Help moving engine for CG....

[Marlhalbrook]

OK-----I need some help...

I'm restoring a VP-1. Have it basically finished (just need to install "new" engine and prop, make cowling and take to hangar....).

Decided to temporarily mount the engine and do a preliminary W&B to see where the empty CG falls as of now........got some interesting results.

Using the front face of the firewall as the Datum...main gear is at 23.8125", tailwheel at 187.875". Leveled the plane and got 48.0 lbs on TW, 235 lb Rt main and 226 lb Left Main. (discrepancy in rt and lt---not sure our leveling dude got the side-to-side level exactly right----for and aft much easier to assure correct!)

I got an empty CG of 39.3"------?? The acceptable CG range in flight is 31 to 39", best CG empty is 30-32". The only variable weights are the Fuel (8 gal at 14") and the pilot at 44. These #s just don't work out..........

So----how do I calculate the distance to move the engine forward to place the empty CG at the forward limit of acceptable range--ie, 31"?? (Engine is 180-186 lbs depending on adding the starter or not----at (-)10.5" without starter and (-)9.75" with--starter moves the CG of the engine itself 1.25 inches aft)

I've just not been able to make the math work......

Here's what we have:

Rt Main --- 235 lbs x 23.8125" = 5596 moment
Lt Main --- 226 lbs x 23.8125" = 5382 moment
TW --- 48 lb x 187.875" = 9018 moment

Total wt = 509 lbs
Total Moment = 19996
Empty CG 39.3"

Best Empty CG range 30-32"
Allowable in flight CG range 31-39"

Engine wt (as weighed in above calc) 180 lbs at (-)10.5"

* Will have a small battery to play with to fine tune CG (for ignition and starter if used)--weight/location are undetermined at this time.

Is there an equation that I just haven't found to help with moving the engine and doing the calculated change?

Help!!

[BoKu]

...main gear is at 23.8125"...

* It's accepted and expected to measure longitudinal positions no more accurately than 0.1". Measuring any finer is for the most part a waste of time and digits. Given the relatively limber nature of lightweight aero structures, you can probably grab your landing gear and flex it by hand a tenth of an inch, and that kind of blows the 0.0125" out of the water.

* One common approach to this situation is to make a free-body diagram, evaluate the masses, arms, and moments, and solve for the engine arm that give it the moment required to place the CG were required.

* Another common approach, and the one easiest to implement with a minimum of algebraic manipulation, is to just make a Google Sheets spreadsheet for the CG calculations and just play around with the engine arm until the summation of moments puts the CG where you want it.

--Bob K.

[martymayes]

In order to get the empty c.g. to 32":


Your number suggest the engine needs to move forward an additional ~20".........so I'd say something is amiss.

48# is a lot of weight on the tailwheel, that's almost 10% of the total airplane wt. If the wt on the tail was halved if would almost nail it.

[cwilliamrose]

I go cross-eyed when there are negative numbers in CG calcs. You might consider using 100" forward of the firewall as the datum so all your numbers are positive and there's less chance of making a error. Back when we did this stuff on paper it made huge sense but even using spreadsheets I still keep everything positive.

[Marlhalbrook]

I'm really starting to doubt our weights..........I weighed the plane myself--saw the numbers on the scale--but I'm REALLLY starting to think that there is a bad apple in the numbers somewhere.

I come up with 22 or 22.3 inches from every calculation that I do.......that cannot possibly be right!

This plane has almost 300 hrs on it with a 1600 cc VW that was mounted 1 inch ahead of the firewall (so CG of that engine was about 6 or 7 inches ahead of the firewall). The paperwork for the plane claims that it was within CG limits with a 240 lb pilot and full fuel. We've added nothing to the airframe---except a coat of paint and varnish (with attention to being as light as possible!) and have actually moved this engine--that is a little heavier--- forward to put it's CG 10.5 inches ahead of the firewall.

Something is just not right.....this has been very frustrating!

[Frank Giger]

Kind of a stupid question - you did raise the tail to flying configuration when weighing, right?

[robert l]

I've always loved the Volks Planes, I & II so I'm following this. Thanks
Bob

[Marlhalbrook]

Frank----Yep-----

[cwilliamrose]

And a follow up question; Did you locate the main gear and the tailwheel yourself or did you get the numbers from the drawings?

[Steve H]

Marl,

A formula for CG change was posted on the Yahoo Volksplane forum.

Here it is:

Engine weight X How far you want to move it divided by airplane gross
weight = CG change

ie. 175 pound engine X 2 inches = 350 moments. 350 moments / 750 pound
gross weight airplane = a CG change of 0.47 inches.


Hope this helps.

Steve H

[martymayes]

Kind of a stupid question - you did raise the tail to flying configuration when weighing, right?

Next question: Was whatever used to raise the tail somehow included in the tail wt? There is too much wt on the tailwheel.

[Marlhalbrook]

cwilliamrose......

Plane is a restoration-----has about 300hrs TT. All we're doing is replacing the engine that was separated from the airframe by the previous owner (actually the guy before the immediately previous---but that's not important..:-) ) and cleaning her up, repaint, minor repairs of hangar rash, through inspection of structure.... The plane is built according to the plans. Maingear and TW are in correct positions per the plans.

[Marlhalbrook]

Next question: Was whatever used to raise the tail somehow included in the tail wt? There is too much wt on the tailwheel.

Marty----

No. Built a tower of concrete blocks--then used thin pieces of ply to shim the scale to level the plane with the tailwheel directly on the scale---no tare involved. Took 5 weights and averaged to 48 lbs....(all were 47.9-48.1)

[cwilliamrose]

cwilliamrose.......... The plane is built according to the plans. Maingear and TW are in correct positions per the plans.

