[Beancounter]

What is the better method of aligning the engine thrust line. Does one shim the dyna mounts or the firewall mounts.

 

[David-aviator]

What is the better method of aligning the engine thrust line. Does one shim the dyna mounts or the firewall mounts.

Not necessary to do either. The off set is designed into the engine and mount.

I just hung the Lycoming and noticed the mount bolts took 3 washers on the right side but just one on the left. The engine is off set to the right.

 

[Beancounter]

David I should have been more specific. My engine is pointed about a 4 degrees down on the vertical plane as compared to the longerons. I have read that for the plane to be most efficient they should be in the same vertical plane.

 

[L.Adamson]

David I should have been more specific. My engine is pointed about a 4 degrees down on the vertical plane as compared to the longerons. I have read that for the plane to be most efficient they should be in the same vertical plane.

I'd double check again, because my 6's engine has the downward offset, and so does the cowl to fit it. In fact, a whole lot of aircraft have the engines slanted downward, as well as many R/C models that I built. I'm sure that Van's engine mount is taking all this into account.

L.Adamson --- RV6A

 

[pierre smith]

It's self-aligning, Matt.......

.....as it comes from Van's. Don't shim it up or to the left. Just mount it. As has been said, it's supposed to have down and right thrust.

Regards,

 

[Beancounter]

Thanks for the suggestions guys. Does anybody shim their engines back up when they sag. Just being cautious before I make the final cut on fitting my cowling.

 

[L.Adamson]

Thanks for the suggestions guys. Does anybody shim their engines back up when they sag. Just being cautious before I make the final cut on fitting my cowling.

I adjusted my cowl, so that the spinner is towards the top for any eventual engine sag. The engine has been on the mount for four years, but only 25 hrs. of use (which is in the last 1 1/2 month). So far, it hasn't sagged at all. But lower firewall mount -- washers can also be installed to make up for sag, if required later.

L.Adamson --- RV6A

 

[az_gila]

Yes....

Thanks for the suggestions guys. Does anybody shim their engines back up when they sag. Just being cautious before I make the final cut on fitting my cowling.

...and the correct washers to use are Lord part numbers J2218-6 (about 1/8 thick) and J2218-61 (about 1/16 thick). AN970's will also work, but their diameter is a little smaller.

However, build it now and worry about shimming later only if it sags....

 

[Steve Sampson]

I am pretty sure the thrust line is meant to be down, compared with the horizontal (canopy rails on the RV) on most aircraft.

The reason I understand, though I dont know where I got it from, is so that when you put power in it reduces the pitch up. In other words it stays close to trim over a wider power range that way.

Not 100% sure of that but close.

 

[Andy_RR]

down-thrust and right-thrust? why do we have it?

I can kind of half buy the pitch trim theory for downthrust, but why do we need right-thrust too? I don't believe torque reaction makes any engineering sense either.

I'm guessing it's because the stagnation point of front of the aircraft is actually below the propellor axis and both angles are there to correct for the unequal angles of attack of the propellor blades caused by the consequent assymetric flow.

 

[L.Adamson]

I can kind of half buy the pitch trim theory for downthrust, but why do we need right-thrust too? I don't believe torque reaction makes any engineering sense either.

I'm guessing it's because the stagnation point of front of the aircraft is actually below the propellor axis and both angles are there to correct for the unequal angles of attack of the propellor blades caused by the consequent assymetric flow.

The right thrust line, is doing the same thing that the offset vertical stab does. Which is to offset the left drift tendencies. And of course, left drift is a combination of a spiral slipstream from the prop, torque, and P-factor. As with lift, the percentages of this combination will be arqued forever!

As to torque, my 6A is a "torque monster"! When doing touch and goes, I have to be careful, and get my right rudder in quickly as power is applied. I might even need some right aileron like some WWII fighters, because it seems to want to "twist" to the left. The 0320 "9A's" I flew were nothing like this. Perhaps my engine is just putting out 300HP!

