We occasionally get inquiries from modelers who are concerned that their engine will not fit on the VMAR engine mounts supplied with VMAR ARF's. Although there is no way to guarantee that the VMAR engine mounts will accomodate every engine on the market we are quite careful to make sure that VMAX and most other popular engines fit on the mounts, provided that the engine is of the recommended size. Discussions with consumers reveals that some modellers occasionally misunderstand the way the engine mount is to be used or do unnecessary work (drilling) when installing their engines.

Resolution:

In Brief: The VMAR clamp style engine mounts do not have to be drilled for most engines. The engine lugs fit between the engine mount beams and the clamping plates. Generally the instruction manuals illustrate how to use the clamping plates.

In Detail: No drilling is normally required to use the VMAR clamp style engine mount. Remove the four black machine bolts and clamping plates from the engine mount beams. Place the engine on the engine mount beams and reinstall the clamping plates and bolts so that the clamping plates are above the engine lugs and the bolts are placed such that one bolt is ahead of each engine lug and one bolt is aft of each engine lug. Align the engine to be pointing straight ahead or slightly to the right and tighten the clamping bolts firmly to hold the engine in place.

Generally the clamping plates have three bolt holts to allow for two different orientations and different fore and aft locating of the engines. Four Stroke engines should be set further foreward if they are too long aft of the engine lugs. In the event that an engine does not fit on the beams with the clamps and bolts in place, the bolts should be removed and the engine mount drilled or drilled and tapped for bolts to pass through the engine and the lugs in the traditional manner.

Some engines such as the OS LA series have narrower cases than others and may be pose difficulties when mounting.  If modelers are determined to install an OS LA they may have to remove the engine mounts and respace them closer together. The engine mounts can be removed by undoing the mounting bolts from the back of the beams i.e. inside the fuel tank compartment slightly aft of the beams or, depending on the model, when blind nuts have been installed behind the firewall the bolts can be loosened from the front.  Alternatively, modellers may wish to leave the engine mounts in place per the factory location and use a plywood adapter plate to accomodate the OS LA. Use 1/8" or thicker plywood. To make a plywood adapter plate, remove the four machine black machine screws and clamping plates from the engine mounts. Make the adapter large enough to completely cover the flat area of engine beams... from left to right and from the beam forward tip to the aft end near the firewall ensuring that the adapter plate is sized to sit flat on the engine beams. Mount the plywood plate to the engine beams with the four screws and washers. Do not reuse the metal engine clamps. Now position your engine on top of the plywood plate, center it and mark the cutout area you will require in the plate center area so that your engine lugs can sit down upon and be supported by the plywood. Cut away the engine clearance area. Center and mount the engine using machine bolts/nuts/washers... the bolts should pass through the engine lug holes and the plywood adapter plate.

Some engines have wider crankcases or in cases where a larger engine is being installed and the beams are not wide enough apart, they beams can be removed and reinstalled with a wider stance OR filing the clamping plates to accomodate the case width may help where only a small additional bit of clearance is required.

Question: How do I position my engine on the engine mounts and on the firewall when I intend on installing the engine at an angle or inverted rather than upright?

Short Answer: Regardless of orientation, the center line of the crankshaft must be positioned over the intersection of the vertical and horizontal thrust lines for your particular model. There are a few exceptions as described below.

Better Answer: Some models pretty much force you to install your engine in an upright orientation. Other models, particularly once you move up from basic trainers, offer you a variety of engine orientation options. Rotated 90 degrees is one such choice that often works well with Pitts style mufflers...  inverted (sometimes called 180 degree rotation) is another choice. Generally these non-upright orientations, hide more of the engine and muffler and result in a more realistic scale like appearance to the model.

The choice of what angle to use is pretty much up to you, limited by the physical size of the engine, muffler, cowl etc. Some engines for example can be oriented at 90 or 135 degrees but run into a physical problem at other angles... they hit something, the muffler won't clear the fuselage, the needle valve is at an awkward location etc. When selecting an angle to mount your engine, you have to balance off appearance, convenience, practicality and most of all make sure it can physically fit. Don't forget to test your planned installation angle with the muffler and carb installed. Also check out your engine manual for any specific guidance from the engine manufacturer. Some manufacturers don't recommend an exactly inverted 180 degree installation. We have found this to be good advice in many cases... 180 degree inversions have a tendency to smother the glow plug in oil and residue if run at low RPM for too long.