Just a thought. I have seen a lot of people go to all the trouble of leveling the airplane to do a W&B and not take a few minutes to measure the weight points. I hope you find the answer.

[Marlhalbrook]

Steve...

That's a simplification of the formula w/W=d/D where "w" is weight to shift, "W" is total weight of aircraft, "d" is change in CG, and "D" is distance the weight is moved.

Look at it another way, D= (W x d)/w if do the calculation, just to move the CG 6 inches forward from 39 to 33, D= (509 x 6)/180 = 16.9 inches (!!)

I think the error in this is that the engine is such a high % of the total wt that moving it with this simple equation doesn't take into account the reduction in the weight of the tail by moving the weight forward. If i move the engine forward, the weight on the maingear will increase and the weight on the tail decrease---as the moment for the tailwheel decreases, the need to balancing weight on the nose decreases also......I don't see how this ratio equation accounts for that-----but it's been a LOOOOOOOOOOOOOONG time since I studied math!


I did find this spreadsheet on the KC Dawn Patrol webpage (www.dawnpatrol.org/Wghtbal.htm). It is VERY handy. Helps crunch the numbers of a lot of variables and keep it all straight.....playing with it last night, I found some really interesting info.

Looks like pilot weight makes minimal change to CG---fuel, on the other hand, is more dramatic.... Also, adding oil and prop, plus placing a battery on the firewall (which now has room for one since the engine is moved forward 3 1/2 inches from the "normal" position.....plus possibly a starter...the CG moves mathematically to a range that is totally acceptable---but in the back half of the allowable CG range....all this---AT WORST CASE OF HEAVY PILOT AND ENGINE NOT MOVED FROM PRESENT LOCATION, FULL FUEL TO EMPTY........the biggest limitation becomes the gross weight of the plane (the original paperwork states gross at 850 lbs with a reduced "G" loading to essentially the equivalent of Utility Catagory (+4/-2 G).) and climb performance.

I've found multiple photos of other VP-1s with engines that appear to have engine mounts up to about 10 inches ----so, obviously, this is not the 1st plane to have to address aft CG. I've read in many places about VP-s tending to be tailheavy----makes me wonder how many are flying that have never had a good examination of the actual CG of that particular plane.......hate to say, but many of these birds were built in a time when folks didn't have all the resources we have today---so many may have just "build it like the plans"(for the most part), hung an engine on it, and ASSUMED that it was good to go. Copy the CG info from the plans for the FAA paperwork requirements and off you go into the wild blue yonder........you KNOW that happened, I just wonder how common that may have been. The benign flight characteristics and very powerful controls of the design prolly hide a multitude of sins on some of these early homebuilts......

[Marlhalbrook]

Just a thought. I have seen a lot of people go to all the trouble of leveling the airplane to do a W&B and not take a few minutes to measure the weight points. I hope you find the answer.


Know what you mean.....I'm old school....don't trust the plans dimensions. Dropped plumb-bobs with plane level---have centerline, datum, CL of mains and tailwheel drawn on my shop floor. (actually still have the same for our Starduster Too, Kolb MK III Classic, Rans S-9, and Long EZ on the shop floor---and EAA Biplane in our garage.......maybe I need to repaint the floor!! ;)

[Frank Giger]

I still have one little piece of tape on the floor of my hangar for when I marked for my W&B...and hollered at someone who was going to pull it up the last week.

[CraigCantwell]

The CG numbers for the engine sound a bit off. From your description, the CG for the engine and prop combination is 9.5 inches from the firewall? That would indicate that the the CG point of the motor would be just about at the trailing edge of the back cylinder. CG should be more like the aft side, or center of the forward cylinder.

[Marlhalbrook]

The CG numbers for the engine sound a bit off. From your description, the CG for the engine and prop combination is 9.5 inches from the firewall? That would indicate that the the CG point of the motor would be just about at the trailing edge of the back cylinder. CG should be more like the aft side, or center of the forward cylinder.


CG of the engine sans prop --- you are very close to spot on. It balances almost exactly between cylinders 1 and 2----actually somewhat closer to the leading edge of 1. (this includes the dizzy, oil pump, y-pipe, Revmaster carb, but not the prop. We actually balanced it on a dowel rod on the welding table to prove it to ourselves....).

As the engine is mounted on our temporary mount---the balance point of the engine is exactly 10.5 inches ahead of the front face of the firewall (our datum) so (-)10.5 inches for the moment.

[CraigCantwell]

What happens with the numbers when you add the prop in?

[Marlhalbrook]

What happens with the numbers when you add the prop in?

Very small movement forward (of the aircraft empty CG-----about 0.2 inch, if I remember correctly.)

Have crunched numbers to the point that I'm blue in the face.....have decided to move the engine forward about 1.5 inches more than is now. That will give just enough clearance to mount a starter on back of Diehl case with a small clearance hole in firewall. (Will relocate the "standard" weight saving cut-out in the ply firewall to accommodate. And use a stainless bowl to provide firewall extension into the hole...).

Am currently building a jig to weld up a mount---located the mount points on the firewall in the extreme corners of the firewall to get the loads off the ply and transfer them more directly to the longerons. Have seen pics of several VP-1s, mostly from England, that have done the same.

Thinking very seriously about a simple "dummy-engine" to mount in the mounts and load it down to simulate g-loading the mount and firewall---not to destruction, but to a reasonable limit. How to do this and not destroy other structure will be my next 'research' project.

Oh--BTW----moving the engine forward 1.5 inches more---adding the prop and starter--and a 12 lb "cart" battery on the firewall moves the CG forward enough that with the lightest and heaviest pilot that we could expect (and that stays below the gross weight), from full fuel to empty, the CG stays in an acceptable range in all cases. Now, if the numbers work that well when we get it actually done and weigh it again, I will be tickled!