Over the years, I could easily see what these three forces do, thanks to many years of R/C. A high powered engine will easily roll a plane inverted when in slow flight, and quick power application. I even seen it happen with a Reno Racer at a local flyin. The engine nearly quit, and then suddenly went to full power during an emergency landing. Now that's the torque issue. Duke Fox, who produced model airplane engines, proved that the spiral slipstream could be compensated by placing a vertical stab below the airplane also. But this just isn't practical for full size planes.

L.Adamson --- RV6A

 

[Andy_RR]

hmmm, yes. I can see that right thrust would add a yaw moment to compensate for the spiral slipstream, but it seems like an inefficient way of doing it.

From what I understand the RV-8 has no VS offset, but the RV-7 does. Does anyone know whether there is additional right thrust on the RV-8 engine frame? Was this a bit of Van developing his ideas with each model?

Also, a slight thread drift, but if we (can) have offset VS for the spiral slipstream, surely there would be some efficiency gain by having wash-in/wash-out or unequal angle of attack on each side of the HS?

As far as torque goes, I can only see it needs counteracting with a roll moment. Obviously the rudder provides a little of this, but as you say, until you get powerful engines, probably the amount of roll trim is unnoticable.

 

[L.Adamson]

hmmm, yes. I can see that right thrust would add a yaw moment to compensate for the spiral slipstream, but it seems like an inefficient way of doing it.

From what I understand the RV-8 has no VS offset, but the RV-7 does. Does anyone know whether there is additional right thrust on the RV-8 engine frame? Was this a bit of Van developing his ideas with each model?

Also, a slight thread drift, but if we (can) have offset VS for the spiral slipstream, surely there would be some efficiency gain by having wash-in/wash-out or unequal angle of attack on each side of the HS?

As far as torque goes, I can only see it needs counteracting with a roll moment. Obviously the rudder provides a little of this, but as you say, until you get powerful engines, probably the amount of roll trim is unnoticable.

Efficiency comes with jet engines. With propellers, we'll always have some inefficiency, because something is sitting in the airstream to maneuver a flight surface one way or the other.

The 6 that I have doesn't have a vertical offset built in. You can just count on a rudder tab/ wedge of some kind...........providing everything else is straight. The un-equal angle of attack or washout that you speak of, is nothing more than adding a tab here or there. Both methods still create small amounts of additional drag.

L.Adamson

 

[Andy_RR]

trim tab drag?

I would suggest that the extra drag of a trim tab is amplified several times by the control surface offset it generates.

In straight and level flight there shouldn't be any need for a rudder deflection, since no yaw moment is needed. However if your VS isn't aligned with the slipstream, you will need rudder deflection to eliminate the yaw moment. A deflected rudder must generate more drag than an aerodynamically symetrical VS/rudder We may be talking small beer in terms of extra drag though. I don't know.

On the other hand, the HS/elevator is always generating negative lift (in steady state flight) so surely optimising its AoA for cruise flight (on both sides) could yield efficiency advantages?

 

[L.Adamson]

In straight and level flight there shouldn't be any need for a rudder deflection, since no yaw moment is needed.

That assumption is incorrect. During the takeoff roll and climbout, you're going to need right rudder on a single engine airplane, with the prop turning clockwise from the pilot's view.

Since my RV6A has a smaller vertical stab & rudder than the newer models, it also takes more right rudder to compensate. However, that smaller rudder is better for rolls..

The idea of the offset stab, offset engine, and trim tabs..............is to have the fuselage aligned with the oncoming airstream at normal cruise speeds. This will keep the "ball" centered, and the airplane won't be in yaw. Without a variable rudder trim tab, it's never going to be perfectly centered in all situations...........without some rudder "foot" work.