Once you've decided on the installation orientation that you want, the actual position of the engine on the firewall is no different with a rotated engine than it is with an upright engine. The engine thrust is always centered around the crankshaft and regardless of orientation, the center line of the crankshaft must be positioned over the intersection of the vertical and horizontal thrust lines for your particular model. The vertical thrust line lies up and down through the middle of the firewall. The horizontal thrust line is often drawn on the firewall or is to be draw on the firewall by you after referencing location information in the model instruction manual.  

There are some exceptions but the exceptions are few and far between... if the mounting face of the engine mounting lugs is offset from the center line of the crankshaft you have to adjust accordingly. This is rare, we have not seen this in standard two stroke engines. Engine manufacturers avoid this if at all possible. If you are working with a large gas engine adapted from another industry or a multi piston engine, examine the mounting system carefully and make sure you understand the relation between the engine mounting lugs and the center line of the crankshaft.

Question: Which engine mounts are supposed to come with my model... Aluminum or Plastic type mounts?

Answer: Either Plastic or Aluminum mounts could be supplied depending on availability when your ARF was produced and packed at the factory.

Better Answer: The Plastic and Aluminum mounts are interchangeable. Both use the heavy duty metal clamping bars and machine bolts and nuts... only the base material of the T-Beam itself is different. Depending on availability from our suppliers we supply either type. Given increases in availability we are planning on moving all production to the plastic mount by the end of 2005 unless the model has a specific need for the aluminum mount.

Question: Is it important that the two T-Beam Engine Mounts be exactly the same in appearance, smoothness and/or surface shape or size?

Answer: No. It does not matter.

Better Answer: No. It does not matter. There are several molds that make the T-Beam Engine Mounts and variations in appearance, smoothness, surface shape and/or size can be found between parts made in the different molds. These are a clamp style mount and the top surface of the T-Beam that lies against the engine lugs is flat. The sides and other appearance attributes of the mount do not affect its performance or suitability.

Question: I have a VMAR ARF model with an Engine Mount with no clamps. I bought my model in the summer or fall of 2004. Why does my engine mount not have clamps and how do I use it with my engine?

Answer: In response to some feedback from modellers we produced a limited number of models in the summer and fall of 2004 with no clamps to gauge public acceptance in advance of our 2005 production schedule. A set of four motor mount bolts, nuts and washers or 4 self tapping screws were included with each such ARF in lieu of the clamps. Position your engine so that the engine lug forward edge is about 15/16" back from the forward end of the engine mount.

If your model came with bolts, mark and then drill the mount with a 5/32" bit and bolt the engine directly to the engine mounts using the 4 bolts, nuts and washers provided.

• We have produced an illustrated Important Information sheet of supplementary instructions for the Stinger and loaded this information on the Stinger portion of our web site. Direct access to this PDF document can be obtained by clicking on http://www.richmondrc.com/vma/common/techinfocommon/s240x/documents/S240X_Important_Info_20041022_11x17.pdf
• Although this document relates directly to the Stinger, it is applicable to other VMAR models using bolts and nuts with the composite engine mount without clamps.

If your model came with self tapping screws, drill the mount with a 1/16" drill bit to make pilot holes and then expand the holes to about 2/3 of the diameter of the screws provided. Do not over-enlarge the holes.

Question: I have already purchased a .40 to .52 size engine. How do I know if my engine will fit properly between the engine mounts that are installed in many VMAR ARF's?

Answer: Most popular .40 to .52 size two stroke engine cases will fit between the engine mounts that are pre-installed in many VMAR ARF's. Most popular two stroke engines of this size have similar case dimensions. For example the VMAX, Thunder Tiger and OS .40-.52 size engines are very similar in size. The OS 40LA and 46LA have non-standard (too narrow) cases and pose difficult installation problems. We do not recommend the use of these engines unless you are able to modify either the engine mount T-beam spacing by removing and reinstalling the engine mounts OR can build and install a shim set (2 shims) to narrow the gap between the pre-installed engine T-Beams OR can cut a 1/8-3/32" plywood mounting plate and install it on top of the T-beams after cutting a horseshoe U-shaped recess for the engine case. 

Question: Can I finish my firewall to enhance it's appearance and seal it against oil and fuel?

Answer: Yes.

Better Answer: The firewalls are made from either 6mm plywood or multiple layers of thinner 2-3mm plywood laminated together. They are more than strong enough for any modelling application. They will withstand oil and fuel without further treatment or finishing. If you wish to seal the edges and faces that is also fine. When finishing, we suggest checking the edges for any voids, fill them with CA or Epoxy and then seal the edges and faces with finishing resin or epoxy. Let dry and then install your engine mounts and tank.

