Engine Mount Modification

[J. C. Wheeler]

Having a Stits Playboy and a VW engine with tube engine mount that does not match the mounting points on the Playboy, I need to modify the mount.

The firewall mount points measures 22 3/4" wide and 18 1/2" high while the mount I got with the engine measures 18 1/2" wide and 13 1/2" high. Now my questions, without a buying/purchasing a new mount;

1. Can I (safely) build the firewall using 3/4" marine plywood attaching it to the mount locations on the fuselage and then mount the smaller engine mount to the firewall (after of course facing the firewall with stainless) using large washers on each side of the firewall similar to the way the VW engine is mounted to the VP-1?

2. Can I simply bring say a 1/4" flat steel plate from the smaller mount points to reach out to the fuselage mount points and weld and gusset them to the smaller mount?

Hope that is not to confusing, I know what I want to express, just not sure I have done that correctly. Open to your thoughts, suggestions and ideas.

Thanks, J.C.

[Kevin Horton]

This discussion probably belongs in the Aircaft Design forum, but we'll keep going here unless Sean moves it.

One thing to keep in mind - if the engine comes off the aircraft in flight, the CG will move very far aft. This usually ends up with a loss of control and a fatal accident, unless you have an aft mounted engine (i.e. not a Playboy), or a very light engine and a very far forward CG. So, whatever mod you make, understand that you are trusting your life to it. Do a proper job, don't just shoot for the cheapest mod that is marginally acceptable. How much is your life worth?

I'm not very enthusiastic about either of your proposed mods. The engine mount takes a lot of force, especially if you ever have a hard landing. The thrust forces are probably the least of your worries. A plywood mod could easily take some damage that wasn't detected, but could lead to an in-flight failure later. A steel mod is a better bet, in my opinion, but it is hard to design one via a web forum.

How much will the installed engine, prop, spinner etc. weigh? How far forward of the firewall will the CG of the engine and prop be? How many g is the rest of the structure designed for, and what in-flight g limit will you use? What does the landing gear have to absorb shocks when landing? The answers to these questions will give us an idea of how much force the mount must be able to withstand.

If it was my aircraft, I would get a very competent welder to either cut off the existing legs on the mount and weld on new ones, or make a new mount.

Do you know how far forward the engine needs to be mounted to make the CG come out in the right place? That is important too. If you haven't got that issue sorted out, post again and we can discuss how to approach solving it.

[Sean_Caranna]

Created a new Firewall Forward forum and moved this topic here.

[J. C. Wheeler]

Good Morning Kevin,

Thanks for the reply and your interest. In trying to answer some of your questions, I will do my best but I am in no way an engineer nor expert.

What I have is a Stits Playboy in which the fuselage and tail components are Chrom Moly steel tubing, very nice welding I might add. The wings are equally well constructed of Spruce with the main spar being 1 1/4 inch thick and, if I remember correctly 5 1/4 inch wide. The rear spar is a full 1 inch thick and I believe 4 inches wide with all ribs well designed, braced and constructed. I do not know what g force it is designed but is classified as I understand it as aerobatic. I believe the aircraft was designed for a pretty substantial 'G' load rating. The wingspan is 282 inches or 23 1/2 feet and a wing cord of 50 inches.

The landing gear is a one piece steel formation designed to flex a couple of inches either way. On this will be mounted the wheels and tires (600x6).

The aircraft I believe was designed with an empty weight of 685 lb. and gross weight of 960 lb. I may be wrong, but I can't believe the aircraft will weigh that much. It was designed for a 100 HP engine with a cruise speed of 130 and max speed of 150 mph.

I want to fly this aircraft in the new Sport Pilot Class which has a weight limit of 1320 lbs and a max speed of 138 mph I believe it is. To get this aircraft down to the speed requirement I want to use a New "0" time, 1835 cc VW engine with a Ray Hegy 54x30 prop which I also have 'New'. I have been told this engine/prop combination would work well on the VP-1 aircraft, which was what I originally set out to build until I ran into the Stits project and purchased it.

I do not have the project or parts here at the house but I believe the engine and prop with accessories will weigh in at around 150 lb. tops which I believe will be considerably less that the 100 hp setup. The engine mount will allow the engine to sit about 8 inches forward of the firewall and the prop will be some 4 inches forward via a casting mounted to the front end of the engine case, 'with bearing in the nose' to allow for more streamlined cowling.

My thoughts are to use the engine mount I have, (which again, measures just 2 to 2 1/2 inches from reaching each and every one of the 4 mount points in the airframe), by first securing say a 1/4 inch thick steel plate to the engine mount using both a bolt as if it were bolted directly to the airframe, then weld this in place on each of the 4 corners of the engine mount itself with gussets for strength, also leaving the bolt in place. Then using the 4 such steel plate tabs drilled to match the airframe attach points and bolt them to the original designed points on the airframe. This would not extend the engine any further forward to any extent.

I have had an A&P look at what I have and he says the aircraft is very well constructed and quality workmanship. The CG, I will have to work on to determine just how/where it is. The fellow in the hanger next to me has a set of scales and I hope to enlist him and his equipment when I get to a point where I can reasonably determing the CG. I believe I can juggle the fuel tank and battery location to help with the CG.

I am coming up on 67 years old and have no plans nor intentions of doing any aerobatics or even tight maneuvering. Likewise, not interested in speed, I just want to get up and fly around in the peace and quite and see some country side from the air. I flew as a Flight Engineer during my Air Force Career (retired almost 30 years ago) and want to get back up there.

I hope what I have said here makes sense to you and I thank you again for your reply and your interest in helping me in this matter.