That "spiral slipstream" is always hitting the vertical stab from the left side (clockwise engines), and it wants to pivot the nose left. At cruise speeds, the small amount of builtin offset, can compensate for the yaw. You wouldn't believe the amount of rudder force that's required to keep the ball centered at around 180 mph, if you don't have rudder trim, or an offset stab. It's a lot, and nothing like the light aileron forces. Yet it only takes a dinky wedge or tab to completely eliminate it.

L.Adamson --- RV6A

 

[Bevan]

On the other hand, the HS/elevator is always generating negative lift (in steady state flight) so surely optimising its AoA for cruise flight (on both sides) could yield efficiency advantages?

Yes, an aft CG situation will require less downforce from the elevators and therefore less drag resulting in higher cruise speed or better fuel economy or both. BUT aft CG also brings less stability and there are strict limits to be observed. I've herad that the load master on the large transports will actually move the load aft during cruise for better economy and then move it back again for landing. Not sure if it's true, anyone know?

Bevan
RV7A wiring

 

[rv9aviator]

Thread hijack! I have a perfectly good set of Lord mounts that have 490 hours since new. I have heard that Lord mounts are better quality than the ones Van's sell. Should I use the ones I have and just fit the cowl for a little sag or go ahead and buy brand new mounts. This is an 0-320 150 HP E3D off a Cherokee. I got the mounts with the engine and it had less than 500 hours on a factory reman.

 

[mahlon_r]

I would replace them....
Good Luck,
Mahlon
?The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."

 

[John Clark]

CG Adjustments

Yes, an aft CG situation will require less downforce from the elevators and therefore less drag resulting in higher cruise speed or better fuel economy or both. BUT aft CG also brings less stability and there are strict limits to be observed. I've herad that the load master on the large transports will actually move the load aft during cruise for better economy and then move it back again for landing. Not sure if it's true, anyone know?

Bevan
RV7A wiring

I don't know of anyone moving the payload in flight (sounds a little hairy to me), but the 747-400 carries 3300 gallons of fuel in the horizontal stab that can be managed to maintain optimal CG.

John Clark
RV8 N18U "Sunshine"
KSBA

 

[Andy_RR]

That assumption is incorrect. During the takeoff roll and climbout, you're going to need right rudder on a single engine airplane, with the prop turning clockwise from the pilot's view.

No, I don't think so. I may have missed the word "cruise" in my claim, but you are correct - you can only tune for one situation without an adjustable tab. I am talking about optimising for your normal cruise speed.

If the aircraft is not yawing, then what need of any yaw moment from the rudder/VS, save for compensating for something else that's badly adjusted?

That "spiral slipstream" is always hitting the vertical stab from the left side (clockwise engines), and it wants to pivot the nose left. At cruise speeds, the small amount of builtin offset, can compensate for the yaw.

I'm talking about making the VS aligned with the spiral slipstream so it has zero AoA at the optimum cruise point. At 180mph, this angle isn't going to be very large at all.

...Yet it only takes a dinky wedge or tab to completely eliminate it.

I don't think the size of the tab should deceive you into thinking the forces are small. It's not the trim tab that's doing the work of your right foot - it's the whole rudder. Think of the extra mechanical advantage the trim tab has compared to your rudder pedals!

 

[Andy_RR]

Yes, an aft CG situation will require less downforce from the elevators and therefore less drag resulting in higher cruise speed or better fuel economy or both.

No, I'm not talking about moving the CG position at all. I'm proposing making the same downforce more efficiently. If two HS/elevator panels are working at the same Cl, then the Cd will be less than if one panel is operating at a higher Cl to correct for the other panel which is doing nothing (or maybe even opposing it, if the helix angle is large enough)

The Cd-v-AoA looks like a quasi-quadratic function, so converging the AoA points for each HS panel to the same point on the curve should yield efficiency gains. If it were linear, it would be a wash and of no consequence.

The other thing to consider too is that unequal downforce on the HS panels should produce a rolling moment which must be counteracted by the ailerons - more drag!

 

[rv9aviator]

It's only money, Huh

Thanks Mahlon, I appreciate the info.