Question: I want to make a new firewall and install a VMAX Engine using the standard VMA-MOUNTUNP componsite Engine Mount. Do you have any Templates that I could use a cutting and drilling guides?

Answer: Yes, please see the attached PDF files below. These are a bit rough and ready but they should do the trick.

Question: I am installing my engine and I'd like to shift the thrust line. Is this OK?

Answer: Well... it depends! The factory location for the vertical and horizontal thrust lines is fairly important and the best option is to not depart from the factory specs regarding thrust lines. However, we recognize that some engines and some engine orientations may look better aesthetically if they are shifted somewhat from the factory thrust line locations.

Our advice in this regard is as follows:

1) Do not shift the left and right location of the engine from the location of the vertical thrust line recommended for the model. i.e. keep the engine centered on the firewall with respect to left and right. If you need a bit of left or right thrust, use the power module nut and bolt spacing system or the engine mounting angle to change the thrust angle but do not offset the engine itself to the left or right.

2) You can shift the engine slightly higher or lower on the firewall than indicated by the location of the horizontal thrust line on the firewall. Don't go crazy... a centimeter maybe is a good maximum guideline depending on the model and how you plan on using it. The more you depart from the recommended location the more risk you are taking that this will adversely and significantly affect the flying characteristics of the model. Again, if all you want to do is induce some up or down thrust, use the the power module nut and bolt spacing system or the engine mounting angle to change the thrust angle rather than offset the entire engine up or down.

To understand what is happening when you shift the thrust lines by moving the engine up or down, left or right on the firewall.... it helps to take things to an extreme in your head. Consider nailing a 48" long 2x4 plank to your firewall so that the plank is pointing up and down and centered over the firewall so that 24" is above the center of the firewall and 24" is below the center of the firewall.

Now imagine mounting your engine to the front of the top end of the 2x4 and think what would happen when you rev up the propeller. The engine thrust would pull the top of the 2x4 forward and torque (twist) the nose of the model downwards. So applying power would result in the model pointing down... not really what you expected but that's what will happen! This will require aggressive amounts of up elevator to correct and the degree of up elevator required will constantly change not only with the engine thrust per RPM and prop selection but with the degree of effectiveness of the elevator depending on the air speed of the model. So every time you change the tprop or the throttle setting (RPM) of the motor or the models air speed you will need to adjust the elevator trim. Talk about a pain in the butt!

Now let's go the other way... imagine mounting your engine to the bottom end of the 2x4 and think would would happen when you rev up the propeller. The engine thrust would pull the bottom of the 2x4 forward and torque (twist) the nose upwards... i.e. applying power will result in the model pointing up. This will in turn require lots of down elevator to compensate for and the amount of down elevator needed will constantly change depending upon engine thrust and the effectiveness of any down elevator setting will in turn vary with the air speed of the model. Every time you change throttle setting of the motor or the prop or the air speed of the model, you will need to adjust the elevator trim. Big time headache!

So taking our imaginary extreme analogy and putting it into context of a typical model without a big honking 2x4 nailed to the front of the firewall... here are the results of moving the engine up or down from the recommended thrust line location:

a) Moving the motor below the recommended thrust line will pitch up the nose somewhat and require varying amounts of down elevator trim depending upon RPM, prop selection and air speed of the model.

b)  Moving the motor above the recommended thrust line will pitch down the nose somewhat and require varying amounts of up elevator trim depending upon RPM, prop selection and air speed of the model. 

c) The pitch up and pitch down forces will increase as you move the engine further from the recommended thrust line.

d) The further you shift the thrust line the more "snaky" and jumpy the model will tend to be as you vary throttle settings and the more difficult it will be to fly in a controlled and predictable manner.

Question: I am installing my engine. Where are the thrust lines on the firewall?

Answer: The horizontal thrust line normally appears on the forward face of the firewall or you draw the horizontal thrust line on the forward face of the firewall yourself following location instructions contained in the documentation that came with your model.

If there is no horizontal line indicated on the firewall and you are unable to locate the location of the line in the reference material there is an easy way to position the horiztonal thrust line youself. To do this, fit your cowl over the fuselage. Look at the cowl from the left and right sides and adjust the vertical angle of the cowl to align with the fuselage. There should not be any sharp drop or rise in the silhouette profile of the fuselage as you move your eye along the fuselage and over the cowl. Once you have the cowl in place, look through the front hole where the crankshaft is going to protrude. Mark the horizontal thrust line on the firewall so that it is evenly located up and down with respect to the hole in the front of the cowl.