Take Care, J.C.

[J. C. Wheeler]

While I still need to do something with my engine mount, I just got word from Mr. Ray Stits after he looked at a series of photos of my aircraft, saying it is definately NOT, a Stits Playboy.

So, on that point, I am again back to Square One on trying to identify my aircraft. Regardless of what it is, the Engine Mount still needs attention.

My communications with Mr. Stits was relayed via a Third Party so please do not ask for his email address, I do not have it, nor will I divulge the name of my third party contact as I consider what I received a nice gesture, hope you can appreciate my consideration. I understand that Mr. Stits no longer provides technical assistance for any of his aircraft. It was only thru the Third Party that I was able to get his opinion on my aircraft.

Thanks, J.C. Wheeler

[Sean_Caranna]

Try posting some pics of it here in the aircraft design forum. Perhaps someone here can help ident it.

[Kevin Horton]

J.C.,

Please excuse the slow reply. I wanted to do some thinking before replying, and yesterday was a crazy day.

There are a number of things to consider here:

Is the VW a suitable aviation powerplant? I have read concerns about cylinder head cooling problems if you run at high power for too long. If true, this means that the VW might be OK if you had an aircraft that allowed you to run at low power except for take-off, but you might have an increased risk of engine failure if you had to run at high power most of the time. You said the aircraft was originally designed for a 100 hp engine. If so, you might need to run that VW pretty hard to get acceptable performance. I’m no VW expert, so I won’t dive in too deep here - I’ll leave this issue to those who know VWs better than I do.

Will a lower hp powerplant deliver satisfactory aircraft performance? Take a look at other aircraft that use the VW engine. How does your aircraft compare in terms of weight and wing area?

Will the CG be in the right place? Originally you thought you had a Playboy, which was a successful design. We could have looked at the engine weight of typical Playboy engines, and figured out how far forward of the firewall the VW needs to be to get a similar CG. But now we don’t know what aircraft it is. It might even be a one-of design. All I can say is to be very careful to do a proper weight and balance, and to do whatever it takes to get the CG in a suitable place. If you’ve got an aircraft of conventional design, with no wing sweep, you should probably shoot for a CG of somewhere between 20% and 25% mean aerodynamic chord for the first flight. The aircraft might be perfectly acceptable at CGs further aft than this, but you want to work back there a little bit at a time, not start there on flight one.

Strength of the engine mount mod - You said that you figure the weight of the engine and prop at 150 lb. I’ll call this value W. The engine mount you have is 13.5� high at the firewall. I’ll call this value H. We need to know how far forward of the firewall the CG of the engine and prop will be. We don’t have this value, but we’ll label it D. Lastly, we need a design g loading. You don’t want to do any aerobatics or tight manoeuvring. Normal category FAR 23 type certificated aircraft are designed to take 3.8 g, with a 1.5 factor of safety, for an ultimate g loading of 5.7. We’ll call this one N.

Looking at the in-flight loads case at high speed, with zero angle of attack. If pull some g, we have a load down on the engine/prop combination of W x N. If we consider moments around where the engine mount lower firewall point is, we have a moment of W x N x D. This moment must be reacted by the upper firewall mounting point, which means the upper engine mount attachments are being pulled away from the firewall with a force of (W x N x D)/H. We’ve got two mounting points, so the load on each one is half that, or (W x N x D)/(2 x H). Just to get an idea of the forces involved, lets say that the CG of the engine/prop combination was 24 inches ahead of the firewall (value D). We get a force of (150 x 5.7 x 24)/(2 * 13.5) = 760 lb pulling each upper engine mounting point away from the firewall. The lower engine mounting points would be pushed into the firewall with a similar force.

We also need to consider the straight downwards force created by the g on the engine. If we assume that the four engine mount points share this load (this is probably not a good assumption, mind you), we have a force of (W x N)/4 = 214 lb straight down on each firewall point. The 760 lb load and the 214 lb load are at right angles to each other, so the total force is the square root of (760 squared + 214 squared), or 790 lb. If we assumed that the whole vertical load was taken by the upper mounts, as a worst case assumption, that gives us a vertical load of 428 lb per upper mounting point, and a total load of about 870 lb on each upper mount. I’m not trying to design an engine mount for you - I’m just shooting these numbers at you so you have some idea of how much force the mount should be able to withstand.

In some cases the landing loads in a hard landing might be even worse than the flight loads. We don’t have enough info to know how well your landing gear will absorb hard landings. But I am going to assume that you won’t deliberately fly off rough strips, and that you will do a very good inspection of the whole aircraft, including engine mount, if you ever have a hard landing.

There is one big caveat about all this - although I graduated as a mechanical engineer over 20 years ago, I went straight into pilot training, and never did any mechanical design work. So, it is quite possible that I have screwed up these calculations, or have missed some other important aspect.

[J. C. Wheeler]

Good Morning Kevin,

Thanks for you valued input, you covered the problem in more depth than I had given it. While I an no mathmatician, I can understand how you determined your answers.

Sorry I did not get back with you sooner, but my wife took a fall Sunday night about 10:00 PM and yesterday morning she was in much pain thinking she may have a few broken ribs. So, I spend yesterday morning at the Emergency Room with her and the rest of the day taking care of her. I am with my first cup of coffee this morning and she is still sleeping so though this a good time to reply.

I have now found out that what I have is not a Stits Playboy and have posted another post under the construction forum looking for help there. I am also looking at the possibility of a little larger engine too.

I want to say thanks for you advice and may contact you again if I have further such questions.

Take Care, J.C. Wheeler Clarksville, Arkansas