empty and then loaded with say, 4 ounces of weight. This would be significant because the plane weighs only 20 ounces. Might have to wait till the spring though. Lots of snow and 10 below here.

[hawaiianspork226]

cool, thanks guys