The vertical thrust line lies up and down through the middle of the firewall. If it is not shown, you can simply draw it into place. Make sure it is in the middle.

You may also find this information on our web site. Look for the links to your model, then the link to the detailed information and browse down to the engine section. The position of the horizontal and/or vertical thrust line may be stated there.

Question: What is a Firewall? 

Answer: The firewall is the flat plate or wall like component that the engine mounts are attached to. All propeller driven model airplanes have a firewall. It is located near the front of the model just behind the engine.

Better Answer: The term firewall comes from the first use of internal combustion engines in cars and aircraft and dates back to even earlier usage in the 1800's. If we take the word apart into its elements it is exactly what is says it is... a Fire Wall.

Internal combustion engines like model airplane engines... burn fuel. They get hot, very hot. This heat is ok for the engine but is not generally a good thing for the rest of the aircraft. The Fire Wall separates the hot, heavy engine from the rest of the aircraft.

In most propellor driven aircraft the firewall is just aft of the engine and is a wall that pretty much caps the forward end of the fuselage. The engine mounts are attached to the forward face of the firewall and the engine in turn is attached to the engine mounts.

In VMAR models the firewall is generally a piece of plywood that looks like a small flat plate. The firewall may be round or elliptical, or be squarish in shape with rounded corners and measure 3 to 8+ inches across. It is usually about 6mm (1/4") thick.

Question: Can I use a 4 Stroke engine on my VMAR ARF model?

Answer: In most cases, Yes.

Better Answer: There are some things to consider here. First of all if the model has a cowl it will usually have a power module consisting of a removeable forward firewall and a set of engine mounts. These engine mounts can be oriented in many different ways so you can select the orientation and separation of the mounts to suit your engine. It is also very easy to work on with this setup because the forward firewall is completely removeable from the model. So... VMAR models with a cowl and power module probably will be able to accomodate a 4 stroke engine. Conversely... models without a cowl and power module usually have a factory installed engine mount and a fixed firewall only and may be more problematic when trying to fit them out with a 4 stroke engine. It is safe to say that if the model does not have a cowl and power module there may be significant work and mucking around in order to install a 4 stroke engine.

4 stroke engines are not as standardized as most 2 stroke engines... some have the carburetors at the front, some at the back. Some have valve pushrods running up the front of the crankcase... some up the back. As a consequence, 4 stroke engines vary quite a bit in length (i.e. the distance from the prop shaft to the back of the engine) within a particular size. Although for example a 2 stroke .46 from one supplier will be very similar in external size to that of a 2 stroke .46 from another supplier... it is not necessarily the case when comparing 4 strokes. A .52 four stroke from one manufacturer can be quite different from a .52 four stroke from another manufacturer. There is no way we can be aware of all the different sizes and shapes of four stroke engines in the market and modelers should be checking with the engine manufacturer for sizes before assuming that the engine will fit into any particular model. Once you know the length of the four stroke engine that you are considering, place the cowl on your model. Then measure from where the back plate of the spinner will be to the front face of the removeable power module firewall. Subtract about 1.25" from this distance and then compare the result with the length of your four stroke engine. Engines that exceed this length will significantly protrude from the front of the cowl and should be trial fitted carefully before purchase.

In some cases, we are pre-cutting cowls to make installation of our recommended 2 stroke engine go as quickly and easily as possible. We do stock blank (not cut) cowls for those who wish to use a differnet 2 stroke engine size (for example a .61 instead of a .46) or a 4 stroke instead of a 2 stroke or want to go with an inverted installation or other orientation when we may have assumed a 90 orientation when pre-cutting the factory cowl.

Lastly, we have some models that do not use a cowl per-se and do not have a power module but may look like they do. For example the F4U Phantom and Skyhawk and Arrow Tiger have a shaped and fitted nose section but not a cowl and we have not recommended 4 stroke engines for these models at all. Again, if we do not use a cowl and power module on a model... we may be significant work and mucking around in order to install a 4 stroke engine.

Please note that to this point this article makes no attempt to explain the differences between the different types of engine systems. For some of the pro's and con's of 2 stroke and 4 stroke engines please the article included below:

Question: What gives with Model Airplanes Engines? What's a 2 stroke? What's a 4 Stroke? What do you mean by Glow as compared to Gas? Now there are electric motors being adapted to replace engines... how the heck do I choose what is right for my model?

Answer: Well, answers to this can take a bit of explaining. Here is an overview.

2 Stroke (sometimes called 2 cycle)

• Is what the majority of current modelers mean when they say "Model Airplane" or "Glow" Engine
• Noisy screaming type of sound that must be carefully muffled and never sounds like the real thing.
• Uses Model Airplane Fuel (alcohol, oil, nitromethane and additives)
• Uses a Glow Plug hence the term "Glow" Engine.
• Generally the cheapest, lightest, most powerful engine for the money.
• Referred to by displacment in hundreds of cubic inches (or in cubic centimeters = cc)
• Imperial measure is cubic inches and a slang has developed
  • .40 cubic inches is referred to as "a 40"
  • .90 cubic inches is referred to as "a 90"
  • 1.20 cubic inches is referred to as "a 120"
• Generally range in size from .049 to 1.80 although some smaller and bigger engines exist.

4 Stroke (sometimes called 4 cycle)

• Has become more popular since the 90's with those wanting quieter more realistic engine sound for use in semi-scale or scale models.
• Quieter more familiar engine sound when in the air. Much more realistic for scale flying models. Sounds much more like the real thing.
• Uses Model Airplane Fuel (alcohol, oil, nitromethane and additives) although special blends with more or less oil and more or less nitro may be required for some engines.
• Uses a Glow Plug hence the term "Glow" Engine can be used for either 2 stroke or 4 stroke engines
• Generally costs more, weighs more and gives less power for more money than a 2 stroke engine of similar size.
• Referred to by displacment in hundreds of cubic inches (or in cubic centimeters = cc)
• Imperial measure is cubic inches and a slang has developed
  • .52 cubic inches is referred to as "a 52"
  • .91 cubic inches is referred to as "a 91"
• Generally range in size from .25 to 2.50 although some smaller and bigger engines exist.
• To get the same power as a 2 stroke, add on about 30-50% in size. This is a very rough rule and does not consider weight and torque issues but it is a good guide.
  • A model calling for a 2 stroke .46 would require roughly a 4 stroke of size .60-.70
  • A model calling for a 2 stroke .61 would require roughly a 4 stroke of size .80-.90

Gas (sometimes called Ignition Engine)

• Has become more popular for use with big models.
• Quieter more familiar engine sound when in the air. Much more realistic for scale flying models. Sounds like the real thing  because it pretty well is the real thing!
• Uses Gasoline. Special oil or additives may be added to the gas in some cases. Gas is 25-50% cheaper than Model Airplane Fuel.
• Uses a Spark Plug hence the term "Ignition".
• Often adapted by the manufacturer from some other purpose such as a weed eater, leave blower etc. Although the engines end up quite different with regards to fuel supply, ignition, prop adapter and flywheel etc, they usually started with an existing product and adapted from there.
• Seldom used on smaller models and nearly always used on big models due to the higher weight of the engine and ignition system.
• Generally costs more, weighs more and gives less power for more money that a 2 stroke engine of similar size. May cost and weigh more than a 4 cycle Glow engine.
• Referred to by displacement in cubic centimeters (cc) (or in cubic inches but almost universally now in cc)
• Metric measure is cubic centimeters (cc) and the cc is nearly always noted in referring to these engines
  • for example "30 cc"
• Generally range in size from 20cc to about 65cc although some smaller and considerable numbers of bigger engines exist.

Electric Motors (both brush type and brushless type)

• Have become more popular since 2000. Increasing in popularity
• Just as a combustion  Engine needs and is succesful depending on the type of fuel it burns, an electric Motor needs batteries.
• As battery technology put more and more power (run time) into batteries of a certain weight (we call this energy density) electric powered flight is starting to take off (literally)
• Generally brushless motors are taking over. They can be very expensive when coupled to a suitable electronic speed control and a good set of NiMh or LiPo batteries.
• Main advantage is quieter and no mess. No fuel to buy.
• Main disadvantage is price and flight time per charge.
• They will continue to improve and be more popular in time.
• The rating system is not yet standardized so it is hard to compare apples with applies. Generally if a model calls for a .40 two stroke engine, modelers need to select an electric motor that can provide similar power in order to have similar flight characteristics. This can vary widely by motor type (brushless is better), battery type, speed control and wiring.

Please note that to this point this article makes no attempt to advise on if a particular model can be adapted to accomodate a different type of power system. For some considerations related to fitting 4 stroke engines into a model please search on "Four Stroke" for further assistance.